Petrogenesis of Early Neogene Magmatism in the Northern Puna; Implications for Magma Genesis and...

JOURNAL OF PETROLOGY VOLUME 43 NUMBER 5 PAGES 907–942 2002

Petrogenesis of Early Neogene Magmatism inthe Northern Puna; Implications for MagmaGenesis and Crustal Processes in the CentralAndean Plateau

PABLO J. CAFFE1∗, ROBERT B. TRUMBULL2, BEATRIZ L. COIRA1

AND ROLF L. ROMER2

1INSTITUTO DE GEOLOGIA Y MINERIA, UNIVERSIDAD NACIONAL DE JUJUY-CONICET, AVDA. BOLIVIA 1661,

SAN SALVADOR DE JUJUY, ARGENTINA2GEOFORSCHUNGSZENTRUM-POTSDAM, TELEGRAFENBERG 14473, POTSDAM, GERMANY

RECEIVED JANUARY 22, 2001; REVISED TYPESCRIPT ACCEPTED NOVEMBER 30, 2001

of UOLM-2 and MM group rocks. Assuming the same end-New compositional data and petrogenetic models are presented formember compositions, the modelling suggests genesis of the MMpre-Upper Miocene volcanism in the northern Puna of Argentinamagmas at higher pressure than the UOLM-2 centres ([10 kbar(22°S–24°S). Two phases of volcanism produced small domevs 7 kbar), which may reflect the influence of crustal thickening incomplexes of mainly silicic andesite to low-SiO2 rhyolite. Thethe plateau region by the mid-Miocene. The felsic dome complexesUpper Oligocene–Lower Miocene phase (UOLM, 20–17 Ma),of this study are compositionally similar to the large-volume, caldera-produced two distinct groups of rocks. The UOLM-1 group issourced felsic ignimbrites that dominated volcanism in the regionmetaluminous and mainly andesitic, with isotopic compositions likefrom 10 to 2 Ma and our results suggest that there is no fundamentalthose of the recent arc ( 87Sr/86SrT >0·706; �NdT −3). Thedifference in magma genesis between them. The differences in theUOLM-2 group is more silicic and peraluminous, and has isotopicvolumes and the mode of eruption reflect changes in the stress andcompositions indicating a substantial crustal contribution ( 87Sr/thermal regime with time.86SrT >0·713; �NdT −8). The Mid-Miocene phase (MM:

15–12 Ma) produced rocks similar in composition to those of theUOLM-2 group ( 87Sr/86SrT >0·710; �NdT −7) but withhigher incompatible element contents. Ratios of Ba/Nb and Zr/

KEY WORDS: Cenozoic volcanism; Central Andes–Puna Plateau; crustalNb in the UOLM group rocks are uniform and similar to those ofassimilation; dacitic magmas; geochemical modellingthe current arc, whereas the ratios in MM centres show a mixed

arc and back-arc affinity. This suggests that the westward shift inthe arc began in the northern Puna in the mid-Miocene. Neitherthe exposed Palaeozoic felsic basement nor the lower-crustal gran-

INTRODUCTIONulites known from xenolith suites are compositionally suitable asprotoliths for the UOLM and MM magmas. The preferred petro- Cenozoic magmas erupted in the high plateau regiongenetic model for the magmas involves hybridization of a depleted (Puna–Altiplano) of the central Andes (Fig. 1) have beenarc basalt with partial melts of the felsic basement. Geochemical the subject of intense investigation since the late 1980s.modelling and thermal arguments rule out magma mixing as the The focus has primarily been on the Upper Miocene toprocess of hybridization. Successful assimilation–fractional crys- Recent magmatism, which includes large-volume cal-tallization (AFC) solutions indicate an increase in crustal as- dera-sourced ignimbrites (Schneider, 1985; de Silva,

1989a; Francis et al., 1989; Ort et al., 1996; Lindsay etsimilation from 15–25% in UOLM-1, to 40–60% in the case

∗Corresponding author. Telephone: 54 388 4221593. Fax: 54 3884232957. E-mail: [email protected] Oxford University Press 2002

JOURNAL OF PETROLOGY VOLUME 43 NUMBER 5 MAY 2002

crustal thickening (up to >60 km; Yuan et al., 2000)beneath the Puna region (Isacks, 1988; Sheffels, 1990;Allmendinger et al., 1997; Kley et al., 1999). It hasalso been suggested that the subduction angle steepenedduring this time, leading to a narrowing and westwardmigration of the volcanic front (Coira et al., 1993; Allmen-dinger et al., 1997, and references therein). The northernPuna Middle Miocene volcanic centres form the southernextension of the Bolivian Tin and Polymetallic Belt andsome of them have a close relationship with Pb–Zn–Ag± Sn ore deposits (Coira, 1994; Caffe & Coira, 1999).Therefore, a better understanding of the genesis anddevelopment of these centres is important for manybroader issues relating to geodynamic controls on Andeanmagma generation, and may have implications for therelationship between magmatism and ore associations.

We present new petrographic, mineralogical, geo-chemical, isotopic and geochronological data for UpperOligocene to Middle Miocene volcanic dome complexesand related igneous rocks from the northern Puna. Thedata are used to identify potential magma source com-ponents within the mantle and crust. We suggest thateven in the most crustal-like magmas the involvement ofmantle sources cannot be ruled out. Geochemical andisotopic modelling favor hybridization of arc magmas inthe lower crust, and are consistent with a broadly felsiccomposition of the lower crust suggested by Lucassen etal. (1999b).

TECTONIC SETTING ANDFig. 1. Regional map of the Andean Central Volcanic Zone showingthe location of pre-10 Ma volcanic centres (Β). The study area is REGIONAL GEOLOGYoutlined by the box. Grey circles are 20–17 Ma centres with arc-like

The northern Puna region of Argentina (22–24·5°S) isBa/Nb and Zr/Nb ratios. Open stars are 14–12 Ma magmas with arc-like signatures; filled stars indicate centres with back-arc affinity. The structurally separated from the southern Puna (25–28°S)El Toro lineament divides the northern and the southern Puna, and by the NW–SE-trending El Toro lineament, whereas itsis marked with a bold continuous line. Other NW–SE lineaments in

border with the Bolivian Altiplano to the north (>21·5°S)the southern Puna are represented as dashed lines.lacks any major structural expression. Volcanism in theperiod from 26 to 18 Ma shows a characteristic dis-

al., 2001), frontal-arc composite volcanoes (Davidson et tribution. North of 22°S in the Bolivian Altiplano volcanical., 1991; Feeley & Davidson, 1994; Matthews et al., centres are spread out across the arc and back-arc regions,1999), and small-volume back-arc or transverse mafic whereas south of 25°S (southernmost Puna) they arecentres (Kay et al., 1994a; Davidson & de Silva, 1995; confined to the arc front. This distribution and the overallRedwood & Rice, 1997). In comparison, the initial phases scarcity of volcanic rocks of this age in the 22–25° segmentof Andean volcanism in the Altiplano–Puna region (Late led to the suggestion of a spatial gap in magmatism inOligocene to Middle Miocene) have not been investigated the northern Puna between the Upper Oligocene andin detail, and have been considered mainly in papers of Lower Miocene (Coira et al., 1993; Allmendinger et al.,a regional or reconnaissance nature (Halls & Schneider, 1997). In contrast, Middle Miocene to Pliocene volcanic1988; Coira et al., 1993; Kay et al., 1994b; Trumbull et centres are spread out over the whole Argentine Punaal., 1999). (Fig. 1). On the basis of age determinations and trace

This study focuses on the Late Oligocene–Middle element geochemistry, Coira et al. (1993) and Kay et al.Miocene magmatism in the northern Puna region of (1999) suggested progressive shallowing of the angle ofArgentina (22–24·5°S). This period coincides with im- subduction from Oligocene to Recent times south ofportant episodes of compressive deformation in the Cent- 28°S, and steepening north of 25°S. This interpretation

was questioned by Kley et al. (1999), who concluded thatral Andes, which are thought to be the main reason for

908

CAFFE et al. EARLY NEOGENE MAGMATISM, NORTHERN PUNA

no major changes took place in the subduction regime with mineralization, being part of the Bolivian Tin orPolymetallic Belts (see Cunningham et al., 1991; Coira,before 10–7 Ma.1994). These complexes and some volcaniclastic unitsinterbedded in the Middle Miocene Tiomayo Formation(Coira et al., 2002) display only slight deformation. How-

Cenozoic volcanism ever, seismic reflection data from the Laguna de PozuelosCenozoic volcanic rocks in the Puna province (22–28°S) basin (Fig. 2) show syntectonic sedimentation structuresand southern Bolivia occur across a broad area underlain and thrusting and folding of mid-Miocene strata (Gangui,by Ordovician sedimentary and volcanic rocks (Acoite 1998), which suggests that there was intense compressiveFormation; Turner, 1960; and Faja Eruptiva de la Puna; strain in the northern Puna at this time.Coira et al., 1999, respectively), scarce Cretaceous to At >11–8 Ma, the Quechua tectonic phase (Table 1)Paleocene rocks (Salta Group; Salfity, 1982), and Eocene resulted in marked elevation of the northern Puna regionto Lower Miocene redbed deposits associated with the and development of the broad San Juan del Oro erosionalsedimentary fill of the first Andean foreland basins ( Jor- surface (Gubbels et al., 1993). The third volcanic phasedan & Alonso, 1987). Cenozoic volcanic phases identified (10–4 Ma) overlaps with the Quechua phase, and isin the Puna (Coira et al., 1993) are roughly coincident with dominated by undeformed, large-volume ignimbritesthose proposed for the Bolivian Altiplano and Eastern (Fig. 2, Table 1). As in the Altiplano, the end of theCordillera by Soler & Jimenez (1993), and comprise four Quechua phase (at 8 Ma) in the Puna is marked bymain periods: Upper Oligocene–Lower Miocene, Mid- changes in strain directions and deformational behaviourMiocene, Upper Miocene–Pliocene, and Pleistocene of the upper–middle crust, passing from a compressional(Table 1). Each of these volcanic phases coincided with regime into one that was dominated by strike-slip orthe end of a major tectonic phase. Thus, the oldest extensional faulting (Cladouhos et al., 1994).Cenozoic magmatic rocks (Upper Oligocene–late LowerMiocene) are interbedded with thick conglomeratic se-quences, indicating that they were erupted before or

DESCRIPTION OF THE UNITSduring the Pehuenche tectonic phase. This phase involvedthrusting of Ordovician metasediments onto Paleogene SAMPLEDredbeds. Thrusting ceased at>18 Ma in southern Bolivia First volcanic phase: Upper Oligocene to(Herail et al., 1994; Kley et al., 1996), whereas activity Lower Miocene (henceforth UOLM centres)continued to >16 Ma some 80 km to the south (Cla-

Casa Colorada dacite dome complex (22°19′S–66°20′W)douhos et al., 1994; Coira et al., 2002).The Casa Colorada dacite dome complex is a smallLike the deformation, the first phase of volcanism alsoextrusive magmatic centre (>1·5 km3) located west ofshows a diachronous distribution from north to south.Sierra de Rinconada at the intersection of north–south-Magmatism in the northern Puna may have started inand east–west-trending faults (Fig. 2). Three successivethe Upper Oligocene (>28 Ma) as in the Altiplano, butvolcanic events have been recognized (Caffe, 1996): de-it was mainly active during the late Lower Mioceneposition of a tuff ring; collapse of growing lava domes;(19–17 Ma). The age of these later eruptions overlapsand effusion of flow-banded, crystal-rich, dacitic lavas.somewhat with the second volcanic phase (Coira et al.,The 17·3 ± 0·7 Ma K–Ar age of the centre (Coira et1993; Allmendinger et al., 1997); however, because theal., 2002) indicates that the main dacite lava dome waseruptions occurred during the Pehuenche tectonic event,extruded contemporaneously with the nearby Cabreriawe ascribe the rocks to the first volcanic phase. TheFormation ignimbrites.eruptive volumes of this early magmatic phase were

similar to, or even higher than, those of the secondMinuyoc dacite dome complex (22°32′S–66°14′W)volcanic phase but volcanism was more restricted geo-

graphically, being confined to the areas around the The Minuyoc dacite dome complex is the smallest earlyLaguna de Pozuelos (Fig. 2). The first volcanic phase Miocene volcanic feature in the northern Puna (0·15 km3).was not accompanied by ore formation. It was emplaced at the intersection of NW–SE and

The second volcanic phase (Fig. 2, Table 1) developed NNE–SSW fracture zones at the eastern border of Sierracompletely within the Middle Miocene (16–12 Ma). Main de Rinconada (Fig. 2). The eruptive sequence at Minuyoccentres erupted within this phase have been classified as includes a basal explosion breccia followed by massivevolcanic dome complexes or dacitic stocks (Caffe & hydromagmatic breccias and tuffs, crystal-rich daciticCoira, 1999), and form a group of regionally extensive, lavas and finally, phreatic tuffs. This complex has notconspicuous magmatic features in the northern Puna, been dated, but on the basis of its trace element andEastern Cordillera and the Bolivian Altiplano (Coira et isotopic composition, it is correlated with other dated

first cycle volcanic rocks.al., 1993). Most of the centres are closely associated

909


Table 1: Summary of Upper Cenozoic tectonic and volcanic phases in northern Puna and

surrounding areas in Chile and Bolivia

Volcanic Tectonic Centres Age (Ma)

phase phase

Pleistocene 4 Chorrillos basaltic andesite 2·0

San Geronimo basaltic andesite

Tuzgle volcano

Tocomar ignimbrite

Diaguita

Upper Miocene 3 Guacha 4·2

Pliocene La Pacana 4·0–4·6

Panizos 7·9–6·7

Coranzuli 6·6

Vilama 9·7–2·5

Quechua

Middle Miocene 2 Chinchillas 13

MM Aguiliri 12·7

Pan de Azucar 12–13

Huayra Huasi 11·8

Poquis 12·9

Tiomayo Formation tuffs 15·7–12·3

South Lipez tuffs and ignimbrites 15

Pehuenche

Upper Oligocene 1 Cabreria Formation ignimbrites 17·4

Lower Miocene UOLM Pirurayo volcanic complex 20

Casa Colorada dome 17·3

Laguna de Pozuelos volcaniclastic sequence 18·6

Minuyoc dome

El Morro

Age sources: Schwab & Lippolt (1974); Coira (1979); Marinovic (1979); de Silva (1987); Linares & Gonzalez (1990); Coira etal. (1993, 2002); Fornari et al. (1993); Ort et al. (1996); Lindsay et al. (2001); P. J. Caffe (unpublished data, 2000).

(>400–1200 m thick) known as the Cabreria FormationPirurayo volcanic complex (22°21′S–65°52′W)(Coira et al., 2002). The upper member (QuebradaThe Pirurayo volcanic complex consists of a successionGrande) is composed of 20 volcaniclastic units, of whichof block and ash flow deposits, lavas and lahar bodiesnine are crystal-rich, poorly welded, dacitic ignimbriteswithin the middle levels of the conglomeratic Moreta(each 2–15 m thick). According to thickness variations ofFormation (Soler, 1996) (Fig. 2). Ages obtained for thethe pyroclastic sequence, the source of the primary de-unit range between 28± 3 Ma and 20± 2 Ma (Linaresposits may be located to the north of Casa Colorada,& Gonzalez, 1990). Soler (1996) estimated the volumenear 22°15′S, from which they thin out both to theof the Pirurayo deposits at >13 km3. The compositionnorth and south. Homogeneous major and trace elementis mainly andesitic to dacitic, although scarce low-silicacompositions suggest that the entire volcanic volumerhyolites are also present. The complex is strongly de-represented in these units (>15 km3, primary plus re-formed, with open folds prevailing in the northern ex-worked) erupted from the same source. Coira et al. (2002)posures, tight folds and thrust faults at central locations,determined a 17·4 ± 0·8 Ma K–Ar age for the middleand a monoclinal form in the south.part of the sequence.

Cabreria Formation ignimbrites and volcaniclastic deposits Laguna de Pozuelos volcaniclastic sequence(probable location of vents 22°14′S–66°20′W) (22°30′S–22°40′S; 66°08′W–66°15′W)

The western margin of Sierra de Rinconada (Fig. 2) This sequence of mixed pyroclastic and reworked volcanicrocks is located SW of the Laguna de Pozuelos basinis flanked by a thick conglomeratic sequence

910


Fig. 2. Geologic map of the northern Puna of Argentina, and location of sampled centres (stars). SVRF, San Vicente–Rinconada Thrust.

(Fig. 2). Primary volcanic units include pumiceous, crys- from an ignimbrite in the middle section is 18·6± 1 Ma(P. J. Caffe, unpublished data, 2000), which is consistenttal-rich dacite to rhyodacite ignimbrites, massive air-fall

tuffs, pyroclastic surge deposits, and lahar bodies up to with its stratigraphic position below mid-Miocene tuffsof the Cara Cara strata (Tiomayo Formation), dated30 m thick. Chernicoff et al. (1996) suggested that a ring

fracture (ancient caldera structure) at the southern border by Cladouhos et al. (1994). The greatest abundance ofpyroclastic material is restricted to exposures to the westof the Laguna de Pozuelos basin could have been the

source of eruptions. A new K–Ar biotite age for pumice of the Pan de Azucar mine (Fig. 2), and the total preserved

911


eruptive volume is>6 km3. This value is probably much de Rinconada. The complex hosts a mineralized hydro-thermal breccia, with ores enriched in Zn and minor Sn,too low, as the base of the sequence is not exposed in

most outcrops, and because post-10 Ma cover in the which were mined intermittently (Caffe & Coira, 1999).The centre has been dated by only a single K–Ar ageLaguna de Pozuelos basin has been extensive. Thus, the

Laguna de Pozuelos volcaniclastic sequence studied here determination of 13± 1 Ma (Linares & Gonzalez, 1990).Volcanic units comprise co-ignimbrite breccias and mas-may correspond to the basal part of an >1200 msive low-volume pyroclastic flow deposits, followed bysequence interpreted from seismic reflection data byblock and ash flow deposits and a lava dome confinedGangui (1998).to the south. Volcanism was related to the same NW–SEfault along which the Minuyoc complex was emplaced.

El Morro (23°11′S–66°54′W)

This centre is located slightly to the east of the Argentina– Aguiliri (23°11′–23°17′S; 66°51′–66°57′W)Chile border near Jama (Fig. 2). It is a small quartz

The Aguiliri lava dome complex is the southernmostandesite stock, which intrudes the Ordovician basementeruptive centre studied (Fig. 2). The centre forms aclose to the mid-Miocene Aguiliri volcanic complex (seecluster of one dacite stock and three dacitic lava domes,below). This centre has not been dated, but a numberwhich intruded basement and Tertiary sediments nearof factors, including greater degree of erosion than the the Chile–Argentina border at Jama. A new K–Ar biotite

Aguiliri dacitic domes, and its overall geochemical char- age indicates a Middle Miocene age of 12·7 ± 1·3 Maacteristics, suggest the El Morro centre has closer affinities (P. J. Caffe, unpublished data, 2000). The Aguiliri in-with magmatic rocks of the first phase rather than the trusive stock has anomalously high concentrations of U,second. and weak anomalies of Pb and Ag. Aniel (1987) concluded

that the U enrichment was hydrothermal, not magmatic.

Second volcanic phase: Middle MiocenePETROGRAPHY AND MINERAL(henceforth MM centres)COMPOSITIONSPan de Azucar (22°32′–22°38′S; 66°01′–66°07′W)The sampled dome complexes and related volcanic unitsThe well-known Pan de Azucar Ag–Pb–Zn ore deposit,in the northern Puna have dominantly dacitic to rhyo-which has been mined from colonial times until the earlydacitic compositions except for the El Morro intrusive1990s, is situated in a volcanic dome complex of therocks and some flows from the Pirurayo complex, whichsame name at the southwestern margin of the Lagunaare andesitic. The dacitic rocks generally share similarde Pozuelos basin (Fig. 2). The complex comprises apetrographic characteristics. Lavas and pumice fragmentsvariety of lava flows, autobreccias, subvolcanic dacites,are porphyritic, with a variably devitrified glassy ground-lava domes and diverse pyroclastic rocks. Available K–Armass. Crystal contents range from 5 to 47 vol. % inages (12 ± 2 Ma, 13 ± 1 Ma; Coira, 1979) constrainpumice and from 19 to 60 vol. % in lavas (Table 2).the eruptive events to the Middle Miocene. Caffe (1999)Shallow intrusive facies have the same crystallinity as theidentified a minimum outcropping volume of 1·5 km3,lavas.which was erupted in three events. The first event began

The dominant mineral (Table 2) in the dacitic andwith plinian eruptions followed by alternating growth–rhyodacitic rocks is plagioclase, followed in abundancecollapse episodes of lava domes. The second event in-by biotite and quartz. Amphibole, clinopyroxene, ortho-volved phreatomagmatic–vulcanian eruptions, latepyroxene, (Ti-) magnetite and ilmenite occur in minordacite-dome lavas and emplacement of a late-stage shal-and variable proportions. Sanidine is extremely rare, andlow subvolcanic body. The third event involved collapseallanite, apatite, zircon and monazite occur as accessories.of a flow-banded lava dome to the south and eruptionSmall (millimetre-sized) clots of the two or three dominantof grey dacitic dome lavas. Eruptions were localized bymineral phases in lavas and pumices are interpretedNW–SE extensional fractures crossing the Laguna deas crystal accumulation textures (Table 2). The mainPozuelos basin, most of which are related to strike-slippetrographic difference shown by andesitic lavas (in Pi-components of the thrust faults bounding the high rangesrurayo and El Morro) is the relative abundance of mineral(Fig. 2) to the east and west of the complex (Chernicoffmafic phases, amphiboles and pyroxenes being moreet al., 1996).abundant than biotite.

Some samples from the mid-Miocene centres containChinchillas (22°30′S–66°15′W) sillimanite, ilmenite, spinel and biotite as inclusions inLike Minuyoc, the Chinchillas volcanic dome complex plagioclase phenocrysts and in the cores of plagio-

clase–biotite clots. These are interpreted as restitic phasesis a small volcanic centre (0·26 km3) located in the Sierra

912


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913


Table 3: Representative analyses of plagioclases from northern Puna volcanic dome complexes

Centre: Minuyoc Casa Colorada Pan de Azucar

Sample: Min-9 L CCl-1 L D-8 HTC PA-6 SEL iso-2 TEL

Analysis: 88 95 120 2 62 63 21 17 23 52 1 5 33 55

Description: ph core ph rim microl ph rim core rim ph core ph rim sieved sil-incl ph core ph rim sieved microl

SiO2 52·09 54·56 56·38 57·87 46·85 51·48 58·37 56·17 55·55 54·06 54·21 55·87 52·99 58·78

TiO2 0·04 0·05 0·04 0·05 0·02 0·02 0·03

Al2O3 30·22 28·30 26·96 26·30 33·51 30·44 25·89 27·14 27·94 29·05 28·76 27·57 29·02 25·42

FeO∗ 0·14 0·07 0·08 0·08 0·38 0·35 0·01 0·08 0·06 0·06 0·06 0·29 0·34 0·31

MnO 0·02 0·04 0·04 0·02 0·04 0·03 0·03

MgO 0·02 0·01 0·02 0·01 0·02 0·01 0·00 0·02 0·06 0·02

SrO 0·04 0·09 0·15 0·14 0·11 0·06 0·14 0·15 0·15 0·16 0·11 0·15 0·27 0·22

BaO 0·03 0·09 0·08 0·02 0·02 0·11

CaO 13·25 10·75 9·17 8·31 17·41 13·60 7·51 9·46 9·78 11·46 11·17 9·87 12·30 7·68

Na2O 4·10 5·42 6·01 6·28 1·79 3·78 6·83 6·02 5·58 5·05 5·12 5·64 4·40 6·83

K2O 0·18 0·24 0·27 0·56 0·08 0·18 0·66 0·47 0·42 0·32 0·28 0·43 0·31 0·60

Total 100·02 99·44 99·09 99·64 100·21 99·95 99·46 99·52 99·58 100·26 99·76 99·86 99·76 100·00

An 35·5 47·0 53·4 55·9 15·6 33·1 59·9 52·1 49·5 43·6 44·6 49·6 38·6 59·6

Ab 63·5 51·6 45·0 40·8 83·9 65·8 36·3 45·3 48·0 54·6 53·8 47·9 59·6 37·0

Or 1·0 1·4 1·6 3·3 0·4 1·0 3·8 2·7 2·5 1·8 1·6 2·5 1·8 3·4

∗Iron as total FeO.L, lava; HTC, holocrystalline tonalitic cumulate; SEL, second event lava; TEL, third event lava; ph, phenocrysts; sieved,intermediate sieved zones; sil-incl, plagioclase with relict inclusions of aluminous phases; microl, microlites.

derived from partial melting of assimilated crustal rocks. MM centres show either moderate normal zonation (inAguiliri, An50–37), or oscillatory to slightly inverse zonationInclusions of oval or blocky holocrystalline quartz-dioritic

to tonalitic enclaves (diameter 15–2 cm) are also common. (in Pan de Azucar; Fig. 3 and Table 3). The highestanorthite contents (>An62–48) in the Pan de AzucarThese inclusions have almost the identical mineral as-

semblage to the host lavas (Table 2) but are finer grained. samples occur in intermediate zones of grains showingsieve texture. Plagioclase from mineral clots (Pan deLike the mineral clots mentioned above, we interpret

them as products of crystal accumulation, probably at Azucar), which include relict aluminous minerals, is morecalcic than lava phenocrysts (Fig. 3).the magma chamber–wallrock contacts (de Silva, 1989b).

Mineral compositions were obtained by electron micro- The Sr contents in plagioclase mirror the prominentwhole-rock Sr variations observed in the UOLM to MMprobe analysis from the UOLM centres Casa Colorada

and Minuyoc, and from the MM Pan de Azucar and volcanic rocks (see below). Thus, plagioclase from MMcentres generally has higher Sr contents than the UOLMAguiliri complexes [the latter from Aniel (1987)]. Rep-

resentative microprobe analyses of plagioclase, biotite, equivalents (Table 3).amphibole, pyroxenes and Fe–Ti oxides are reported inTables 3–7. Biotite

The mg-number [100 × Mg/(Mg + Fe)] of biotitePlagioclase phenocrysts from UOLM centres ranges between 45 and

49 (Table 4), except for biotite rims around amphibolePlagioclase compositions vary from sodic andesine tosodic labradorite in the various units. Plagioclase in (in Minuyoc), which are more magnesian (mg-number

56–62). In MM centres, biotite mg-numbers are in thecumulate clots and tonalitic inclusions is significantlyricher in the anorthite component (Fig. 3). Phenocrysts range of 50–60 for Aguiliri (Aniel, 1987) and 45–62 for

Pan de Azucar. Biotite from tonalitic to dioritic inclusionsin UOLM complexes are normally zoned from An76–55

cores to An55–35 rims. Plagioclase phenocrysts from the has mg-numbers between 57 and 61. The TiO2 content

914


Table 4: Representative analyses of biotites from northern Puna dome complexes


Sample: Min-9 D-8 PA-6 FSEL iso-2 TEL

Analysis: 112 92 87 73 78 28 41 27 11 16 46 26 14

Description: reaction ph core ph rim inclus crystal ph ph includ relict ph ph inclus inclus

rim in Hbl pl cryst pl inclus sieved pl pl rims

SiO2 38·37 35·70 35·37 37·25 36·85 35·50 35·64 35·52 34·76 35·71 35·78 36·22 35·09

TiO2 3·64 3·46 3·20 3·58 3·55 4·25 3·93 4·19 4·08 4·33 4·24 4·71 4·28

Al2O3 12·74 15·80 15·37 14·68 15·22 17·49 17·12 16·89 17·14 15·82 15·70 14·95 15·67

Cr2O3 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. — 0·01 0·01 —

FeO∗ 18·37 20·49 21·13 15·72 16·74 18·63 19·71 19·60 19·31 19·96 19·77 16·81 20·29

MnO 0·27 0·26 0·23 0·20 0·21 0·13 0·19 0·23 0·22 0·25 0·27 0·17 0·18

MgO 12·94 10·49 10·22 13·23 13·42 9·48 9·60 9·30 9·35 9·98 10·57 12·65 9·89

BaO 0·03 0·20 0·27 0·09 0·33 0·17 0·27 0·28 0·15 0·28 0·16 0·43 0·17

CaO 0·21 0·01 0·10 0·01 0·02 0·01 0·07

Na2O 0·31 0·31 0·24 0·36 0·52 0·28 0·31 0·40 0·29 0·36 0·36 0·43 0·71

K2O 7·94 9·09 9·16 8·98 9·05 9·38 9·26 9·28 9·40 9·38 9·26 9·25 9·12

F n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0·25 0·07 0·43 0·29

Cl n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0·05 0·05 0·01 0·07

F + Cl = O −0·12 −0·04 −0·18 −0·14

Total 94·83 95·81 95·18 94·19 95·89 95·30 96·02 95·69 94·71 96·26 96·22 95·90 95·69

Ti p.f.u. 0·207 0·199 0·186 0·204 0·201 0·243 0·225 0·241 0·237 0·249 0·242 0·268 0·248

mg-no. 56 48 46 60 59 48 46 46 46 47 49 57 47

∗Iron as total FeO.Ti p.f.u. from 11 O normalization. Nomenclature as in Table 3; n.a., not analysed.

of biotite is higher in MM centres [Ti 0·20–0·27 cations samples lacking reaction texture suggested pre-eruptiontemperatures of 830–900°C (Caffe, 1999).per formula unit (p.f.u.)] than in UOLM lavas (Ti 0·18–

0·21 cations p.f.u.; Table 4), suggesting lower crys-tallization temperatures (Patino Douce, 1993) for the

PyroxenesUOLM group.Pyroxene is a rare phase in the dome complexes (Table2). Orthopyroxene (En68–47 Fs51–29 Wo1·4–2·5) was noted in

Amphibole Minuyoc lavas, and it shows decreasing mg-number fromAmphibole in the UOLM centres classifies as tscher- reaction rims on amphibole to phenocryst cores to rimsmakite to Mg-hornblende and commonly displays dis- (Table 6). Augitic clinopyroxene (En41Fs16Wo43; mg-num-equilibrium textures. For example, hornblende from ber 73) is restricted to rare occurrences in lavas fromCasa Colorada lavas is corroded, and in Minuyoc Pan de Azucar and Aguiliri dome complexes.samples, hornblende is replaced by a reaction rim com-posed of biotite–orthopyroxene–ilmenite–plagioclase.

Fe–Ti oxidesAmphibole phenocrysts from the MM samples classifymainly as Mg-hastingsite or Mg-hornblende (Table 5) Magnetite is the predominant oxide phase in UOLM

lavas and holocrystalline cumulates (Table 7); ilmeniteand, in contrast to the UOLM rocks, they show no signsof instability in the melt before eruption. Amphibole mg- is subordinate. Among the MM domes, Pan de Azucar

contains abundant magnetite (TEL in Table 7), eithernumbers are generally higher in UOLM than in MMlavas (72–84 vs 58–75, respectively; Fig. 4). in the groundmass or included in plagioclase and biotite,

whereas ilmenite is restricted to inclusions in feldsparHornblende–plagioclase geothermometry (Holland &Blundy, 1994) performed on rim compositions from MM cores (SEL and TEL in Table 7). The Aguiliri intrusive

915


Table 5: Representative analyses of amphiboles from northern Puna volcanic dome complexes


Sample: Min-9 L D-8 HTC iso-2 TEL

Analysis: 81 82 73 78 21 51 52

Description: ph core ph rim ph core ph rim clot core ph core ph rim

SiO2 41·99 42·11 47·10 45·58 46·41 41·22 39·98

TiO2 3·14 2·91 0·82 1·02 0·89 3·49 2·75

Al2O3 12·81 13·13 7·97 9·37 7·99 12·57 13·90

Cr2O3 n.a. n.a. n.a. n.a. 0·18 — 0·02

FeO∗ 10·02 10·86 13·21 14·89 13·14 9·38 14·03

MnO 0·11 0·11 0·53 0·56 0·52 0·09 0·30

MgO 13·92 12·69 14·07 12·88 14·01 14·86 11·06

BaO — — 0·04 0·05 — — —

SrO — 0·05 — — — — —

CaO 11·51 11·58 11·23 11·21 11·80 11·83 11·52

Na2O 1·56 1·66 0·90 1·22 1·32 2·12 1·83

K2O 1·10 1·13 0·38 0·46 0·61 1·44 1·38

F n.a. n.a. n.a. n.a. 0·20 0·24 0·06

Cl n.a. n.a. n.a. n.a. 0·02 — 0·01

F + Cl = O −0·09 −0·10 −0·03

Total 96·16 96·24 96·24 97·22 96·81 97·14 96·79

cations

Si 6·157 6·231 6·836 6·621 6·784 6·040 5·980

AlIV 1·843 1·769 1·164 1·379 1·216 1·960 2·020

AlVI 0·368 0·519 0·198 0·223 0·159 0·208 0·429

Ti 0·346 0·324 0·090 0·111 0·097 0·385 0·310

mg-no. 81 72 83 78 66 74 58

∗Iron as total FeO.Si, AlIV, AlVI and Ti (cation p.f.u.) were obtained by 13 cation normalization. n.a., not analysed.

stock lacks magnetite, but lavas from the dacitic domes hematite ranges from >7 to 19 mol %. Lavas from theMM Pan de Azucar complex contain nearly pure ilmenitein Aguiliri (Cerro Chingolo–Norte) contain both oxides.(XHem >2–6 mol %), whereas in the holocrystalline min-Almost all magnetite crystals have developed ox-eral cumulates (AH-51 in Table 7) ilmenite crystals areidation–exsolution lamellae. Among the UOLM volcanicreplaced by a hemo-ilmenite–rutile intergrowth. Anieldomes, the best preserved magnetite compositions (Casa(1987) reported ilmenite from the MM Aguiliri intrusiveColorada) have ulvospinel contents up to 18–21 mol %.rocks, which exhibits variable oxidation textures andAmong the MM centres, non-exsolved, primary grainshematite contents ranging from 7 to 50 mol %.were observed in a few lavas and dioritic cumulates from

Pan de Azucar (TEL in Table 7), ranging from 23 to29 mol % ulvospinel. Reintegrated primary Ti-magnetite

GEOCHEMISTRYcompositions [method of Mathison (1975)] suggest ori-Major elementsginal ulvospinel contents of up to 36–44 mol %.

UOLM rocks show variable composition of primary In general, the UOLM to MM volcanic rocks in theilmenite. In Minuyoc, ilmenite is poor in the hematite northern Puna classify (Fig. 5) as high-K dacites to low-

SiO2 rhyolites in the K2O–SiO2 diagram of Peccerillo &component (>3 mol %; Table 7), but in Casa Colorada

916


The overall major element characteristics of the studiedTable 6: Representative pyroxene analysesrocks are typical for central Andes back-arc silicic volcanicrocks, with Al2O3 ranging from 14 to 17%, K2O >2·5%,

Centre: Minuyoc Pan de K2O/Na2O ratios generally >1 and the sum of the alkalisAzucar >4–9%. TiO2 concentrations are low (<1·6%), and MgO

Sample: Min-9 iso-2 contents are <3% (Table 8). Concentrations of Al2O3,TiO2, FeOt, MgO and CaO decrease with increasing

Analysis: opx-86 opx-114 opx-118 opx-117 opx-113 cpx-29 silica (Fig. 5), K2O increases with increasing SiO2, andDescription: reaction ph ph ph ph ph Na2O is variable, except for the Pan de Azucar matrix

rim in core core rim rim core glasses, where it correlates negatively with SiO2. TheHbl Harker diagrams in Fig. 5 show that, for the same SiO2

content, MM rocks are slightly richer in Na2O (especiallySiO2 52·17 53·85 52·45 51·25 50·57 52·56 Aguiliri, whose K2O/Na2O ratios are >1). Chinchillas

is an exception, and its low Na2O (and CaO) contentsTiO2 0·19 0·19 0·28 0·15 0·13 0·18

probably reflect hydrothermal remobilization. The P2O5–Al2O3 1·18 1·93 1·70 1·82 0·68 0·38

SiO2 variation trends (not shown) are specific to eachFeO∗ 20·29 18·00 19·60 23·94 27·93 9·77centre, with kinked trends in Aguiliri, strong negativeMnO 0·44 0·32 0·26 0·51 1·06 0·38correlation in Pozuelos, and constant P2O5 level in theMgO 22·66 24·16 22·85 19·71 16·64 14·74remaining centres, which suggest different apatite frac-CaO 1·00 1·21 1·13 0·86 0·87 21·11tionation histories. The dioritic and tonalitic enclavesNa2O — 0·03 0·01 0·03 — 0·34show similar variation trends to their volcanic hosts,K2O 0·02 — 0·03 0·02 0·03 0·02the differences reflecting the particular mineralogyTotal 97·97 99·69 98·30 98·28 97·91 99·55accumulated (e.g. high P2O5 indicates apatite

Wo 2·1 2·5 2·3 1·8 1·9 42·6accumulation).

En 64·7 68·4 65·7 57·9 49·6 41·4

Fs 33·2 29·1 32·0 40·3 48·5 16·0

Jd — 0·2 0·1 0·2 — —

mg-no. 67 71 68 59 52 73 Trace elementsAs for the major elements, the trace element char-

∗Iron as total FeO. acteristics of the early Neogene volcanic rocks in theEnd members calculated after 6 oxygen normalization. northern Puna are typical of Andean back-arc mag-

matism (see Kay et al., 1994b), in particular with respectto Ba (370–1224 ppm), Sr (153–849 ppm), Rb (65–328Taylor (1976). A few matrix glass analyses from Panppm), Th (6–22 ppm), U (1–7 ppm), the rare earthde Azucar lavas have high-silica rhyolite compositions.elements (REE)∗ (La 21–56 ppm, Ce 33–107 ppm, SmDioritic enclaves plot in the medium- to high-K andesite4·3–9·4 ppm, Eu 1·1–2·7 ppm, Yb 1·4–3·3 ppm) andfields, as do the El Morro intrusive rocks and the leasthigh field strength elements (HFSE) Hf (3·4–9·8 ppm),evolved rocks from the Pirurayo volcanic complex (high-Zr (110–240 ppm) and Nb (7–21 ppm).SiO2 andesites).

Selected trace element variation diagrams are plottedIn terms of the alumina saturation index [A/CNK =in Fig. 7. Rb concentrations exhibit an overall weakmolar Al2O3/(CaO+ Na2O+ K2O)], both the UOLMpositive correlation with SiO2. Sr concentrations are(A/CNK = 0·83–1·26) and MM groups (A/CNK =more variable and specific to each centre, but for similar0·91–1·39) are highly variable (Fig. 6). Among theSiO2, MM rocks (14–12 Ma) are richer in Sr (430–849UOLM centres, two sub-groups can be differentiatedppm) than those from the older centres (153–480 ppm).using A/CNK ratios. The UOLM-1 sub-group, formedBa and Zr concentrations are also rather scattered and,by El Morro and Pirurayo (Fig. 6), is characterizedlike Sr, concentrations are higher in MM rocks than inby A/CNK ratios that increase with increasing SiO2

UOLM centres. Within each centre, Nb shows a flat(58–71%) from metaluminous quartz-andesites and da-variation trend with SiO2 and tends toward higher valuescites to peraluminous dacites and low-SiO2 rhyolites (A/in the MM group than in UOLM volcanic rocks. InCNK up to 1·14). On the other hand, UOLM-2 centrescontrast, for the same SiO2 level, Y contents decrease(Pozuelos, Casa Colorada, Cabreria Formation and Mi-from the older, UOLM volcanic rocks to the MM rocks.nuyoc), have A/CNK ratios that show no correlation

The Ba/Nb and Zr/Nb ratios (or equivalent Ba/Tawith SiO2 (Fig. 6), and many more samples are per-and Zr/Ta) have been used to distinguish between supra-aluminous. The MM group of rocks from the northernsubduction zone (frontal arc) and back-arc magmas (Hil-Puna have the same characteristics as UOLM-2 rocks

with respect to A/CNK and SiO2. dreth & Moorbath, 1988; Kay et al., 1994b; Davidson &

917


Table 7: Representative analyses of Fe–Ti oxides from northern Puna volcanic dome complexes


Sample: Min-9 L CCl-1 L D-8 HTC PA-6 iso-2 TEL AH-51 HQDC

SEL

Analysis: Mag Ilm Mag Mag Ilm Ilm Mag Mag Ilm Mag Ilm Ilm

90 85 54 60 65 25 47 41/42 42 30a 42 33

Description: includ hbl included included included includ ground reconstr ground ground ground Ti-hem

bt react rim pl qtz qtz pl

SiO2 0·13 0·14 — — 0·70 0·43 0·79 0·38 1·92 1·29 1·92 5·13

TiO2 0·91 48·03 5·67 6·03 42·02 50·34 10·20 12·84 43·38 7·46 43·38 13·60

Al2O3 1·23 0·08 2·23 2·97 0·45 0·23 3·23 5·09 0·26 0·52 0·26 0·33

Cr2O3 — — — — — — 0·08 0·04 0·02 0·02 0·02 0·04

FeO∗ 30·10 40·89 41·21 36·14 38·13 42·65 37·93 41·82 40·04 37·17 40·04 17·64

Fe2O3∗ 62·12 2·92 46·09 53·75 18·27 1·7 44·33 38·01 8·02 46·74 8·02 59·45

MnO 0·46 1·29 0·21 0·28 0·10 1·24 2·16 0·48 0·30 0·31 0·30 0·27

MgO 0·12 0·57 0·14 0·51 0·23 0·89 1·21 1·31 0·45 0·14 0·45 0·12

CaO — 0·16 0·36 0·12 — 0·32 0·06 0·04 0·11 0·14 0·11 0·23

BaO 0·05 0·07 0·11 0·07 — 0·16 — 0·05 — 0·07 — 0·13

Total 95·12 94·15 96·02 99·88 99·90 97·95 100·00 100·05 94·51 93·79 94·51 96·80

Nusp 0·03 0·21 0·18 0·29 0·36 0·23

Xilm 0·92 0·80 0·92 0·86 0·86 0·29

Xhem 0·03 0·19 0·02 0·11 0·11 0·70

Xgk 0·02 0·01 0·03 0·02 0·02 —

Xprph 0·03 — 0·03 0·01 0·01 0·01

∗From microprobe analyses iron was obtained as FeOt. FeO and Fe2O3 calculated from stoichiometry of Droop (1987).L, lava; HTC, holocrystalline tonalitic cumulate; SEL, second event lava; TEL, third event lava; HQDC, holocrystallinequartz-dioritic cumulate; Nusp, normative ulvospinel. Abbreviatures from Kretz (1983), except for gk (geikielite) and prph(pyrophanite).

de Silva, 1995). In terms of the Ba/Nb ratio and Nb there is no significant correlation between LREE contentsor LREE/heavy REE (HREE) ratios and SiO2 values.concentration (Fig. 8 and Table 8), the northern Puna

centres show both a temporal difference (i.e. UOLM vs The dioritic and tonalitic enclaves have higher HREEconcentrations and flatter chondrite-normalized REEMM) and a difference according to geographic location.

The UOLM centres have Ba/Nb ratios and Nb values patterns than the lavas (Table 8, Fig. 9). Negative Euanomalies are weak, which at a first glance seems in-in the field of CVZ arc magmas (Fig. 8) and the eastern

UOLM centres have slightly higher Nb contents than compatible with major-element fractionation models (seebelow), which suggest that plagioclase was a major frac-the western UOLM, at similar Ba/Nb ratios. The MM

samples show generally higher Nb contents than both tionating phase. As suggested by Davidson & de Silva(1995), the weak Eu anomalies could be due to high f O2UOLM groups, and Ba/Nb ratios that vary with geo-

graphic position. Those erupted eastward (i.e. Pan de conditions in Andean magmas and subsequently lowEu2+/Eu3+ ratios. An outstanding feature of the northernAzucar, and the >12 Ma Huayra Huasi porphyry, loc-

ated east of Aguiliri) have a more back-arc affinity, with Puna volcanism is a steepening in slope of the REEpatterns from the UOLM to the MM centres (Fig. 10a).Ba/Nb generally <50, and those erupted to the west

(Chinchillas and Aguiliri) have ratios >50, which are This steepening involves both a slight increase in LREEcontents, raising the La/Sm ratio from 4–7 for UOLMtypical for the frontal arc.

Figure 9 shows a comparison of the REE contents of samples to 5–7·5 for MM rocks, and a decrease in totalHREE, producing a change in the Sm/Yb ratio (fromthe UOLM and MM rocks. Generally, the chondrite-

normalized patterns are light REE (LREE) enriched, and 2–3·3 for UOLM to 3·5–5 for MM). The overall effect

918


Fig. 3. Composition of plagioclase in lower Miocene (UOLM) and middle Miocene (MM) dome complexes. Filled symbols indicate corecompositions; open symbols represent rims. Grey circles for Pan de Azucar data represent analyses of sieve-texture zones. Inverted trianglesshow compositions of plagioclase cores with inclusions of relict Al2SiO5 phases. FSEL and TEL are Pan de Azucar first–second and third eventlavas, respectively. Grey diamonds in Aguiliri represent groundmass plagioclase.

used for dating). Analytical details are given in AppendixA.

UOLM volcanic rocks can be divided into two groupsaccording to their Sr–Nd isotopic compositions (Fig. 11;Table 9), and these correlate with the UOLM-1 andUOLM-2 groups as distinguished by A/CNK ratios(compare Fig. 6). The Pirurayo volcanic complex(UOLM-1) has the lowest 87Sr/86SrT values and thehighest �NdT (>0·706 and −3, respectively), very closeto values reported for Neogene andesites from the CentralVolcanic Zone (CVZ base-line compositions; Davidsonet al., 1991). The UOLM-1 rocks represent the isotopicallymost depleted signatures (in terms of same compositions)reported from the northern Puna except for the Plio-Pleistocene monogenetic mafic centres (Kay et al., 1994a).

Fig. 4. Mg-number versus TiVI compositions of amphiboles from The two Pirurayo samples represent the full range ofUOLM and MM volcanic domes of the northern Puna.

chemical variation at the complex (see Table 9) andtheir nearly identical 87Sr/86SrT and 143Nd/144NdT ratiossupport the concept of closed-system fractional crys-is expressed in the La/Yb ratios, which are 11–23 fortallization during the magmatic evolution of this complex.the UOLM rocks and 23–34 for the MM centres (Fig.The UOLM-2 group (Fig. 6) has much more radiogenic10b).initial Sr isotopic ratios, ranging between 0·7128 and0·7140, and lower �NdT values (–7·6 to −9), similar tothose of the Panizos ignimbrites (Ort et al., 1996). Samplesfrom the mid-Miocene dome complexes have ratios inter-Radiogenic isotopesmediate between the two UOLM groups ( 87Sr/86SrTRadiogenic Nd, Sr and Pb isotopic data from northern> 0·709–0·710, �NdT −6·9 to −8·4) and they morePuna UOLM and MM volcanic rocks are listed inclosely resemble the Chilean APVC ignimbrites (Fig. 11).Table 9. Selected samples represent the full range of

Figure 12 shows the variation of Sr content vs 87Sr/compositions present at each centre, except in the case86Sr ratio. Negative correlations in this diagram are typicalof the Pozuelos volcaniclastic sequence and the Cabreriafor CVZ volcanic rocks and are generally taken asFormation ignimbrites, where only one analysis for each

centre was obtained (from the same pumice samples as evidence for upper-crustal contamination of arc magmas

919


Fig. 5. Variation diagrams of selected major elements vs SiO2 for the northern Puna volcanic rocks.

(Harmon et al., 1984; Hildreth & Moorbath, 1988; Dav- Huasi porphyry (12 Ma) and the 14–15 Ma TiomayoFormation ignimbrites (B. L. Coira & Ch.-H. Chen,idson et al., 1991; Feeley & Davidson, 1994). Following

this interpretation, if we take the 87Sr/86Sr ratios of the unpublished data, 2000). Rogers & Hawkesworth (1989)also observed increasing 87Sr/86SrT with rising Sr con-Pirurayo complex as the most primitive in the northern

Puna, the isotopically more enriched UOLM-2 magmas centration in CVZ magmas sampled on a west-to-easttraverse and attributed it to increased influence of ra-show a crustal assimilation trend typical of CVZ centres,

with lower Sr and higher 87Sr/86SrT ratios. On the other diogenic, late-Proterozoic subcontinental mantle litho-sphere as volcanism migrated progressively eastward. Inhand, the northern Puna MM rocks show an increase in

87Sr/86SrT relative to Pirurayo, but they also have higher the case of the UOLM and MM volcanic rocks, the locusof magmatism was roughly the same, so this explanationSr concentrations (>90–150 ppm more Sr than Pi-

rurayo). This relationship between high Sr contents and is unlikely. Instead, we consider that this trend suggeststhat Sr was not removed by fractional crystallizationhigh 87Sr/86SrT ratios is also seen in Aguiliri, the Huayra

920


The Pb-isotope ratios from the studied centres arerather radiogenic in contrast to typical Nazca Plate basalts(Fig. 13). Like other northern Puna igneous rocks (e.g.6–7 Ma Panizos ignimbrites and shoshonite minorcentres), their compositions correspond to the EasternCordillera–Southern Altiplano Pb domain identified byAitcheson et al. (1995). Metamorphic rocks of the southernPuna basement (Becchio et al., 1999) have similar 207Pb/204Pb ratios (18·1–18·7), but lower 208Pb/204Pb (<38·6),suggesting a crustal component with higher time-integrated Th/Pb ratios in the northern Puna magmas.Felsic granulite xenoliths (Lucassen et al., 1999b) plot inthe lower range of our 206Pb/204Pb data (Fig. 13). ThePb isotopic provinciality in Andean magmas is generallyinterpreted to reflect a dominance of crustal lead overmantle-derived lead (Worner et al., 1992; Aitcheson et al.,1995; Kay et al., 1999) and this is also the case in ourstudy, thus the Pb isotopic variations in the 20–12 Manorthern Puna magmas probably reflect mixing of iso-topically variable crustal source rocks.

DISCUSSION: MAGMA SOURCESFig. 6. Whole-rock A/CNK [molar Al/(Ca + Na + K)] vs SiO2 AND EVOLUTIONdiagram for the northern Puna volcanic rocks. The field for S typemagmas corresponds to the Macusani ignimbrite (Pichavant et al., Petrographic and geochemical results obtained may be1988). The open box represents the field of 6–7 Ma Panizos ignimbrite

briefly summarized as follows. The first-phase UOLM(Ort et al., 1996) for comparison.volcanic rocks (20–17 Ma) form two distinct groupings.The metaluminous to weakly peraluminous UOLM-1group is mainly andesitic but includes minor dacitic

during crustal assimilation and remained incompatible compositions and shows the isotopic signatures within the melt. As discussed in the modelling section below, the least crustal affinity, similar to the ‘CVZ base-line’the incompatible behaviour of Sr in this case can be composition of Davidson et al. (1991). The UOLM-2explained by suppressed fractionation of plagioclase as a group is composed exclusively of dacitic to rhyolitic rocksresult of a higher pressure of crystallization. It should that have variable A/CNK ratios and crustal-like isotopicbe noted that the large-volume Upper Miocene dacitic ratios.ignimbrites (dashed oval in Fig. 12), share the more usual MM volcanic centres from the second volcanic phasetrend of more radiogenic Sr isotope ratios accompanied (14–12 Ma) make up a third compositional group. Theseby decreasing Sr concentrations. are dacitic to rhyolitic in composition and are similar in

The Nd isotope composition of the UOLM-1 Pirurayo many respects to the UOLM-2 group. Their radiogeniccentre is considerably more radiogenic, with an �NdT isotopic ratios are intermediate between those of UOLM-value of −3, than both the UOLM-2 and MM centres, 1 and -2. The A/CNK ratios are variable, as in thewhich overlap considerably in the range of �NdT −6·7 UOLM-2 group. Concentrations of Y, Sc and the HREEto−9). The latter values are similar to reported values for are lower, and the slopes of chondrite-normalized REEdacitic to rhyolitic Upper Miocene–Pliocene ignimbrites patterns are steeper than in UOLM magmas, and the(Ort et al., 1996; Schmitt et al., 2001). Calculated Nd incompatible trace element contents in MM rocks aremodel ages (tDM) for the studied rocks are Proterozoic the highest of all the rocks examined in this study.(>1–1·56 Ga; Table 9) and indistinguishable from those The petrogenetic modelling discussed in this sectionof Late Palaeozoic granites from northern Chile (Lucassen aims to explore two different aspects of the pre-Upperet al., 1999a) but somewhat lower than the tDM values Miocene magmatism in the northern Puna: (1) the rolederived for Puna lower-crustal felsic granulites (Lucassen of upper-crustal fractional crystallization or assimilationet al., 1999b), Chilean basement gneisses (Lucassen et al., in causing the observed compositional variability; (2)1999a), and southern Puna Lower Palaeozoic meta- identification of possible source constraints for the three

magma groups of northern Puna.morphic basement (Becchio et al., 1999).

921


Table 8: Representative major and trace element analyses of northern Puna early Neogene magmatic rocks

Sample: Ce-2 Ce-5 Agui-19 Agui-21 CCl-32∗ CCl-38∗ CCl-37∗ CClD-2 Po-R∗ CCl-1† P-1∗

Group: UOLM-1 UOLM-1 UOLM-1 UOLM-1 UOLM-2 UOLM-2 UOLM-2 UOLM-2 UOLM-2 UOLM-2 UOLM-2

Eruptive Pirurayo Pirurayo El Morro El Morro Cabreria Cabreria Cabreria Cabreria Cabreria Casa Casa

centre: ignimbrite ignimbrite ignimbrite ignimbrite ignimbrite Colorada Colorada

Rock type: lava lava intrusive intrusive pumice pumice pumice pumice pumice lava lava

SiO2 60·14 69·79 60·96 62·08 60·42 63·74 60·41 63·04 61·96 64·88 67·49

TiO2 0·85 0·62 0·67 0·73 0·68 0·69 0·74 0·77 0·56 0·63 0·50

Al2O3 17·82 14·53 16·88 16·43 15·53 15·86 16·37 16·42 14·99 14·68 14·48

Fe2O3t 6·86 5·16 6·99 6·50 5·88 5·55 5·80 5·60 3·92 4·03 3·97

MnO 0·13 0·10 0·09 0·08 0·06 0·06 0·08 0·05 0·08 0·04 0·05

MgO 2·85 0·74 2·81 2·48 2·20 2·30 2·14 2·57 1·49 1·72 0·88

CaO 6·00 3·24 5·13 5·79 4·12 4·18 4·99 4·95 3·86 3·12 3·63

Na2O 2·78 2·08 3·54 3·36 2·11 2·16 2·68 2·30 2·24 1·84 2·49

K2O 1·96 3·20 2·54 2·28 3·14 3·19 2·79 3·15 3·41 4·57 3·42

P2O5 0·24 0·23 0·33 0·28 0·26 0·22 0·23 0·21 0·22 0·20 0·23

LOI 3·13 2·69 2·29 5·30 4·47 2·89

Total 99·63 99·67 99·94 100·00 97·53 100·64 98·52 99·06 98·04 100·18 100·03

Sn 4 15

Pb 28 35 8 26 10

Sc 14·0 10·0 13·0 11·0 13·0 15·0 14·0 10·0 10·0 8·9

Rb 65 115 70 66 130 140 120 114 170 150 110

Ba 797 530 478 427 380 370 400 475 500 479

Sr 480 409 306 322 340 423 154

Nb 15·0 13·0 7·6 7·5 11·0 13·0

Hf 3·9 5·0 4·4 3·8 3·7 3·8 3·7 5·0 5·0 3·4 5·0

Zr 169 131 110 113 125 125 139

Y 22 16 23 24 23 23 26 24

Th 11·8 9·0 6·1 6·7 8·8 9·1 8·6 10·0 12·0 8·4 10·0

U 3·1 3·0 1·7 2·2 2·7 3·0 2·3 3·0 5·2 3·4 3·9

La 38 28 21 29 30 27 28 44 28 36

Ce 70 49 33 59 62 57 52 100 58 71

Pr 8 6 5 7

Nd 30 21 19 20 29 28 27 41 23 26

Sm 5·9 4·3 4·1 5·1 5·6 5·2 5·7 7·5 5·2 5·8

Eu 1·5 1·1 1·1 1·2 1·3 1·3 1·3 1·9 1·1 1·4

Gd 5·2 3·6 3·7 4·8

Tb 0·81 0·57 0·61 0·90 0·77 1·20 0·70 0·80

Dy 4·3 3·0 3·5 4·1

Ho 0·83 0·59 0·66 0·73

Er 2·4 1·7 2·0 1·9

Tm 0·36 0·25 0·30 0·29

Yb 2·2 1·6 1·9 1·8 2·2 1·9 1·8 3·3 2·0 2·3

Lu 0·35 0·26 0·30 0·23 0·35 0·27 0·27 0·49 0·28 0·44

Ba/Nb 53 41 63 57 43 37

Zr/Nb 11·3 10·1 14·5 15·1 11·4 10·7

La/Yb 17 18 11 16 14 14 16 13 14 16

922


Sample: CClD-4 CCl-14 Min-9 Min-6 Min-5 PV-1 Uqui-1 Uqui-6 Puca-4 Puca-5 PBl-3

Group: UOLM-2 UOLM-2 UOLM-2 UOLM-2 UOLM-2 UOLM-2 UOLM-2 UOLM-2 UOLM-2 UOLM-2 UOLM-2

Eruptive Casa Casa Minuyoc Minuyoc Minuyoc Pozuelos Pozuelos Pozuelos Pozuelos Pozuelos Pozuelos

centre: Colorada Colorada sequence sequence sequence sequence sequence sequence

Rock type: lava lava lava lava breccia pumice dacite dacite pumice pumice pumice

block block block

in lahar in lahar

SiO2 64·46 66·45 64·75 64·36 64·79 63·29 64·54 67·13 65·75 69·51 66·25

TiO2 0·61 0·51 0·70 0·60 0·59 0·55 0·70 0·65 0·82 0·66 0·91

Al2O3 15·25 15·49 16·40 15·83 15·94 15·71 15·36 15·93 15·99 12·73 15·71

Fe2O3t 3·71 4·99 4·38 4·73 4·79 4·48 6·17 4·76 4·91 6·16 5·70

MnO 0·07 0·12 0·08 0·06 0·13 0·08 0·07 0·07 0·06 1·41 0·04

MgO 2·26 1·36 1·83 2·03 1·46 1·88 1·29 2·22 2·09 1·60 2·20

CaO 2·97 2·57 4·09 2·65 2·17 4·30 5·96 4·61 4·77 2·78 3·67

Na2O 1·45 1·67 2·65 2·62 2·95 2·45 2·18 1·99 1·64 0·85 1·74

K2O 5·54 4·98 3·55 3·57 3·53 3·11 3·51 2·47 3·71 4·05 3·58

P2O5 0·24 0·21 0·25 0·24 0·24 0·24 0·22 0·19 0·25 0·25 0·22

LOI 3·93 3·01 1·32 2·49 2·98 3·56

Total 100·49 101·35 100·00 99·19 99·56 99·64 100·00 100·00 100·00 100·00 100·00

Sn 3 3 2 4 2 1

Pb 9 13 54 53 39 32 20 22 22 151 17

Sc 9·5 12·0 11·00

Rb 183 188 129 129 117 140 111 117 194 158 165

Ba 614 543 591 690 673 577 472 546 507 479 549

Sr 153 285 360 300 307 378 241 382 309 424 286

Nb 12·4 12·4 11·9 12·5 12·3 11·8 10·4 11·5 11·4 12·6 12·0

Hf 3·8 5·3 5·6 5·3 5·3 5·4 3·6 4·8 5 6·2 4·7

Zr 120 128 135 129 135 130 114 143 137 150 158

Y 22 23 26 32 27 25 25 23 24 22 23

Th 7·7 7·1 9·9 9·5 11·7 8·1 9·4 9·1 9·4 13·3 10·2

U 3·8 2·1 3·6 4·1 4·4 4·3 3·0 2·8 4·2 4·3 3·3

La 29 32 35

Ce 53 61 69

Pr 7 8 9

Nd 26 29 32

Sm 5·3 5·8 6·3

Eu 1·2 1·4 1·4

Gd 4·5 5·0 5·3

Tb 0·74 0·77

Dy 4·2 4·7 4·6

Ho 0·80 0·89 0·84

Er 2·2 2·4 2·3

Tm 0·31 0·35 0·34

Yb 2·0 2·2 2·1

Lu 0·30 0·34 0·33

Ba/Nb 50 44 50 55 55 49 45 47 44 38 46

Zr/Nb 9·7 10·3 11·3 10·3 11·0 11·0 11·0 12·4 12·0 11·9 13·2

La/Yb 15 15 17

923


Table 8: continued

Sample: Agui-10 Agui-13 Agui-14 Agui-28 Agui-32 Agui-36 Agui-38 MCh-12 MCh 53-C MCh-55 MCh-3A

Group: MM MM MM MM MM MM MM MM MM MM MM

Eruptive

centre: Aguiliri Aguiliri Aguiliri Aguiliri Aguiliri Aguiliri Aguiliri Chinchillas Chinchillas Chinchillas Chinchillas

Rock type: intrusive intrusive intrusive lava lava lava dioritic lava lava lava dioritic

cumulate cumulate

SiO2 68·83 67·43 67·14 66·46 66·51 65·75 60·52 68·18 70·02 67·69 58·03

TiO2 0·64 0·62 0·67 0·67 0·73 0·69 1·18 0·68 0·54 0·57 1·36

Al2O3 16·84 16·58 16·04 16·55 16·68 16·75 17·29 15·08 15·07 15·27 16·79

Fe2O3t 2·82 4·04 3·84 4·40 4·84 4·66 7·40 3·10 2·87 3·18 8·74

MnO 0·04 0·05 0·04 0·03 0·05 0·06 0·05 0·03 0·04 0·04 0·08

MgO 1·41 1·48 1·34 1·14 0·79 1·62 2·35 1·26 1·17 1·33 3·85

CaO 2·91 2·98 3·48 3·48 3·38 3·75 5·06 1·19 1·29 1·79 0·13

Na2O 3·35 3·38 3·58 3·20 3·47 3·50 3·09 1·63 1·44 2·07 0·22

K2O 3·53 3·40 3·63 3·38 3·55 3·11 2·65 5·77 5·64 5·76 5·49

P2O5 0·18 0·22 0·24 0·31 0·31 0·32 0·41 0·29 0·23 0·24 0·46

LOI 1·90 1·70 1·29 4·17

Total 100·52 100·17 100·00 99·62 100·30 100·22 100·00 99·12 100·01 99·23 99·32

Sn 2 1 5 4 5 8

Pb 46 16 20 17 16 52 191 41 1364

Sc 7·1 6·7 14·0 7·8

Rb 135 126 131 124 123 97 92 250 254 240 236

Ba 914 830 801 837 948 814 1118 1006 955 839 1510

Sr 759 717 670 622 636 656 681 770 645 493 557

Nb 18·43 15·86 16·43 16·58 17·45 16·86 21·4 14·5 14·0 14·5 16·6

Hf 9·4 9·5 9·3 9·5 9·4 9·8 9·2 9·8 9·3 7·7 8·1

Zr 240 237 219 220 221 223 209 201 191 173 189

Y 20 20 22 24 23 23 32 18 20 21 18

Th 14·4 13·5 13·1 13·1 14·5 13·5 9·5 22·3 15·1 12·3 32·9

U 2·6 1·8 2·2 3·1 2·7 2·9 1·8 3·5 5·0 5·2 1·1

La 47 50 46 48 44

Ce 74 86 84 82 63

Pr 10 10 10 11 9

Nd 35 38 39 38 35

Sm 6·3 6·7 7·6 7·0 6·6

Eu 1·5 1·5 1·6 1·6 1·7

Gd 4·9 5·3 7·0 5·3 5·2

Tb 0·64 0·73 1·10 0·81

Dy 3·5 3·8 6·3 4·0 4·0

Ho 0·62 0·69 1·20 0·71 0·70

Er 1·5 1·7 3·1 1·9 2·0

Tm 0·23 0·26 0·44 0·26 0·28

Yb 1·4 1·6 2·6 1·7 1·9

Lu 0·21 0·22 0·37 0·26 0·29

Ba/Nb 50 52 49 50 56 48 52 69 68 58 91

Zr/Nb 13·0 14·9 13·3 13·3 12·7 13·2 9·8 13·9 13·6 11·9 11·4

La/Yb 34 31 18 28 23

924


Sample: MCh-3B PA-16 PA-6 PA-52 PA-38 PA-46 iso-2 AH-51 iso-3 PA6gl-20‡ iso2gl-36‡

Group: MM MM MM MM MM MM MM MM MM MM MM

Eruptive Chinchillas Pan de Pan de Pan de Pan de Pan de Pan de Pan de Pan de Pan de Pan de

centre: Azucar Azucar Azucar Azucar Azucar Azucar Azucar Azucar Azucar Azucar

Rock type: tonalitic pumice lava lava vesiculated vesiculated lava dioritic tonalitic matrix matrix

cumulate FEL SEL SEL lava–TEL lava–TEL TEL cumulate cumulate glass SEL glass TEL

SiO2 68·01 66·28 66·33 67·72 62·61 67·88 64·23 58·09 62·63 75·81 75·19

TiO2 0·69 0·73 0·69 0·67 0·76 0·72 0·80 1·54 1·04 0·03 0·20

Al2O3 15·59 16·10 16·44 14·80 16·47 14·40 14·55 15·86 14·81 12·34 11·39

Fe2O3t 2·74 4·38 3·94 3·36 4·95 3·77 4·66 9·39 6·51 1·24 0·33

MnO 0·03 0·06 0·06 0·02 0·08 0·05 0·06 0·09 0·08 0·08 0·04

MgO 1·00 1·62 1·39 0·78 1·55 1·57 1·69 3·47 3·36 0·11 0·06

CaO 0·15 2·78 2·84 2·37 3·08 3·23 3·54 5·29 4·39 0·48 0·43

Na2O 0·63 2·56 3·00 2·65 2·50 1·68 3·18 2·22 2·60 1·08 1·18

K2O 8·28 3·52 3·82 4·06 4·01 3·85 3·92 3·20 2·78 2·23 5·47

P2O5 0·27 0·24 0·27 0·31 0·26 0·28 0·31 0·38 0·32

LOI 2·44 2·72 1·62 1·27 2·94 2·56 1·80 0·70 0·52

Total 99·82 100·99 100·39 98·00 99·22 99·99 98·74 100·22 99·03 93·40 94·28

Sn 3 5 4 3 3

Pb 882 29 25 24 22 20 20 8 9

Sc 7·9 7·8 8·1 9·0 18·0 12·0

Rb 314 147 149 181 146 171 150 102 111

Ba 954 814 830 694 763 726 751 945 873

Sr 512 513 486 429 483 573 544 504 529

Nb 14·3 21·0 21·2 20·3 20·4 19·7 20·5 13·6 18·7

Hf 8·1 5·3 5·6 5·4 5·7 6·1 7·7 5·6 7·1

Zr 177 173 173 162 176 178 195 140 178

Y 13 17 18 15 17 17 20 25 22

Th 28·0 10·7 10·1 9·8 10·7 9·0 10·8 5·1 5·6

U 4·1 3·7 4·6 3·3 4·6 3·8 4·0 2·5 2·9

La 47 45 47 52 32 42

Ce 87 86 86 98 64 79

Pr 10 10 10 12 8 10

Nd 38 37 37 42 33 36

Sm 7·4 7·2 6·9 7·6 7·5 7·0

Eu 1·5 1·6 1·8 1·7 1·7 1·6

Gd 5·4 5·7 5·5 5·9 6·7 5·7

Tb 0·76 0·83 0·83 0·88 1·10 0·87

Dy 3·8 3·9 3·9 4·2 5·6 4·4

Ho 0·64 0·70 0·69 0·71 1·00 0·80

Er 1·6 1·8 1·8 1·8 2·6 2·1

Tm 0·24 0·25 0·25 0·27 0·35 0·29

Yb 1·5 1·6 1·5 1·6 2·1 1·7

Lu 0·22 0·24 0·23 0·23 0·33 0·27

Ba/Nb 67 39 39 34 37 37 37 69 47

Zr/Nb 12·4 8·2 8·2 8·0 8·6 9·0 9·5 10·3 9·5

La/Yb 31 28 31 33 15 25

∗Trace elements from Activation Laboratories, Canada, by ICP and INAA.†Analyses of REE, Sc, Th and U by INAA at Cornell University.‡Major elements obtained by electron microprobe analysis at Universidad Complutense de Madrid, Spain.Major and trace elements (except REE and Sc) obtained by XRF at Instituto de Geologia y Mineria–Universidad Nacional deJujuy, Argentine. REE and Sc by ICP-AES at GeoForschungsZentrum-Potsdam, Germany (methods in Appendix A). FEL, SELand TEL are Pan de Azucar first, second and third eruptive events, respectively.

925


Fig. 7. Variation diagrams of selected trace elements vs SiO2 for samples of the northern Puna first (UOLM) and second cycle (MM) centres.It should be noted that for a given SiO2, MM magmas (encircled, continuous line) are consistently more enriched in incompatible trace elementsthan the UOLM counterparts.

Fractionation in upper-crustal magma for selected centres of the UOLM (Pirurayo, Cabreria)chambers and MM (Pan de Azucar) groups. The results suggest

that 20–50% crystallization can account for the majorThe petrographic characteristics and major element com-positional variations in pre-Upper Miocene volcanic element variations observed in these centres. The model

mineral proportions are similar to those observed incentres from this study, along with the limited isotopicvariation within individual centres, are compatible with diorite inclusions, supporting the inference that the

inclusions represent crystal accumulations. The frac-fractional crystallization as the dominant process inmagma evolution. tionation probably took place in upper-crustal

magma chambers, judging from the dominance ofTable 10 shows the results of least-squares modellingdesigned to test the fractional crystallization hypothesis plagioclase in the fractionating assemblages (Table

926


Fig. 8. Ba/Nb vs Nb diagram for northern Puna magmas. The plotted arc field combines data from Worner et al. (1988), Feeley & Davidson(1994), Matthews et al. (1999) and Trumbull et al. (1999). The field of Bolivian back-arc mafic centres includes the minor centres from centraland eastern Altiplano (>67°W; Davidson & de Silva, 1995). The Bolivian Tin Belt trend is plotted for comparison (Halls & Schneider, 1988).

10) and its abundance as a phenocryst phase in the characteristics of basement rocks and mantle in therocks. Central Volcanic Zone. This information is at present

Ideal fractional crystallization is an oversimplification limited by the lack of detailed basement studies in thein some cases. The Pan de Azucar centre, in particular, northern Puna and, more fundamentally, by a lack ofshows petrographic and compositional evidence for local deep exposures and paucity of mantle-derived rocks.disequilibrium (zones with sieve texture in plagioclase Nevertheless, existing information allows some usefulcontaining high-Mg biotite), which we attribute to influx constraints to be made, as follows:of a more mafic magma before eruption. The minor (1) outcrops of pre-Cenozoic basement in the southerndecrease in initial Sr isotope ratios and increase in Nd Puna and in northern Chile show dominantly felsicisotope ratio between the second-event and third-event compositions, with an estimated abundance of <5 vol.lavas at Pan de Azucar is consistent with this hypothesis % metabasites (Becchio et al., 1999; Lucassen et al., 1999a,(PA-6 and Iso-2 in Table 9, respectively). Caffe (1999) 2001). This is also expected to be the case for the lowershowed that the isotopic variations are consistent with crust, on the basis of the dominance of felsic compositions5–7 wt % admixing of a mafic magma similar to the among lower-crustal xenoliths recovered from Salta RiftMaquinas basalt (see below). Therefore even in this case, volcanic rocks (Lucassen et al., 1999b) and from gravityfractional crystallization is the controlling process for and seismic velocity data (Wigger et al., 1994; Zandt etwithin-centre chemical variability. al., 1994; Gotze & Kirchner, 1997; Graeber & Asch,

1999; Swenson et al., 2000). Compilations of chemicaland isotopic data from well over 200 analyses of basement

Source constraints: arc magma granitoids and felsic gneisses (Lucassen et al., 2001) givea good estimate of mid- to upper-crust composition incontamination vs crustal meltingthis region.Fractional crystallization in pre-eruptive magma cham-

(2) The basement in the northern Puna is also mostlybers can explain most of the within-centre chemicalintermediate to felsic in composition and of Lower Palaeo-variations, but differences in trace element contents andzoic age but it has been comparatively poorly studied inisotope ratios between centres, and in particular theterms of its geochemical and isotopic composition. Thedifferences between the UOLM and MM magmaticfew data available (Coira & Barbieri, 1989; Becchio etgroups as a whole, must represent specific characteristicsal., 1999; Coira et al., 1999), suggest low Sr contentsof magma source and/or magma evolution in the north-(25–200 ppm) and radiogenic 87Sr/86Sr ratios (0·720–ern Puna back-arc. A meaningful appraisal of potential0·760) that are similar to the average values of Lucassensources and/or contaminants for these magmas depends

on information about the compositional and isotopic et al. (2001). The only reported Nd isotopic data for

927


Fig. 9. Chondrite-normalized REE diagrams for UOLM (a) and MM (b) samples. Shaded pattern show the range of all data, and symbolsshow representative individual samples.

northern Puna basement rocks are from crustal xenoliths to assess the composition of the mantle component in-volved in magma genesis. Mafic magmas erupted afterincluded in ignimbrites from Coranzuli (Becchio et al.,

1999) and Panizos (Ort et al., 1996), which range between 20 Ma typically have basaltic andesite compositions, andtheir trace element and isotopic compositions show clear�Nd −8 and −15. The few Sr and Nd values available

are similar to those of the better-studied basement in the evidence for crustal contamination (Kay et al., 1994a;Davidson & de Silva, 1995). The least-contaminatedsouthern Puna (Becchio et al., 1999) and contiguous

region of northern Chile (Fig. 11). Further evidence for mafic magmas known in the greater region pre-date theAndean orogenic cycle (Upper Oligocene Chiar Khollua broadly homogeneous felsic upper-crustal composition

in this region is given by a study of siliciclastic sedimentary basalt, Bolivia: Davidson & de Silva, 1995; Cretaceousbasanites from the Salta Rift, Argentina, and the Oli-rocks from Ordovician basins (Bock et al., 2000) in the

northern Puna and Altiplano. gocene Maquinas alkaline basalt from 31°S in Chile:Kay et al., 1999). The compositions of these rocks suggest(3) The felsic lower-crustal granulites from the Salta

Rift reported by Lucassen et al. (1999b) have moderate that the mantle source was isotopically depleted at thebeginning of the Andean orogenic cycle (see Fig. 11).87Sr/86Sr ratios (0·7131–0·7143) and very low Rb/Sr

(>0·04) compared with the outcropping basement, but On the basis of the evidence cited above, neither theexposed basement rocks nor the felsic granulite xenoliths,similar Nd and Pb isotope ratios ( 144Nd/143Nd= 0·5121;

206Pb/204Pb = 18·4–18·5; 208Pb/204Pb = 39·5). which may reflect lower-crustal compositions, could rep-resent the sole source for the northern Puna volcanic(4) Neogene basalts are exceedingly rare in the Central

Volcanic Zone and there is little direct evidence available rocks from this study. The main problem with the mid-

928


melting (>60–70%) would be needed if these lithologieswere the sole magma source. It seems unlikely that thisextent of melting could be achieved thermally, apartfrom the likelihood that melt would separate from itssource well before 50% melting is achieved (Vigneresseet al., 1996).

The 87Sr/86Sr ratios of the MM centres Pan de Azucarand Chinchillas (0·709–0·710) are considerably lowerthan those of the UOLM-2 rocks (Figs 11 and 12),making a pure crustal source even less likely. The raremetabasic rocks from the basement overlap with the MMrocks in terms of Sr isotope ratios but their initial 144Nd/143Nd ratios are much higher (>0·5124, Becchio et al.,1999; Lucassen et al., 1999b) and rule them out as apotential source.

Therefore we conclude that none of the magmasinvolved in the northern Puna volcanic centres could bederived from the basement by crustal melting alone.They must be hybrid magmas and the next sectionpresents bulk mixing and assimilation–fractional crys-tallization (AFC) models to explore probable componentsinvolved and their relative proportions.

Bulk mixing and AFC models

In the context of this study, the bulk mixing process isenvisioned to involve mantle-derived arc basaltic magmasinteracting with partial melts of the lower- or mid–upper-crustal lithologies. Pure mixing is a limiting case unlikelyto occur in practice, as the crustal melts will be coolerthan basaltic magma and some crystallization of the latteris expected to accompany the mixing. The AFC modelstake this crystallization into account.

The mafic end-member composition used in the mod-elling is based on the Maquinas basalts (Kay et al., 1999)Fig. 10. (a) La/Sm–Sm/Yb diagram of northern Puna volcanic rocks

showing the increase in both ratios from the UOLM (20–17 Ma) to except for the Pb isotope ratios, which are considered tothe MM (14–12 Ma) groups. (b) La/Yb vs SiO2 diagram, showing that have been affected by crustal contamination. For Pb, wethe changes in La/Yb do not correlate with differentiation. Plotted for

use isotope ratios and concentrations from the Cretaceouscomparison are the 15 Ma dacites erupted in the Altiplano–Punaboundary region (South Lıpez–northern Tiomayo tuffs; Fornari et al., basanites from the Salta Rift (Kay et al., 1999). Two crustal1993; Coira et al., 2002), the 10–7 Ma Vilama ignimbrite rocks (Coira components are considered. One represents partial meltset al., 1996), and the 7–6 Ma Panizos ignimbrite (Ort et al., 1996). of the mid- to upper crust and for this we use the mean

composition of Palaeozoic granites from northern Chilefrom Lucassen et al. (1999a), which are considered toto upper-crustal basement rocks is their highly radiogenicrepresent crustal melts of the local basement. The secondSr isotopic compositions. The felsic granulite xenolithscomponent is based on an average composition of felsicfrom the Salta Rift have appropriate Sr isotope ratiosgranulites from the Salta Rift, which may be morefor potential source material for the UOLM-2 volcanicappropriate for assimilation of lower crust. The tracerocks, but other compositional features of the granuliteselement concentrations and isotope ratios chosen for theare inconsistent with this hypothesis. For example, thecrustal components are shown in Appendix B and TableNd and Pb isotopic ratios of the volcanic rocks ( 144Nd/11, respectively.143Nd >0·5122; 206Pb/204Pb >18·7; 208Pb/204Pb >38·9)

Mixing models using these end-member compositionsdiffer from those of the granulites ( 144Nd/143Nd= 0·5121;can match the isotopic ratios of the different groups of206Pb/204Pb = 18·4–18·5; 208Pb/204Pb = 39·5).northern Puna volcanic rocks (e.g. 30 wt % crustalFurthermore, the UOLM-2 rocks have only slightlycomponent in UOLM-1, 50 wt % in MM and 70 wt %higher incompatible element contents than the felsic

basement and xenoliths, and a very high degree of partial crust in UOLM-2 samples; see Table 11, models D, G

929


Table 9: Whole-rock Sr, Nd, and Pb isotope data for northern Puna volcanic rocks

Sample Group Age 87Sr/86Sr 87Sr/86Sr(T)143Nd/144Nd �Nd(0) �Nd(T) tDM

206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

(Ma) measured measured (Ga) measured measured measured

Ce-2 UOLM-1 20 0·706899 (± 8) 0·706553 0·512466 (± 5) −3·4 −3·2 0·95 18·886 15·685 39·159

Ce-5 UOLM-1 20 0·706965 (± 8) 0·706606 0·512455 (± 4) −3·6 −3·4 1·01 18·935 15·663 39·136

Pv-1 UOLM-2 18 0·713063 (± 7) 0·712777 0·512171 (± 5) −9·1 −8·9 1·39 18·541 15·677 38·710

CCL-1 UOLM-2 17 0·713686 (± 8) 0·713027 0·512241 (± 5) −7·7 −7·6 1·56 18·965 15·704 39·174

CCLD-4 UOLM-2 17 0·715379 (± 5) 0·714570 0·512231 (± 6) −7·9 −7·8 1·35 18·928 15·681 39·079

CCLD-2 UOLM-2 17 0·713287 (± 8) 0·713060 0·512202 (± 5) −8·5 −8·4 1·46 18·626 15·683 38·775

Min-9 UOLM-2 17 0·713246 (± 6) 0·713004 0·512184 (± 4) −8·9 −8·7 1·39 18·560 15·694 38·766

PA-6 MM 12 0·710249 (± 7) 0·710104 0·512256 (± 5) −7·5 −7·3 1·25 18·787 15·691 38·983

PA-16 MM 12 0·710306 (± 8) 0·710169 0·512276 (± 6) −7·1 −6·9 1·22 18·764 15·681 38·947

PA-38 MM 12 0·709892 (± 8) 0·709758 0·512277 (± 5) −7·0 −6·9 1·16 18·767 15·699 39·005

iso-2 MM 12 0·709161 (± 8) 0·709029 0·512287 (± 4) −6·9 −6·7 1·11 18·713 15·669 38·893

MCh-55 MM 13 0·710919 (± 8) 0·710667 0·512242 (± 5) −7·7 −7·6 1·22 18·656 15·652 38·814

Analytical details in Appendix A.

Fig. 11. Initial 87Sr/86Sr and �Nd values for northern Puna volcanic rocks. Symbols as in Fig. 5. Also plotted for comparison are: Cenozoicbasalts from the CVZ (Maquinas and Segerstrom basalts: Kay et al., 1999; Chiar Khollu basalt: Davidson & de Silva, 1995), volcanic rocksfrom Southern Volcanic Zone (SVZ) (Hickey et al., 1986; Hildreth & Moorbath, 1988), Puna basaltic andesites (Kay et al., 1994a), Maricungabelt and southern Central Volcanic Zone (Kay et al., 1994b; Trumbull et al., 1999), Chilean APVC ignimbrites (Lindsay et al., 2001; Schmitt etal., 2001) and Panizos ignimbrite (Ort et al., 1996). Isotopic data for basement rocks (measured ratios) are from Becchio et al. (1999) and Lucassenet al. (1999a, 1999b); variations as a result of in situ growth of radiogenic daughter isotopes over the last 20–12 Ma are negligible.

930


Fig. 12. Variation diagram of Sr vs 87Sr/86Sr for Cenozoic intermediateto acidic volcanism (>60% SiO2) in the northern Puna. Most of thenorthern Puna rocks plot along two trends (thin dashed and continuousarrows) consistent with upper- or lower-crust contamination. The boldarrow shows a different variation trend linking the MM samples withthe field for other Mid-Miocene centres. Huayra Huasi porphyry,Tiomayo and Aguiliri from B. L. Coira & Ch.-H. Chen (unpublisheddata, 2000). Upper Miocene volcanic rocks compiled from Coira &Barbieri (1989) and Ort et al. (1996) for Panizos, Vilama–Coruto andCoranzuli magmatic systems. Basement rocks as in Fig. 11.

Fig. 13. Pb isotopic ratios of UOLM and MM northern Puna magmasand J, and Table 12) but they generally fail to account and, for comparison, Arequipa massif (Tilton & Barreiro, 1980), Nazca

basalts and sediments (Unruh & Tatsumoto, 1976), Plio-Pleistocenefor the observed trace element concentrations and webasaltic andesites and Cretaceous basanites from the Puna (Kay et al.,consider mixing to be an unsatisfactory explanation for1994a), Panizos ignimbrite (Ort et al., 1996), southern Altiplano and

the hybridization process. A possible exception to this is Eastern Cordillera basement, and 17–19°S frontal volcanic arc (com-pilation by Aitcheson et al., 1995). Also shown: Pan de Azucar ore datathe UOLM-2 rocks, where the mixing and AFC results(Zentilli et al., 1988), southern Puna basement (Becchio et al., 1999) and(below) both yield reasonable solutions.Puna lower-crustal granulites (Lucassen et al., 1999b).Conventional AFC modelling followed the equations

of De Paolo (1981) and the refinement by Aitcheson &Forrest (1994). The bulk distribution coefficients (D val- a lower angle than for UOLM-1 and the estimates for rues) used in the calculations were based on the stable and � are allowed to vary from 0·3 to 0·7. To avoidmineral assemblages predicted for crystallization of the confusion, we point out that the values for crustal as-Maquinas basalt composition using the MELTS program similation given below express the mass of assimilantof Ghiorso & Sack (1995). Specifically, D values were relative to the mass of original magma [Ma/Mm° in thederived for the solid assemblage after 40–60 wt % crys- nomenclature of DePaolo (1981)]. The amount of crustaltallization at various pressure conditions and oxygen material in the final hybrid magma is greater thanfugacity fixed at the Ni–NiO buffer (Appendix B). The this by a factor depending on the degree of fractionalbulk D values of Sr, in particular, are sensitive to the crystallization.pressure of crystallization, as this affects the proportion The AFC model for Pirurayo rocks, as representativeof plagioclase in the assemblage. The graphical method for the UOLM-1 group, is consistent with derivation fromof Aitcheson & Forrest (1994) was used to find reasonable a Maquinas-type basalt with 15–25% crustal assimilation.estimates of the variables r (ratio of assimilation to crys- Both the felsic granulite (lower-crust) and granite (mid-tallization rates) and � (proportion of assimilated material) to upper-crust) compositions yield similar solutions (Tableby simultaneous solution of the AFC equations for several 11, models A–C). The bulk D values for Sr, Ba, Rb andtrace elements and isotope ratios. Figure 14 shows results Nd that fit the Pirurayo data are those for a crystallizationfor the UOLM-1 samples, where mutual intersections of assemblage at a pressure of 7 kbar, where the ratio ofthe AFC curves indicate values of both r and � between clinopyroxene to plagioclase is about 4:1. At lower pres-0·2 and 0·3. The compositions of UOLM-2 and MM sure (<5 kbar), the proportion of plagioclase increases at

the expense of clinopyroxene and this results in lowgroup samples are such that the AFC curves intersect at

931


Table 10: Major element fractional crystallization models

Centre: Pan de Azucar (MM)

Parent Daughter Calc. Residual Solution Parent Daughter Calc. Residual Solution

Ro-4 PA-52 parent phases PA-46 iso2-gl36 parent phases

Rock type: TEL SEL TEL mtx-glass

SiO2 65·64 70·25 65·90 −0·10 Pl 56% 69·94 79·77 69·94 0·00 Pl 50%

TiO2 0·8 0·69 0·92 −0·12 Bt 35% 0·74 0·21 0·68 0·06 Bt 21%

Al2O3 16·74 15·35 16·49 0·12 Mag 6% 14·84 12·08 14·85 −0·01 Hbl 16%

FeO 4·74 3·14 4·73 0·01 Hbl 2% 3·50 0·32 3·50 0·00 Qtz 6%

MnO 0·06 0·02 0·03 0·03 Ap 1% 0·05 0·04 0·05 0·00 Mag 3%

MgO 1·64 0·81 1·57 0·07 F = 0·83 1·62 0·06 1·64 −0·02 Ap 2%

CaO 3·42 2·46 3·40 0·02 �r 2 = 0·20 3·33 0·46 3·32 0·01 F = 0·68

Na2O 2·67 2·75 2·66 0·01 1·73 1·25 1·74 −0·01 �r 2 = 0·005

K2O 3·97 4·21 3·57 0·40 3·97 5·80 3·96 0·01

P2O5 0·31 0·32 0·33 −0·02 0·29 0·00 0·30 −0·01

Centre: Pirurayo complex (UOLM-1) Cabrerıa Fm. (UOLM-2)

Parent Daughter Calc. Residual Solution Parent Daughter Calc. Residual Solution

Ce-2 Ce-5 parent phases CClD-2 Po-r parent phases

Rock type: AL RL Basal Top

SiO2 60·78 70·37 60·86 −0·03 Pl 69% 64·00 67·10 64·02 −0·01 Pl 48%

TiO2 0·86 0·63 0·81 0·04 Opx 17% 0·78 0·61 0·68 0·11 Opx 27%

Al2O3 18·01 14·65 18·09 −0·04 Hbl 6% 16·67 16·23 16·83 −0·08 Bt 23%

FeO 6·24 4·68 6·25 −0·01 Cpx 3% 5·12 3·82 5·12 −0·01 Ap 1%

MnO 0·13 0·10 0·11 0·02 Mag 5% 0·05 0·09 0·13 −0·08 Mag 1%

MgO 2·88 0·75 2·85 0·03 F = 0·54 2·61 1·61 2·58 0·03 F = 0·85

CaO 6·06 3·27 5·98 0·09 �r 2 = 0·070 5·03 4·18 4·95 0·07 �r 2 = 0·057

Na2O 2·81 2·10 2·78 0·03 2·34 2·43 2·21 0·13

K2O 1·98 3·23 1·74 0·24 3·20 3·69 3·14 0·06

P2O5 0·21 0·24 0·30 −0·08

Nomenclature as in Table 3. Basal and Top indicate position of ignimbrites within Cabrera Fm. volcanic sequence. F, wt %volume of remaining parent magma after fractional crystallization; �r 2, sum of the squares of residuals. Mineral abbreviationsafter Kretz (1983).

model Sr concentrations and high Ba, Rb, Nd con- (Table 11, model E) for the lower-crustal assimilant,corresponding to >55–60 wt % crustal component incentrations (Table 11, model C, and Appendix B). The

data for Sm, Zr and Nb are not satisfied by any of the the hybrid magma, or >40 wt % (�>0·75) if the moreradiogenic, Sr-poor granite composition is used for theAFC models. Modelled Sm is too low, perhaps because

bulk D has been overestimated, and the modelled Zr assimilant. Like the UOLM-1 solutions, bulk D valuescalculated for fractionation at 7 kbar give the best fit toand Nb contents are too high, perhaps because the

crystallization of accessory minerals is not considered. data overall. The AFC solutions for La, Nd, Yb, Ba andRb match or slightly exceed observed values regardlessThe UOLM-2 compositions can be reasonably ex-

plained by AFC models using the same end-members as of whether granulitic or granitic assimilants are assumed(see Table 12) but the model Sr concentrations are toofor UOLM-1, but with a higher ratio of assimilation to

fractionation (r = 0·6–0·8). The Aitcheson & Forrest low if granite is used as the assimilant (except for CasaColorada, which has very low Sr). As in the UOLM-1(1994) solution suggests values of � between 1·2 and 1·6

932


Tab

le11

:A

FC

and

bulk

mix

ing

mod

elling

ofea

rly

Cen

ozoi

cvo

lcan

icro

cks

from

the

nort

hern

Pun

a

For

UO

LM-1

mag

mas

For

UO

LM-2

mag

mas

For

MM

mag

mas

AB

CD

Ce-

2E

FG

CC

lD-2

HI

Jis

o-2

F(l

iqu

idre

mai

nin

g)

0·5–

0·6

0·6

0·5

0·7

0·5–

0·6

0·5

0·3

0·4–

0·5

0·4

0·5

r(r

ate

assi

mila

tio

n

frac

tio

nal

crys

talli

zati

on

)0·

40·

30·

30·

7–0·

80·

60·

60·

5

Sr

par

ent

500

500

500

520

500

500

520

500

500

520

Sr

con

tam

inan

t30

010

010

019

430

010

019

432

010

019

4

Sr

pro

du

ct(p

pm

)39

3–47

845

325

742

248

029

4–37

020

029

234

054

3–68

640

335

754

487

Sr/

86S

rp

aren

t0·

704

0·70

40·

704

0·70

40·

704

0·70

40·

704

0·70

40·

704

0·70

487

Sr/

86S

rco

nta

min

ant

0·71

50·

736

0·73

60·

725

0·71

50·

736

0·72

50·

715

0·73

60·

725

87S

r/86

Sr

pro

du

ct0·

7071

–0·7

061

0·70

590·

7071

0·70

690·

7066

0·71

33–0

·712

50·

7134

0·71

370·

7131

0·71

03–0

·709

30·

7099

0·70

970·

7090

Nd

par

ent

1616

1616

1616

1616

1616

Nd

con

tam

inan

t38

4040

3330

3033

3030

33

Nd

pro

du

ct30

–26

2430

2030

40–3

734

2627

42–3

237

2342

�Nd

par

ent

44

44

44

44

44

�Nd

con

tam

inan

t−

10−

14−

14−

11−

10−

10−

11−

10−

10−

11

�Nd

pro

du

ct−

2·5

to−

3·5

−2·

4−

3·4

−2·

7−

3·2

−8·

0to−

8·4

−8·

9−

8·2

−7·

6−

6·7

to−

6·9

−7·

9−

5·8

−6·

7

Pb

par

ent

1212

1212

1212

1212

1212

Pb

con

tam

inan

t25

3535

3525

3535

2535

35

Pb

pro

du

ct20

–16

2124

1928

29–2

832

2826

26–2

536

2420

206 P

b/20

4 Pb

par

ent

19·1

019

·10

19·1

019

·10

19·1

019

·10

19·1

019

·10

19·1

019

·10

206 P

b/20

4 Pb

con

tam

inan

t18

·50

18·4

018

·40

18·6

018

·50

18·4

018

·60

18·5

018

·40

18·6

020

6 Pb

/204 P

bp

rod

uct

18·7

9–18

·82

18·7

318

·68

18·8

818

·86

18·5

5–18

·57

18·5

319

·04

18·6

118

·65–

18·6

218

·57

18·9

718

·69

�(m

ass

of

cru

st/m

ass

of

ori

gin

alm

agm

a)0·

33–0

·27

0·17

0·21

1·16

–1·6

00·

750·

90–0

·75

0·60

Par

enta

lmag

ma

com

po

siti

on

sco

rres

po

nd

toM

aqu

inas

bas

alt

[(M

B)

Kay

etal

.,19

99,A

pp

end

ixB

],ex

cep

tfo

rP

bis

oto

pes

,wh

ich

wer

eta

ken

fro

mu

nco

nta

min

ated

bas

anit

esfr

om

the

Pu

na

(see

text

).A

FCm

od

ellin

g:

assi

mila

nts

are

low

er-c

rust

alg

ran

ulit

icxe

no

lith

s(L

uca

ssen

etal

.,19

99b

)fr

om

the

Sal

taR

ift

(SR

X),

or

Pal

aeo

zoic

gra

nit

oid

s(P

zG:

sam

ple

qtj

-7in

Ap

pen

dix

B;

Bec

chio

etal

.,19

99).

Mo

del

sA

and

E:c

on

tam

inat

ion

of

MB

wit

hS

RX

at7

kbar

.M

od

els

Ban

dF:

assi

mila

tio

no

fP

zGat

7kb

ar.M

od

elC

:as

sim

ilati

on

of

PzG

at5

kbar

.Mo

del

H:c

on

tam

inat

ion

of

MB

wit

hS

RX

at10

kbar

.Mo

del

I:co

nta

min

ant

isP

zGat

10kb

ar.P

aram

eter

su

sed

inA

FCm

od

ellin

gar

efr

om

DeP

aolo

(198

1),

wh

ere

Fis

the

pro

po

rtio

no

fm

agm

are

mai

nin

gaf

ter

assi

mila

tio

n–f

ract

ion

alcr

ysta

lliza

tio

n;

r=

(Ma/

Mc)

:ra

teo

fas

sim

ilati

on

/rat

eo

fcr

ysta

lliza

tio

n.

Valu

eso

fK

dan

db

ulk

Dar

eg

iven

inA

pp

end

ixB

.T

he

par

amet

er�

(mas

so

fcr

ust

/mas

so

fo

rig

inal

mag

ma)

was

calc

ula

ted

follo

win

gA

itch

eso

n&

Forr

est

(199

4).

Wh

entw

ova

lues

are

giv

enth

eyre

pre

sen

tth

era

ng

eo

bta

ined

fro

md

iffe

ren

tF

or

ru

sed

.B

ulk

mix

ing

mo

del

ling

:M

od

els

D–G

and

Jcr

ust

alco

mp

on

ent

isav

erag

eP

alae

ozo

icg

ran

ites

fro

mLu

cass

enet

al.

(199

9a)

and

Bec

chio

etal

.(1

999)

,th

ou

gh

tto

rep

rese

nt

mid

-to

low

er-c

rust

alm

elts

.T

he

par

amet

erF

isth

ep

rop

ort

ion

of

ori

gin

alm

afic

mag

ma

(MB

)p

arti

cip

atin

gin

the

mix

ing

pro

cess

.Fo

rco

mp

aris

on

:C

e-2,

and

esit

efr

om

Pir

ura

yo;

CC

lD-2

,d

acit

icp

um

ice

fro

ma

Cab

reri

aFm

.ig

nim

bri

te;

Iso

-2,

thir

dev

ent

dac

itic

lava

fro

mP

and

eA

zuca

r(T

able

8).

933


Table 12: Trace element AFC and bulk mixing modelling for northern Puna magmas

Rb Sr Ba Zr Nb La Nd Sm Yb

For UOLM-1 magmas

AFC SRX 7 kbar 61 466 478 305 32 31 25 3·4 1·9

r = 0·4

AFC PzG 7 kbar 91 400 520 233 27 31 25 3·8 1·6

r = 0·3

AFC PzG 5 kbar 120 303 652 266 31 36 31 4·6 1·9

r = 0·3

Mixing 70 MB–30 CM 70 407 337 140 13 21 25 4·8 2·4

Average UOLM-1 96 394 546 132 12 30 25 5·0 2·1

For UOLM-2 magmas

AFC SRX 7 kbar 116 413 900 613 60 66 36 4·0 2·6

r = 0·7

AFC PzG 7 kbar 218 277 989 340 36 62 38 5·2 2·1

r = 0·6

Mixing 30 MB–70 CM 118 256 509 170 14 34 32 6·2 3·2

Average UOLM-2 144 329 631 130 12 32 28 5·9 2·1

For MM magmas

AFC SRX 10 kbar 116 680 937 456 57 69 34 3·6 1·5

r = 0·6

AFC PzG 10 kbar 233 480 1074 298 40 68 37 4·7 1·2

r = 0·5

Mixing 50 MB–50 CM 93 332 423 155 13 28 28 5·5 2·7

Average MM 161 591 850 191 18 50 39 7·4 1·6

AFC and mixing parameters from Table 11. Mafic (original) magma is Maquinas basalt (Appendix B).AFC SRX: the average result for Salta Rift granulites as assimilant (compositions in Appendix B). UOLM-1: with A-55 and4-302, F = 0·6; 4-304a, F = 0·5. UOLM-2: with 4-302 and 4-304a, F = 0·6; A-55, F = 0·5. MM: with 4-302 and 4-304a, F =0·5; A-55, F = 0·4.AFC PzG: the average result for Palaeozoic granitoids (Appendix B) as assimilant. UOLM-1: with qtj-7 and 4/84, F = 0·6(7 kbar); with 4/84, F = 0·5 (5 kbar). UOLM-2: with qtj-7, F = 0·5; 4/84, F = 0·6. MM: with av. qtj-7, F = 0·4; 4/84, F = 0·5.Mixing models: CM is crustal melts (average PzG, Appendix B). MB is mantle basalt (Maquinas basalt, Appendix B).

case, the model Sm contents are too low, and Zr and Maquinas basalt at 10 kbar (F = 0·5) is 0·94 clino-pyroxene, 0·03 plagioclase and 0·03 garnet (AppendixNb are too high.

The AFC solutions for the MM rocks (Table 11, B). The corresponding values for DSr, DNd, DPb and DRb

produce a good fit to the observed MM data (Table 12).models H and I) indicate proportions of crustal materialin the hybrid magma of >50% using a granulite com- The model results are reasonable for both granulitic or

granitic assimilants but the granulite composition yieldsposition for the assimilant and >40% for the moreradiogenic granitic composition. One important differ- a better fit for Yb and Sr.

In all the AFC models calculated for UOLM and MMence between the MM and UOLM-2 models concernsthe bulk D values for Sr. The Sr concentrations in MM groups, the solutions failed to match Zr, Nb and Sm

concentrations although the fit to other trace elementsrocks are higher than UOLM-2, and if we assume thesame end-member compositions for both, the values for is acceptable (Table 12). We suggest that this is due to

an oversimplification in formulating the bulk distributionDSr must be lower for the MM case than for UOLMmodels (appropriate ranges of DSr are 0·54–0·73 for coefficients. Only clinopyroxene, plagioclase and garnet

were considered (Appendix B), and neglecting accessoryMM, 0·90–1·13 for UOLM, Appendix B). The MELTScalculations show that the fractionating assemblage of phases or oxides that host these elements would lead to

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Fig. 14. Example of AFC solution curves plotted following the equations from Aitcheson & Forrest (1994) for UOLM-1. Components used area pristine mafic magma similar to Maquinas basalt and a lower-crustal granulite. D values used are given in Appendix B (see text).

bulk D values that are too low, and thus melt con- region. The D values for models that best fit the observedcentrations that are too high. data correspond to a fractionation assemblage stable at

>7 kbar according to the MELTS solution for Maquinasbasalt, which implies roughly the same mid- to lower-crustal depths as for the UOLM-1 group.Preferred models

The MM dacite compositions cannot be explained byThe model results show that the composition of Pirurayobulk mixing and the acceptable AFC solutions (Tablesandesites (UOLM-1) can be explained by fractional crys-11 and 12) are similar to those for the UOLM-2 rockstallization of a mafic arc magma like the Maquinas basalt(40–50% assimilation). The modelling is consistent withwith a moderate degree assimilation (15–25%) of crustalthe concept that both magma groups could have beenmaterial similar compositionally to the Palaeozoic gran-generated from the same basaltic end-member and similarites or lower-crustal granulites of the known local base-crustal components. If this is the case, the observedment. Simple mixing of basaltic and crustal componentsdifferences in trace element concentrations can be in-does not fit the observed data. The El Morro quartz-terpreted in terms of different physical conditions of theandesitic intrusive rocks have the same trace elementAFC process. According to the MELTS crystallizationcharacteristics as the Pirurayo andesites and may havemodelling, the higher degree of incompatibility for Sr,formed in a similar way. The observed UOLM-1 com-Ba and LREE in the MM group magmas compared withpositions are best fitted by an AFC model with low rthe UOLM-2 group, and the greater compatibility ofvalues (0·3) and pressures (7 kbar) representing mid- toHREE and Y, can be caused by fractionation of parentallower-crustal depths.basalts at higher pressures ([10 kbar vs 7 kbar). TheThe model results for the UOLM-2 group do not allow15 Ma South Lıpez ignimbrites [Morokho and Bonetea clear distinction between AFC and bulk mixing. Bothcentres at 21°30′S, Fornari et al. (1993) and Fig. 10] andsolutions give a reasonable fit to the data and the pro-the northern Tiomayo Formation tuffs at 22°S (Coira etportion of assimilated crust is roughly similar for bothal., 2002), have similar values of Sr, Ba, Nb, Zr, La/Sm(40–60% and 70%, respectively). However, AFC is con-and Sm/Yb to those of the contemporary MM rockssidered the more realistic process because the temperaturestudied here, and we suggest that their origin may bedifference between pristine basaltic magma and partialsimilar.melts of the crust should cause the former to cool and

It should be emphasized that the AFC models used tocrystallize during the mixing process. The AFC modellinggenerate Tables 11 and 12 are simple in the sense thatindicates a high assimilation to fractionation rate (r =they consider only mass-balance and assume that the0·6, Table 11), which implies a small thermal contrast

between mafic magma and assimilated crust in the source rate of assimilation and fractionation (the r parameter)

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is constant during the AFC process. Alternative and back-arc magmas is well established (e.g. Kay et al.,1994a; Davidson & de Silva, 1995) but its application topotentially more realistic AFC models by Spera & Bohr-the UOLM and MM rocks is problematic because theirson (2001) consider energy constraints as well as masstrace element ratios may reflect variations in degree orbalance. In these energy-constrained AFC models (EC-type of crustal assimilation as well as source magmaAFC), assimilation is assumed to occur only via partialvariations. Nevertheless, the UOLM and MM samplesmelts of the wallrocks, and therefore the r parameterdo show systematic differences in Ba/Nb (Fig. 8) anddepends on the solidus temperature and melt productivityZr/Nb (Table 8) ratios as described above. In particular,in the wallrocks. A full comparison of the two methodsthe UOLM magmas have Ba/Nb ratios in the range ofis beyond the scope of this paper and may not bethe recent CVZ arc and this is also true for those MMmeaningful for the northern Puna example because thecentres located in the western part of the study area.EC-AFC models require constraints on melt productivityThe eastern MM centres (Pan de Azucar and Huayraand extraction for the crustal component, and these areHuasi) have lower Ba/Nb ratios in the range of recenthighly uncertain. We performed a limited comparison ofback-arc mafic centres. If the effects of crustal assimilationthe two methods based on Sr concentration and isotopeand fractionation on the Ba/Nb ratios are minor or ifratios for the cases presented in models B (UOLM-1)they affected the UOLM and MM magmas equally, thenand F (UOLM-2) of Table 11. For the EC-AFC cal-these difference in Ba/Nb are evidence that the frontalculation heat capacity and enthalpy values for basalticarc migrated westward with time and that the back-arcmagma and crustal assimilant were taken from Bohrsonsetting which the study area now occupies (Fig. 1) became& Spera (2001, table 4), the basaltic magma temperatureestablished in the mid-Miocene. This conclusion is some-was 1280°C and the initial crustal temperature waswhat speculative but we note that Ba/Nb ratios of600°C. The crustal assimilant was assumed to haveandesitic UOLM-1 rocks are similar to those of daciticsolidus and liquidus temperatures of 700°C and 900°C,UOLM-2 centres with at least 50% crustal material.respectively, and a linear increase in melt productionThis suggests that the effect of crustal assimilation andbetween them. Bulk DSr for crustal melting was set atfractionation on the Ba/Nb ratio may in fact be minor.unity, as the assimilant composition used (Palaeozoic

The AFC models discussed above imply that the MMgranite average) already represents a crustal partial melt.group magmas may have undergone hybridization in theThe EC-AFC solution indicates >20% crystallizationcrust at a higher pressure than the UOLM-1 and UOLM-before assimilation begins. This causes slight changes in2 magmas (>10 kbar vs 7 kbar). This inference dependselement concentrations in the magma, and once as-on several assumptions that cannot be directly testedsimilation begins, the compositional trends of the hybridbut it is consistent with tectonic interpretations, whichmagma are similar to those for conventional AFC models.consider the Middle Miocene as the main time forA useful parameter for comparing the two model resultsintracrustal shortening and thickening of the Andeanis the degree of assimilation needed to achieve a givenorogen (Gubbels et al., 1993; Cladouhos et al., 1994;isotopic ratio in the hybrid magma. For the AFC modelHerail et al., 1994; Kley et al., 1996; Allmendinger et al.,B (Table 11), a ratio of 87Sr/86Sr = 0·7059 is reached1997). On the other hand, MM rocks lack the highafter 15 wt % assimilation (proportion of crust added toLREE/HREE ratios found in several post-Miocenethe original magma) and the EC-AFC solution yieldscentres from the Central Andes, which are suggested to20 wt % assimilation. For model F in Table 11, the AFCindicate extreme crustal thickness (e.g. the Vilama Cal-calculation gives 87Sr/86Sr = 0·7134 after 40 wt %dera in northern Puna with La/Yb >26–45: Coira et al.,crustal contamination and the EC-AFC solution requires1996; or the Cordillera Blanca batholith in southern Peru55 wt % assimilation to reach the same ratio. Consideringwith La/Yb >27–116: Petford & Atherton, 1996). Thethe uncertainty of assumptions inherent in both models,moderate ratios of MM dacites (La/Yb = 23–33) maywe conclude that the agreement between them is goodindicate that current crustal thicknesses were reachedand that the conventional AFC model appears adequateafter the Late Middle Miocene (Isacks, 1988; Cladouhosfor this application. More insights on the hybridizationet al., 1994; Allmendinger et al., 1997; Okaya et al., 1997)process might be gained from a full EC-AFC treatment,or, as argued by McMillan et al. (1993), that the effectbut this would require a better understanding of theof crustal thickening is recorded in hybrid magma com-composition and thermal state of the crust than is nowpositions only after a significant delay.available.

The large-volume Upper Miocene to Pliocene daciticignimbrites erupted in the Puna plateau and southernAltiplano are compositionally similar to the UOLM and

Regional implications MM centres studied here and they have also been sug-The use of incompatible trace element ratios (Ba/Nb, gested to form through AFC processes of crustal as-

similation by mafic arc magma (e.g. de Silva et al., 1993;Ba/Ta, La/Ta, Zr/Nb) to discriminate frontal-arc from

936


Ort et al., 1996; Lindsay et al., 2001; Schmitt et al., 2001). ratios of UOLM-2 and MM rocks overlap and haveThe degree of assimilation concluded by different workers considerable crustal affinity ( 87Sr/86SrT >0·710–0·713;varies, but there is consensus that the hybrid magmas �NdT −7 to −8). Pb isotope ratios are similar in thecontain of the order of 50% crust or more. The Upper three groups and are dominated by crustal Pb. ValuesMiocene Panizos ignimbrite (Ort et al., 1996), for example, overlap with those of other CVZ volcanoes and basementhas geochemical and isotopic compositions almost ident- in the Eastern Cordillera–Southern Altiplano domain ofical to those of the UOLM-2 magmas (Fig. 11). Other Aitcheson et al. (1995).caldera-sourced dacitic ignimbrites in the area have some- (4) Partial melting of known basement lithologies can-what different geochemical and isotopic features [i.e. not be the sole process for generating the UOLM orVilama Caldera in Fig. 10 and in the study by Coira et MM magmas. Most isotopic and trace element char-al. (1996)] but the first-order difference between the acteristics of the magmas can be explained by an AFCnorthern Puna centres from this study and the Upper process involving different degrees of fractional crys-Miocene–Pliocene ignimbrites from the same region is tallization of an arc basaltic magma similar in compositionthe enormous contrast in magma volumes. The increase to the Oligocene Maquinas basalt (Kay et al., 1999)in volume of hybrid magmas produced since the late and assimilation of felsic crustal melts. Successful AFCMiocene could be a consequence of several processes solutions suggest that the proportion of crustal materialthat are interrelated: (1) a progressive increase in the increased from 15–25% in UOLM-1 magmas to 40–60%volume of asthenospheric mantle and extension in the for UOLM-2 and MM magmas, but cannot, in general,back-arc, both caused by arc migration to the west and distinguish between a felsic granulite (lower crust) orsteepening of the subduction dip and/or by lithospheric granitic assimilant (mid- to upper crust).delamination following crustal thickening (Coira et al., (5) The dependence of bulk D values on pressure,1993; Allmendinger et al., 1997); (2) changes in regional especially for Sr, suggests that the depth of magma genesisstress conditions (from compressive to tensional or strike- by AFC was greater for the MM group magmas thanslip) in the northern Puna crust since 10–8 Ma (Allmen- for the UOLM group (10 vs 7 kbar). This increase indinger et al., 1997; Riller et al., 2001), which enhanced

pressure may reflect the influence of tectonic shortening,and focused emplacement of mafic magmas into thewhich was active since at least the mid-Miocene (Allmen-crust; (3) bulk intracrustal convection causing efficientdinger et al., 1997).heat transport into compositionally fertile regions of the

(6) Ratios of Ba/Nb and Zr/Nb in the UOLM groupmid-crust (Babeyko et al., 2002).are fairly homogeneous and resemble values from thepresent arc, whereas the MM centres show arc-like ratiosin a western group and intraplate affinity in an easterngroup. This suggests that the westward shift of the arcCONCLUSIONStowards its present position in the western CordilleraThe pre-Upper Miocene volcanism in the northern Punabegan in this region from 20–17 Ma and 14–12 Ma, andof Argentina produced small dome complexes, which arethat the back-arc position of the northern Puna wasgrouped by age into the UOLM (20–17 Ma) and MMprobably established since the mid-Miocene.(15–12 Ma) volcanic phases. The main results of geo-

We suggest that there is no fundamental difference,chemical and isotopic analyses of representative centrescompositionally, between magma genesis of the daciticfrom these two phases can be summarized as follows:centres reported here and the large ignimbrite sheets that(1) the UOLM phase produced two distinct groups oferupted from caldera complexes in the northern Punarocks, one (UOLM-1) mostly andesitic and metaluminoussince the Upper Miocene. The difference in volume andand the other (UOLM-2) more silicic and with higher andmode of eruption may instead reflect differences in crustalmore variable A/CNK ratios. The MM phase (14–12 Ma)stress and thermal state. In the pre-Upper Miocene, theproduced dacitic to low-Si rhyolitic rocks similar tostress pattern was uniformly compressive and this maythe UOLM-2 group but with higher concentrations ofhave restricted intrusion of mafic magmas to the lowerincompatible trace elements. Some of these centres arecrust. In contrast, the extension and transpressive stressesassociated with Ag–Pb–Zn mineralization.that prevailed since >8 Ma allowed mafic magmas to(2) Petrography, mineral chemistry and whole-rockintrude at more variable depths within a thickened crust.compositions indicate that the chemical variations withinRecent thermal–mechanical modelling by Babeyko et al.individual dome complexes reflect low-pressure fractional(2000, 2002) showed that mafic intrusions alone arecrystallization before eruption. This is consistent withnot enough to explain the large-scale crustal meltingminor isotopic variability within single centres.represented by the ignimbrites and present-day geo-(3) Sr- and Nd-isotopic compositions of UOLM-1physical anomalies in the Altiplano–Puna crust. Instead,rocks are distinctive and like those of present CVZ arc

andesites ( 87Sr/86SrT >0·706; �NdT −3), whereas the a higher mantle heat flow is needed, and possible reasons

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melting beneath the Altiplano–Puna plateau. Earth and Planetaryfor this include slab retreat and detachment of over-Sciences Letters (submitted).thickened mantle lithosphere (Kay et al., 1994).

Bacon, C. R. & Druitt, T. H. (1988). Compositional evolution of thezoned calcalkaline magma chamber of Mount Mazama Crater Lake,Oregon. Contributions to Mineralogy and Petrology 98, 224–256.

Becchio, R., Lucassen, F., Kasemann, S., Franz, G. & Viramonte, J.ACKNOWLEDGEMENTS (1999). Geoquımica y sistematica isotopica de rocas metamorficas

del Paleozoico inferior: Noroeste de Argentina y Norte de ChileWe thank C. Casquet and J. Erzinger, who permitted(21°–27°S). Acta Geologica Hispanica 34, 273–299.the access to some of the analytical facilities used for this

Bock, B., Bahlburg, H., Worner, G. & Zimmermann, U. (2000).work. P. Zambrana, O. Orosco, H. Justi and A. J. PerezTracing crustal evolution in the southern Central Andes from Late

are gratefully acknowledged for their help in field trips, Precambrian to Permian using Nd and Pb isotopes. Journal of Geologyas are E. G. Baldo, P. Flores, P. Cachizumba, R. Liquın, 108, 515–535.E. Kramer, C. Schulz and K. Hahne for their assistance Bohrson, W. A. & Spera, F. J. (2001). Energy-constrained open-

system magmatic processes II: application of energy-constrainedin various parts of the analytical work. We are alsoassimilation–fractional crystallization (EC–AFC) model to magmaticgrateful to J. Lindsay, A. Schmitt, W. Siebel and W.systems. Journal of Petrology 42, 1019–1041.Schnurr for many helpful discussions about magmatic

Caffe, P. J. (1996). La dacita de Casa Colorada. Complejo volcanicoprocesses and evolution of the Andean CVZ. The Ger-domico del Terciario superior en Puna Norte, Argentina. Memoriasman exchange program DAAD supported the stay ofdel XII Congreso Geologico de Bolivia (Tarija, Bolivia) 3, 1019–1030.

P.J.C. in Potsdam during 1998. The fieldwork and part of Caffe, P. J. (1999). Complejos volcanicos domicos del Terciario superiorthe geochemical analyses were financed by the Argentine de Puna Norte: sus implicancias magmatotectonicas y meta-CONICET (National Council of Scientific and Tech- logeneticas. Ph.D. thesis, Universidad Nacional de Cordoba, Ar-

gentina, 421 pp.nological Research), both with Ph.D. fellowships to P.J.C.Caffe, P. J. & Coira, B. (1999). Complejos de domos volcanicos deland a grant to B.L.C. (PIP-5017). Additional funding for

Mioceno medio de Puna Norte. Un modelo geologico y meta-geochemical studies came from the Agencia de Pro-logenetico para yacimientos epitermales de metales de base ricos enmocion Cientıfica y Tecnologica de Argentina (PICT-Ag (Sn). In: Zappettini, E. (ed.) Recursos Minerales de la Republica00511, to B.L.C.), and the Special Research ProgramArgentina. Instituto de Recursos Minerales, SEGEMAR. Anales 35, 1569–

SFB-267 ‘Deformation processes in the Andes’ supported 1578.by the Deutsche Forschungsgemeinschaft (DFG). Reviews Chernicoff, C. J., Garea, G., Hongn, F., Seggiaro, R., Zappettini, E.,by Shan de Silva, Todd Feeley and Chris Nye, as Coira, B., Caffe, P. J., Chayle, W., Rodrıguez, G. A., Perez, A.,

Soler, M. M. & Rankin, L. (1996). Interpretacion geologica delwell as comments and editorial suggestions by Marjorierelevamiento aeromagnetico de la Puna Septentrional, Jujuy y Salta.Wilson, highly improved the quality of the original manu-Direccion Nacional del Servicio Geologico, Serie Contribuciones Tecnicas 1, 46script. Finally, we wish to thank the people from thepp.Puna of Jujuy for their hospitality, especially the personnel

Cladouhos, T. T., Allmendinger, R. W., Coira, B. & Farrar, E. (1994).and teachers of many rural schools. Late Cenozoic deformation in the Central Andes: fault kinematicsfrom the Northern Puna, Northwestern Argentina and SouthwesternBolivia. Journal of South American Earth Science 7, 209–228.

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comparados con otros depositos metalıferos en los Andes del N de Whole-rock Sr, Nd and Pb isotopic compositions (TableChile y Argentina. V Congreso Geologico Chileno, Santiago, pp. 331–369. 9) were determined at the GeoForschungsZentrum

Zuleger, E. & Erzinger, J. (1990). Determination of REE and Y in laboratories using procedures described by Romer et al.silicate materials with ICP-AES. Fresenius Zeitschrift fur Analytische

(2001). Samples (0·116–0·123 g) were dissolved with 52%Chemie 332, 140–143.HF for 4 days at 160°C on a hot plate. Digested sampleswere dried and taken up in 6N HCl. Sr and Nd wereseparated and purified using cation-exchange chromato-

APPENDIX A: graphy. Pb was separated using the HBr–HCl anion-ANALYTICAL TECHNIQUES exchange procedure of Tilton (1973). 87Sr/86Sr and 143Nd/

144Nd were obtained on a Finnigan MAT262 multi-col-Major and trace elementslector mass spectrometer operated in static mode. RatiosElectron microprobe analyses of minerals (Tables 3–7)were normalized to 86Sr/88Sr = 0·1194 and 146Nd/were performed in a JEOL-JXA 8900-M Superprobe144Nd = 0·7219, respectively. Multiple measurement ofat the Universidad Complutense de Madrid (Spain).NBS 987 Sr reference material and La Jolla Nd referenceOperating conditions were 15 kV and 20 nA, with 10 smaterial gave 0·710249 ± 0·000004 (n = 12) andcounting times and a beam diameter of >5 �m. Stand-0·511892 ± 0·000007 (n = 13), respectively. Staticards were from the Smithsonian Institution. The ZAF143Nd/144Nd values were adjusted to the value obtainedcorrection was applied.for dynamic measurements (0·511850± 0·000004, n=Most of the whole-rock major and trace element data14). Analytical uncertainties are reported as 2� of the(Table 8) were obtained by X-ray fluorescence (XRF) atmean. 87Sr/86Sr(T) and �Nd(T) were calculated for knownthe laboratory of the Instituto de Geologia y Mineria,K–Ar ages (except sample Min-9, for which an age ofUniversidad Nacional de Jujuy, Argentina, on a Rigaku17 Ma was assumed), using �87Rb = 1·42E −11 y−1FX2000 spectrometer with a Rh tube. Ground andand �147Sm = 6·54E −12 y−1, ( 147Sm/144Nd)0CHUR =homogenized samples were fused with lithium tetraborate0·1966, and ( 143Nd/144Nd)0CHUR= 0·512638, respectively,as a flux for major element analyses. Ba, Sr, Rb, Zr, Nb,and the concentration data given in Table 8. Rb and SrHf, Y, Th, and U determinations were performed ondata were determined with XRF with an estimatedrock powder pellets mixed with methyl methacrylate,precision in Rb/Sr better than 1%. Sm and Nd wereand pressed at 20 t. Operating conditions were 50 kVdetermined with ICP-AES and the estimated precisionand 45 mA. Major and trace elements were analysed byis better than 0·5% for Sm/Nd. Lead isotope analysesstandard methods, using standards from the US Geo-were performed on a Finnigan MAT262 mass spec-logical Survey and the Japan Geological Survey. Thosetometer using static multicollection. Data were correctedsamples whose LOI calculations are not shown have beenfor mass discrimination with 0·1%/a.m.u. Re-analysed for major elements following the fundamental

parameters method (Rousseau, 1984). producibility at the 2� level is better than 0·1%.

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APPENDIX B

Table B1: Partition coefficients (KD) and D values calculated for AFC models

Selected KD for the fractionating assemblage 5 kbar 7 kbar 10 kbar

F = 0·5 F = 0·6 F = 0·5 F = 0·5 F = 0·4

Cpx Pl Grt 67% Cpx 86% Cpx 78% Cpx 94% Cpx 72% Cpx

33% Pl 14% Pl 22% Pl 3% Grt 17% Grt

3% Pl 11% Pl

D D D D D

Sr 0·50 3·40 1·46 0·91 1·14 0·57 0·73

Pb 0·87 0·61 0·78 0·83 0·81 0·84 0·69

Rb 0·03 0·30 0·12 0·07 0·09 0·04 0·05

Ba 0·10 0·27 0·16 0·12 0·14 0·10 0·10

Nb 0·05 0·045 0·05 0·05 0·05 0·05 0·05 0·05

Zr 0·29 0·20 2·12 0·26 0·28 0·27 0·34 0·59

La 0·28 0·18 0·25 0·27 0·06 0·01 0·22

Nd 0·86 0·09 0·03 0·61 0·75 0·69 0·81 0·63

Sm 1·60 0·06 0·13 1·09 1·38 1·26 1·51 1·18

Yb 2·00 0·10 8·00 1·37 1·73 1·58 2·12 2·81

Partition coefficients for andesites and basalts. KD values for Cpx and Pl are from Bacon & Druitt (1988) and Ewart & Griffin(1994). For Grt KD values are from Irving & Frey (1978) and Hauri et al. (1994). Bulk D calculated according to proportionsof minerals indicated, which were obtained using the MELTS program (Ghiorso & Sack, 1995) after crystallizing Maquinasbasalt at pressures of 5, 7 and 10 kbar, assuming H2O = 1·2%, and that f O2 followed the nickel–nickel oxide buffer. Liquidsin equilibrium with mineralogies separated ranged from basaltic to low–SiO2 andesitic. F, fraction of liquid remaining.

Table B2: Compositions of the mantle and crustal end-members used in the AFC and bulk mixing trace

element modelling for northern Puna magmas

ppm Mantle-derived Contaminant SRX Palaeozoic granitoids

magma

4-302 4-304a A-55 qtj-7 4/84 Av. Pz G

Rb 25 21 13 16 114 183 125

Sr 520 353 327 463 102 93 194

Ba 208 320 162 526 400 744 533

Zr 118 273 405 233 172 222 159

Nb 13 21 13 16 8 15 12

La 11·0 45 14 34 30 50 28

Nd 15·9 38 11 27 30 45 23

Sm 3·8 8·1 4·4 5·2 6·7 9·6 4·9

Yb 2·0 2·7 5·3 6·9 3·2 4·9 2·5

Mantle-derived magma is Maquinas basalt. All trace element data are from Kay et al. (1999), except Rb, which was notanalysed by those workers. Concentration for this element was taken from the Chiar Khollu basalt, considered one of theleast contaminated arc magmas in the region (Davidson & de Silva, 1995). SRX, Salta Rift granulitic xenoliths [data fromLucassen et al. (1999b)]; qtj-7, Quebrada de Tajamar Granitoid (Becchio et al., 1999): 4/84, Sierra de Moreno Granitoid; Av.Pz G, mean north Chilean granitoid (Lucassen et al., 1999a).

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