Contrib Mineral Petrol (1995) 119:43-55 ~_? Springer-Verlag 1995 J.E. Mungall. R.F. Martin Petrogenesis of basalt-comendite and basalt-pantellerite suites, Terceira, Azores, and some implications for the origin of ocean-island rhyolites Received: 29 June 1994 / Accepted: 20 October 1994 Abstract Petrogenetic modeling of the Recent lava suc- cession of Santa Barbara and Pico Alto volcanoes and associated basaltic lavas indicates that there are two dis- crete lava series present, one erupted from the axial rift linking the two central volcanoes and one associated with monogenetic cones scattered around the flanks of Santa Barbara. The felsic lavas of both volcanoes are peralkaline and appear to be derived from associated basalts by fractional crystallization of an assemblage in- cluding essential amphibole. Trace element abundances in the felsic lavas, particularly those of Sr and REE, can- not be reconciled with an origin through partial melting of basaltic material at the base of the volcanic pile. The difference between the comenditic and pantelleritic dif- ferentiation trends of Santa Barbara and Pico Alto is at- tributed primarily toJO 2 control of the crystallizing as- semblage, probably related to thermal dissociation of magmatic water in the Santa Barbara magma chamber. This effect may be augmented by minor differences in parent basaltic compositions, the Pico Alto pantellerites being derived from the rift basalts whereas the Santa Barbara comendites are derived from the off-rift basalts. A compositional gap between 54 and 64% silica content in the lavas is not present if the suite is extended to in- clude co-magmatic hypabyssal xenoliths, leading to the in%rence that the gap in this and other bimodal suites results solely from a relative inability of magma of inter- mediate composition to erupt. J.E. Mungall (ES~]) ~ - R.F. Martin Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, QC H3A 2A7, Canada Present address: Bayerisches Gcoinstitut, Universilfit Bayreuth, D-95440 Bayreuth, Germany Edilorial responsibility: W. Schreyer Introduction Terceira is an oceanic island in the Azores archipelago, formed on young oceanic crust at a slowly spreading ex- tensional plate boundary. Like many other volcanic cen- ters formed in this tectonic environment, it possesses a distinct compositional gap between mafic and felsic modes. Rhyolite and trachyte form extensive flows, pumice fall deposits and even ignimbrites which can be strongly peralkaline. The large volume of felsic rocks on Terceira and similar oceanic islands presents a petroge- netic puzzle: rhyolite flows account for more than 50% of the total volume of recently erupted rocks (Self and Gunn 1976). There is no obvious source for such a large vol- ume of felsic rock. Self and Gun n (1976) considered two possible sources, direct extraction of felsic magmas from the mantle (Nicholls and Ringwood 1972; Green et al. 1974; Yoder 1976), and fractional crystallization of a mantle-derived basaltic liquid. The hypothesis of direct extraction of rhyolitic magma from the mantle is current- ly in disfavor, because it is contradicted by experimental data (e.g., Mysen et al. 1974; Wyllie 1984). Felsic magmas may also be formed by partial melting of recta-basaltic or meta-gabbroic rocks deep under Ter- ceira, as is proposed by others for Pantelleria (Lowen- stern and Mahood 1991) and for Iceland (O'Nions and Gr6nvold 1973; Gunnarsson 1987; Sigmarsson et al. 1991). A wide range of felsic to intermediate composi- tions can be produced by partial melting of dry or hy- drated basaltic material at pressures typical of the mid- dle or lower crust (Helz 1975; Spulber and Rutherford 1983; Macdonald et al. 1987; Thy et al. 1990; Rushmer 1991; Beard and Lofgren 1991; Skjerlie and Johnston 1992). Our objective is to elucidate the relationship be- tween the mafic and felsic lavas of Tcrceira. Stable iso- tope geochemistry, radiogenic isotope geochemistry and trace element modeling, might provide ways to discrimi- nate between the two modes of origin. Although studies of stable isotope geochemistry (e.g., Nicholson et al. 1991) have shown that Iceland{c rhyo-
Transcript
Contrib Mineral Petrol (1995) 119:43-55 ~_? Springer-Verlag 1995
J.E. Mungal l . R.F. Martin
Petrogenesis of basalt-comendite and basalt-pantellerite suites, Terceira, Azores, and some implications for the origin of ocean-island rhyolites
Received: 29 June 1994 / Accepted: 20 October 1994
Abstract Petrogenetic modeling of the Recent lava suc- cession of Santa Barbara and Pico Alto volcanoes and associated basaltic lavas indicates that there are two dis- crete lava series present, one erupted from the axial rift linking the two central volcanoes and one associated with monogenetic cones scattered around the flanks of Santa Barbara. The felsic lavas of both volcanoes are peralkaline and appear to be derived from associated basalts by fractional crystallization of an assemblage in- cluding essential amphibole. Trace element abundances in the felsic lavas, particularly those of Sr and REE, can- not be reconciled with an origin through partial melting of basaltic material at the base of the volcanic pile. The difference between the comenditic and pantelleritic dif- ferentiation trends of Santa Barbara and Pico Alto is at- tributed primarily toJO 2 control of the crystallizing as- semblage, probably related to thermal dissociation of magmatic water in the Santa Barbara magma chamber. This effect may be augmented by minor differences in parent basaltic compositions, the Pico Alto pantellerites being derived from the rift basalts whereas the Santa Barbara comendites are derived from the off-rift basalts. A compositional gap between 54 and 64% silica content in the lavas is not present if the suite is extended to in- clude co-magmatic hypabyssal xenoliths, leading to the in%rence that the gap in this and other bimodal suites results solely from a relative inability of magma of inter- mediate composition to erupt.
J.E. Mungall (ES~]) ~ - R.F. Martin Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, QC H3A 2A7, Canada
Terceira is an oceanic island in the Azores archipelago, formed on young oceanic crust at a slowly spreading ex- tensional plate boundary. Like many other volcanic cen- ters formed in this tectonic environment, it possesses a distinct compositional gap between mafic and felsic modes. Rhyolite and trachyte form extensive flows, pumice fall deposits and even ignimbrites which can be strongly peralkaline. The large volume of felsic rocks on Terceira and similar oceanic islands presents a petroge- netic puzzle: rhyolite flows account for more than 50% of the total volume of recently erupted rocks (Self and Gunn 1976). There is no obvious source for such a large vol- ume of felsic rock. Self and Gun n (1976) considered two possible sources, direct extraction of felsic magmas from the mantle (Nicholls and Ringwood 1972; Green et al. 1974; Yoder 1976), and fractional crystallization of a mantle-derived basaltic liquid. The hypothesis of direct extraction of rhyolitic magma from the mantle is current- ly in disfavor, because it is contradicted by experimental data (e.g., Mysen et al. 1974; Wyllie 1984).
Felsic magmas may also be formed by partial melting of recta-basaltic or meta-gabbroic rocks deep under Ter- ceira, as is proposed by others for Pantelleria (Lowen- stern and Mahood 1991) and for Iceland (O'Nions and Gr6nvold 1973; Gunnarsson 1987; Sigmarsson et al. 1991). A wide range of felsic to intermediate composi- tions can be produced by partial melting of dry or hy- drated basaltic material at pressures typical of the mid- dle or lower crust (Helz 1975; Spulber and Rutherford 1983; Macdonald et al. 1987; Thy et al. 1990; Rushmer 1991; Beard and Lofgren 1991; Skjerlie and Johnston 1992). Our objective is to elucidate the relationship be- tween the mafic and felsic lavas of Tcrceira. Stable iso- tope geochemistry, radiogenic isotope geochemistry and trace element modeling, might provide ways to discrimi- nate between the two modes of origin.
Although studies of stable isotope geochemistry (e.g., Nicholson et al. 1991) have shown that Iceland{c rhyo-
44
lites must be derived by partial melting of meta-basaltic rock that has interacted with meteoric water, the pre- dominantly submarine nature of the Azores platform prevents use of this kind of discrimination on Terceira (Mungall 1993). Similarly, contamination of basalt or rhyolite by partial melting of older basaltic crust cannot be detected in radiogenic isotope ratios because of the extreme variability in the mantle source under Terceira (Dupr6 et al. 1982; Davies et al. 1989).
Discrimination between competing theories for the origin of the felsic rocks must therefore depend on de- tailed examination of their chemical compositions.
Regional and tectonic framework
The Azores archipelago sits atop the Azores platform in the mid- Atlantic Ocean near latitude 40~ (Fig. 1), straddling the triple junct ion of the North American, Afr ican and Eurasian plates. The main trend of the islands forms the Afr ica-Euras ia plate boundary, which is undergoing slow oblique extension along a spreading axis oriented approximately NNW (Roest and Srivastava 1991). The lavas of the islands and thosc dredged from the crest of the mid-At- lantic Ridge on the Azores platform show characteristics of hotspot volcanism (Schill ing 1975; Flower et al. 1976; White et al. 1976, 1979; Hawkesworth et al. 1979; Dupr6 et al. 1982; Davies et al. 1989).
Previous work on Terceira includes that of Self (1974, 1976) and Self and Gunn (1976), who established a detailed volcanic stratigraphy for the past 23.1 ka. A small sampling of lavas f rom Terceira was used as an example of a volcanic suite undergoing tractional crystallization by All6gre el al. (1977) and Minster et al. (1977).
Terceira is composed of four overlapping volcanic centers at the intersection of a rift trending N W - S E toward Sa6 Miguel and a l ineament striking toward Graciosa (Fig. 1 ). The landward exten- sion of this l ineament is an array of monogenet ic basalt ic cones and explosion pits, here collectively referrcd to as the rift, which trends E - W through the truncated stratocone of Santa Barbara in the west of the island and the southern end of the Pico Alto mass i f in the north-central part of thc island. The rift shows evidence of exten-
sional tectonics on the ground, possibly related to stress concen- tration between the two large silicic centers at either cnd (cf. Muller and Pollard 1977; Gudmundsson 1988, 1990), al though in the regional framework it appears to belong to a t ransform fault system. Flows of rhyolite containing abundant basaltic enclavcs, and a composite flow consisting of an agglomerate of pillow-like masses of basalt and blocks of rhyolite, appear along strike with the rift within the Pico Alto massif; thus the rift persists eastward of its surface cxpression. A series of monogenet ic cones, compris- ing the "of f - r i f t " series, are scattered throughout the south half of the island. Two extinct felsic volcanic centers lie in the southern and eastern sectors of the island.
The Pico Alto massif possesses an oblong caldera containing rhyolite flows and domes, largely vented from the caldera rim. Several extensive sheets of rhyolitic ignimbrite, found in the cen- tral and eastern portions of the island, form a thin drape over higher topography and deep, valley-filling deposits centered on the Pico Alto caldera. Santa Barbara is a t runcated stratovolcano that only recently (ca. 15 ka) lost its summit owing to a catastrophic caldera-forming eruption. It still reaches over 1000 m above sea level, and falls away steeply to the sea in the north, west and south. Since the caldera formed, rhyolite and trachyte lava have erupted both from within the caldera and from radial fissures in the f lanks; basaltic eruptions have been limited to the lower slopes. Where the rift runs into the east f lank of Santa Barbara, it is expressed as a shallow graben that is partially filled with recent endogenous domes of trachyte; to the west of the caldera it reap- pears as a line of domes and coul4es whose vents extend down below sea level.
Description of the suite
Sampling
Most of the flows sampled were erupted following the eruption of the Lajes ignimbrite 22 ka ago according to the volcanic stra- tigraphy of Self (1974, 1976). Tephra deposits that mantle most of the surface of the island are excluded from the petrogenetic study to follow because some were found to have undergone extensive alteration due to leaching, apparently by meteoric water at ambient conditions (Mungall and Mart in 1994). Sam- pling of fclsic material was restricted to massive, glassy rocks.
Fig. 1 Sketch map of the Azorcs cast of the Mid-At- lantic Ridge (M.A.R.). Alter- nating rift basins and central volcanoes form Eurasian- Afr ican plate boundary be- tween North Azores Fracture Zone and Gloria Fracture Zone, both of which are trans- form faults. East Azores Frac- ture Zone is an extinct fracture zone predating activity along Terceira Rift. l ,ocations of tec- tonic features inferred from bathymetry in Searle (1980, Fig. 1). Numbers in margins give nor th latitude and west longitude
31 30 29 28 I i Northl i
| Azores / Fracture
~_ Graclosa
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39
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45
The sampled lavas span a range from basalt to strongly peralka- line pantellcrite and comendite; rock namcs were assigned using the alkali-silica diagram for basaltic rocks (LeBas et al. 1986) and the Fe-AI diagram for felsic rocks (Macdonald 1975).
Plutonic xenoliths arc common in lelsic pyroclastic deposits of both Santa Barbara and Pico Alto. They have miarolitic textures indicative of shallow emplacement and display variable degrees of hydrothermal alteration. We have collected about thirty of these and included Ihe less altered ones in the suites corresponding to the sources of their host deposits.
Compositional data
Analytical methods
Major and most trace element concentrations in whole rocks were determined by X-ray fluorescence (XRF) (Bennet and Oliver 1992) at McGill University. Ferrous ion concentration was deter- mined by metavanadatc titration. Analytical error expressed as coefficient of variation for all XRF data reported here is tess than 2%. A small subset of rocks was analyzed for rare-earth elements (REE) and Hf, Ta and Th by instrumental neutron activation analy- sis (INAA) (Gibson and Jagam 1980) at the University of Water- loo, and by inductively coupled plasma - mass spectrometry (ICP- MS) analysis at Memorial University of Newfoundland.
Compositions of glassy melt inclusions were determined by electron microprobe at McGilt using glass reference standards. Difficulties with charge-compensating diffusion of Na were over- come by reducing beam current to t 0 nA, defocussing the beam to nearly 20 Ixm, and ensuring that standardization was accomplished under precisely the same conditions (including dwell time before counting) as during analysis of unknowns. The compositions of several hundred phenocrysts are tabulated in Mungall (1993); space limitations prevent their presentation:here.
Whole rocks
Results of whole-rock analyses of selected lavas and melt inclusions are summar ized in Table 1. The basalts range from mildly silica undersaturated to or thopyroxene nor- mative, whereas all but one of the hawaiite samples have or thopyroxene in the norm. Most samples of mugeari te and benmoreite, and all samples of trachyte and rhyolite are quartz normative, although quartz never appears as a phenocryst phase. Values of the agpaitic index, plotted in Fig. 2 against magnes ium number [ M g / ( M g + F e *), cations], range f rom 0.3 in the basalts through 1.0 in relatively mafic trachyte of Santa Barbara, and up to 1.2 and 1.6 in the most strongly peralkaline samples of rhyo- lite of Santa Barbara and Pico Alto, respectively. This increase in agpaici ty is reflected by an abrupt decline in normative anorthite between basaltic rocks (mugearite, benmorei te) and trachyte, followed by the appearance first of normative aegirine in the least agpaitic rhyolite of Santa Barbara, and further by abundant normative sodi- um metasil icate in the rhyolite of Pico Alto. The compo- sition of the most felsic liquid attaincd on Terceira prob- ably is represented by the most felsic of the melt inclu- sions, at 69.0 wt% SiO2, 12.6 wt% Fe203, and 5.5 wt% A1203, with an agpaitic index of at least 2.4. The agpaitic index is a minimum because reported Na20 data are min- imum values owing to Na loss f rom glass under the clec-
tron beam. Since both Na and Fe are major constituents of the melt, the analytical total does not contain any in- formation concerning abundances of elements not ana- lyzed such as H20.
The composit ions of felsic rocks arc plotted in Fig. 3, showing Macdonald 's (1975) classification of peralka- line lavas. All of the felsic lavas of Pico Alto are pantel- lerite, whereas those of Santa Barbara are comenditic trachytc and comendite.
Figure 4 is a series of variation diagrams showing concentrations of major and trace elements, plotted against silica content. Symbols arc described in the fig- ure captions. The rift basalts contain greater amounts of Fe, Mn, Na, Nb, R Ba, Y and Ce, and notably less A1. Similarly, the pantelleritic suite of Pico Alto consistently shows higher abundances of Fc, Mn, Na, Nb, Zr, Rb, Y and Ce, and lower abundances of A1 at given silica con- tents, compared with the comenditic Santa Barbara suite. The persistence of relations among incompatible ele- ments such as Zr and Nb suggests that the Pico Alto and rift lavas belong to a common magmat ic suite, whereas the Santa Barbara lavas can be grouped with the off-rif t basalts.
A large gap in silica content, containing only two lava samples, lies between the abundant nmgeari tes (54% SiO2) and comenditic trachytc (64% SiO2). The plutonic xenoliths of the Santa Barbara suite greatly reduce this gap, to about 2%. The plutonic xenoliths of the Pico Alto suite also extend the range of the pantellerite, albeit much less so.
The melt inclusions analyzed f rom the Pico Alto suite vary between a low silica range coinciding with the plu- tonic xenoliths of Pico Alto and also with the comenditic trachyte of Santa Barbara, to a high silica extreme with remarkably high total Fe, Mn, and low AI.
In Fig. 5, a plot of P / Z r against Ce/Zr , the two suites of basalt are clearly distinct. The differences cannot be related to crystal sorting, since these are incompatible elements (removal of P from the most differentiated basalts will only blur rather than sharpen the distinction). The differences therefore reflect different characteris- tics of the source in the mantle, a finding consistent with the recognized heterogeneity of the mantle under the Azores (Davies et al. 1989). Self and Gunn (1976) noted the presence of two distinct basaltic suites on Terceira, one defining a sil ica-undersaturated trend associated with one of the extinct centers, and another, defining a silica-saturated trend, associated with the off-r if t and rift eruptive series. The two basaltic suites that we recog- nize here both belong to the recent, saturated suite of Self and Gunn (1976) despite the presence of slight sili- ca-undersaturation in a few of the most mafic members. There are thus at least three distinct suites of basalt on the island.
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99
.57
23
4
1257
11
5
2 18
7 29
19
36
7 39
1
178
26
20
341
39
2 15
5 22
19
29
3 39
7
142
22
16
279
39
101.
63
97.4
1
%- g
v
+
z
f )
2 i/ /
] ....
0 ~ % ~ * * * ~ * , 0
I I O.2 0.4 O.6 Mg/(Mg+Fe*) (cations)
Fig. 2 Agpaicity versus magnesium number. All data in cation units. (Symbols in this and subsequent figures: x rift basalt and Pico Alto lava, o cognate xenolith from Pico Alto, �9 melt inclusion fiom fayalite in Pico Alto lava, + off-rill basalt and Santa Barbara lava, * cognate xenolith from Santa Barbara) All data from Pico Alto encircled with dotted line; all lavas frmn Santa Barbara encir- cled with solid line
19
17
o~ 15 13
cO O~ tl 9
7 f . . . . 7 7 ~ 0 2 4 6 8 10
FeO (wt.%)
Fig. 3 Iron-aluminum classification diagram for felsic lavas, after Macdonald (1975). Symbols as in Fig. 3. Felsic lavas from Santa Barbara trend from comenditic trachyte to comendite; at Pico Al- to, all rhyolite is pantellcritic
Petrogenetic models
Mafic lavas
The results of modeling of fractional crystallization of the two basaltic suites are superimposed on the data in Fig. 4. The model is based on the quasi-thermodynamic relations of Nielsen and Dungan (1983) and Neilsen (1985, 1988); however, we did not follow their approach in calculating the proportions of crystallizing phases. Instead we lbrced the model to extract a set proportion of phases whose compositions were controlled by Nielsen's relations, given a liquidus temperature calculated to give an olivine activity of unity in Nielsen's scheme. Details and a program listing of the model were given by Mun- gall (1993). Mineral/melt partition coefficients used in this and subsequent models are given in Table 2.
47
Felsic lavas
We have modeled derivation of the Santa Barbara lava suite by either partial melting of a hypothetical meta- basaltic parent belonging to the same suite, or fractional crystallization of the off-rift basalts. We make two as- sumptions: (1) that the extraction of a felsic partial melt can be accomplished only at high melt fractions, on the order of 20 to 30%, leading to essentially equilibrium melting (cf. Arzi 1978); (2) that melts, once separated from their sources, evolve by perfect fractional crystal- lization.
The elements Zr and Rb remain highly incompatible throughout the range of magma compositions found on Terceira (Mungall 1993). In contrast, values of KD for Ba, Sr and Eu show great dependence on the composi- tions of the phenocrysts and melts, particularly in feldspar minerals (Mahood and Stimac 1990; Blundy and Wood 1991; Mungall 1993). Although the compositional dependence is most likely due to changing crystal-chem- ical parameters in the solid phases, with the melt follow- ing Henry's Law behavior throughout the range from basalt to rhyolite (Blundy and Wood 1991), the composi- tions of the phenocrysts do depend on melt composition. It is thus convenient to treat the varying partition coeffi- cients as being dependent, albeit secondarily, on melt composition. This approach permits quantitative model- ing of partial melting and fractional crystallization using concentrations of trace elements whose Kr, varies contin- uously with composition.
Discussion
Basalt evolution
Model proportions of crystallizing phases appear in Table 3, and the modeled liquid line of descent for each suite appears in Fig. 4. The model rift basalts fractionate a gabbroic assemblage dominated by plagioclase, with lesser amounts of clinopyroxene and olivine. As crystal- lization progresses, the proportion of olivine drops; at 8.19% Mg, spinel and ilmenite appear on the liquidus, bringing down Fe and Ti contents, followed by apatite at 6.09% Mg, whereupon P contents rapidly drop.
In marked contrast, the off-rift basalts initially frac- tionate an assemblage dominated by clinopyroxene, which rapidly drives the residual liquids to high A1 con- tents until at 8.73% Mg, the proportion of plagioc/ase climbs to resemble that observed in the rift basalts. Ox- ides become saturated at 6.58% Mg, at which point olivine apparently stops crystallizing. The scarcity of olivine in lavas in this range of composition suggests that the hiatus is real. At 4.4% Mg, olivine reappears as a very minor constituent, and apatite reaches saturation.
In both suites, the early evolution of the basaltic mag- mas takes place at nearly constant silica content, but when oxide minerals appear on the liquidus silica rises rapidly.
48
Fig. 4 Variation diagrams, SiO2 on abscissa. Oxides in wt%; others in ppm. Symbols as in Fig. 3; two-sigma errors of replicate analyses smaller than symbols. Solid line shows model liquid line of descent for off-rift Santa Barbara suite; dotted line shows model liquid line of descent for rift Pico Alto suite
20
15
7<
10
10
o c~ 0 5
O ~3
Z
( ~
+ 4- *
4 ~ , * *
"\,', "\,, "j ",,.,
' \ ',%
'"',.-:i} I .... I , I , , , I , I
0 L---,
9
8
7
6
5
4
3
2 ' 6
4-
"-,,.,.~
_. �9
/-,.,
I I , I ~ _ f , I , I , I
,..s . . . . . . ?
I , t I , I , I , I , l 48 52 56 60 64 68 72
Si02
15
10 0 LI -
5
0.5
0.4
0.3 0
0.2
0.1
0
4
3
02 2 I--
1
0
8
7
6
5
% 4
3
2
1
' .{ I , I , I , I . I
/" /'
�9 ~/ ,~....' 0 t +
D,*'
f ; + . * . " �9 ,'-%~
, I , I , I , I , I , I , I
I i i ' ' " i ' ~ : : " N ~ " ~''
< 4~2".
48 52 56 60 64 68 72
Si02
Inc lus ion o f a m p h i b o l e in the f r ac t iona t ing a s sem- b lage , c o n s t r a i n e d by ac tua l a m p h i b o l e c o m p o s i t i o n s in the xenol i ths , was n e c e s s a r y in o rde r to mode l the l iquid l ine o f descen t f rom m u g e a r i t e th rough to the fe l s i c m a g - mas o f both sui tes . A n h y d r o u s so l id a s s e m b l a g e s l ead i n e s c a p a b l y to a much lower a g p a i c i t y in the r e s idua l t r achy t i c l iquid, w h i c h sugges t s that a m p h i b o l e c rys ta l - l iza t ion , r e su l t i ng f rom high vola t i le and high a lka l i con- cen t ra t ions in the t r a n s i t i o n a l l y a lka l ine basa l t i c m a g m a , is a n e c e s s a r y p r econd i t i on for the genera t ion o f p e r a l k a -
l ine r e s i d u a by f rac t iona l c rys ta l l i za t ion . The fa i lu re o f the m o d e l to r e p r o d u c e Ce, Nb and Sr abundances in the r i f t sui te at i n t e r m e d i a t e c o m p o s i t i o n s p r e s u m a b l y re- sul ts f r om i n a c c u r a c c y in the p a r t i t i o n i n g coe f f i c i en t s used.
The ear ly m o d e l c r y s t a l l i z i n g a s s e m b l a g e s in the two sui tes o f basa l t are very d i f ferent . M a h o o d and B a k e r (1986) found that in s imi l a r basa l t i c rocks f rom Pante l le - r ia , c rys t a l l i za t ion at one a t m o s p h e r e p r o d u c e s a phe - noc rys t a s s e m b l a g e s imi l a r to the m o d e l resu l t s for the
Fig. 4
oY
1.5
0.5
0
1500
1000
500
0
800
700
600
500
400
300
200
100
0
150
100
50
X .:.x
X / I ~ X , ,,. X
',, X
if- ",e + X
X X +
4. :-1 ..., '"
+ . .
'k,..+
I , I I , I I ' ' ; " 1
~x* x ,~ X + X
I
2
2000
1500
1000
500
300
200
100
0
200
150
lOO
50
0
400
300
200
100
, I , I , I , I , I , I ,
:o'"~ i :
, I , I , I , I , , I
o"•
, I , I I , I , I , I ,
I _ , I , B , I , I , - L , h 0 , I
48 52 56 60 64 68 72 72 SiQ2
L _
/ /
, I , I , I , I , I , I
48 52 56 60 64 68 Si02
49
h
rift basa/ts, whereas crystallization at 8 kbar pressure induccs massive clinopyroxene fractionation with sub- sidiary amounts of plagioclase and olivine. The model results thus suggest that the rift basalts began to fraction- ate at low pressure, and underwent a relatively small amount of differentiation before being erupted. On the other hand, the off-rift basalts appear to have undergone an initial period of high-pressure fractionation. Later model phenocryst assemblages in the off-rift suite are intermediate between Mahood and Baker 's (1986) high-
and low-pressure trends, suggesting continuing differen- tiation at lower but still considerable pressures, consis- tent with the inferred stability of amphibole in the inter- mediate members of the model series.
The differences between the two suites, initially ex- pressed primarily in minor element variation, were therefore enhanced during ascent through the crust. The controls may lie in the tectonic and thermal regimes in the rift, as opposed to those of the flanks of the central volcanoes. Extensional tectonics in the rift should pro-
50
o o o o I " -
x
a.
70
60
5O
4O
30
20
10 ' 0.2
.,,," x / ,,," x X ,,' /
..." . / X :"
.," X YX xX /." (X ' ' , X ,r
, X / X X/:
I I I I I I
0.25 0.3 0 .35 0.4 0 .45 0.5
Ce /Z r
Fig. 5 P/Zr versus Ce/Zr. All basaltic rocks plotted; the rift basalts fall in an entirely different range from the off-rift basalts, indicating different characteristics of the source
mote rapid ascent o f m a g m a along dykes; fu r thermore , the frequent passage o f m a g m a pulses along closely spaced dykes will result in local heat ing o f the crust. These factors should result in relatively little cool ing and differentiat ion dur ing m a g m a ascent. In contrast , pulses o f m a g m a released below the remainder o f the volcanic pile will rise solely as a result o f their buoyancy through relatively cold, brittle crust, so that they will begin to cool and different iate at greater depth.
A c c o r d i n g to the mode l results, it is reasonable to p ropose that weakly me ta luminous to mildly peralkal ine t rachyte can be genera ted by simple fract ional crystal- l ization o f either the rif t or off- r i f t basalts.
Felsic lavas
Figures 6 and 7 show the actual data for benmorei tes , t rachytes and rhyolites o f the Santa Barbara suite, a long
Table 3 Model proportions a of crystallizing phases in basalts (Ol olivine, Cpx clinopyroxene, Pl plagioclase, Spl spinel, llm il- menite, Ap apatite, Amp amphibole)
with the mode l results. Shown are a t ract ional crystal- l ization mode l start ing at benmore i te (solid line), an equi l ibr ium part ial mel t ing model using mugear i t e as a source (dashed line), and a mode l l iquid line o f descent p roduced by perfect fract ional crystal l izat ion o f a l iquid that or iginated by 35% mel t ing o f mugear i t e (dotted line). The restite in the mel t ing mode l was assumed to be 63% p/agioclase, 20% cl inopyroxene, 15% oxide miner- als, and 2% apatite, to resemble the results o f both mod- el ing (above) and exper iment (Helz 1975; Spulber and Ruther ford 1983; Thy et al. 1990; Rushmer 1991; Beard and Lofg ren 1991; Skjerlie and Johnston 1992). In the fract ional crystal l izat ion models , the par t i t ioning of Eu and Ba depends l inearly on Zr, whereas in the mel t ing model , the plagioclase in the restite is a ssumed to have relatively constant (and small) distr ibution coeff ic ients (KD) for these elements.
The fract ional crystal l ization model provides a re- markably g o o d fit to the data in all cases. The Santa Barbara suite o f comendi t ic t rachyte can be derived through about 35% fractional crysta l l izat ion of a ben- morei te , and the lava series is cont inuously represented
Table 2 Mineral melt partition coefficients used in modeling. All values are weight partitioning coefficients. The given values from Blundy and Wood (1991 ) for plagioclase are the ones used in trace element fractionation modeling of melting and fractional crystal- lization in basaltic rocks. Modeling of Ba, Sr and Eu partitioning behaviour is discussed in the text for the felsic rocks, in major element modeling of the basaltic rocks we have used the full ex- pressions given by Blundy and Wood (1991) for variations in Ba
and Sr partitioning coefficients in plagioclase. Partition coeffi- cient for Nb in spinel is assumed to be equal to that in ilmenite. Literature sources indicated by abbrevations as follows [LM LcMarchand et al. 1987, F Fujimaki et al. 1984, NN Nielsen et al. 1992, H Hart and Dunn 1993, I Irving 1978, W Watson and Green 1981, BW Blundy and Wood 199t; IF Irving and Frey 1984, A Adam et al. (1993)1
O1 Cpx Ilm Spl Ap P1 Amp
Rb LM 0.02 LM 0.03 LM 0.07 LM 0.07 LM 0.04 A 0.07 Ba H 0.00 BW 0.75 A (I.46 Nb H 0.007 I 0.81 (0.81) A 0.21 Ce F 0.002 H 0.085 ~ 5.75 F 0.027 IF 0.3 Sr H 0.128 ~ 4.8 BW 1.5 A 0.35 Sm F 0.003 H 0.291 NN 0.009 NN 0.007 W 9.8 F 0.013 IF 1.2 Zr LM 0.04 H 0.123 LM 0.14 LM 0.14 LM 0.05 A 0.35 Eu LM 0.01 F 0.255 LM 0.1 LM 0.1 ~ 4.8 F 0.022 IF 1.1 Y NN 0.003 H 0.467 NN 0.004 NN 0.003 " 6.7 A 1.3
a Interpolated from Watson and Green 1991
51
1000
B a
500
t _ _ , _ _ 1 0,.5 ......... X r
. . . . . . . . . :
Rb
200
100
, / crystallization of benmoreite I ..--'""*" ....
I ,-'"" melt of mugearite i i i t i i i ~ i J i i i i i
Sr
500
400
300
200
100
i
0 500
+ " , ,
+ " '4 .
§ + "'"--.~ ....................
r 1 i i 1000 1500 2000
Zr(ppm)
Fig. 6 Trace element modeling of Santa Barbara rhyolite petroge- nesis; levels ofBa, Rb, Sr versus Zr. Three different model trends are identified in the Rb diagram; pointers in Ba diagram indicate liquid fraction for each nmdel in this and following variation dia- grams. All data are fit well by fractional crystallization throughout the range from benmoreite to comenditc. Simple partial melting model does not fit Ba or Sr; addition of a fractional crystallization step after 35% partial melting allows fit to some of the Ba data but not Sr
by a single liquid line of descent up to about 60% crystal- lization to form comendite. Barium rises to a maximum in the samples of comenditic trachyte, upon which it falls, rapidly at first as the KD reaches a peak of 5, then progressively more slowly as the K D drops to around 2. Strontium drops immediately, and has fallen well below 100 ppm in all of the rhyolite samples. Both Rb and Sm are incompatible, and rise steadily throughout, whereas Eu is initially slightly compatible but later becomes slightly incompatible, following the relationship shown by Mahood and Stimac (1990). As an important conse- quence of this behavior in the fractional crystallization model, the Eu "anomaly" , here represented by the ratio Eu/Sm, increases in the range of rhyolite compositions, i.e., Eu drops relative to Sin.
The partial melting model cannot be stretched to fit the data. The Sr abundances predicted by the melting model are much too high, even if a relatively primitive
a o o . . . . . . . . . l o o o hsd0 ' ' ' ~ o o ' o
Zr (ppm)
Fig. 7 Trace clement modeling of Santa Barbara rhyolite petroge- nesis; levels of Sm, Eu, Eu/Sm versus Zr. Data are modeled well by fractional crystallization, and do not fit the partial melting model, even with subsequent fractional crystallization
trachyte magma is allowed to undergo extensive frac- tional crystallization after extraction from the mugeari- te source. This problem is not rectified by proposing more mafic basaltic source-rocks, because all have hundreds of ppm Sr. The REE abundances are also too high, especially in the case of Eu. If Eu concentrations in the melt are controlled by equilibrium with plagioclase, the result is an increase in Eu with increasing Zr (decreasing melt fraction), exactly counter to what is ob- served. Allowing fractional crystallization of the partial melt does not correct the mismatch. As a result of this, the Eu "anomaly" predicted by the partial melting model is much smaller than what is observed (i.e., Eu/ Sm is too large),
A further argument can be advanced based on field relations. Even if the melting model were to produce an adequate fit to the data for the rhyolites (e.g., see Ba plot, Fig. 6), there remains a range of primitive trachyte with very high Ba contents that would have to be generated by small degrees of continued fractional crystallization of benmoreite. The partial melting model for the rhyolites would then require that such mafic trachyte mix with rhyolite to produce the range of intermediate rhyolite compositions. There is no compelling evidence to sug-
52
Table 4 Model Proportions of crystallizing phases in trachyte and rhyolite (abbreviations as for Table 3, Sa sanidine)
gest that this p a r t i c u l a r pa i r o f c o m p o s i t i o n s p r o d u c e d b y such d i f fe ren t p r o c e s s e s should c o m m o n l y coexis t , or that they should be able to mix easi ly.
M o d e l i n g b a s e d on t r ace e l emen t f r ac t iona t ion there- fore s t rong ly s u p p o r t s the hypo the s i s that the fe l s ic m a g - mas o f Santa B a r b a r a are de r ived by f r ac t iona l c rys ta l - l i za t ion o f a s s o c i a t e d basa l t i c m a g m a s , and equa l ly s t rong ly re jec t s the sugges t ion that they are p r o d u c e d by pa r t i a l me l t i ng o f o lder ba sa l t i c crust . The r e s t r i c t e d c o m p o s i t i o n a l r ange o f the P ico A l t o lava sui te makes such a s t rong d i s t inc t ion imposs ib l e , but the da ta a re con- s is tent wi th a f rac t iona l c rys t a l l i za t ion mode l .
Leas t - squa re s m i x i n g m o d e l s
A l eas t - squa res m i x i n g m o d e l can be used to assess the l ike ly ident i t i es and p r o p o r t i o n s o f c ry s t a l l i z i n g phases (e.g. , B r y a n et al. 1968). Bo th o f the sui tes o f rhyol i te a re s ucce s s fu l l y m o d e l e d f rom t r achy te to the i r mos t d i f fe r - en t i a t ed m e m b e r s by remova l o f an anhydrous a s sem- b l age i n c l u d i n g san id ine or a n o r t h o c l a s e , c l i n o p y r o x -
Sum of squared oxide residuals: 0.49 (excluding NaaO)
ene, fayal i te , and ox ide mine ra l s (Table 4). B e c a u s e N a 2 0 da ta tbr me l t i nc lus ions are semi -quan t i t a t ive , Na20 was e x c l u d e d f rom the f i t in the invers ions for the P ico A l to suite. A m p h i b o l e canno t be i n c l u d e d in any o f the c rys - t a l l i za t ion steps; a t t empts to do so inva r i ab ly resu l t in add i t ion o f a m p h i b o l e by the m a s s - b a l a n c e ca lcu la t ion . This impl i e s that even in the sui te o f pan t e l l e r i t e of P ico Al to , a m p h i b o l e and a e n i g m a t i t e a p p e a r on the l iqu idus on ly at the very la tes t s tages o f f rac t iona t ion , and do not p lay a role in the evolut ion f rom the less p e r a l k a l i n e to the mos t d i f f e r en t i a t ed m e m b e r s o f the suite. In the P ico A l to sui te , the m o d e l ox ide is i lmen i te , whe reas in the San ta B a r b a r a suite, on ly the spinel phase s eems to be f rac t iona t ing . These mode l resu l t s a cco rd exac t ly wi th obse rva t ions o f the ac tua l p h e n o c r y s t a s s e m b l a g e s both on Terce i ra (Munga l l 19931 and e l s e w h e r e (cf. Car- m i c h a e l 1967). The a b s e n c e o f a m p h i b o l e sugges t s that the fe l s ic m a g m a s evolve under lower p r e s s u r e than the i r a m p h i b o l e - b e a r i n g i n t e r m e d i a t e - c o m p o s i t i o n pa ren t m a g m a s .
c- --n c- o
~0.5
LL
LI_
.... +q
+ ++%}
*+ + ~i-+-"-/ ................... .7;4",
I I I I I I I I I I I I
0 1000 1500
Zr (ppm)
Fig. 8 Molar ratio of ferric iron to total iron in rhyolite and tra- chyte. Santa Barbara lavas show wide scatter, presumed to result from thermal dissociation of water and diffnsive loss of H> where- as compositions of pantellerites from Pico Alto cluster tightly at reduced compositions. Tie line connects two inter-banded glasses with identical major- and trace-element compositions
Comendite versus pantellerite; effects of H20
Both rhyolite suites originate at very similar trachyte compositions; their divergent evolution results from the presence of different oxide minerals in the fractionating assemblage. The comenditic magma of Santa Barbara fractionates spinel in model proportions ranging from 2.45 to 8.20%, whereas at Pico Alto, the pantelleritic magma fractionates much lower model proportions of ilmenite, ranging from 0.37 to 0.60%. The consequence of this difference is a trend toward iron and manganese enrichment in the Pico Alto series.
The reason for the difference in the precipitating as- semblage of oxide minerals might lie in the oxygen fugacity at which each series is evolving. Figure 8 pre- sents the molar ratio of oxidized to reduced iron plotted against Zr for samples of glassy rhyolite. Since the sam- ples are glassy, they can be assumed to have retained the redox states they had as liquids. The Santa Barbara series ranges to much more oxidized compositions than does the Pico Alto series. Of particular note is the difference between the compositions of green and black members of a banded trachyte from Santa Barbara, shown in Fig. 8 joined by a tie-line. The black trachyte is significantly more oxidized than the green, despite the fact that the two were intimately comingled during eruption and have identical major- and trace-element geochemistry. Large- scale disequilibrium oxidation of trachytic liquids has taken place under Santa Barbara, but not under Pico Al- to. Magmas resist changes to their oxidation state; a pos- sible means of altering the redox state would be de- gassing of H 2 following thermal dissociation of water (cf. Sato 1978; Carmichael 1991).
Experimental petrology shows that the effect of in- creasing JO 2 is to promote spinel fractionation and iron depletion, whereas at low j~O 2 ilmenitc is the liquidus oxide mineral; if ilmenite crystallization is controlled by
53
low activity of Ti, then an iron-enrichment trend should be expected, even in very felsic magma such as panteller- ite (Roeder and Osborne 1966; Snyder et al. 1993; Toplis 1994).
We suggest that the difference between the comen- ditic trend of Santa Barbara and the pantelleritic trend of Pico Alto results in large part from early degassing of small but significant amounts of water and H 2 from the Santa Barbara trachytes; at Pico Alto, the trachytic mag- ma is inferred to have retained its volatiles. Unfortunate- ly we have little information regarding the volatile con- tents of the various mafic and felsic suites of Terceira, and these ideas cannot be tested rigorously.
The compositional gap
Although both the lava suites show large compositional gaps between about 54 and 64% SiO> the Santa Barbara suite as a whole (including its cognate xenoliths) does not contain any significant gap. We suggest that at Santa Barbara, and by extension, in bimodal lava suites else- where, the presence of the large compositional gap re- sults not from a real absence of intermediate composi- tion, but rather from an inability of these magmas to erupt. The concept of ~a "density filter" (Stolper and Walker 1980; Walker 1989) cannot be applied here, be- cause the voluminous Fe- and Ti-rich basalts are the densest members of either suite. Instead, it is likely that rising viscosity of the morc evolved lavas coupled with a very rapid crystallization through a short temperature range acts to impair the ascent of benmoreite along dykes (cf. Marsh 1981; Delaney and Pollard 1982; Pet- ford et al. 1994). With continued evolution to rhyolitic composition the magmas gain access to another mecha- nism, namely large volative-driven overpressures, suffi- cient to drive even very viscous magma relatively large distances through vents (Reches and Fink 1988).
The abundance of felsic lava (more than 50% of recent eruptive volume) and virtual absence of intermediate lava is therefore probably controlled more by eruption dynamics than by the actual volume of magma available within the volcano. Large parts of the magmatic series are probably present in dikes, magma chambers, and cu- mulate sequences that never reach the surface of the growing volcano (Francis et al. 1993), making attempts to constrain subsurface evolution by statistical analysis of eruptive volumes risky at best.
Conclusions
The results of our modeling excercise suggest that there are two distinct magmatic systems active on Terceira, one leading from the off-rift basalts through to the Santa Barbara comendites, the other leading from the rift basalts through to the pantellerite of Pico Alto. The two basaltic suites originate from distinct mantle sources, and evolve along distinct P-T paths during ascent. The
54
o f f - r i f t sui te unde rgoes an in i t i a l p e r i o d o f h igh -p re s su re f rac t iona t ion b e t b r e m o v i n g to h ighe r c rus ta l levels , whe rea s the r i f t su i te appea r s to evolve en t i re ly at low p res su res .
The fe l s i c m a g m a s o f San ta B a r b a r a are de r ived f rom the o f f - r i f t basa l t s by ex t ended f rac t iona l c rys t a l l i za t ion , not b y p a r t i a l me l t ing . A t P ico A l t o the rhyol i te is p roba - bly de r ived f rom the r i f t basa l t s in the same way. Bo th fe l s ic m a g m a t i c sui tes d iverge f rom ve ry s imi l a r m a f i c t r achy tes ; the pan te l l e r i t i c P i co A l to sui te shows an i ron- e n r i c h m e n t t r end cha rac t e r i s t i c o f sui tes evolv ing by f r ac t iona l c ry s t a l l i z a t i on at low oxygen fugaci ty , w he re a s the c o m e n d i t i c San ta B a r b a r a sui te appea r s to r e f l e c t h i g h e r J O 2. This may be a t t r i bu tab le to loss o f H 2 du r ing d e g a s s i n g o f San ta B a r b a r a m a g m a .
The ex i s tence o f a l a rge c o m p o s i t i o n a l gap be tw e e n 54 and 64% SiO 2 in the o f f - r i f t Santa B a r b a r a lava sui te is i n fe r r ed to resu l t f r om the re la t ive i n a b i l i t y o f i n t e rme- d ia te lavas to erupt , de sp i t e the i r ev iden t a b u n d a n c e as in t rus ive rocks in the vo lcan ic ed i f i ce .
Acknowledgements This work is distilled lu Part I of J,E. Mun- gall's doctoral dissertation. A previous version was greatly im- proved by helpful reviews by Drs. Hawkesworth and W6rner. The work was supported by NSERC operating grants to R.F. Martin. We are grateful to I.L. Gibson for supplying several REE analyses.
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