Festschrift zum 60. Geburtstag von Helfried MostlerGeol. Paläont. Min. Innsbruck, ISSN 0378-6870, Bd. 20, S. 67-86, 1995

A STUDY ON A DIKE SWARM RELATED TO THE KÖNIGSPITZE (GRAN ZEBRU)PLUTON, ORTLER-CAMPO-CRYSTALLINE (VENOSTA VALLEY, W SOUTH TYROL):

IMPLICATIONS ON MAGMA EVOLUTION AND ALTERATION PROCESSES

Volkmar Mair & Fridolin Purtscheller

With 7 figures and 3 tables

Abstract:A dike swarm related to the Königspitze (Gran Zebru) pluton of oligocene age intruded quartzphyllites and Triassiccover of the Ortler-Campo-Crystalline following pre-existent alpine structures. Two different types of-intrusions are tobe recognized: Type A are two phase intrusions like "sheeted dikes" following NNW-SSE fractures; Type B are ande-sitic dikes concordant to the EW striking schistosity of the quartzphyllites. Detailed field observation, pétrographiework and mineral- and bulk rock chemistry show that these typical postcollisional intrusions are products of successivemagma pulses originated from the same evolving magma chamber within a short time. The first magma pulse em-placed basalts, the second one andésites. Magma evolution through fractionation of amphibole, Al-poor clinopyrox-ene, magnetite and minor plagioclase is documented by the occurrence of cumulate xenoliths and xenocrysts of amphi-boles and diopsides as well by major and trace element chemistry.The dikes show different degrees of postmagmatic alteration, such as hydration of primary minerals and glassy matrixand changes in major and trace element chemistry due to fluid transport. The estimate of this secondary alteration al-lows the correct chemical classification even of the most altered samples using common classification diagrams, devel-oped for fresh, unaltered rocks.

Zusammenfassung:Ein Schwärm von magmatischen Gängen im Gefolge des oligozänen Königspitz (Gran Zebru) Plutons intrudierte indie Quarzphyllite und triassischen Dolomite des Ortler-Campo-Kristallins. Die Intrusionen folgen vorgegebenen alpi-nen Stukturen und lassen sich in zwei Typen klassifizieren: Typ A sind zweiphasige Intrusionen in Form von „sheeteddikes" welche in NNW-SSE-streichende saigere Klüfte intrudierten, Typ B sind andesitische Sills, konkordant zurEW-streichenden Schieferung des Quarzphyllits. Geländebefunde, Pétrographie, sowie Mineral-und Gesamtgesteins-chemie belegen, daß diese typischen postkollisionalen Gänge Produkte von schnell aufeinanderfolgenden Magma-injektionen einer evolvierenden Magmenkammer sind. Die erste Injektion lieferte Basalte, die zweite Andésite. EineMagmenentwicklung durch Fraktionierung von Hornblende, AI-armen Clinopyroxen, Magnetit und wenig Plagioklasist durch das Auftreten von Kumulatxenolithen, Xenokristallen von Hornblende und Diopsid, sowie durch die Haupt-und Spurenchemie belegt.Die Intrusionen zeigen unterschiedlich starke postmagmatische Alteration, wie Hydratisierung des primären Mineral-bestandes und der glasigen Matrix, sowie Änderungen in Haupt- und Spurenchemie durch Fluidtransport. Eine genaueAbschätzung dieser Alterationsprozesse erlaubt eine korrekte Klassifikation auch der stark alterierten Gesteinsprobenmittels der üblichen Klassifikationsdiagramme, welche nur für frische Proben entwickelt wurden.

67

1. Introduction

Oligocene, postcollisional intrusions, em-placed along the Periadriatic suture, havebeen studied by several authors in attempt toreconstruct a geodynamic model of the East-ern Alps (GATTO et al., 1976; BECCALUVA etal., 1983; PURTSCHELLER & MOGESSIE, 1988;

DAL PIAZ et al., 1989; and others). The vari-ous plutons, stocks and dikes are different inage and chemistry.

The typical orogenic character of calcalca-line/shoshonitic nature of these intrusions iswell-documented, even if chronology and distri-bution of the magmatic bodies do not fit spaceand time versus composition relationships,which characterize most modern consumingplate margins. Geochemical and isotope data aredocumented for most of the large intrusions, butnot for the numerous dike swarms. Beside thatthere is a great scatter of the chemical data sincechemical differences do not only reflect the na-ture of the source region and magma generationbut are strongly affected by later processes suchas magmatic differentiation and/or accumula-tion. These processes are not known in detaileven for the larger intrusions, except for the Ad-amello batholith and the Bergell intrusion (DALPIAZ et al., 1979; LAUBSCHER, 1983; ULMER et

al., 1983). In addition, there do not exist estimatesof the influence of secondary alteration andweathering processes on these rocks.

2. Geological outlines

The intrusions of the Ortler-Campo-Crystal-line were first described by STÄCHE & JOHN,

1879, as "suldenite" and "ortlerite". Most intru-sive bodies are mapped on the Mt. Cevedalesheet (ANDREATTA, 1951). They are undeformed,cut the regional schistosity, the alpine foldingand shear zones and intruded basement and Tri-assic covers. According to GATTO et al., 1976,they get older from SE (32 m.y.) towards NW(87 m.y.). While PURTSCHELLER & MOGESSIE,

1988, refer a chemical variation of the dikesfrom basaltic in the west (Ortler-Cevedale) torhyolitic in the east (Hoher Dieb), DAL PIAZ etal., 1989, recognized two groups of magmaticbodies with calc-alkaline and high-K-calcalka-line/shoshonitic affinity in two adjacent and sep-arate belts north and more or less parallel to thePejo Line, which separates the overlapping To-nale Unit from the underlying Ortler Basement.The high-K-calcalkaline/shoshonitic group isrepresented by the Grünsee- and Mare Valley in-trusive complexes and the Gavia Valley dikes.The calcalkaline group includes the Forno Valleyandesitic dikes and the Gran Zebrù quartzdioriticpluton, discontinuously exposed in the glacierareas at the base of the southern wall of the Kö-nigspitze (Gran Zebru), between the upperZebrù Valley, the Pale Rosse and the BottigliaPass (fig. 1).

Fig. 2 shows the investigated dikes outcrop-ping NE of the main intrusion, E of the GranZebrù between the upper Sulden- and MartellValley. The dikes intruded the triassic dolomitesand the quartzphyllite complex which is a se-quence of metapelites with beds and lenses ofmarbles, quartzites and metabasites. Dikescrosscut schistosity and an eo-Alpine (THÖNI,

1983), north-verging shear zone, marked bycarbonate-rich cataclasites, dolomite-, gypsum-and serpentinite lenses. Thermally overprintedxenoliths of the shear zone, together with thoseof the country rocks, forthcome in most of thedikes.

Field observation shows that magma em-placement followed two different systems:

Type A: In the western region in the Triassicdolomites and in the quartzphyllites at the base ofthe dolomites the magma intruded in NNW-SSEstriking, more or less vertical fractures. These1.5 m to 4 m thick dikes are very interesting be-cause they document a two-phase intrusion inform of a sheeted dike: the 50-70 cm thickouter parts consist of a dark „primitive", theinner part of a light grey, more differentiatedmaterial. The boundaries between the basalt

68 Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995

Vinschgau Valley

A A damei'lo

Fig. 1: Tectonic map showing the position of postcollisional intrusions.The frame indicates the studied area; Austroalpine units: 1 Ötztal Stubai Altkristallin. 2 micaschists and paragneisses of Ortlernappe, 3 quartzphyllites and retrogressed paraschists, 4 sedimentary cover, 5 Tonale unit (high grade metamorphic); SouthernAlps: 6 undifferentiated cover and basement sequences. 7 Adamello batholith; Postcollisional intrusions: 8 Rumo and Samoclevolamellae, 9 major plutons, stocks and apophyses of calcalkaline (CA) and high-K calcalkaline/shoshonitic (HC) affinity (divided bystippled line). After DAL PIAZ et al., 1988.

rims and the younger andésite core are markedby chilled margins and fluidal structures due tomechanical mingling of the two phases (fig. 3a).

Type B: Intrusion of the dikes in the easternpart of the area followed the WSW-ENE striking

and 35°^5° south-dipping schistosity and verti-cal WSW-ENE fractures of the quartzphyllites.Thus, the magmatic bodies seem to cover both,the aspect of sills, when following the schistos-ity, and of short dikes, where they follow thefractures (fig. 3b).

Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995 69

A SchöntaufspitzeHintergráthu

AMadritsc ispitze

chaubachhutte

Rif. 5° Alpini1=1 Cima Miniera

AQuartzphylliteMarbleTriassic Sediments

Eisseespitze

SerpentiniteGypsumCataclasite

ASuldenspitze^ H ] two-phase dikeH I basaltic Andésite^ B l AndésiteI I DaciteRif. Casati

cont. met. Quartzphyllitecont. met. Dolomite

Fig. 2: Sketch map of the Königspitz (Gran Zebru) Pluton with related dikes. The frame indicates the investigated dikes.

3. Petrography and mineralogy

The thickness of the dikes varies from a fewcentimeters to 5 m, but only dikes thicker than0.5 m are considered. The contacts to the coun-try rocks are sharp and marked by chilled mar-gins. Because of the small size of the dikes nosign of contact metamorphism is detected.

Based on field observation and detailed pétro-graphie work, two rock types with different tex-tures and mineral assemblages can be distin-guished:

Type I is represented by the inner parts of thesheeted dikes and by a 1 m thick dike outcrop-ping at the Eissee Pass. These dikes are verysimilar to those described by PURTSCHELLER &MOGESSIE, 1988. The paragenesis of these por-phyric, black to dark-green basalts/basaltic an-désites is characterised by large phenocrysts of

hornblende, clinopyroxene and plagioclase in aglassy matrix, sometimes containing minute pla-gioclase crystals. Magnetite and apatite are themost common accessories. Chlorite, calcite se-ricite and rare epidote are alteration products ofthis matrix. Calcite is not only dispersed in thegroundmass but also occurs in small cavities.

The up to 2 cm long hornblendes are brown togreen and occur as idiomorphic crystals andvery often in agglomerates. All hornblendes dis-play a slight oszillatory zoning. Some show acorroded rim but are rarely altered to calcite,chlorite, epidote and opaques.

Macroscopically the clinopyroxenes are notdetectable. In thin sections they are unzoned, butoften show a rim of amphiboles. No sign of al-teration can be observed.

Plagioclase phenocrysts are rare and muchsmaller than the hornblendes and pyroxenes.Occurring as single crystals or as agglomeratessome of them are optically unzoned, others are

70 Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995

hilled margin

Fig. 3b

Fig. 3: Different intrusion mechanisms.3a: Type A: two-phase intrusions: phase 1 (rim zone) basalt.phase 2 (inner zone) andésite.3b: Type B: "common" intrusions of the eastern part of the area:concordant and discordant to the surrounding quartzphyllite.

hg. .ia

zoned. Very often the cores are altered to sericiteand calcite. In the more altered samples the pla-gioclases are totally altered. Relictic anorthitesare not visible, but sometimes the primary zona-tion of the plagioclase is preserved by zones ofdifferent sericitization.

The dikes bear rare xenoliths of country rockswith slight thermal overprint. Xenoliths of mag-matic origin, such as cumulates, vesicles orother magmatic rocks are not detectable.

Type II represents the dikes of the eastern partand the outer parts of the two-phase intrusions.These dikes are the most frequent ones and verysimilar to those described by DAL PIAZ et al.,1989; they are considered to be products of themain intrusion phase.

The dikes are porphyritic with black, up to5 cm long hornblende and white plagioclase-phenocrysts in a light grey glassy matrix withfeldspar and magnetite. The alteration of the dif-

ferent dikes varies from very fresh rocks whereonly the Ca-rich cores of the plagioclases areslightly sericitized, to samples where the pheno-crysts and the whole matrix are altered to sericite,chlorite, calcite, minor epidote and opaques.

The plagioclase phenocrysts occur as singlecrystals, but more frequently as agglomerates ofnormal-zoned individual graines. While thelarger crystals are idiomorphic, sometimes withcorroded rims and sericitized cores, the smallerones are totally sericitized.

The olive-green to brown-green hornblendesare always idiomorphic and show the sameweak optical oszillatory zoning.

A second type of amphiboles occurs as acces-sory mineral. The crystals are idiomorphic butmost of them have corroded rims. They showoszillatory zoning from a yellow-green core to adark green rim. These amphiboles are xeno-crysts of cumulitic origin.

Geol. Palciont. Mitt. Innsbruck. Bd. 2Ü. 1995 71

Green pyroxenes with a diameter up to 2 cmare detectable in all dikes. They occur as idio-morphic, sometimes corroded grains and as rel-ics in amphiboles and amphibole agglomerates.

All dikes bear contact-metamorphic xenolithsof the surrounding rocks and magmatic inclu-sions, cognate mafic nodules according to DALPiAzetal, 1989.

4. Magmatic inclusions

Magmatic inclusions with different texturesand parageneses occur in all intrusions ofType B. Cognate inclusions are widespread buthardly visible in the field, as they show the sameparagenesis as the dikes and differ only due totheir fine-grained porphyritic texture. The con-tacts between inclusion and dike material arenever sharp but show fluidal textures and rimsof chemical/physical reactions between inclu-sion and dike material. The xenoliths display achemistry between that of Type A and B dikes,sometimes with a higher amount of alkalies (seechapter bulk rock chemistry). They may beinterpreted as fragments detached and broughtup from the rim of the crystallizing magmachamber or as products from crystallization pro-cesses occurring within the host magma .

The most common inclusions (up to 15 cm insize) are rounded, coarse-grained hornblende-gabbros with cumulate texture. The contact to thedike material is marked by a rim of fine- to me-dium-grained amphiboles. The paragenesis ofthe cumulates is characterized by long-prismatichornblendes + plagioclase + accessory magnetite± clinopyroxene. The amphiboles are never al-tered, but the plagioclases sometimes show slightalteration to sericite and clinozoisite. The brownamphiboles are slightly zoned while the feldsparsare unzoned. According to the bulk rock chemis-try, these inclusions can be classified as nephe-line-normative monzogabbros. Texture, paragen-esis, chemistry and the elevated anorthite-contentof the plagioclases strongly suggest that the horn-blende gabbros originated from a basaltic melt by

fractional crystallization. The rounded to ellip-soidal forms of the inclusions may be caused by:an eruption of the magma chamber before a solidcumulate stratus developed, or convection in astratified magma chamber due to repeated injec-tion and migration of melts in and from themagma chamber (DAL PIAZ et al., 1979; LAUB-

SCHER, 1983; ULMER et al., 1983; CONRAD &

KAY, 1984), or the formation of "drops" duringthe magma-ascent due to different viscosity ofcumulates and melt (BACON, 1986).

5. Mineral Chemistry

Mineral compositions were analyzed using anARL-SEMQ electron microprobe with fourwavelength dispersive spectrometers and aNORAN energy dispersive system at the Insti-tute of Mineralogy and Petrography, Universityof Innsbruck using standard conditions. Repre-sentative analyses are given in table 1.

AmphibolesThe brown-green amphiboles of the cumulates

are unzoned or slightly zoned pargasites to mag-nesio-hastingsites (IMA classification, followingLEAKE, 1978, and calculated with the computerprogram „EMP-AMPH" of MOGESSIE, TESSADRI

& VELTMAN, 1990). Where these cumulitic horn-blendes occur as single xenocrysts in the dikes,they show weak optical and chemical zonationfrom magnesio-hastingsite in the core to ferrian-tschermakitic hornblende in the rim, document-ing a diminution of the edenite vector caused byuprising of the cumulitic inclusions.

The brown to green hornblendes of Type Aand B are tschermakite to ferri-tschermakite incomposition and display slight oszillatory zon-ing due to Fe(Mn)-Mg exchange, probablycaused by cooling of the magma .

ClinopyroxenesThe pyroxenes occurring as relics in the cu-

mulates and cumulitic xenocrysts of hornblendeare almost pure, unzoned diopsides.

72 Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995

Macroscopically the pyroxenes of Type A arenot detectable. In thin section they are unzonendand often have a rim of tschermakitic horn-blende. Composition slightly varies from purediopside to salite.

Type B pyroxenes occur with diameters up to2 cm as idiomorphic, sometimes corroded grainsand as relics in amphiboles and amphibole ag-glomerates. They are unzoned diopsides, com-parable to those described by DAL PIAZ et al.,1989, from the Bottiglia Pass, and by ULMER etal., 1983 (Type 3, Monte Mattoni).

FeldsparsThe feldspar of the cumulates is an almost

pure, unzoned anorthite with An 95 to An 90,sometimes slightly altered to sericite.

In the dikes of Type A feldspar phenocrystsare much rarer than phenocrysts of hornblendeor pyroxene. Only a few of the plagioclases areunzoned, most of them show optical and chemi-cal zonation. Chemical profiles show a zonationfrom An 90 in the core to An 66 in the rim.

In the dikes of Type B feldspars occur as sin-gle crystals and, more frequent, as agglomeratesof normal zoned individual grains. Chemical zo-nation ranges from bytownite (An 85) in thecore to andesine (An 50) in the rim.

ween 51 wt% and 54 wt%; rocks of Type B areandésites with SiO2 between 56.5 wt % and 60wt %. A comparison with published data showsthat Type A is similar to the Ortler basalts de-scribed by PURTSCHELLER & MOGESSIE, 1988and Type B corresponds to the chemistry pub-lished by DAL PIAZ et al., 1988 for the König-spitz (Gran Zebru) Pluton. For comparative pur-poses representative analysis for cognate inclu-sions and a cumulate are plotted. While thehornblende gabbro cumulate falls into the picritebasalt field, the cognate inclusion displays achemistry between that of Type A and Type B.

The Harker diagrams (fig. 5) imply that thetwo different phases are products of a conti-nuous magma evolution. Increasing SiO2 corre-lates with increasing alkalies and decreasingFe2O3 (Fe tot), MgO and CaO. The AFM-dia-gram, the K2O vs SiO2 diagram after PECCERIL-

LO and TAYLOR, 1976 and the TiO2 vsFeO*/MgO diagram after MYASHIRO, 1973show a typical calcalkaline differentiation trend.The TiO2 values are below 1 wt% and decreasewith increasing SiO2. According to MYASHIRO,

1973; PEARCE & CANN, 1973 this is typical forcalcalkaline rocks. The A12O3 contents displaya range from 16 wt% to 19 wt%. MnO, P2O5

and SO3 do not show indicative trends.

6. Bulk rock chemistry

Major elements have been analysed on fusedrock samples with an ARL-SEMQ microprobeusing standard conditions. Trace elements weremeasured with ICP-AES (Philips PU 7000)using LiBO2-flux technique. Representativeanalyses are given in table 2.

Major elementsThe two different rock types observed in the

field differ from each other chemically. Accord-ing to the TAS diagram (LE MAÎTRE, 1984;IUGS-Comission, 1988) (Fig. 4) Type A repre-sents basalts /basaltic andésites with SiO2 bet-

Trace elementsWhile the compatible elements Co, Cr and V

decrease with increasing SiO2, the elements Ba,Rb and Sr are strongly enriched, with a greatscatter in the andésites due to various degrees ofalteration. The elements of the Ti-group, Zr, Nband Y display low values, Zr is enriched, Nb de-creases during magma evolution. According toGATTO et al., 1976; PECCERILLO et al, 1976;

BECCALUVA et al., 1979; GILL, 1981; BECCALUVA

et al., 1983 these trends of trace and major ele-ments are typical of calcalkaline magmatism atconvergent plate margins.

Magmatic differentiation trends based ontrace element chemistry are consistent with theresults of discrimination using major elements.

Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995 73

oes6CMO

16'

14

12

10

8

6

4'

2

0

Foidite

/

/ Tep

Basanite

9Picro-basalt

. / V Phonolite /

Xjephriphonolite y \ Trachyte

NÇhonotephrite \ ^ \

\ / \ Steffi \T r

/basdi \^^^îja% _ °•

Basalt

basalticAndésite

Andésite

• Type A: basalts

A Type A: andésites

• Type B: andésites

• magmatic enclaves

O data Purtscheller & Mogessie, 1988

D data Dal Piazetal., 1988

achydacite |^ ^ - - " ^ Rhyolite

\

Dae ite \

\

\

37 41 45 49 53 57 61 65 69 73 77

Fig. 4: The Total Alkali-Silica Diagram (LE BAS et al., 1992). The two intrusion phases are clearly separated. All dikes of the ea-stern part fall in the typical area for postcollisional magmatites of calcalkaline series. Although there are different degrees of altera-tion, the scatter of the samples is quite low. For comparative purposes two magmatic inclusions and data from PURTSCHELLER &MOGESSIE. 1988, and DAL PIAZ et al., 1988. are plotted. All analyses recalculated to 100% loss-free.

7. Influence and degree of secondary altera-tion processes

Macroscopically most dikes appear veryfresh, but slightly sericitized feldspars or chlori-tized amphiboles and loss on ignition (L.O.I.)values ranging from 1.8 wt% to 2.5 wt% indi-cate a slight secondary alteration of the rocks.L.O.I, values from 1 wt% to 7 wt% are reportedfor similar rocks by several authors (GATTO etal., 1976; BECCALUVA et al., 1983; DEUTSCH,

1984; VENTICELLI et al., 1984; DAL PIAZ et al.,

1988; PURTSCHELLER & MOGESSIE, 1988); theyare considered to be typical. A chemical classifi-cation of these dikes using the different discrim-ination diagrams is problematic, because the di-agrams are developed exclusively for freshrocks (e.g. the TAS-diagram is limited for rockswith L.O.I.-values < 2 wtÇf; LE BAS et al.,

1992). Therefore an estimate of various degreesof secondary alteration on petrography andchemistry of these dikes is needed!

An andesitic dike outcropping at the NW-ridge of the Eisseespitze, approximately in thecenter of the investigated area, turned out to besuitable for such an estimate. Crosscutting thequartzphyllites this approximately 3 m thickdike splits into two separate, parallel dikes, asshown in fig. 6. The main dike remains as thickas before the sharing, the second becomes thin-ner. After the distance of approximately 70 m itis only 30 cm thick. While the main dike suf-fered no alteration, the second shows increasingalteration with decreasing thickness. Thereforethe alteration of the second dike is clearly causedby elevated penetration of postmagmatic fluidsat one end of the dike and not by fluid infiltrationof the whole area.

74 Geol. Falcioni. Miti. Innsbruck. Bd. 20. 1995

1 4 -

oO)s "•

•

••

1

50 55

SiO2

Fig. 5: Harker Diagrams for the most significant major and trace elements. For comparative purposes cumulates and a intermediatemagmatic inclusion are plotted.

Geol. Paläont. Mitt. Innsbruck, Bd. 20. 1995 75

Fig. 6: The dike studied for the estimate of increasing secon-dary alteration with decreasing thickness. From the geometryof the dike it is clear, that the alteration processes are postmag-matic and related to the dike end and not to a regional fluid in-filtration. Sample locations are indicated by their numbers.

Ten samples (SST I-SST X), collected everyten meters from the center of the dike (fig. 6),are analyzed for petrography, mineralogy, inclu-sions and chemistry. Changes in petrographyand mineralogy of the respective samples, de-pendent on decreasing thickness (= increasingalteration), are listed in detail in table 3. Chemis-try of the samples is determined after cuttingthem into 1 cm thick slices and removing the in-clusions, that are analyzed separately.

The autohydrothermal fluids are rich in SO4

and CO2. This is proved by the presence of cav-ities from 0.5 cm to 5 cm in diameter, filled withcelestite and baryte and with calcite, laumontiteand quartz. With increasing fluid flow the horn-blende-phenocrysts are replaced by chlorite, epi-dote, calcite and pyrite, the feldspars by sericiteand calcite. This replacement begins with chlor-ite growth along the cleavage of the amphibolesand the growth of sericite and laumontite in theCa-rich feldspar cores, and ends with pseudo-morph phenocrysts. The glassy matrix is in-creasingly replaced by chlorite, rare epidote anddispersed calcite and laumontite. Sometimescalcite and laumontite occur in form of ag-glomerates and in small cavities dispersed inthe matrix.

The major elements do not show indicativetrends.

The trace elements show different behavior(fig. 7): The mobile elements Sr and especiallyBa and Rb are countinuously enriched untilbeing elevated enough to crystallize in specific

minerals such as baryte and celestite, exclusive-ly occurring in cavities. Comparing table 3 withfig. 7 it is easy to note that the values of theseand other trace elements are depleted in sam-ples where these inclusions have been removed.The elements Zn and Zr, normally assumed tobe immobile, show slight enrichment. Other ele-ments, Co, Cr, Cu, Ni, Nb, V, and Y do not showsignificant trends, but scattering values. No de-pletion of elements due to alteration is ob-served.

Where dikes are altered due to other fluids,perhaps of non-magmatic origin, cavities withbaryte, celestite, laumontite and quartz havenever been observed, only small cavities filledwith calcite and chlorite are found. Trace ele-ments do not show significant enrichment or de-pletion.

8. Discussion

The basalts are products of an early stage ofmagma evolution and cannot be considered asprimitive parental magma, due to their trace ele-ment chemistry and the low MgO and highA12O3 content. According to ULMER et al., 1983,such basalts result from a fractionation of oli-vine, spinel and Al-poor clinopyroxene in adeep-seated magma chamber. The crystalliza-tion of almost pure diopside and anorthite andthe absence of biotite and K-feldspar in the ba-salts suggest Ca-rich primary melt, further de-pleted in Ti, P, Y, and Zr.

Amphibole xenocrysts and cumulates of am-phibole + plagioclase in the andésites indicate amagma evolution dominated by amphibolefractionation. This may be proved by the lack ofolivine, because according to KUSHIRO, 1974,plagioclase in presence of amphibole and pyrox-ene is stable only after the disappearance of oli-vine. Therefore, we assume two different frac-tionation processes. While the first, describedabove, produced the basalts, the second pro-duced the andésites through segregation of am-phibole + plagioklase ± diopside. The fractiona-tion of amphiboles causes depletion of Mg, Fe,

76 Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995

o

cN

N

SST I SST II SST III SST IV SST V SST VISST VIISST IX SST X

SST I SST II SST III SST IV SST V SST VISST VIISST IX SST X

com

nce

SST I SST II SST III SST IV SST V SST VISST VIISST IX SST X

SST I SST II SST III SST IV SST V SST VISST VIISST IX SST X

SST I SST II SST III SST IV SST V SST VISST VIISST IX SST X • SST I SST II SST III SST IV SST V SST VISST VIISST IX SST X

sample sample

Fig. 7: The postmagmatic alteration is shown by the behavior of FeO* and trace elements, which increase with increasing alterati-on (decreasing dike thickness = decreasing sample number). The location of the samples is shown in fig. 6. Note that values muchlower than the expected trend (especially of the samples SST I - SST III) arise from removing the filled cavities before analysingbulk rock chemistry.

Ca, Al and minor Ti an Y and strong enrichmentof Si in the residual melt (CAWTHON, 1976),trends documented by the bulk rock chemistry.The depletion of Al is too slight, in regard to theother elements. The fractionation of Al-free cli-nopyroxene, which occurs in some cumulates, inthe basalts and as xenocrysts in all andésitesmay be an explanation for this. The occurrenceof amphibole- and diopside xenocrysts with cor-roded rims may document résorption of cumu-litic material through magma mixing or min-gling; perhaps due to convection in a stratifiedmagma chamber as proposed by DAL PIAZ et al.,

1979; LAUBSCHER, 1983; ULMER et al., 1983, for

the Adamello.

9. Conclusions

Field evidence indicates that the two-phasedikes originated from successive pulses ofevolving magma following the ascent path pre-pared by the early intrusion. The followingmagma was emplaced when the foregoing onewas still hot and incompletely solidified. This

Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995 77

explains the chilled margins and fluidal struc-tures observed at the contact between the twophases and needs the existence of an evolvingmagma chamber.

The older magma-pulse emplaced basalts/ba-saltic andésites, the younger one andésites.Magma evolution through fractionation of am-phibole, magnetite, Al-poor clinopyroxene andminor plagioclase in a deep-seated magmachamber is implied by the occurrence of cumu-late-xenolithes and xenocrysts of amphibolesand diopsides in the andésites, and by mineraland rock chemistry as well. The observed occur-rence of basalts and andésites, generated fromthe same magmatic source at nearly the sametime in a restricted area suggests that the mod-els of chronology, geochemistry and distribu-tion of the periadriatic intrusions proposed byGATTO et al., 1976, and PURTSCHELLER & Mo-

GESSiE, 1988, are too simple.An estimate of the varying degrees of altera-

tion shows that autohydrothermal, CO2- andSO4- bearing fluids slightly enrich the rockswith Zn and Zr, and strongly with Ba, Rb, Sr,until baryte, celestite together with laumontite,calcite and quartz crystallize in the numerouscavities. At the same time phenocrysts and ma-trix are hydrated and replaced by water- bearingminerals (increasing L.O.I.). Good chemicalclassification of these rocks is possible aftercareful sampling and sample preparation (re-moving of filled cavities) and recalculating allmajor elements anhydrous to 100%.

10. Acknowledgements

We would like to thank E. Mersdorf and R.Tessadri for their assistance and greatful helpduring mineral and rock analyses. R. Tessadriand A. Mogessie are thanked for reviewing thedraft manuscript, and for critical comments andhelpful discussions.We are greatful to M. Tessadri-Wackerle for re-viewing the english version.

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Authors ' address:Mag. Volkmar Mair, Univ.-Prof. Dr. Fridolin Purtscheller, In-stitut für Mineralogie und Pétrographie, Innrain 52, A-6020Innsbruck, Austria.

Manuscript submitted: October 5, 1994

Table 1: representative mineral analyses

Table 2: representative bulk rock analysesTable 3: changes in mineralogy and petrography with increasing alteration.

80 Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995

o gTa

ble

1: r

epre

sent

ativ

e m

iner

al a

nal

yses

, fel

dspa

rs

03 a.

sam

ple

SiO

2Ti

O2

AI2

O3

FeO

MnO

MgO

CaO

Na2

OK

2Oto

tal

cum

ulat

e

Bt 2

0a R

44,3

50,

0235

,68

0,35

0,00

0,11

19,5

50,

580,

0410

0,68

catio

ns (8

Ox)

Si Al

Fe Mg

Ca

Na

K Ti Mn

tota

l

albi

tean

orth

iteor

thoc

lase

2,04

11,

936

0,01

30,

008

0,94

60,

052

0,00

20,

001

0,00

04,

999

5,08

94,6

90,

23

Sf 1

cor

e

43,9

20,

0335

,21

0,56

0,00

0,16

18,6

70,

970,

1199

,63

2,04

51,

932

0,02

20,

011

0,93

10,

088

0,00

70,

001

0,00

05,

037

8,54

90,8

20,

64

basa

lt

Sf 1

rim

SF

45,1

80,

0434

,91

0,57

0,00

0,16

17,9

91,

470,

1410

0,46

2,08

21,

896

0,02

20,

011

0,88

80,

131

0,00

80,

001

0,00

05,

039

12,7

886

,42

0,80

Vco

re

46,2

10,

0134

,47

0,36

0,00

0,16

17,1

11,

720,

4410

0,48

2,11

91,

863

0,02

40,

011

0,84

10,

153

0,02

60,

000

0,00

05,

037

15,0

082

,47

2,53

SF V

rim

48,3

90,

0331

,44

0,49

0,01

0,16

14,1

83,

600,

4998

,79

2,25

01,

723

0,01

90,

011

0,70

60,

325

0,02

90,

001

0,00

05,

064

30,6

266

,64

2,74

basa

ltic

Ep

2a

66,1

10,

0122

,38

0,00

0,02

0,15

2,47

9,78

0,08

101,

00

2,87

11,

145

0,00

00,

010

0,11

50,

823

0,00

40,

000

0,00

14,

969

87,3

412

,19

0,47

andé

site

Ep

2b

62,9

20,

0121

,68

0,07

0,01

2,47

3,72

8,99

0,04

99,9

1

2,78

61,

132

0,00

30,

163

0,17

60,

772

0,00

20,

000

0,00

05,

034

81,2

018

,57

0,24

Ep2

c

64,4

80,

0123

,49

0,06

0,00

0,79

3,74

6,50

0,07

99,1

4

2,83

31,

216

0,00

20,

052

0,17

60,

554

0,00

40,

000

0,00

04,

837

75,4

724

,00

0,54

Sf II

cor

e

50,8

50,

0031

,07

0,18

0,00

0,09

13,3

24,

270,

1499

,92

2,31

91,

670

0,00

70,

006

0,65

10,

378

0,00

80,

000

0,00

05,

039

36,4

362

,79

0,79

andé

site

Sf II

rim

Sf

54,6

40,

0029

,04

0,21

0,03

0,15

10,7

65,

850,

2310

0,91

2,45

01,

535

0,00

80,

010

0,51

70,

509

0,01

30,

000

0,00

15,

043

48,9

749

,77

1,27

III c

ore

47,3

20,

0333

,95

0,20

0,02

0,14

16,2

62,

480,

2010

0,60

2,16

31,

829

0,00

80,

010

0,79

60,

220

0,01

20,

001

0,00

15,

040

21,3

977

,48

U4

Sf II

I rim

52,2

80,

0330

,50

0,19

0,03

0,10

12,7

34,

260,

1410

0,26

2,36

71,

627

0,00

70,

007

0,61

70,

374

0,00

80,

001

0,00

15,

009

37,4

161

,78

0,81

00 K)

Tabl

e 1

: re

pres

enta

tive

min

eral

ana

lyse

s, h

ornb

lend

es

cum

ulat

e

sam

ple

SiO

2Ti

O2

AI2

O3

Cr2

O3

FeO

MnO

MgO

CaO

K2O

Na2

Oto

tal

Bt

20

core

B

t 20

rim

Ci

o_ "ti S? 3 Mil 1^ a ï P~ ca CL

O 199

Si AI'

VA

l V

IF

e3+

Ti Cr

Mg

Fe

2+

Mn

Ca

K Na

tota

l

40,9

01,

6814

,37

0,03

13,7

30,

1411

,66

11,9

80,

921,

9797

,38

catio

ns (

FM

= 1

3 )

6,03

91,

961

0,54

00,

506

0,18

60,

000

2,56

31,

188

0,01

81,

898

0,17

70,

568

15,6

44

41,2

01,

6614

,90

0,00

13,0

50,

1812

,03

12,0

90,

721,

9897

,81

6,01

71,

983

0,57

70,

564

0,18

40,

000

2,61

31,

032

0,02

61,

895

0,13

20,

561

15,5

84

92

/18

39,4

11,

7215

,19

0,01

13,9

40,

2112

,42

12,0

80,

661,

7897

,42

5,75

72,

243

0,37

31,

092

0,19

30,

000

2,70

40,

611

0,02

61,

887

0,12

30,

500

15,5

09

Sf a

cor

e Sf

a c

ente

r

bas

alt

Sf a

rim

Sf

d c

ore

Sf d

cen

ter

42,0

41,

5013

,00

0,00

12,5

70,

1914

,28

11,2

50,

481,

6896

,99

6,03

91,

961

0,23

91,

389

0,16

40,

000

3,05

40,

121

0,02

61,

734

0,08

60,

466

15,2

79

41,3

1

13,1

20,

0414

,41

0,23

12,2

811

,78

0,55

I79

97,2

6

6,05

31,

947

0,31

40,

940

0,19

40,

009

2,68

30,

829

0,02

61,

847

0,10

60,

510

15,4

58

42,0

01,

9213

,17

0,04

11,8

00,

1413

,56

11,9

40,

621,

8597

,04

6,12

61,

874

0,38

70,

674

0,21

00,

009

2,94

40,

763

0,01

81,

867

0,11

40,

526

15,5

12

42,6

51,

9112

,67

0,02

12,3

30,

2213

,61

12,1

30,

561,

8297

,92

6,16

71,

833

0,33

00,

726

0,20

90,

000

2,93

60,

769

0,02

61,

876

0,10

40,

513

15,4

89

41,7

61,

6913

,32

0,01

14,4

30,

2212

,58

11,8

20,

531,

9098

,26

6,05

51,

945

0,32

90,

932

0,18

30,

000

2,71

80,

818

0,02

61,

838

0,09

60,

531

15,4

71

Sf d

rim

42,0

21,

9213

,53

0,05

11,7

00,

0913

,91

12,0

10,

591,

9497

,76

6,06

61,

934

0,36

60,

766

0,20

80,

009

2,99

40,

649

0,00

91,

857

0,11

30,

547

15,5

18

basa

ltic

andé

site

Ep2

core

Ep2

cen

ter

Ep2

rim

41,6

81,

9513

,92

0,00

12,9

20,

1713

,48

11,9

80,

601,

7298

,42

5,98

72,

013

0,34

20,

968

0,20

70,

000

2,88

10,

585

0,01

71,

846

0,11

20,

483

15,4

41

40,7

91,

8013

,99

0,00

14,8

30,

2512

,43

11,9

80,

571,

6198

,25

5,90

92,

091

0,29

31,

150

0,20

00,

000

2,68

00,

643

0,02

51,

890

0,11

70,

610

15,6

08

41,2

11,

9313

,58

0,04

12,5

70,

1613

,41

11,8

90,

581,

6096

,97

5,99

82,

002

0,32

40,

978

0,21

00,

009

2,91

20,

552

0,01

71,

853

0,10

50,

455

15,4

15

Paläon % . Innsl 03 to o oo

U>

Tab

le 1

:

sam

ple

SiO

2T

iO2

AI2

O3

Cr2

O3

FeO

Mn

OM

gO

Ca

OK

2ON

a2

Oto

tal

catio

ns (

Si Allv

AIV

I

Fe

3+

Ti Cr

Mg

9

Fe

2+

Mn

Ca

K Na

tota

l

rep

rese

nta

tive

min

eral

Bt 5

cor

e B

t 5 c

ente

r

43,8

91,

6512

,15

0,01

12,1

50,

1814

,25

10,6

30,

511,

9297

,34

FM=

13)

6,26

31,

737

0,30

51,

190

0,18

00,

000

3,03

80,

260

0,02

61,

630

0,09

40,

532

15,2

55

41,9

11,

5713

,49

0,00

15,0

70,

3011

,77

10,6

70,

541,

9797

,29

6,07

41,

926

0,41

41,

223

0,17

40,

000

2,54

10,

605

0,03

51,

653

0,09

60,

557

15,2

98

anal

yses

, h

orn

ble

nd

es

and

ésit

e

Bt 5

rim

42,4

51,

5713

,30

0,00

14,8

40,

3311

,78

10,6

60,

491,

8397

,25

6,15

61,

844

0,43

21,

146

0,17

40,

000

2,54

60,

659

0,04

41,

657

0,08

70,

514

15,2

59

Bt 3

cor

e

43,7

71,

4912

,39

0,02

10,5

70,

1715

,02

10,6

90,

461,

9496

,52

6,25

91,

741

0,34

81,

154

0,16

30,

000

3,20

70,

110

0,01

71,

642

0,08

60,

542

15,2

69

and

pyro

xene

s

Bt 3

rim

41,0

41,

5114

,45

0,00

15,9

40,

3110

,67

10,5

30,

511,

9096

,86

6,01

71,

983

0,51

01,

188

0,16

70,

000

2,33

40,

767

0,03

51,

656

0,09

70,

537

15,2

91

cum

ulat

e

sam

ple

SiO

2T

iO2

AI2

O3

Fe2O

3 *

Fe

O*

Mn

OM

gO

Ca

OK

2ON

a2

Oto

tal

catio

ns (

6 O

x]

Si Al

Fe

3+

Fe

2+

Mg

Ca

Na

K Ti Mn

tota

l

enst

atite

ferr

osi l

itew

olla

sto

nite

92

/20

Px

46,8

10,

797,

785,

153,

650,

1413

,45

21,4

60,

040,

2899

,55

1,74

50,

342

0,14

50,

114

0,74

70,

857

0,02

00,

002

0,02

20,

004

3,99

8

43,5

06,

6249

,88

basa

lt

Ep2

Px

3

47,9

80,

715,

516,

501,

770,

1314

,63

22,4

50,

020,

2699

,96

1,77

90,

241

0,18

10,

055

0,80

90,

892

0,01

90,

001

0,02

00,

004

4,00

1

46,0

73,

1350

,81

basa

ltic

andé

site

Ep2

Px 1

50,5

40,

604,

390,

009,

280,

2812

,70

21,8

30,

030,

1199

,76

1,89

30,

194

0,00

00,

291

0,70

90,

876

0,00

80,

001

0,01

70,

009

3,99

8

37,8

015

,50

46,7

0

Ep2

Px 2

50,0

90,

683,

853,

915,

580,

2614

,49

21,7

50,

030,

1910

0,83

1,85

10,

168

0,10

90,

172

0,79

80,

861

0,01

40,

001

0,01

9,

0,00

84,

001

43,5

89,

4147

,01

andé

site

ESA

1 P

x ES

A

51,3

90,

343,

492,

232,

800,

1415

,60

23,3

30,

030,

2299

,57

1,89

30,

151

0,06

20,

086

0,85

60,

921

0,01

60,

001

0,00

90,

004

3,99

9

45,9

64,

6349

,40

2P

xl

52,4

20,

142,

322,

091,

810,

0717

,10

23,0

40,

030,

1899

,20

1,92

40,

100

0,05

80,

056

0,93

60,

906

0,01

30,

001

0,00

40,

002

4,00

0

49,3

22,

9347

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00 Os

<—

incr

easi

ng a

lter

atio

n<—

de

crea

sing

dik

e th

ickn

ess

sam

ple

dik

e th

ickn

ess

colo

r

text

ure

thin

sec

tion

SS

TI

0.3

m

dark

gree

n-bl

ack

mas

sive

fille

d ca

vitie

sw

ith q

uart

z,ca

lcite

, bar

yte,

cele

stite

, la

u-m

ontit

e

the

few

fsp

and

hbl

phen

-oc

ryst

s in

the

glas

sy m

atrix

are

pseu

do-

mor

phic

ally

repl

aced

by

cc, c

hi, e

p an

dm

t. Th

e zo

ning

of th

e pr

imar

ym

iner

als

ispr

eser

ved

byzo

ning

of t

heva

rious

amou

nts

ofse

cond

ary

min

eral

s.

SS

TII

0.5

m

dark

gree

n-bl

ack

mas

sive

fille

d ca

vitie

sw

ith q

uart

z,ca

lcite

, bar

yte,

cele

stite

the

few

fsp

and

hbl

phen

-oc

ryst

s in

the

glas

sy m

atrix

are

pseu

do-

mor

phic

ally

repl

aced

by

cc, c

hi, e

p an

dm

t. T

he h

bl

are

corr

oded

with

dar

kbr

own

rims.

The

fsp

are

se-

rie it

ized

and

part

ly re

-pl

aced

by

cal-

cite

. Som

e as

-

sim

i lat

ea m

a-te

rial

of s

ur-

roun

dinq

quar

tzph

yllit

e.

SST

III

0.8

m

dark

gre

y

fine-

grai

ned

only

few

fille

dca

vitie

s

smal

l fsp

and

hbl p

hen-

ocry

sts

are

rel-

ativ

ely

wel

lpr

eser

ved.

Fsp

are

seria

lized

only

in th

eco

res;

hbl

are

alte

red

only

alon

g th

eir

clea

vage

to c

c,e

p a

nd

chi.

Lots

of

smal

lca

vitie

s fil

led

wit

h cc

, ch

i,la

umon

tite.

SST

IV

1 m

grey

porp

hyrit

ic

fille

d ca

vitie

sw

ith q

uart

z,ca

lcite

all h

bl a

re w

ell

pres

erve

d,w

hile

the

fsp

are

tota

lly r

e-pl

aced

by

seri-

cite

and

cc.

Lots

of

smal

lca

vitie

s fil

led

with

cc,

quar

tz, a

ndba

ryte

(see

nega

tive

valu

ein

fig

. 7).

SS

TV

i.5m

light

gre

y

porp

hyrit

ic

...

the

mat

rix is

char

acte

rized

by li

ght

and

dark

are

as(d

iffer

ent d

e-gr

ees

of a

lter-

atio

n).

Hbl

are

wel

l pre

serv

ed,

fsp

are

tota

llyre

plac

ed b

yse

ricite

and

calâ

tes;

mal

lca

vitie

s fil

led

with

chi

and

SST

VI

2 m

light

gre

y

redd

ish

porp

hyrit

ic

...

the

mat

rix is

char

acte

rized

by li

ght

and

dark

are

as(d

iffer

ent d

e-gr

ees

of a

lter-

atio

n). H

bl a

rew

ell p

rese

rved

,fs

p ar

e to

tally

repl

aced

by

seric

ite a

ndca

lcite

; sm

all

cavi

ties

fille

dw

ith c

hi a

nd

SST

VII

dike

sha

ring

light

gre

y

porp

hyrit

ic

• the

port

ion

ofm

atrix

toph

enoc

ryst

sde

crea

ses.

Few

, but

wel

lpr

eser

ved

hbl,

all f

sp s

eric

i-ti

zed.

SST

IX

3 m

light

gre

y

porp

hyrit

ic

...

Hbl

zon

ed a

ndw

ell p

rese

rved

,fs

p zo

ned

but

slig

htly

ser

ici-

tized

. M

atrix

unal

tere

d.

Tabl

e 3:

cha

nges

of

pet

rog

rap

hy

and

min

eral

og

y w

ith

alte

rati

on

cc =

cal

cite

; chi

= c

hlor

ite; e

p =

epi

dote

; fs

p =

fel

dspa

r;

blen

de;

mt

=m

agne

tite.

SSTX

3 bi

s 4

m

light

gre

y

porp

hyrit

ic

...

Hbl

, fsp

and

mat

rix v

ery

fresh

. Onl

yfe

w o

f th

e fs

psl

ight

ly s

eric

i-tiz

ed.

incr

easi

ng

hbl =

hor

n-