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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
,76
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Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1994 85
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-
Recommended