Petrology of an Eclogite-Metarodingite Suite At
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American Mineralogisr, Volume 64, pages l5-31, 1979 Petrologyof an eclogite-metarodingitesuite at Cima di Gagnone,Ticino, Switzerland BrRNnnoW. EveNs Departmentof GeologicalSciences AK-20, Uniuersityof Washington Seattle, llashington 98I 95 VoLxnaa.nTnouusnonrr Institutfur Kristallographieund Petrographie, Eidgenbssische Technische Hochschule ETH-Zentrum. C H -8092Zilrich. Switzerland eNn Wor-RRe*rRrcHrrn M ineralog isch- P etrog raphisches I nst i tut der U niuer si tiit Wien A-1010 Wien.Austria Abstract Mafic rocks composed essentiallyof garnet * clinopyroxeneoccur in associationwith metamorphicultramafic rocks in the PenninicAdula-Cima Lunga Nappe around the summit of Cima di Gagnone,Ticino, Switzerland.The mafic rocks are interpretedas basalticvolcan- ics and minor intrusivesand their mildly and strongly rodingitizedproducts, later metamor- phosedunder eclogitefacies conditions. They show continuousvaiiation in their bulk chem- ical, mineralogical,and textural propertiesfrom eclogite(metabasaltic, Cpx with Jd > l0 percent)to metarodingite(CaO o 24 weight percent,NarO ( 0. I weight percent).Garnets in this seriesvary continuouslyfrom 84 percentpyralspite, l6 percentugrandite (in eclogite),to 42 percentpyralspite, 58 percentugrandite; some highly calcicmetarodingites contain garnet with 80 percentugrandite. Clinopyroxenes vary correspondinglyfrom omphacite(45 percent Jd) to diopside ((l percent Jd) and eventuallyto fassaite(23 percent Cats). The metaro- dingitesdiffer from grospyditesand kyanite eclogitesin kimberlitesby the absenceof kyanite and presenceof diopsiderather than omphacite;they also differ in bulk composition. Abundancesof some trace and minor elementsappear to have been little affectedby the processof rodingitization or by the succeedingepisodes of eclogite and amphibolite facies metamorphism.They suggestsea-floor tholeiitic basalt affinitiesand a possibleoceanic origin for the ultramafic-mafic rock suite at Cima di Gagnone. An eclogitefacies garnet lherzolite assemblageis preservedin one of the ultramafic bodies. The Kp for Fe1o1u/Mgpartitioning betweengarnet and clinopyroxeneaverages 6 in the eclogites,and rises,with increasein grossularcomponent in the garnet, to l5 in the more calcic metarodingites.The Riheim and Green temperaturecalibration applied to eclogites with garnets containing up to 25 percent grossular component, and a new calibration by Ganguly suitablefor low-Na pyroxene-calcicgarnet pairs applied to the metarodingites.give comparablevalues and suggestconditions for the eclogitefacies metamorphism were likely to havebeen800"C,Px25 kbar.SimilarvalueshavepreviouslybeenobtainedflortheGagnone garnet lherzolite. Theseconditions do not overlap those estimatedfor the Tertiary Central Alpine Barrovian-stylemetamorphism in the area (600*100'C, P ( l0 kbar). The Central Alpine metamorphicoverprint did not reequilibrateFe and Mg betweengarnet and pyroxene. However, it partially amphibolitized the eclogitesand was responsiblefor hornblende and epidote growth in the metarodingites.Contact reaction zones (probably polymetamorphic)between metarodingite boudins and ultramafic rock contain hornblende, diopside, epidote, sphene, and chlorite; contact zones against eclogite contain the same minerals,except for garnet insteadof diopside.Phlogopite and staurolitehave beenfound in amphibolitizedeclogite. 0003404x/7 9 /0 102-00I 5$02.00 l5 l6 EVANS ET AL.: ECLOCITE-METARODINGITE SUITE Assuming metasomaticrodingitization to be an exclusiveaccompaniment of serpentiniza- tion, the mafic-ultramafic rock suite at Cima di Gagnone must, at an early stage in its metamorphic history, have been in a low I, low P environment. This was lollowed by subductionand eclogitefacies metamorphism, then tectonic upriseto shallowerdepths, and regional Barrovian-stylemetamorphism. General introduction dins, amphibolites (some eclogitic), and semi-pelitic gneisses.Alpe Arami, l5 km SE of Cima di Gagnone, This is a petrologicalstudy of a suiteof mafic rocks is probably the best-knownlocality in this zone.This surrounding and enclosedin lensesof metamorphic suite has been tentatively interpreted as an associa- ultramafic rock in amphibolite faciesgneisses in the tion of highly metamorphosedoceanic rocks belong- vicinity of Cima di Gagnone,Valle Verzasca,Ticino, ing Tethyan plate.The lherzolitic composition Switzerland.The mafic rocks compriseamphibolites, to the (which include garnet peridotite) eclogites,metamorphosed rodingites, and rocks tran- of the ultramafics the hand, correlationwith the sitional betweenthem, together with the products of has suggested,on other the south European plate. contact reaction betweenmafic and ultramafic rock. subcontinentalmantle of The the suite-Mesozoic or pre-Mesozoic-is This paper describesthe completelygradational set of age of unknown. Dal Vesco (1953) gave specialatten- bulk chemical,textural, and mineralogicalproperties still mafic/ultramafic componentsof the suite. of the eclogite-metarodingitesuite. These mafic rocks tion to the regionalgeology may together with associatedultramafics record geologic, Additional information on the (1943, p. 418-430), Evans metamorphic,and tectonicevents pre-dating the Ter- be found in Wenk 1967, (1974), Heinrich (1978), and tiary Central Alpine, Barrovian-styleregional meta- and Trommsdorff (1978). morphism. St?iuble The grade of metamorphism in the Gagnone area Regional introduction has beendetermined in the courseof numerous stud- ies on progressive regional metamorphism in the The Cima di Gagnone areais locatedin the Lepon- Alps (Trommsdorff and Evans, 1974, and tine Alps 25 km N of Locarno, betweenthe Ticino Central The kyanite * staurolite * mus- and VerzascaValleys. It is situatedin the heart of the others). assemblage 4 quartz the metapelitic gneisses, CentralAlpine metamorphicbelt closeto the sillima- covite characterizes AlzSiOupolymorphs nite isograd. Structurally, the area lies within the whereasin quartz veins all three (Heinrich, kyanite lower Pennine Nappes, between the N-S trending can be found 1978),although early late andalusiteand fibrolite (e.g.,at axial culmination of the Ticino (LeventinaNappe) to dominates over Fig. 2). Metacarbonaterocks con- the E and the steeplyplunging Maggia Zone to the W Passodi Gagnone, dolomite, tremolite,diopside, quartz, cal- (Fig. l). It includesthe boundary zone betweenthe tain calcite, plagioclase,and scapolite.Almandine-andesine Simano and the Cima Lunga (:Adula) Nappes cic in to have (Preiswerk et al., 1934} Isoclinally folded rocks in amphibolite can be shown many examples assemblage.Ultra- this area possessa flat-lying regional foliation and formed from an earlier eclogitic or axial planes that dip progressivelysteeper south to- mafic rocks are composedof olivine, anthophyllite chrome-spinel, wards the S, where they merge into the steep,E-W cummingtonite, tremolite, chlorite, generationsof enstatite.The meta- trending zones close to the Insubric Line (Wenk, and at least two r9ss). morphism involved at least an earlier high-pressure facies(defined by the paragenesesomphacite Boundaries between structural units (nappes) in eclogite * pyralspiteand olivine * pyrope) and a later lower- this generalarea have traditionally beendrawn along pressure faciesevent. This last event,the metacarbonatezones, on the assumption,based on amphibolite was Tertiary in age, the lower-gradePennine nappes, that the metacarbo- Central Alpine metamorphism, recently nate rocks are metamorphosedMesozoic sediments. as has long been known from field and more (Jiiger 1967; At Cima di Gagnone, they are impure calcareous from geochronological studies et al., sedimentsvarying in thicknessfrom a few centimeters Frey et al., 1974). to severaltens of meters.The metacarbonaterocks Field relations of the mafic rocks are accompaniedby a distinctive suite of rock types that are largely absent from the monotonous This study concentrateson the mafic rocks occur- quartzo-feldspathicgneieses of the over- and under- ring in contact associationwith the ultramaficbodies lying nappes, uiz. ultramafic lensesand macrobou- around Cima di Gagnone(Fig. 2). Layersof variably EVANS ET AL: ECLOGITE-METARODINGITE SUITE '../r,t', '/,, 4 .., /'.r,, , U , , "'./-y' JT'',i i UTHERN ALP ffiffi LEvENTTN^ ilAppe f-_l BELLTNzoNA-DAscrozoNE l-]-ll srraNo NAppE Fl mERcoscrA-ARBEDozoNE VZ touLl /ctrA LuNGAr{appE [-*l r oN a L I rE liJl ,acGrA zoNE TnErAcaRBoNArEs Fig. l. Tectonicmap of the southernLepontine Alps. Smallareaaround Cima di Gagnoneis enlargedin Fig 2 Alpe Arami is at A amphibolitizedeclogite occur along the margins of commonly cross-cutby irregular thin veinsof garnet, the ultramafic bodies and in the nearby semi-pelitic pyroxene(less common), and secondaryhornblende. gneisses.Metamorphosed rodingites (i.e., calc-silicate Compared to the eclogites,the metarodingitesform rocks formed by Ca-enrichmentof dikes or country thinnerbodies, seldom exceeding one meterin thick- rock during serpentinization)and rocks transitional ness, and are strongly boudinaged. Recognizable betweeneclogite and metarodingiteoccur inside the metarodingiteboudins may be as smallas oneor two ultramafics. They form thin boudinaged sheets,al- centimetersacross. most everywhereparallel to the compositionalband- Dark, amphibole-rich contact reactionzones (Fig. ing and foliation in the ultramafics. 4) surround all metarodingiteboudins. There are The eclogites,which we will define here as con- normally three such reactionzones around eachbou- sistingof pyralspiteand pyroxenewith more than l0