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Transmission Electron Microscopy Study of Very Low-Grade ClayMinerals (2003)38, 459–481 Transmissionelectronmicroscopystudyof verylow-grademetamorphicevolutionin NeoproterozoicpelitesofthePuncoviscana formation(CordilleraOriental, NWArgentina) 1, 2 M . D O C A M PO * AN D F. N I E TO 1 Institutode Geocronolog ´ga y Geolog´gaIsoto´picaand Facultadde Ciencias Exactas y Nautrales,U.B.A., Pabello ´n INGEIS,Ciudad Universitaria, 1428-Buenos Aires, Argentina, and 2 Departamentode Mineralog ´gayPetrolog ´ga and I.A.C.T.,Universidadde Granada-CSIC, Avda. Fuentenueva s/ n,18002-Granada, Spain (Received 17 April 2003; revised 19 May 2003 ) ABSTRACT:The Puncoviscana Formation, largely cropping out in NWArgentina, is mainly composed of apelite-greywacke turbidite sequence affected by incipient regional metamorphism and polyphase deformation. Metapelites, composed mainly of quartz, albite, dioctahedral mica and chlorite, were sampled in the Lules-Puncoviscana and Choromoro belts. Lattice-fringe images, selected area electron diffraction and analytical electron microscopy analyses, coupled with previous data from white mica crystallinity index, indicate astate of reaction progress for Puncoviscana slates consistent with medium anchizone- to epizone-grade metamorphism. The 2 M polytype prevails in dioctahedral micas, coexisting in afew cases with the 1 Md polytype as aconsequence of lack of equilibrium. The 2 M polytype coexists with 3 T in two slates and long-range four-layer and ten-layer stacking sequences were identified in another sample. Samples with 3 T and long-range stacking sequences present b values characteristic of intermediate–high pressure metamorphism and ordered chlorites (1 L, 2L, 3L and 7L) prevail. Based on the Sicontents of dioctahedral micas and considering peak temperatures of ~350 –400ºC, pressures from 5kbar and 5 –7kbar were derived for metapelites from the Lules- Puncoviscana and Choromoro belts, respectively. These values agree with facies series derived from the b values. Micas with awide range of phengitic substitution, as evidenced by Fe +Mg and Si contents, coexist. These variations could not arise from the disturbing effect of detrital white K-mica because TEMevidence indicates that they are absent or represent <10% of the mica population. Thus, compositional variations suggest that dioctahedral micas of individual slates crystallized at different pressure conditions in response to the P-T path of the metamorphism. Moreover, in several biotite-free slates the illite crystallinity (IC) values lead to an underestimation of the metamorphic grade attained in these rocks. The coexistence of IC corresponding to anchizone and the occurrence of biotite in some slates and View metadata, citation and similar papers at core.ac.uk brought to you by CORE felsic metavolcanicprovided by Digital.CSIC rocks intercalated in the Puncoviscana metasediments are interpreted to be the result of ametamorphic path including arelatively high-pressure/ low-temperature (H P/LT) event, followed by alower-pressure overprint possibly at higher temperatures than the H P/LT event. Small micas formed during the high-pressure stage would prevail in the <2 mmfraction, producing anchizone IC. KEYWORDS:CentralAndes, Puncoviscana Formation, intermediate Na-K dioctahedral micas, wonesite, mixedlayers, illite crystallinity. *E-mail: [email protected] DOI: 10.1180/0009855033840109 # 2003The Mineralogical Society 460 M. Do Campo and F.Nieto Thereconstruction of P-T pathsfor medium- to datawould pro videa constraintfor furthe r high-graderock sisa commonobject ivein geodynamicmodels of the area. petrologicalstudies. However, in clastic sequences TheHRTEM andAEM techniqueshave been affectedby very low- to low-grade metamorphism employedin recent years to study numerous thesevariables and particula rlytheir temporal sequencesaffected by incipient metamorphi sm evolutionare difficult to establish, often because (Lee et al.,1984;Lee & Peacor,1985; Lee et al., ofa lackof reliable indicator minerals. Moreover, 1986;Dalla Torre et al.,1996;Dong & Peacor, considerablecontrov ersyexists about whether 1996;Lo ´pezMunguira & Nieto,2000; Warr & equilibriumconditions are reached in incipient Nieto,1998; Schmidt and Livi, 1999). A compre- metamorphism.Merriman&Peacor(1999) hensivereview can be found in Merriman & Peacor compiledconsiderable evidence of chemical and (1999). texturaldisequilibrium in pelites and metapelites formedat sub-greenschist conditions that invalidate GEOLOGYANDSAMPLE theuse of clay-mineral-rel ated‘ geothermometers’ DESCRIPTIONS foraccurated eterminationoftemperature. However,the same a uthorsrec ognizedthat ThePuncoviscana Formation (Turner, 1960), which mineraltransitio nsin pelites generall yfollow constitutesthe basement of thestudy area, is mainly well-constrainedsequ encesas a functionof composedof a pelite-greywacketurbiditesequence, metamorphicgrade. They pointed out that there is withsubordinate sandstone and locally interbedded ageneraltrendto de creasingdisord erwith conglomerates,shelf limestone and volcanic rocks increasinggrade, which is consistent with a series (Omarini,1983; Omarini & Baldis,1984; Jezek, ofprogradetransitions in whichpelites pass through 1990).This unit was affect edby polyph ase asequenceof metastable states as they approach deformationand has eastern and western tectonic stablech emicalequili brium.In this contex t, units,tending NS whichshow contrasting tectonic Merriman& Peacor(1999) postulated that the evolution(Mon & Hongn,1991, 1996) (Fig. 1). The specificstate of a givensystem can be characterized easternLules-Puncoviscan abeltis characterized by ascorresponding to a ``stateof reaction progress ’’ asymmetricfolds overturned to the west, with definedby data obtained by X-ray diffraction concomitantd evelopmento fana xialplane (XRD) ortransmission electron microscopy (TEM). cleavage.A secondsup erposedde formation Fine-grainedrocks, in which intergrown phyllo- produceda crenulationcleavage and associated silicatesare the dominant constituent s,require microfolding,which affects the bedding and the sophisticatedtechniques for study, such as high- axialplane cleavage. The western Choromoro belt resolutiontransmissionelectronmicroscopy ismarked by tight chevron folds overturned to the (HRTEM) andanalyti calelectro nmicroscopy east,accompanied by an east-dipping axial plane (AEM). foliation.Superimposed on these folds are other Thepurpose of this paper is to establish the P-T cleavagesurfaces that are best developed in the conditionsformetapeliticrocksofthe westernpart of the Choromoro belt. The main NeoproterozoicPuncoviscana Formation, affected deformationof the Pu ncoviscanaFormation byincipient regional metamorphism, based on the occurredduring the latest Proteroz oic–earliest detailedstudy of representative samples at TEM CambrianBraziliano orogeny (Ramos & Basei, scalecoupled with previously published data on the 1997;Do Campo, 1999b). crystallinityindex and b parameterof white mica TheSanta Rosa de Tastil Granitic Complex is an (Do Campo et al.,1998;Do Campo, 1999a,b). epizonalpost-tectonic body that developed a narrow ThePuncoviscana Formation crops out exten- contactzone that comprises an inner hornfels sivelyin the Cordillera Oriental and to a lesser aureoleand an outer zone of semi-metamorphose d extenton the eastern border of Puna (Salta and pelites(Kilmurray et al.,1974).However, only part JujuyProvinces), in NW Argentina.The compre- ofthis complex batholith belongs to the basement, hensionofits metamo rphice volutionis an becauserecent investigations have demonstrated importantke yforth eunderstandingoft he thatthe red granite facies intrude Eopalaeozoic Neoproterozoic-CambricevolutionofSouth sediments(Hongn et al.,2001b).The Puncoviscana America.In addition, a betterknowledge of its Formationis uncomformably overlain by Cambrian metamorphicevolution outlined from mineralogical sandstoneof the Meson Group. TEMstudy of pelites of the Puncoviscana Formation 461 67º 66º 65º BOLIVIA PARAGUAY BRASIL B O L I V I A Tarija N URUGUAY 22º E L I H C T L LOCATION E MAP B REPUBLICA ARGENTINA O 60º R 74º 25 º O M 23º 0 500 Km 9 0º T O L R C H I L E E O La Colorada B H El Niño 42 C Muerto Hill 39 A 36 N A C 24º S Jujuy I 26 V A R G E N T I N A O Mojotoro C Range N U 20 51 Puncoviscana 48 P Formation Salta - Precambrian- S 25º Cambrian granites E Ordovician L magmatism U L Fault Guachipas Range 3 Sample 0 50 100 Km 3 4 26º FIG.1. Geological sketch and location map. Illitecrystallinity values for Lules Puncoviscana 0.25ºD2y),(Table1). The medium anchizonal to beltand the Puna sector of Choromoro belt are in epizonalgrade indicated by IC datais in agreement therange of high anchizone and the anchizone– withprevious results (Toselli & Rosside Toselli, epizoneboundary (Table 1, data from Do Campo et 1982;Toselli, 1990). al.,1998;Do Campo, 1999a) whereas values for the Thepresence of prograde biotite in several CordilleraOriental section of Choromoro belt metapelitesfrom the Sierra del Cobre area and correspondto anchizone showing a slightlyhigher felsicmetavol canicrocks interc alatedin the averagevalue and more scattering (range 0.36 to sedimentarysequence at El Nin˜oMuertoHill 462 M. Do Campo and F.Nieto indicatesthat greenschist-fac iesconditions were northernmostMojotoro Range. Metalimolite PU51 reachedin the eastern border of Puna. However, showscurved crystals of trioctahedral mica up to accordingto Hongn et al.(2001a),Ordovician 350 mmlongand interleaved phyllosilicate grains magmaticbodies produced a strongmetamorphic composedof chlorite + trioctahedralmica
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