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dc.contributor.authorCampo, Margarita do-
dc.contributor.authorNieto, F.-
dc.date.accessioned2009-11-25T08:27:19Z-
dc.date.available2009-11-25T08:27:19Z-
dc.date.issued2003-12-
dc.identifier.citationClay Minerals 38(4): 459-481 (2003)en_US
dc.identifier.urihttp://hdl.handle.net/10261/18986-
dc.description.abstractThe Puncoviscana Formation, largely cropping out in NW Argentina, is mainly composed of a pelite-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 a state of reaction progress for Puncoviscana slates consistent with medium anchizone- to epizone-grade metamorphism. The 2M polytype prevails in dioctahedral micas, coexisting in a few cases with the 1Md polytype as a consequence of lack of equilibrium. The 2M polytype coexists with 3T in two slates and long-range four-layer and ten-layer stacking sequences were identified in another sample. Samples with 3T and long-range stacking sequences present b values characteristic of intermediate–high pressure metamorphism and ordered chlorites (1L, 2L, 3L and 7L) prevail. Based on the Si contents of dioctahedral micas and considering peak temperatures of ~350–400°C, pressures from 5 kbar and 5–7 kbar 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 a wide 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 TEM evidence 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 felsic metavolcanic rocks intercalated in the Puncoviscana metasediments are interpreted to be the result of a metamorphic path including a relatively high-pressure/low-temperature (HP/LT) event, followed by a lower-pressure overprint possibly at higher temperatures than the HP/LT event. Small micas formed during the high-pressure stage would prevail in the <2 µm fraction, producing anchizone IC.en_US
dc.format.extent10752 bytes-
dc.format.mimetypeapplication/octet-stream-
dc.language.isoengen_US
dc.publisherMineralogical Society (Great Britain)en_US
dc.rightsopenAccessen_US
dc.subjectCentral Andesen_US
dc.subjectPuncoviscana Formationen_US
dc.subjectWonesite, mixed layersen_US
dc.subjectIntermediate Na-K dioctahedral micasen_US
dc.subjectIllite crystallinityen_US
dc.titleTransmission electron microscopy study of very low-grade metamorphic evolution in Neoproterozoic pelites of the Puncoviscana formation (cordillera Oriental, NW Argentina).en_US
dc.typeartículoen_US
dc.identifier.doi10.1180/0009855033840109-
dc.description.peerreviewedPeer revieweden_US
dc.relation.publisherversionhttp://dx.doi.org/10.1180/0009855033840109en_US
dc.type.coarhttp://purl.org/coar/resource_type/c_6501es_ES
item.openairetypeartículo-
item.cerifentitytypePublications-
item.languageiso639-1en-
item.grantfulltextopen-
item.openairecristypehttp://purl.org/coar/resource_type/c_18cf-
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