Transmission electron microscopy study of very low-grade metamorphic evolution in Neoproterozoic pelites of the Puncoviscana formation (Cordillera Oriental, NW Argentina)

The 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 Lul...

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Autores principales:	Do Campo, M., Nieto, F.
Formato:	JOUR
Materias:	Central Andes Illite crystallinity Intermediate Na-K dioctahedral micas Mixed layers Puncoviscana formation Wonesite Crystallization Deformation Electron diffraction Mica Quartz Slate Transmission electron microscopy Metamorphisms Clay low grade metamorphism metamorphism pelite Proterozoic Argentina Jujuy South America
Acceso en línea:	http://hdl.handle.net/20.500.12110/paper_00098558_v38_n4_p459_DoCampo
Aporte de:	Biblioteca Digital - Facultad de Ciencias Exactas y Naturales (UBA) de Universidad de Buenos Aires

id	todo:paper_00098558_v38_n4_p459_DoCampo
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spelling	todo:paper_00098558_v38_n4_p459_DoCampo2023-10-03T14:08:41Z Transmission electron microscopy study of very low-grade metamorphic evolution in Neoproterozoic pelites of the Puncoviscana formation (Cordillera Oriental, NW Argentina) Do Campo, M. Nieto, F. Central Andes Illite crystallinity Intermediate Na-K dioctahedral micas Mixed layers Puncoviscana formation Wonesite Crystallization Deformation Electron diffraction Mica Quartz Slate Transmission electron microscopy Metamorphisms Clay low grade metamorphism metamorphism pelite Proterozoic Argentina Jujuy South America The 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 Md 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 intertmediate-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 phengitie 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 biotile-free slates the illite crystallinity (IC) Values lead to all 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. JOUR info:eu-repo/semantics/openAccess http://creativecommons.org/licenses/by/2.5/ar http://hdl.handle.net/20.500.12110/paper_00098558_v38_n4_p459_DoCampo
institution	Universidad de Buenos Aires
institution_str	I-28
repository_str	R-134
collection	Biblioteca Digital - Facultad de Ciencias Exactas y Naturales (UBA)
topic	Central Andes Illite crystallinity Intermediate Na-K dioctahedral micas Mixed layers Puncoviscana formation Wonesite Crystallization Deformation Electron diffraction Mica Quartz Slate Transmission electron microscopy Metamorphisms Clay low grade metamorphism metamorphism pelite Proterozoic Argentina Jujuy South America
spellingShingle	Central Andes Illite crystallinity Intermediate Na-K dioctahedral micas Mixed layers Puncoviscana formation Wonesite Crystallization Deformation Electron diffraction Mica Quartz Slate Transmission electron microscopy Metamorphisms Clay low grade metamorphism metamorphism pelite Proterozoic Argentina Jujuy South America Do Campo, M. Nieto, F. Transmission electron microscopy study of very low-grade metamorphic evolution in Neoproterozoic pelites of the Puncoviscana formation (Cordillera Oriental, NW Argentina)
topic_facet	Central Andes Illite crystallinity Intermediate Na-K dioctahedral micas Mixed layers Puncoviscana formation Wonesite Crystallization Deformation Electron diffraction Mica Quartz Slate Transmission electron microscopy Metamorphisms Clay low grade metamorphism metamorphism pelite Proterozoic Argentina Jujuy South America
description	The 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 Md 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 intertmediate-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 phengitie 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 biotile-free slates the illite crystallinity (IC) Values lead to all 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.
format	JOUR
author	Do Campo, M. Nieto, F.
author_facet	Do Campo, M. Nieto, F.
author_sort	Do Campo, M.
title	Transmission electron microscopy study of very low-grade metamorphic evolution in Neoproterozoic pelites of the Puncoviscana formation (Cordillera Oriental, NW Argentina)
title_short	Transmission electron microscopy study of very low-grade metamorphic evolution in Neoproterozoic pelites of the Puncoviscana formation (Cordillera Oriental, NW Argentina)
title_full	Transmission electron microscopy study of very low-grade metamorphic evolution in Neoproterozoic pelites of the Puncoviscana formation (Cordillera Oriental, NW Argentina)
title_fullStr	Transmission electron microscopy study of very low-grade metamorphic evolution in Neoproterozoic pelites of the Puncoviscana formation (Cordillera Oriental, NW Argentina)
title_full_unstemmed	Transmission electron microscopy study of very low-grade metamorphic evolution in Neoproterozoic pelites of the Puncoviscana formation (Cordillera Oriental, NW Argentina)
title_sort	transmission electron microscopy study of very low-grade metamorphic evolution in neoproterozoic pelites of the puncoviscana formation (cordillera oriental, nw argentina)
url	http://hdl.handle.net/20.500.12110/paper_00098558_v38_n4_p459_DoCampo
work_keys_str_mv	AT docampom transmissionelectronmicroscopystudyofverylowgrademetamorphicevolutioninneoproterozoicpelitesofthepuncoviscanaformationcordilleraorientalnwargentina AT nietof transmissionelectronmicroscopystudyofverylowgrademetamorphicevolutioninneoproterozoicpelitesofthepuncoviscanaformationcordilleraorientalnwargentina
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Transmission electron microscopy study of very low-grade metamorphic evolution in Neoproterozoic pelites of the Puncoviscana formation (Cordillera Oriental, NW Argentina)

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