Dating protracted fault activities: microstructures, microchemistry and geochronology of the Vaikrita Thrust, Main Central Thrust zone, Garhwal Himalaya, NW India

Montemagni, Chiara; Montomoli, Chiara; Iaccarino, Salvatore; Carosi, Rodolfo; Jain, Arvind K.; Massonne, Hans-J.; Villa, Igor M. (2019). Dating protracted fault activities: microstructures, microchemistry and geochronology of the Vaikrita Thrust, Main Central Thrust zone, Garhwal Himalaya, NW India. Geological Society Special Publications, 481(1), pp. 127-146. London: Geological Society, London 10.1144/SP481.3

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The timing of shearing along the Vaikrita Thrust, the structurally upper boundary of the Main Central Thrust zone (MCTz), was constrained by combined microstructural, microchemical and geochronological investigations. Three different biotite-muscovite growth and recrystallisation episodes were observed: a relict mica-1; mica-2 along the main mylonitic foliation; mica-3 in coronitic structures around garnet during its breakdown. Analyses of biotite by electron microprobe show chloritization, and bimodal composition of biotite- 2 in one sample. Muscovite-2 and muscovite-3 differ in composition from each other. Biotite and muscovite 39Ar-40Ar age spectra from all samples give both inter-sample and intra- sample discrepancies. Biotite step ages range between 8.6 and 16 Ma, muscovite step ages between 3.6 and 7.8 Ma. These ages cannot be interpreted as "cooling ages", as samples from the same outcrop cooled simultaneously. Instead, Ar systematics reflect sample-specific recrystallisation markers. Intergrown impurities were diagnosed by Ca/K ratios. Age data of biotite were interpreted as a mixture of true biotite-2 (9.00±0.10 Ma) and two alteration products. The negative Cl/K-age correlation identifies a Cl-poor muscovite-2 (>7 Ma) and a Cl-rich, post-deformational, coronitic muscovite-3 grown at ≤5.88±0.03 Ma. The Vaikrita Thrust was active at least from 9 to 6 Ma around 600 °C; its movement ceased by 6 Ma.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Other Institutions > Emeriti, Faculty of Science
08 Faculty of Science > Institute of Geological Sciences

UniBE Contributor:

Villa, Igor Maria

Subjects:

500 Science > 550 Earth sciences & geology

ISSN:

0305-8719

Series:

Geological Society Special Publications

Publisher:

Geological Society, London

Funders:

[UNSPECIFIED] Ministero dell’Istruzione, dell’Università e della Ricerca (Italy)

Projects:

[UNSPECIFIED] PRIN project 2015EC9PJ5

Language:

English

Submitter:

Igor Maria Villa-Toscani

Date Deposited:

15 Jan 2020 15:29

Last Modified:

04 May 2020 11:49

Publisher DOI:

10.1144/SP481.3

Additional Information:

Titel of vol. 481: Crustal Architecture and Evolution of the Himalaya-Karakoram-Tibet Orogen. - ISBN 978-1-78620-403-5

Uncontrolled Keywords:

Geochronology, Microstructures, Vaikrita Thrust, Garhwal Himalaya

BORIS DOI:

10.7892/boris.137808

URI:

https://boris.unibe.ch/id/eprint/137808

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