Geochronology of Himalayan shear zones: unravelling the timing of thrusting from structurally complex fault rocks

Montemagni, Chiara; Villa, Igor Maria (2021). Geochronology of Himalayan shear zones: unravelling the timing of thrusting from structurally complex fault rocks. Journal of the Geological Society, 178(6) Geological Society of London 10.1144/jgs2020-235

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Dating structurally complex fault rocks often results in internally inconsistent ages, as several mineral generations are intergrown at scales << 10 µm and almost always altered to various degrees. We describe here 39Ar-40Ar stepheating using the combination of two independent indicators that allow the discrimination of coexisting mica generations from each other and from the ubiquitous retrogression/alteration phases. A necessary first step is electron probe microanalysis to assess both inventory and spatial distribution of the mineral phases that need to be distinguished a posteriori by 39Ar-40Ar systematics. One indicator is based on mica stoichiometry, which can be proxied by the 39Ar concentration in combination with the 37Ar/39Ar and 38Ar/39Ar (i.e. Ca/K and Cl/K) ratios. The other indicator is the furnace temperature, at which a degassing peak accompanying dehydration and structural collapse is observed. As dehydration rates depend on the average bond strength in the crystal structure, it is predicted (and indeed observed) that the temperature of the differential Ar release peak is variable among different minerals. As the Ca/Cl/K signatures of pure micas coincide with the Ar release peak, their combination identifies the isochemical steps that correspond to the degassing of pristine micas. Only these should be used to date the activity of shear zones.
This procedure should become routine in analysing polydeformed metamorphic rocks.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences
08 Faculty of Science > Institute of Geological Sciences > Isotope Geology

UniBE Contributor:

Villa, Igor Maria

Subjects:

500 Science > 550 Earth sciences & geology
500 Science

ISSN:

0016-7649

Publisher:

Geological Society of London

Language:

English

Submitter:

Igor Maria Villa-Toscani

Date Deposited:

23 Jan 2023 09:42

Last Modified:

23 Jan 2023 23:28

Publisher DOI:

10.1144/jgs2020-235

Uncontrolled Keywords:

Himalaya, Main Central Thrust zone, 40Ar/39Ar geochronology, Ar Differential Release Plot, Isochemical Ages

BORIS DOI:

10.48350/176697

URI:

https://boris.unibe.ch/id/eprint/176697

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