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Argon behaviour in an inverted Barrovian sequence, Sikkim Himalaya: the consequences of temperature and timescale on 40Ar/39Ar mica geochronology

Mottram, Catherine M.; Warren, Clare J.; Halton, Alison M.; Kelley, Simon P. and Harris, Nigel B. W. (2015). Argon behaviour in an inverted Barrovian sequence, Sikkim Himalaya: the consequences of temperature and timescale on 40Ar/39Ar mica geochronology. Lithos, 238 pp. 37–51.

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DOI (Digital Object Identifier) Link: https://doi.org/10.1016/j.lithos.2015.08.018
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Abstract

40Ar/39Ar dating of metamorphic rocks sometimes yields complicated datasets which are difficult to interpret in terms of timescales of the metamorphic cycle. Single-grain fusion and step-heating data were obtained for rocks sampled through a major thrust-sense shear zone (the Main Central Thrust) and the associated inverted metamorphic zone in the Sikkim region of the eastern Himalaya. This transect provides a natural laboratory to explore factors influencing apparent 40Ar/39Ar ages in similar lithologies at a variety of metamorphic pressure and temperature (P–T) conditions.

The 40Ar/39Ar dataset records progressively younger apparent age populations and a decrease in within-sample dispersion with increasing temperature through the sequence. The white mica populations span ~ 2–9 Ma within each sample in the structurally lower levels (garnet grade) but only ~ 0–3 Ma at structurally higher levels (kyanite-sillimanite grade). Mean white mica single-grain fusion population ages vary from 16.2 ± 3.9 Ma (2σ) to 13.2 ± 1.3 Ma (2σ) from lowest to highest levels. White mica step-heating data from the same samples yields plateau ages from 14.27 ± 0.13 Ma to 12.96 ± 0.05 Ma. Biotite yield older apparent age populations with mean single-grain fusion dates varying from 74.7 ± 11.8 Ma (2σ) at the lowest structural levels to 18.6 ± 4.7 Ma (2σ) at the highest structural levels; the step-heating plateaux are commonly disturbed.

Temperatures > 600 °C at pressures of 0.4–0.8 GPa sustained over > 5 Ma, appear to be required for white mica and biotite ages to be consistent with diffusive, open-system cooling. At lower temperatures, and/or over shorter metamorphic timescales, more 40Ar is retained than results from simple diffusion models suggest. Diffusion modelling of Ar in white mica from the highest structural levels suggests that the high-temperature rocks cooled at a rate of ~ 50–80 °C Ma− 1, consistent with rapid thrusting, extrusion and exhumation along the Main Central Thrust during the mid-Miocene.

Item Type: Journal Item
Copyright Holders: 2015 The Authors
ISSN: 0024-4937
Project Funding Details:
Funded Project NameProject IDFunding Body
PhD studentshipNE/1528018/1NERC
Advanced FellowshipNE/ HO16279/1NERC
Not SetIJP 2008/R3Royal Society International Joint Project
Keywords: 40Ar/39Ar; inverted metamorphism; Himalaya; cooling; ductile thrusting
Academic Unit/School: Faculty of Science, Technology, Engineering and Mathematics (STEM) > Environment, Earth and Ecosystem Sciences
Faculty of Science, Technology, Engineering and Mathematics (STEM)
Faculty of Science, Technology, Engineering and Mathematics (STEM) > Physical Sciences
Faculty of Wellbeing, Education and Language Studies (WELS)
Item ID: 44744
Depositing User: Nigel Harris
Date Deposited: 06 Nov 2015 10:30
Last Modified: 05 Apr 2019 19:12
URI: http://oro.open.ac.uk/id/eprint/44744
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