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

作者：Mottram Catherine M^*; Warren Clare J; Halton Alison M; Kelley Simon P; Harris Nigel B W

来源：Lithos, 2015, 238: 37-51.

DOI：10.1016/j.lithos.2015.08.018

• Ar-40/Ar-39
• Inverted metamorphism
• Himalaya
• Cooling
• Ductile thrusting

Ar-40/Ar-39 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 Ar-40/Ar-39 ages in similar lithologies at a variety of metamorphic pressure and temperature (P-T) conditions.
The Ar-40/Ar-39 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 similar to 2-9 Ma within each sample in the structurally lower levels (garnet grade) but only similar to 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 (2 sigma) to 13.2 +/- 13 Ma (2 sigma) 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 (2 sigma) at the lowest structural levels to 18.6 +/- 4.7 Ma (2 sigma) at the highest structural levels; the step-heating plateaux are commonly disturbed.
Temperatures >600 degrees 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 Ar-40 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 similar to 50-80 degrees C Ma(-1), consistent with rapid thrusting, extrusion and exhumation along the Main Central Thrust during the mid-Mikene.

• 出版日期2015-12-1