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 ~2–9Ma within each sample in the structurally lower levels (garnet grade) but only ~0–3Ma at structurally higher levels (kyanite-sillimanite grade). Mean white mica single-grain fusion population ages vary from 16.2±3.9Ma (2σ) to 13.2±1.3Ma (2σ) from lowest to highest levels. White mica step-heating data from the same samples yields plateau ages from 14.27±0.13Ma to 12.96±0.05Ma. Biotite yield older apparent age populations with mean single-grain fusion dates varying from 74.7±11.8Ma (2σ) at the lowest structural levels to 18.6±4.7Ma (2σ) at the highest structural levels; the step-heating plateaux are commonly disturbed.Temperatures >600°C at pressures of 0.4–0.8GPa sustained over >5Ma, 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°CMa−1, consistent with rapid thrusting, extrusion and exhumation along the Main Central Thrust during the mid-Miocene.
Highlights
40Ar/39Ar dating is a tool commonly used to investigate the cooling and exhumation history of metamorphosed terranes
Samples were collected through a transect of this zone of deformation and inverted Barrovian metamorphism developed in the Main Central Thrust (MCT) zone, within the Daling Formation Lesser Himalayan Sequence (meta)sediments (LHS) rocks (Fig. 1; Mottram et al, 2014a,b)
Overall the white mica cooling ages obtained for the leading-edge of the MCT in the Sikkim Himalaya overlap within error with the ~13 Mnz/Zrn age (Ma) cooling age for the South Tibetan Detachment (STD) in the north of the Sikkim Himalaya (Kellett et al, 2013), suggesting that exhumation and cooling occurred simultaneously on both the MCT and STD in the region
Summary
The mathematical formulation only strictly applies for a cooling path that conforms to a shape of 1/T Factors such as excess Ar incorporated during crystallisation, most likely facilitated by fluids (Arnaud and Kelley, 1995; Baxter et al, 2002; Di Vincenzo and Palmeri, 2001; Foland, 1979; Halama et al, 2014; Harrison et al, 1985, 2009; Itaya et al, 2009; Roddick et al, 1980; Ruffet et al, 1995), mixing of two age populations (Beltrando et al, 2009; Chopin and Maluski, 1980; Dempster, 1992; Di Vincenzo et al, 2001; Hames and Cheney, 1997; Hammerschmidt and Frank, 1991; Viete et al, 2011), and the "openness" of the grain boundary network (Kelley, 2002; Smye et al, 2013; Warren et al, 2012a) can make the interpretation of 40Ar/39Ar dates as representing crystallisation, cooling, or the partial resetting of older ages problematic. Micas appear to require prolonged periods at moderately high metamorphic temperatures to yield robust cooling ages that can be used to estimate a cooling rate and exhumation history
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
