High resolution determination of orogenic cooling rates: an eastern Himalayan example

Warren, C. J.; Singh, A.K.; Roberts, N.M.W; Halton, A. M. and Singh, B.K. (2013). High resolution determination of orogenic cooling rates: an eastern Himalayan example. In: Building Strong Continents Conference, 2-4 Sep 2013, Portsmouth, p. 48.

URL: http://www.port.ac.uk/special/buildingstrongcontin...

Abstract

Cooling and exhumation rates of high-grade metamorphic rocks are commonly determined from differences between temperature-time pairs. It is analytically straightforward to determine high precision ‘dates’, via U-Pb (monazite or zircon) or Ar/Ar (muscovite or biotite) to constrain higher or lower temperature events respectively. What is generally more difficult is to link these ‘dates’ to temperatures, thereby linking geologically meaningful ‘ages’ to metamorphic ‘stages’. New advances in petrogenetic modelling and trace element fingerprinting techniques pave the way for improvements in the interpretation of ages yielded by high temperature chronometers. For the lower temperature chronometers, high spatial resolution laserprobe analyses and diffusion models allow cooling rates to be determined with higher precision.

The Greater Himalayan Sequence (GHS) in the central Himalaya has reportedly experienced relatively synchronous metamorphism and cooling during the Oligocene and Miocene (ca. 32-20 Ma). In contrast, recent and new monazite U-Th-Pb and muscovite Ar/Ar data from the eastern Himalayan regions of Sikkim, Bhutan and Arunachal Pradesh, suggest that the timing (and grade) of peak metamorphism and the timing of exhumation-related cooling of units at the highest structural levels of the GHS appears to young eastwards. Monazites associated with peak metamorphism and/or melting reactions yield ages of 26-23 Ma in N. Sikkim, yet yield much younger ages between 15-13 Ma, in NW Bhutan. New data from the upper structural levels of the GHS in Arunachal Pradesh show that monazite ages from similar lithologies at high structural levels yield ages of ca. 16-11 Ma. Muscovite Ar/Ar ages mirror this younging trend, yielding 13-12 Ma in Sikkim, 13-11 Ma in Bhutan and ca. 7 Ma in Arunachal Pradesh.

Together these data suggest a higher degree of complexity in the architecture and mechanisms of formation and exhumation of the eastern Himalayan GHS than is reported for the central portions of the orogen. We show how a combination of high precision dating, detailed petrology and diffusion modelling can yield powerful insights into the evolution and cooling history of high-grade rock units.

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