The timing of prograde metamorphism in the Garhwal Himalaya, India

Prince, Christophe Iain (2000). The timing of prograde metamorphism in the Garhwal Himalaya, India. PhD thesis The Open University.



The Himalaya provide the most significant example of present-day orogenesis and consequently have been extensively studied to gain an understanding of the principle controls on the response of the crust to continental collision. However, our understanding of the prograde metamorphic evolution of the orogen remains poor. This thesis builds on recent advances in the study of PTt paths, using garnet chronometry, to better constrain the thennobarometric evolution of the Garhwal section of the Indian Himalaya. Results show that the metamorphic core of the Garhwal Himalaya - the High Himalayan Crystalline Series (HHCS) - records a complex, continuous prograde thermal history from initial burial -10 Ma after continental collision at -50 Ma, up to cooling and exhumation at 20-16 Ma BP. PT paths obtained from garnets indicate that prograde metamorphism occurred during crustal thickening and "peak" thermobarometric estimates show that the presently exposed HHCS records temperatures of -700 °C throughout the section accompanied by a decrease in pressures from --13 kbar at the base to -6 kbar at the top. However, chronometric information shows that reorganisation of the orogenic wedge resulted in the juxtaposition of rocks which attained different PT conditions at different times and places during orogenesis. Additionally, temperatures were sufficient in the early stages of orogenesis for the development of small leucogranitic bodies to form by fluid-present melting. The HHCS in Garhwal, therefore, cannot be considered as a single coherent crustal slice. Furthermore, the continued reorganisation of the orogen since collision also means the heat generation within the overthickened orogenic wedge is sufficient for anatexis of the crust to form the well-studied melts intruding the upper levels of the HHCS.

However, interpretation of the results is complicated by the isotopic systematics involved in garnet chronometry and by the role of small inclusions with high concentrations of the critical elements of- Nd, Pb, Sr. The systematics of the Sm-Nd system in garnet has been investigated by a comparison of concentrations obtained insitu by LA-ICP-MS with those obtained by isotope dilution. Results show that while such inclusions can pose a problem to chronometry, their effects can be identified and constrained.

In the course of such work data was obtained on the trace-element zonation in garnet, which acts as a monitor of the chemical evolution of the rock. While the controls on such zonation are still poorly understood the data presented here emphasise the importance of fractionation of the chemical system from which the garnet grows by both accessory minerals and by garnet itself. Furthermore, different minerals fractionate distinctly different elements this can be recognised in the trace-element zonation preserved in garnet.

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