Trace-element behaviour during high-grade metamorphism and anatexis of the Himalayas

Ayres, Michael William (1997). Trace-element behaviour during high-grade metamorphism and anatexis of the Himalayas. PhD thesis The Open University.



Rates of physical processes within the continental crust may be constrained by modelling disequilibrium geochemical signatures observed in both metamorphic and magmatic rocks formed within young orogens such as the Himalayas. The High Himalayan Crystalline Series (HHCS) in the Zanskar region of NW India is a fault-bounded sequence of high-grade metasedimentary and anatectic rocks with a structural thickness of >40 km. Miocene intrusive leucogranite sheets, comprising biotite leucogranites and cross-cutting tourmaline leucogranites, have been emplaced into the upper structural levels of the HHCS. 87Sr/86Sr and 143Nd/144Nd data indicate that the leucogranites were derived from HHCS metasediments. Trace-element modelling of potential melting reactions in pelitic assemblages indicates that these granites result from low-degree melting by vapour-absent muscovite breakdown. REE modelling of accessory phase behaviour during anatexis suggests that apatite and monazite jointly control the REE chemistry of anatectic melts. Partial apatite and monazite dissolution during anatexis is also responsible for the Sm/Nd fractionation and 143Nd/144Nd disequilibrium observed in the leucogranites relative to their protoliths. Such disequilibrium can constrain the timing of early events in polymetamorphic terrains.

LREE and Zr concentrations indicate temperatures of anatexis of 690-750°C for the biotite leucogranites and 650-690°C for the tourmaline leucogranites. Tourmaline leucogranites are undersaturated with respect to the LREE, suggesting that these magmas did not fully equilibrate with restitic monazite prior to extraction and implying source residence times of <180 ka. Sr-isotope systematics suggest that high-grade metamorphism and melt extraction occurred over a timescale of> 100 ka.

Cation interdiffusion data have been calculated at 700±30°C from forty HHCS metapelitic garnets by inverse modelling of diffusively modified manganese growth profiles that are indicative of Rayleigh partitioning during garnet growth. Diffusion modelling suggests that the duration of peak metamorphism affecting the Himalayan garnets was a least an order of magnitude shorter than that experienced by garnets from the Scottish Dalradian.

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