Metamorphism of high-P metagreywacke from the Eastern Himalayan syntaxis: phase equilibria and P-T path

摘要

High-pressure (HP) metagreywacke from the Namche Barwa Complex, Eastern Himalayan Syntaxis (EHS), consists of garnet, biotite, plagioclase, quartz, rutile and ilmenite with or without K-feldspar, sillimanite, cordierite, spinel and orthopyroxene. Two types of metagreywacke are recognized: medium-temperature (MT) and high-temperature (HT) types. Garnet in the MT metagreywacke shows significant growth zoning and contains lower MgO than the weakly zoned garnet in the HT metagreywacke. Petrographic observations and phase equilibria modelling for four representative samples indicate that both types of metagreywacke experienced clockwise P-T paths subdivided into three stages: stage I is the pre-peak prograde to pressure peak (P-max) stage characterized by progressive increase in P-T conditions. The P-max conditions are estimated using the garnet composition with maximum CaO, being 12.5-13.5kbar and 685-725 degrees C for the MT metagreywacke, and 15-16kbar and 825-835 degrees C for the HT one. Stage II is the post-P-max decompression with heating or near-isothermal to T-max stage and the T-max conditions, constrained using the garnet compositions with maximum MgO, are 11kbar and 760 degrees C for the MT metagreywacke, and similar to 12kbar and 830-845 degrees C for the HT one. The modelled mineral assemblages at T-max are garnet+biotite+K-feldspar+rutile+plagioclase +/- ilmenite in the presence of melt for both types of metagreywacke, consistent with the petrographic observations. Stage III is the post-T-max retrograde metamorphism, characterized by decompression and cooling. The modelling suggests that the melts with high Na/K ratios (1.7-5.2) have been produced during stages I and II, which could be responsible for the formation of sodium-rich leucogranites. This study and previous results indicate that the Higher Himalayan Crystallines in the EHS consist of MT-HP and HT-HP metamorphic units separated by a speculated tectonic contact. Petrological and structural discontinuities within the EHS cannot be easily interpreted with tectonic aneurysm' model.