Clockwise exhumation path of granulitized eclogites from the Ama Drime range (Eastern Himalayas)

The metamorphic evolution of a granulitized eclogite from the Phung Chu Valley (Eastern Himalaya) was reconstructed combining microstructural observations, conventional thermobarometry and quantitative pseudosection analysis. The granulitized eclogite consists of clinopyroxene, plagioclase, garnet, brown amphibole, and minor orthopyroxene, biotite, ilmenite and quartz. On the basis of microstructural observations and mineral relationships, four metamorphic stages and related mineral assemblages have been recognized: (i) M1 eclogite-facies assemblage, consisting of garnet, omphacite (now replaced by a clinopyroxene + plagioclase symplectite) and phengite (replaced by biotite +plagioclase symplectite); (ii) M2 granulite-facies assemblage, represented by clinopyroxene, orthopyroxene, garnet, plagioclase and accessory ilmenite; (iii) M3 plagioclase + orthopyroxene corona developed around garnet, and (iv) M4 brown amphibole + plagioclase assemblage in the rock matrix. Because of the nearly complete lack of eclogitic mineral relics, M1 conditions can be only loosely constrained at > 1.5 GPa and > 580 degrees C. In contrast, assemblage M2 tightly constrains the peak granulitic stage at 0.8-1.0 GPa and > 750 degrees C. The second granulitic assemblage M3, represented by the plagioclase + orthopyroxene corona, formed at lower pressures (similar to 0.4 GPa and similar to 750 degrees C). During the subsequent exhumation, the granulitized eclogite experienced significant cooling to nearly 700 degrees C, marked by the appearance of brown amphibole and plagioclase (M4) in the rock matrix. U-Pb SHRIMP analyses on low-U rims of zircon from an eclogite of the same locality suggest an age of 13-14 Ma for the M3 stage. The resulting decompressional clockwise P-T path of the Ama Drime eclogite is characterized by nearly isothermal decompression from > 1.5 GPa to similar to 0.4 GPa, followed by nearly isobaric cooling from similar to 775 degrees C to similar to 710 degrees C. Modelling of phase equilibria by a calculated petrogenetic grid and conventional thermobarometry on a biotite-garnet-sillimanite metapelite hosted in the country rock granitic orthogneiss extends the inferred P-T trajectory down to similar to 630 degrees C and similar to 0.3 GPa.