Elusive Himalayan eclogites: evidence that they were there from zircon U-Pb geochronology and trace element geochemistry

Mafic and pelitic granulites exposed in the eastern Himalayan kingdom of Bhutan preserve textural evidence for a precursor high-pressure metamorphic event, the precise conditions of which are generally unrecoverable due to the later high temperature overprint. As high pressure metamorphism is rare in the Himalayas, especially in the eastern parts of the orogen, their thermobarometrical and geochronological evolution place important constraints on the geodynamic evolution of the Himalaya in particular and continental collisions in general. We report SHRIMP-RG trace element (REE) and U-Pb zircon geochronological data, collected by the same instrument and on adjacent spots of the same crystal. These data suggest that zircons crystallized at 14-15 Ma over a temperature range of ca. 705-815 ° C. This age is interpreted to indicate the timing of HP metamorphism due to the lack of negative Eu anomaly, the depleted heavy REE signature and the low temperatures of crystallization. Zr-in-rutile electron microprobe data from similar rocks suggests that rutile crystallized over a larger and higher range of temperatures (ca. 620-870 ° C), interpreted as rutile growth during the eclogite-granulite transition and/or during granulite facies metamorphism. These data will help to establish better links between accessory mineral crystallisation and metamorphism and will also help to resolve the conditions and timing of eclogite facies metamorphism in the Himalaya.