Direct zircon U-Pb evidence for pre-Himalayan HT metamorphism in the Higher Himalayan Crystallines, eastern Garhwal Himalaya, India

The high-grade Higher Himalayan Crystallines (HHC), located between the South Tibetan Detachment System and Main Central Thrust in the collision zone between the Indo-Australian and Eurasian plates, have been subject to at least four significant phases of deformation and metamorphism. The earliest of those significantly predates the Cenozoic continent-continent collision, but has been difficult to date isotopically because of later overprinting. Migmatitic paragneiss from the Badrinath Formation in the Dhauliganga Valley, northern Uttarakhand, some of the highest-grade rocks in the HHC, preserves direct evidence of mid-Ordovician metamorphism in the form of 465.8 +/- 6.4 Ma zircon overgrowths with extremely low Th/U (0.0038-0.0074). The overgrowths have formed on ca. 2.63-0.71 Ga detrital zircon cores and are themselves overgrown by two generations of Miocene metamorphic zircon with mean Pb/U ages of ca. 21.5 and 18.2 Ma. Monazite from the same sample has a mean Pb/Th age of 19.4 +/- 0.2 Ma. The oxygen isotopic compositions of the monazite (delta O-18: 7.69 +/- 0.08 parts per thousand) and youngest zircon overgrowths (delta O-18: 7.95 +/- 0.12, 8.24 +/- 0.09 parts per thousand) are consistent with mineral growth in a metasediment, but either of the two minerals did not grow in isotopic equilibrium with each other, or the original composition of the monazite has not been preserved. If the quartz (delta O-18: 13.29 +/- 0.11 parts per thousand) equilibrated with the youngest zircon and its composition has been preserved, then the last episode of zircon growth took place at low temperature, ca. 420 degrees C, after the migmatization. The protolith of the Badrinath migmatite was a Neoproterozoic or Early Palaeozoic metasediment partially melted (and probably migmatized) in the Middle Ordovician. The strong planar foliation currently present in the migmatite is probably the result of mid-crustal extrusional channel flow and HT decompressional partial melting in the Miocene.