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 (δ¹⁸O: 7.69 ± 0.08‰) and youngest zircon overgrowths (δ¹⁸O: 7.95 ± 0.12, 8.24 ± 0.09‰) 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 (δ¹⁸O: 13.29 ± 0.11‰) 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°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.