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

• Published in: Geological journal (Chichester, England) Vol. 57; no. 1; pp. 133 - 149
• Main Authors: Prabha Mohan, Shashank, Williams, Ian S., Singh, Sandeep
• Format: Journal Article
• Language: English
• Published: Hoboken, USA John Wiley & Sons, Inc 01.01.2022; Wiley Subscription Services, Inc
• Subjects: Cenozoic; Channel flow; Composition; Crystal structure; Crystallinity; Deformation; eastern Garhwal Himalaya; Higher Himalayan Crystallines; Isotopes; Lead; Low temperature; Metamorphism; Metamorphism (geology); Minerals; Miocene; Monazite; Ordovician; oxygen isotopes; Palaeozoic; Paleozoic; pre‐Himalayan HT metamorphism; Thorium; Zircon; zircon U–Pb ages
• ISSN: 0072-1050; 1099-1034
• DOI: 10.1002/gj.4287

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. Zircon in migmatite from the Higher Himalayan Crystallines, Dhauliganga Valley, has metamorphic zircon overgrowths with a Pb isotopic age of 465.8 ± 6.4 Ma, providing direct evidence of pre‐Himalayan (mid‐Ordovician) HT metamorphism. Cooling of partial melt and the exsolution of fluids associated with periods of post‐peak decompression resulted in two episodes of Himalayan zircon growth, ca. 21.5 and 18.2 ± 0.4 Ma. Monazite records an age of 19.4 ± 0.2 Ma. Each of the three generations of metamorphic zircon growth has a distinct, crustal, oxygen isotopic composition, consistent with the migmatite having a sedimentary protolith. The monazite and zircon are not in oxygen isotopic equilibrium. The isotopic fractionation between quartz and the youngest zircon suggests an equilibration temperature of ca. 420°C, much lower than that required to produce partial melting during migmatization.