Low-delta O-18 mantle-derived magma in Panjal Traps overprinted by hydrothermal alteration and Himalayan UHP metamorphism: Revealed by SIMS zircon analysis

We report two generations of low-delta O-18 zircons from the Himalayan eclogites and their host gneisses. In situ SIMS delta O-18 analyses on single zircon crystals (with known age and Hf isotope ratios) from two populations of chemically distinct zircons demonstrate a complex history: (1) an early low-delta O-18 mantle-derived magma, (2) followed by post-emplacement high-temperature meteoric-water alteration and finally (3) crystallization of new, low-delta O-18 minerals during the ultrahigh-pressure metamorphism. Magmatic zircon,(269 Ma) in Group I eclogites yielded delta O-18 values from 1.9 to 4.6 parts per thousand VSMOW with an average value of 4.0 +/- 0.2 (n = 35, the error is 2SD analytical precision and n represents number of analyzed spots), which is lower than the typical mantle values (5.3 +/- 0.6, 2SD). In contrast, metamorphic zircons (45 Ma) in Group II eclogites preserve unusually low, negative delta O-18 values from -3.9 to -2.7 parts per thousand (average: -3.4 +/- 0.4, n = 35, 2SD). Zircons in felsic gneiss that surround Group II eclogites have inherited magmatic cores (ca. 260 Ma) with delta O-18 values of ca. 2.9 parts per thousand, which decrease to -0.1 parts per thousand in metamorphic (ca. 45 Ma) rims. These zircons preserve lower delta O-18 values than would be equilibrated with typical mantle. The low-delta O-18 values in magmatic zircons suggest that the mafic protolith to these eclogites formed from a hydrothermally altered subducted oceanic crust and the negative delta O-18 values in metamorphic zircons indicate hydrothermal alteration after crystallization of the mafic magmas but before growth of metamorphic zircons. This study reports evidence for melting of subducted low-delta O-18 ocean crust to form low-delta O-18 mantle-derived mafic magmas as previously proposed by Cartwright and Valley (1991, Geology 19, 578-581) for Proterozoic Scourie Dikes. (C) 2017 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.