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

Abstract

We report two generations of low-18O zircons from the Himalayan eclogites and their host gneisses. In situ SIMS 18O 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-18O mantle-derived magma, (2) followed by post-emplacement high-temperature meteoric-water alteration and finally (3) crystallization of new, low-18O minerals during the ultrahigh-pressure metamorphism. Magmatic zircon (269 Ma) in Group I eclogites yielded 18O values from 1.9 to 4.6‰ 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 18O values from −3.9 to −2.7‰ (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 18O values of ca. 2.9‰, which decrease to −0.1‰ in metamorphic (ca. 45 Ma) rims. These zircons preserve lower 18O values than would be equilibrated with typical mantle. The low-18O values in magmatic zircons suggest that the mafic protolith to these eclogites formed from a hydrothermally altered subducted oceanic crust and the negative 18O 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-18O ocean crust to form low-18O mantle-derived mafic magmas as previously proposed by Cartwright and Valley (1991, Geology 19, 578-581) for Proterozoic Scourie Dikes.