Low-£_18O mantle-derived magma in Panjal Traps overprinted by hydrothermal alteration and Himalayan UHP metamorphism: Revealed by SIMS zircon analysis
Journal
Gondwana Research
Journal Volume
56
Pages
12-22
Date Issued
2018
Author(s)
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 ¡Ó 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 ¡Ó 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 ¡Ó 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¡V581) for Proterozoic Scourie Dikes. ? 2017 International Association for Gondwana Research
Subjects
Eclogites
Himalaya
In situ SIMS £_18O analysis
Kaghan
Panjal Traps
Zircon oxygen isotopes
Type
journal article