Oxygen isotopes in Indian Plate eclogites (Kaghan Valley, Pakistan): Negative δ18O values from a high latitude protolith reset by Himalayan metamorphism

Abstract

Oxygen isotope compositions are reported for the first time for the Himalayan metabasites of the Kaghan Valley, Pakistan in this study. The highest metamorphic grades are recorded in the north of the valley, near the India– Asia collision boundary, in the form of high-pressure (HP: Group I) and ultrahigh-pressure (UHP: Group II) eclogites. The rocks show a step-wise decrease in grade from the UHP to HP eclogites and amphibolites. The protoliths of these metabasites were the Permian Panjal Trap basalts (ca. 267 ± 2.4 Ma), which were emplaced along the northern margin of Indiawhen it was part of Gondwana. After the break-up of Gondwana, India drifted northward, subducted beneath Asia and underwent UHP metamorphism during the Eocene (ca. 45±1.2Ma). At the regional scale, amphibolites, Group I and II eclogites yielded δ18O values of +5.84 and +5.91‰, +1.66 to +4.24‰, and −2.25 to +0.76‰, respectively, relative to VSMOW. On a more local scale, within a single eclogite body, the δ18O values were the lowest (−2.25 to−1.44‰) in the central, the best preserved (least retrograded) parts, and show a systematic increase outward into more retrograded rocks, reaching up to +0.12‰. These values are significantly lower than the typical mantle values for basalts of +5.7 ± 0.3‰. The unusually lowor negative δ18O values in Group II eclogites potentially resulted from hydrothermal alteration of the protoliths by interactions with meteoric water when the Indian plate was at southern high latitudes (~60°S). The stepwise increase in δ18O values, among different eclogite bodies in general and at single outcrop-scales in particular, reflects differing degrees of resetting of the oxygen isotope compositions during exhumation-related retrogression.