STUDY ON OXYGEN ISOTOPE OF HIGH-PRESSURE VEINS AND HOST-ROCKS FROM WESTERN TIANSHAN IN CHINA: IMPLICATIONS FOR DEEP FLUIDS FLOW AND THE CHARACTERISTIC OF SUBDUCTION

Abstract

The western Tianshan high-pressure (HP) metamorphic belt, which is probably to represent paleosubduction mélange of Paleozoic south Tianshan ocean between the Yili-central Tianshan and Tarim plates, is situated in the south Tianshan Mountains in China. High-pressure veins are extensively developed in this HP belt. Analyses of oxygen isotope of main HP-metamorphic mineral separates and whole rocks are conducted for HP veins and host rocks in order to get inquiring into the sources and flows of deep fluids in the paleo-subduction zone and the characteristic of the subduction of the slab. The δ18O values of high-pressure vein are from 8.28‰ to + 10.70‰, most of which are at the range of +9.50‰ ± 1. The metabasic host rocks have the similar range of δ18 O values (+ 9.25‰ - + 10.10‰) to the high-pressure veins. Both of veins and host rocks don't show large variations in δ 18O values. The variations in oxygen isotope ratios between a vein and its host rocks or between the center and the rim of a single vein are irregular. Generally, these variations are not significant. For most pairs, the variations in δ18O values are less than 1‰. Contrary to whole rocks, mineral separates show a very large range of oxygen isotopic composition. The δ18O values of quartz, garnet and omphacite separates are + 11.40‰ - + 15.20‰, + 3.59‰ - + 11.60‰ and + 8.30‰ - + 13.05‰ separately. Phengites and glaucophanes show narrow ranges of δ18O value, which are + 10.00‰ - + 11.10‰ and + 9.26‰ - + 9.94‰ respectively. The oxygen isotope fractionations between various HP metamorphic minerals in eclogitic rocks from western Tianshan are extensively disequilibria at peak metamorphic temperature. The data of oxygen isotope of whole rocks indicates that the deep fluids in the paleosubduction zones mainly originated from adjacent host-rocks. The extensively disequilibria of oxygen isotope fractionations between minerals probably shows that the subduction and exhumation of the subducted slap are rapidly, and external fluids incorporated into the deep fluids in the paleosubduction zones.