P-T path of high-pressure/low-temperature rocks and tectonic implications in the western Tianshan Mountains, NW China

The Chinese western Tianshan high-pressure/low-temperature (HP-LT) metamorp hic belt, which extends for about 200 km along the South Central Tianshan s uture zone, is composed of mainly metabasic blueschists, eclogites and gree nschist facies rocks. The metabasic blueschists occur as small discrete blo cks, lenses: bands, laminae or thick beds in meta-sedimentary greenschist f acies country rocks. Eclogites are intercalated within blueschist layers as lenses, laminae, thick beds or large massive blocks (up to 2 km(2) in plan view). Metabasic blueschists consist of mainly garnet, sodic amphibole, ph engite, paragonite, clinozoisite, epidote, chlorite, albite, accessory tita nite and ilmenite. Eclogites are predominantly composed of garnet, omphacit e, sodic-calcic amphibole, clinozoisite, phengite, paragonite, quartz with accessory minerals such as rutile, titanite, ilmenite, calcite and apatite. Garnet in eclogite has a composition of 53-79 mol% almandine, 8.5-30 mol% grossular, 5-24 mol% pyrope and 0.6-13 mol% spessartine. Garnet in blueschi sts shows similar composition. Sodic amphiboles include glaucophane, ferrog laucophane and crossite, whereas the sodic-calcic amphiboles mainly compris e barroisite and winchite. The jadeite content of omphacite varies from 35- 54 mol%. Peak eclogite facies temperatures are estimated as 480-580 degrees C for a pressure range of 14-21 kbar. The conditions of pre-peak, epidote- blueschist facies metamorphism are estimated to be 350-450 degrees C and 8- 12 kbar. All rock types have experienced a clockwise P-T path through pre-p eak lawsonite/epidote-blueschist to eclogite facies conditions. The retrogr ade part of the P-T path is represented by the transition of epidote-bluesc hist to greenschist facies conditions. The P-T path indicates that the high -pressure rocks formed in a B-type subduction zone along the northern margi n of the Palaeozoic South Tianshan ocean between the Tarim and Yili-central Tianshan plates.