P-T evolution of glaucophane-omphacite bearing HP-LT rocks in the western Tianshan Orogen, NW China: new evidence for 'Alpine-type' tectonics

摘要

The late Palaeozoic western Tianshan high-pressure/low-temperature belt extends for about 200 km along the south-central Tianshan suture zone and is composed mainly of blueschist, eclogite and epidote amphibolite/greenschist facies rocks. P-T conditions of mafic garnet omphacite and garnet-omphacite blueschist, which are interlayered with eclogite, were investigated in order to establish an exhumation path for these high-pressure rocks. Maximum pressure conditions are represented by the assemblage garnet-omphacite-paragonite-phengite-glaucophane-quartz-rutile. Estimated maximum pressures range between 18 and 21 kbar at temperatures between 490 and 570 degreesC. Decompression caused the destabilization of omphacite, garnet and glaucophane to albite, Ca-amphibole and chlorite. The post-eclogite facies metamorphic conditions between 9 and 14 kbar at 480 570 degreesC suggest an almost isothermal decompression from eclogite to epidote-amphibolite facies conditions. Prograde growth zoning and mineral inclusions in garnet as well as post-eclogite facies conditions are evidence for a clockwise P-T path. Analysis of phase diagrams constrains the P-T path to more or less isothermal cooling which is well corroborated by the results of geothermobarometry and mineral textures. This implies that the high-pressure rocks from the western Tianshan Orogen formed in a tectonic regime similar to 'Alpine-type' tectonics. This contradicts previous models which favour 'Franciscan-type' tectonics for the southern Tianshan high-pressure rocks. [References: 55]