Petrology and phase equilibrium of newly found eclogites from Kekesu Valley in eastern part of southwest Tianshan HP-UHP metamorphic belt, China, and its tectonic significance

Eclogites and omphacite-bearing blueschists have been newly found in the eastern segment of the southwest Tianshan orogenic belt, Xinjiang, northwest China. After detailed petrological study, three samples including one fresh eclogite TK003, one blueschist sample TK026-8 and one retrograded eclogite TK027, were selected for phase equilibrium modeling under NC(K)MnFMASHO (N₂O-CaO-K₂O-MnO-FeO-MgO-Al₂O₃-SiO₂-H₂O-O) system, by thermocalc 3.33 software. Composition analyses of garnets in these three samples show typical growth zoning with Xpy and Xgrs increasing, Xspss decreasing from core to rim. Pseudosection modeling of the garnet zonation reflects that the eclogites and blueschist experienced a similar P-T evolution trajectory, with a near iso-baric heating in the early stage, and reached eclogite facies metamorphic field with peak P-T regime of 480–515°C, 2.00–2.30 GPa. Subsequently the rocks experienced an early iso-thermal decompression retrograde stage with P-T conditions of 515–519°C, 1.78–1.93 GPa. Variations of mineralogy and modes of these rocks are probably due to different retrograde paths as a consequence of different bulk-rock composition, as well as a variation in fluid activity during exhumation. P-T calculation and a peak geothermal gradient of 6–7°C/km indicate HP rocks in the Kekesu Valley experienced cold subducted eclogite facies metamorphism. Thus a huge oceanic subduction eclogite facies metamorphic belt in southwest Tianshan has been recognized, extending from the Kekesu Valley in the east to the Muzhaerte Valley in the west for nearly 200 km. However, UHP evidence has not been found in the Kekesu terrane, perhaps because the slab in east part of southwest Tianshan did not subduct into such a great depth.