A BRIEF REVIEW OF UHP META-OPHIOLITIC ROCKS, SOUTHWESTERN TIANSHAN, WESTERN CHINA

Abstract

We here summarize recent studies of the occurrence, petrology, geochemistry, and geochronology of a new UHP metamorphic belt in the southwestern Tianshan. Eclogites here are grouped into three types: (1) lenticular eclogite bodies surrounded by blueschists; (2) eclogites with relict pillow structures; and (3) banded eclogite layers within marbles. They contain inclusions of coesite pseudomorphs in garnet, coesite exsolution rods in omphacite, and the possible stable association of aragonite + magnesite after dolomite. Rocks record three discrete stages of recrystallization: a peak UHP eclogite stage at 560-600°C and 4.9-5.0 GPa, a main UHP eclogite stage at 598-496°C and 2.6-2.7 GPa and retrograde overprinting under epidote blueschist-facies conditions. Protoliths possess geochemical characteristics of ocean-island basalts that originated from an enriched mantle with εNd = -1.4 to -0.4, NMORB from depleted mantle with εNd = +6.7 to +7.4, and EMORB from enriched mantle with εNd = -2.5 to +3.2. SHRIMP dating of zircon separates shows that the oceanic basaltic protoliths formed in a pre-Carboniferous (>310 Ma) seamount environment, began northward subduction beneath the Yili-central Tianshan craton at the end of the Permian (280-290 Ma), and were subjected to HP-UHP metamorphism in the Triassic (220-230 Ma). Based on the occurrence of this Triassic HP-UHP belt together with discovery of coeval low-P granulite-facies rocks to the north, a paired metamorphic belt tectonic model is proposed for the southwestern Tianshan, western China.