Geochemistry and zircon U-Pb ages of granulite xenolith from Tuoyun basalts, Xinjiang: Implications for the petrogenesis and the lower crustal nature beneath the southwestern Tianshan

The granulitic xenoliths discovered in the late Cretaceous basaltic rocks from Tuoyun basin, western Xin-jiang Province contain the assemblage of Opx + Cpx + PI +/- Grt +/- Qtz +/- Kfs. Mineral chemistry, petrochemistry and zircon U-Pb ages from the xenoliths have been reported in the paper. Petrogenesis, the lower crustal nature and the mantle-crust interaction reflected by the granulites are discussed as well. Tuoyun granulites are mainly mafic with few intermediate components, and represent the magmatic products underwent granulite-facies metamorphism in lower crustal condition. Equilibrium temperature and the maximum pressure estimates of the granulites are 910 +/- 35 degrees C and 13.5 X 10(8) Pa respectively, indicating that the crust beneath southwestern Tianshan had the thickness of less than 44 km and high geotherm (e.g. > 80 mW/m(2)) when the host magma erupted. During their formation, the fractional crystallization of olivine, pyroxene, plagioclase and ilmenite, plagioclase accumulation and the contamination of felsic components or fluid metasomatism processes had happened. Metamorphic zircon U-Pb age of 253 Ma may record the crust-mantle interaction caused by the orogenic root delamination beneath the southwestern Tianshan.