Chemical composition and ultrahigh-P metamorphism of garnet peridotites from the Sulu UHP terrane, China: Investigation of major, trace elements and Hf isotopes of minerals

The Zhimafang peridotite body recovered by a pre-pilot borehole (PP1) of the Chinese Continental Scientific Drilling Project occurs in gneiss from the southern Sulu ultrahigh-P (UHP) terrane, east-central China. Detailed petrographic observation of the freshest lherzolites reveals the existence of pre-peak mantle relics, rare enstatite inclusions (Opx1) in garnet, and coarse-grained diopside (Cpx1) in the peridotites. Matrix phases 01+Grt+Opx+Cpx +/- Phl represent the assemblage of the peak-stage. At least > 1.5 GPa pressure increase is recognized from the mantle setting to the peak-stage of subduction-zone UHP metamorphism at similar to 800 degrees C and 6.8 GPa for the peridotites. All peridotitic minerals have high Mg# (0l, 91-93, Cpx, 93-94 and Opx, 92-94). Garnets are depleted in LREE, and have increased abundances from Pr through the MREE to a plateau of heavy REE (HREE) at about 10 times chondrite. In contrast, clinopyroxenes display humped REE patterns with high (La/Yb)(N) values of 27-68. Phlogopite is a major carrier of Ba (5205 ppm) and Th (5.5 ppm). Partition coefficients (D-Cpx/Grt) for REE between clinopyroxene and garnet decrease from LREE (similar to 100) to HREE (< 1 but > 0.1), suggesting that clinopyroxene and garnet equilibrated at low-T (< 1100 degrees C). Zircons from three phlogopite and/or magnesite-bearing peridotites show very low Lu-176/Hf-177 values (between 10(-3) and 10(-6)), and a wide range in Hf-176/Hf-177 ratios (0.28221-0.28250). The zircons with low Hf-176/Hf-177 and relatively high Lu-176/Hf-177 document that metasomatism affected the peridotite protolith. The model ages (T-DM) of zircons mostly cluster around 1.3-1.4 Ga, which probably reflects the metasomatic source. These chemical data combined with previous published bulk-rock major and trace element data indicate that the Zhimafang peridotites probably represent depleted and metasomatized subcontinental lithospheric mantle (SCLM) beneath the Sino-Korean craton. Investigation of the Sulu UHP metamorphic garnet peridotites provides additional insights into the Earth's geochemical and tectonic evolution, and enlarges our understanding of the exhumation of orogenic peridotites. (C) 2008 Elsevier B.V. All rights reserved.