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 Ol + 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 ~ 800 °C and 6.8 GPa for the peridotites. All peridotitic minerals have high Mg# (Ol, 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 (DCpx/Grt) for REE between clinopyroxene and garnet decrease from LREE (~ 100) to HREE (< 1 but > 0.1), suggesting that clinopyroxene and garnet equilibrated at low-T (< 1100 °C). Zircons from three phlogopite and/or magnesite-bearing peridotites show very low 176Lu/177Hf values (between 10− 3 and 10− 6), and a wide range in 176Hf/177Hf ratios (0.28221-0.28250). The zircons with low 176Hf/177Hf and relatively high 176Lu/177Hf document that metasomatism affected the peridotite protolith. The model ages (TDM) 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.