Origin and tectonic implication of an UHP metamorphic mafic–ultramafic complex from the Sulu UHP terrane, eastern China: Evidence from petrological and geochemical studies of CCSD-Main Hole core samples

Abstract

Study of the Maobei garnet peridotite and pyroxenite are significant, because these rocks were subjected to subduction-zone UHP metamorphism that is indicative of deep continental subduction. In order to better understand the origin of the Maobei peridotite and pyroxenite and the processes of continental subduction, we performed geochemical analyses on whole-rock and mineral samples. The studied samples were recovered in the main-hole of the Chinese Continental Scientific Drilling in the Sulu UHP terrane, east-central China. The garnet peridotites occur as lenses or layers in pyroxenite; they comprise garnet wehrlite and garnet dunite with minor interlayers of garnet websterite and garnet. Mini-bands, nodules or veins of eclogite and garnet clinopyroxenite (“internal”) also irregularly occur in garnet wehrlite. The peridotites are composed of olivine (Fo 83–84), garnet (Prp 51–56), ± diopside ± enstatite (En 86–87) ± titanoclinochumite, and have lower Mg/(Mg + Fe total) values of 0.79–0.83 and higher Al 2O 3 and CaO contents in comparison with the gneiss-hosted Sulu mantle peridotites. Garnet wehrlite displays variable REE patterns, from LREE-enriched to slightly LREE depleted relative to MREE and HREE; whereas, other ultramafic rocks show roughly flat REE patterns. In general, Rb, Ba and LREE increase, and compatible element (e.g. Cr, Co and Ni) abundances decrease from peridotite, pyroxenite to “internal eclogite” and “internal Grt-clinopyroxenite”. The calculated REE partition coefficients (D REE) between Cpx and Grt of the Maobei peridotite and pyroxenite decrease regularly from LREE (D Ce = 32–708) to HREE (D Yb = 0.01–0.10), which is comparable with natural mantle peridotite and eclogite crystallized at 800–1100 °C. The geochemical data and occurrence of these rocks suggest that the protoliths of the Maobei peridotites are cumulates derived by differentiation of basaltic magma at the lower continental crust. Thus, the “Internal eclogite and Grt-clinopyroxenite” result from post-cumulus crystallization of the trapped melt. Ti-clinohumite orthopyroxenite may have formed by reaction between peridotitic olivine and indigenous melt or country rock-derived (“exotic”) TiO 2-bearing fluids at high-P condition during continental subduction to mantle depths. The peak P−T estimates of 795–840 °C and 5.3–6.8 GPa of the Maobei peridotites and the available petrological data of country rocks indicate that the continental crust was subducted to a depth of ∼ 200 km.