Petrogenesis of UHP Metamorphic Crustal and Mantle Rocks from the Chinese Continental Scientific Drilling Pre-pilot Hole 1, Sulu Belt, Eastern China

Abstract

The Pre-pilot Hole No. 1 of the Chinese Continental Scientific Drilling Project (CCSD-PPH1) in the southern Sulu terrane recovered a continuous core of eclogite, garnet peridotite, orthogneiss, paragneiss, and minor schist and quartzite. Geochemical characteristics indicate that the garnet peridotite was derived from depleted mantle; all other rock types are metamorphosed supracrustal rocks with continental affinities. The eclogite consists of garnet, omphacite, phengite, quartz (coesite), amphibole, rutile, and zircon; P-T estimates of peak metamorphism are 785-820°C and >2.7 to 3.7 GPa. The gneisses show common amphibolite-facies mineral assemblages consisting of plagioclase, K-feldspar, muscovite, and quartz, with minor garnet, epidote (or zoisite), and biotite; coesite inclusions in zircon indicate that the gneisses together with eclogites were subjected to an early UHP metamorphism prior to amphibolite-facies retrogression. The Garnet peridotites show porphyroblastic textures, and consist mainly of garnet, clinopyroxene, orthopyroxene, and olivine with minor phlogopite. Garnet and clinopyroxene porphyroblasts show significant compositional zoning. Applying relevant geothermobarometers, core compositions of the minerals indicate P-T conditions of 6.0-7.0 GPa and 1100-1200°C. For the rim, similar or slightly higher P-T conditions compared to the eclogites were obtained. We suggest that the former represent crystallization conditions of garnet peridotite in the upper mantle, whereas the latter reflect reequilibrium conditions during incorporation of mantle rocks into the subducted slab. We conclude that the garnet peridotite may have been derived from the mantle wedge above the subduction zone. If so, these mantle-derived rocks were sandwiched between continental-derived country rocks.