Garnet Peridotites in UHP Mountain Belts of China

Abstract

Ultrahigh-pressure (UHP) garnet peridotites occur as blocks, lenses, or layers within granuliteamphibolite-facies gneiss in the Altyn, northern Qaidam, and Dabie-Sulu UHP belts of China. Most garnet peridotites from northern Qaidam and Sulu represent pieces of a depleted, residual mantle (Type A), whereas those from the Altyn and Dabie Mountains formed by crystal cumulates in crustal mafic-ultramafic layered intrusions (Type B). LREE enrichment, high 87Sr/86Sr (0.7071-0.7100) and low 143Nd/144Nd ratios (0.5122-0.5128) indicate that some peridotites experienced metasomatism and/or crustal contamination. Both types of garnet peridotites were subjected to UHP metamorphism at 750-980°C and 3.5-6.5 GPa with an extremely low thermal gradient of ≤5°C/km. Representative UHP assemblages of the Altyn Mountains, Ol + Opx + Cpx + Grt ± magnesite (Mgs), and of northern Qaidam, Ol + Opx + Cpx + Grt, together with eclogite within gneiss formed at ~460-500 Ma. Garnet peridotite of the Dabie-Sulu belt, with the assemblage Ol + Opx + Cpx + Grt ± Phl ± Mgs ± Ti-clinohumite ± pargasite, formed at ~220-240 Ma. UHP assemblages are only preserved in less than 30 vol% of exposed Grt peridotites because most have been obliterated by serpentinization. Exsolution microstructures are widespread in UHP minerals. Exsolved phases can either be mono- or multi-mineralic, such as ilmenite and magnetite in olivine; quartz, ilmenite, phlogopite, and Grt + Ilm ± Amp in diopside; and Rt ± Cpx + Ap in garnet. Occurrence of clinoenstatite and orthoenstatite intergrowths in many peridotites suggests that the polymorphic transformation of orthopyroxene is common during subduction and exhumation. These exsolution and transformation structures imply that garnet peridotites in the huge mountain belts of China might have come from great depths (>150-200 km).