Two types of peridotite in North Qaidam UHPM belt and their tectonic implications for oceanic and continental subduction: A review

Two types of peridotites are recognized in the North Qaidam continental-type UHP metamorphic belt. (1) Garnet peridotite, which includes garnet lherzolite, garnet-bearing dunite, garnet-free dunite and garnet pyroxenite, is one of the most informative lithologies in a continental-type subduction zone. Observations such as diamond inclusion in a zircon crystal and decompression exsolutions in garnet and olivine, plus thermobarometric calculations, argue that this garnet peridotite must have derived from mantle depths in excess of 200 km. Geochemical data reveal that the protolith of the garnet peridotite is largely of cumulate origin from high-Mg melts in a sub-arc mantle wedge environment rather than a abyssal peridotite. (2) Oceanic lithospheric mantle harzburgite, which occurs together with a meta-cumulate complex (including garnet pyroxenite and kyanite-eclogite) and with eclogite of MORB protolith. They are interpreted as exhumed blocks of the subducted oceanic lithosphere formed in the Cambrian (∼500–550 Ma). The presence of these two types of peridotites in the same continental-type subduction belt is unique and they allow a better understanding of the tectonic history of the North Qaidam continental-type UHP belt in particular and processes of plate tectonic convergence from oceanic lithosphere subduction to continental collision/subduction in general.