The Luliangshan garnet peridotite massif of the North Qaidam UHPM belt, NW China - a review of its origin and metamorphic evolution

The Luliangshan garnet peridotite massif is an ultramafic complex in the North Qaidam UHPM belt, NW China. The strongly layered complex comprising garnet-bearing dunite, garnet-harzburgite, garnet-lherzolite and garnet-pyroxenite and garnet-free dunite, occurs together with eclogite embedded in various continental gneisses. The geological setting, the internal structure, bulk-composition, rare earth elements, isotopic and mineral composition data show that the garnet peridotite derives from a middle Ordovician Alaskan-type layered sub-arc cumulate intrusion of ascending mantle wedge melts. An abyssal peridotite protolith can be excluded. During the Ordovician-Silurian continental collision, thickening and foundering, the Luliangshan peridotite complex was exposed to ultrahigh pressures (UHP) reaching 5.5 GPa possibly > 6 GPa at temperatures of 900 degrees C (perhaps up to 1000 degrees C) corresponding to a depth of similar to 200 km. The extreme pressure conditions have been derived from thermobarometry using mineral compositions of the garnet peridotite assemblages, but they are supported by a wealth of decompression-induced mineral exsolutions in UHP minerals and by diamond inclusion in zircon. The Luliangshan garnet peridotite has experienced four stages of retrograde overprint during exhumation that lasted into the Devonian: (i) decompression-induced unmixing of the UHP minerals; (ii) garnet kelyphitisation; (iii) amphibole overprinting and (iv) serpentinization. Hydrous minerals occurring within peak metamorphic assemblage represent pseudo-inclusions, that is reaction products of reactions related to various stages of decompression and cooling rather than prograde inclusions during porphyroblast growth.