P–T–t evolution of garnet pyroxenites in the western Qaidam Block, West China: Insights into the tectonic transition from an Andean-type continental arc to backarc extension in the northern margin of East Gondwana

The continental arc evolution from crustal thickening to root foundering can make the lower crustal metamorphism evolve from the HT eclogite/HP granulite facies to low-P/T type HT-UHT conditions. However, this metamorphic evolution has been poorly constrained from natural examples. The Huatugou terrane in the western Qaidam Block contains garnet pyroxenites comparable to arclogitic xenoliths from the modern Andean arc at Mercaderes. Detailed petrographic observation, phase equilibrium modelling, and geothermometric calculation show that the garnet pyroxenites recorded a clockwise P–T path, evolving from pre-peak compression-heating to peak conditions of >1000 °C and >2.0 GPa to post-peak decompression under UHT conditions down to ~1.0 GPa, followed by cooling to ~800 °C/0.6 GPa. In-situ U-Pb dating yields cooling ages of ~526 Ma for zircon and 480–487 Ma for rutile and apatite. These results, together with existing zircon U-Pb ages from associated UHT rocks, reveal that the prograde metamorphism may have started from ~560 Ma, and the post-peak decompression with UHT overprinting was followed by cooling under granulite facies conditions between 540 and 507 Ma before cooling to low amphibolite facies conditions at ~487 Ma. Combined with other regional geological data, we infer that the Qaidam Block was an Andean-type continental arc in the northern margin of East Gondwana, in response to the southward subduction of the Proto-Tethys Ocean. The Qaidam continental arc was thickened to more than 70 km, followed by backarc extension that was triggered by the foundering of arc root before ~540 Ma.