Origin and metamorphic evolution of Chachahe eclogites, North Qaidam UHP metamorphic Belt, NW China: Implications for fate of overriding plate material in subduction channel

Although numerical modelling and marine seismic data show that overriding plate material can enter subduction channel and even recycle to mantle, and finally contribute to mantle heterogeneity and arc magmas, protoliths of the majority of high pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks on the earth's surface originate from the subducting plate. Whether overriding plate material can enter subduction channel and their fate in subduction channel remain enigmas. The North Qaidam UHP Metamorphic Belt (NQUB), located in northwestern China, was formed by north-dipping subduction of the Qaidam Block beneath the Quanji block in the Early Paleozoic. In this paper, we report that some eclogites in Chachahe area, NQUB experienced a 1.92 Ga low-grade metamorphism prior to eclogitization at similar to 450 Ma. This Paleoproterozoic metamorphic event is not reported in the subducted Qaidam Block but widespread in the overriding Quanji Block, indicating that protoliths of Chachahe eclogites originated from the overriding plate. A clockwise P-T path from 12 kbar/520 degrees C to near UHP conditions of 25.5 kbar/690 degrees C, followed by isothermal decompression, was estimated for the Early Paleozoic metamorphism using a combination of garnet major and trace element zoning, phase equilibrium modelling and thermobarometry. This P-T path pattern is similar to other eclogites in this area, protoliths of which originated from the subducted plate. These results clearly show that overriding plate material can enter subduction channel and be carried to mantle depths of at least 85 km and exhume fast along subduction channel with other slices from subducted slab.

Automated summary

The fate of overriding plate material in subduction channels remains unknown. This study shows that some eclogites in the North Qaidam UHP Metamorphic Belt originated from the overriding plate, unlike the eclogites from the subducted plate. The results reveal that the overriding plate material can reach mantle depths of at least 85 km in the subduction channel and exhumed along with other slices from the subducted slab.