Metamorphic evolution of overriding plate reveals the deep geodynamic process during continental subduction and collision: Insights from the North Qaidam tectonic belt, northern Tibetan Plateau

Abstract

The timing and mechanism of the transition from continental subduction to exhumation (TSE) are important perspectives to decipher the tectonic evolution of orogenic belt. The difficulties in discriminating prograde, peak and retrograde eclogite-facies zircon in subducted continental rocks make it hard to constrain the timing of TSE. Actually, the TSE would cause the extension of the overriding plate and thus theoretically influence the metamorphic pressure–temperature evolution of rocks from the overriding plate, which, in turn, provides a potential approach to recover the TSE. An integrated study of petrography, mineral chemistry, zircon and monazite geochronology and geochemistry, conventional geothermometer and phase equilibrium modeling was carried out on the pelitic gneisses collected from Oulongbuluke block of the North Qaidam tectonic belt. The pelitic gneisses preserve distinct mineral assemblages, textures and compositions corresponding to prograde, peak, and retrograde metamorphism. Trace element signature and U–Pb geochronology of zircon and monazite record remarkable abundance changes of garnet and plagioclase at ca. 434 Ma. The data, coupled with P–T results obtained from conventional geothermometer and phase equilibrium modeling, reveal a clockwise pressure–temperature-time path, which is characterized by a peak P–T condition of 5.7–6.5 kbar and 713–736 °C at ca. 434 Ma, and a retrograde decompression and cooling process to 3.8–4.2 kbar and 659–672 °C; the metamorphic peak at ca. 434 Ma undoubtedly formed in the context of continental subduction and collision, and the retrograde decompression and cooling process of rocks from the overriding Oulongbuluke block fits well with the numerical results of deep slab-breakoff. The results link the first reported Silurian peak metamorphism of the Oulongbuluke block to the deep geodynamic process, and would help to understand the tectonic evolution of the North Qaidam tectonic belt and zircon behavior during metamorphism.