Late Neoproterozoic–Cambrian eclogites and high-pressure granulites in the Central Qilian terrane (China) record the earliest subduction of Proto-Tethyan Ocean in the eastern Tethysides

Abstract

The Central Qilian terrane (CQT) of the northern Tibetan Plateau played a key role in the tectonic evolution of the Proto-Tethyan Ocean in the Tethysides, but its formation and tectonic attribution have been hotly debated. Here, we report the discovery of eclogites and HP mafic granulites in the Beidahe Complex of the western CQT. These occur as blocks of various sizes within a sequence of metavolcanic–sedimentary rocks, exhibiting typical a ‘block-in-matrix’ and thrust imbrication structure. The eclogite facies metamorphic rocks preserve distinct mineral assemblages and textures corresponding to prograde, peak, and retrograde metamorphism. By combining phase equilibrium modelling with SHRIMP and LA-ICPMS U–Pb dating of metamorphic zircon, sphene, and rutile, the Late Neoproterozoic–Cambrian (c. 553–516 Ma) eclogite facies peak with low thermal gradients of 10–14°C/km, Cambrian (c. 515–506 Ma) post-peak decompression and Ordovician (c. 495–455 Ma) cooling histories for these metabasic rocks have been restored. These constitute hairpin-type clockwise pressure–temperature–time (P–T–t) paths depicting in detail the sequence of deep subduction and subsequent exhumation in Central Qilian during the Late Neoproterozoic to Early Palaeozoic. Our new findings suggest that the CQT represents a Japan-type arc-accretionary system that formed as a result of the North Qilian oceanic plate, one of the major branches of the Proto-Tethys Ocean, being subducted southward. Eclogites in the Beidahe Complex in western CQT offer the earliest (c. 553 Ma) metamorphic record of subduction in the Qilian orogen, indicating that the North Qilian Ocean commenced subducting southward prior to the Late Neoproterozoic.