Northward subduction of the South Qilian ocean: Insights from early Paleozoic magmatism in the South-Central Qilian belts

Abstract

The Qilian orogen consists of the North Qilian belt, the Central Qilian block and the South Qilian belt and is regarded as part of the northern margin of the Proto-Tethys Ocean. The tectonic mechanism responsible for the early Paleozoic magma generation in the Qilian orogen is crucial to understanding the ocean-continent configurations and the closure of the Proto-Tethys Ocean. This paper compiles the early Paleozoic mafic and granitic intrusions in the South Qilian belt and the Central Qilian block, to constrain the subduction and accretion of the South Qilian ocean. The early Paleozoic mafic intrusions in the Central Qilian block indicate that the influence of slab-derived fluids dominates their petrogenesis, while the mafic intrusions in the South Qilian belt were mostly affected by involvement of significant sediment-derived melts in their formation. They are supportive of a mélange model, in which melting of mélange diapirs originating at the slab-mantle interface generates fluid-metasomatised magma source in the deep mantle under the Central Qilian block and sediment-derived melts-metasomatised magma source under the Hualong block in the South Qilian belt, consistent with northward subduction of the South Qilian ocean. It is proposed that northward intra-oceanic subduction of the South Qilian Ocean occurred at 530–470 Ma in front of the Hualong block and continued at 470–446 Ma but was then accompanied by the development of a mature volcanic arc. Continuous subduction resulted in accretion of the Hualong block northwards onto the Central Qilian block, and this collision started at ca. 446 Ma and lasted to ca. 420 Ma. The final closure of the South Qilian ocean was marked by the onset of continental collision between the Qaidam-Quanji block and the amalgamated Hualong-Central Qilian block at ca. 440 Ma, and also the cause of the A-type granitoids after final collision occurred at ca. 420 Ma.