Multiple subduction processes of the Proto-Tethyan Ocean: Implication from Cambrian intrusions along the North Qilian suture zone

Cambrian mafic to felsic intrusive rocks intruding Precambrian rocks are recognized along the southern margin of the North Qilian suture zone. These intrusive rocks predate final closure of the North Qilian Ocean and consideration of these rocks is essential to any reconstruction of the tectonic framework of the Qilian Orogen. They include 517 Ma transitional IS type granite, 510 Ma I-type calc-alkaline diorite, 509–505 Ma calc-alkaline gabbro, and 498 Ma shoshonitic monzonite as well as highly fractionated syenogranite. Distinctive geochemical signatures and Sr-Nd-Hf isotopic compositions indicate derivation from multiple magma sources associated with subduction. Moreover, tectonostratigraphic correlation indicates that the Precambrian rocks along the southern margin of the North Qilian suture zone were associated with the Central Qilian microcontinent. These Cambrian mafic to felsic intrusive rocks are likely to be magmatic products of a continental arc built on the northern margin of the Central Qilian microcontinent during the processes of southward subduction of the North Qilian Ocean. Together, a Cambrian Andean-type arc, Cambrian-Ordovician Mariana-type intra-oceanic arc and multiple Early Paleozoic accretionary complexes within the North Qilian suture zone attest to various subduction-accretion processes that occurred within the Proto-Tethyan Ocean from Cambrian to Middle Ordovician time prior to terminal continental collision.