Tectono-thermal evolution of the Qilian orogenic system: Tracing the subduction, accretion and closure of the Proto-Tethys Ocean

Abstract

Abstract The Qilian orogenic system in the northern Tibetan Plateau records the complete tectonic cycle from opening of Proto-Tethys Ocean, initial subduction, to continental subduction and collision, and extensional collapse during Neoproterozoic to Devonian. Based on a compilation of recently published high-quality data, this contribution provides an overview of the composition, distribution, nature and ages of the principal tectonic elements, including ophiolites, multiple magmatic rocks, as well as high-grade metamorphism and anatexis, with a view to trace the tectonic evolution of the Qilian orogenic system with respect to the opening, subduction, accretion and closure of Proto-Tethys Ocean. Three main suture zones resulting from amalgamation involving microcontinental blocks within the Proto-Tethys Ocean are identified in the Qilian orogenic system, including the North Qilian subduction-accretion suture (NQLS), South Qilian subduction-accretion suture (SQLS) and North Qaidam subduction-collision zone (NQDS) from north to south that sandwich the Alxa, Qilian, Oulongbuluke and Qaidam Blocks. Two contrasting types of convergent orogeny in the Early Paleozoic includes the accretionary orogeny between Alxa, Qilian and Oulongbuluke Blocks (NQLS and SQLS), and the collisional orogeny between the Oulongbuluke Block and Qaidam Block (NQDS). Both Mariana-type and Andean-type subduction systems were identified in the NQLS based on the occurrence of Early Paleozoic accretionary complexes, ophiolites, high-pressure/low temperature metamorphic rocks and arc-related volcanic rocks and intrusions. The northward subduction beneath a Mariana-type intra-oceanic subduction system is represented by the Cambrian-Ordovician island-arc related volcanic rocks, back-arc volcanic rocks and ophiolite complex, and the southward subduction of the Proto-Tethys North Qilian Ocean beneath the Qilian Block generated an Andean-type continental arc, characterized by coeval arc magmatism and high-temperature metamorphism. Although oceanic subduction-related high-pressure metamorphism has not yet been recognized, the SQLS is also a typical accretionary orogen composed of island arc rocks, back-arc magmatic assemblages, ophiolite and accretionary complexes. The NQDS is a typical collisional orogen, characterized by ultrahigh pressure pelitic/granitic gneisses intercalated with minor eclogites and garnet peridotite blocks, although the previous subduction of oceanic crust can not yet be precluded, supported by the existence oceanic-type eclogite and arc-related magmatism and high-temperature metamorphism. Integrating the geological, petrological, geochemical and geochronological evidence, we reconstruct the Neoproterozoic-Paleozoic tectonic history of the Qilian orogenic system with respect to Proto-Tethys Ocean, which includes opening of the Proto-Tethys Qilian Ocean (580-550 Ma), subduction (520-460 Ma), and closure with subsequent deep continental subduction (455-430 Ma), and multi-stage exhumation of deeply subducted continental slab (425-400 Ma), as well as final extensional collapse (400-360 Ma)