Early Paleozoic collision and amalgamation of multiple terrains in the northern Qinghai-Tibet plateau: Implications for collisional tectonics and the formation of ultrahigh pressure metamorphic belts

The northern Qinghai-Tibet plateau (NQTP) is a mosaic of multiple terrains and island/continental arcs that consists of the Qilian, Qaidam, northern part of the eastern Kunlun, southern part of the eastern Kunlun and Altyn terrain. Each of these terrains is bounded by either a subduction complex (SC) of ophiolite, melange, and HP-UHP metamorphic rocks or a continental arc complex (CAC) of island arc volcanic rocks, fore-arc accretionary wedge, and back-arc rocks. Geochronological studies on various rocks from these terrains yielded the following results: (1) Ophiolitic gabbros and HP eclogites of the northern Qilian have U/Pb zircon ages of ~500 Ma, and 463±6 Ma and 468±13 Ma, respectively, similar to the formation age of the blue schist; (2) northern Qilian Shan arc formed at 486-438 Ma similar to 469-454 Ma for the back-arc volcanic rocks; (3)the ophiolite of the northern Qaidum hasa U/Pb zircon age of 514.2±8.5 Ma,older than 496.3±6.3 Ma for gabbros, 496-440 Ma for the arc rocks have ages, and 495-440 Ma for the peak metamorphism. (4) Pyroxenites from Qingshuiquan ophiolite have U/Pb zircon ages of 530±26 Ma and 518±36 Ma; (5) the Wanbaogou OIB has a Rb-Sr whole-rock age of 648±54 Ma. These analyses show that the ophiolite representing the proto-Tethyan basin has an age of 650-500 Ma, and the subduction of this basin and formation of the continental arc complex occurred at about 500-440 Ma. The proto-Tethys was present at the southern hemisphere between the Gondwanaland, Siberian, North America , and Baltic plate at ~550 Ma, while the North China (NCP), South China (SCP) and a series of other small plates formed a chain of archipelago close to the northern margin of Gondwanaland. Most of the terrains that constitute the NQTP may have originated from this archipelago chain and assembled by "Central Asia type" subduction and accretion processes. During 480-460 Ma, the proto-Tethys basin was subducted beneath the NCP to 30 km deep and resulted in the northern Qilian HP belt. During 500-440 Ma, continuous subduction of the proto-Tethyan plate beneath the Qilian terrain to ~100 km deep and led to the formation of the northern Qaidam UHP belt. Later arc-arc, arc-continent and continent-continent collisions at 440-400 Ma led to the juxtaposition and amalgamation of these terrains. Such collisional events were responsible for a number of geologic features in this area. Eventually, these terrains were welded with the NCP and SCP to form the "Early Paleozoic China collage". Two lines of evidence suggest that the ATF might originated from a transform fault in the Early Paleozoic. First, a number of elongated Early Paleozoic ophiolitic relicts distribute along the ATF, which may represent highly disassembled oceanic crust along a transform fault. Second, regional seismic tomography studies across the ATF have imaged a high velocity block beneath the ATF which might be the detached Early Paleozoic subducted slab. Interestingly, the Altyn terrain, ATF, and Qilian terrain together form a curved zone in the NQTP. The Qilian and Altyn HP-UHP metamorphic belts are located to the syntaxis at either end of this curved zone, which resemble structurally the syntaxises in the Himalya and Alps. Such syntaxises are excellent sites for the exhumation of HP-UHP rocks due to intensive interactions of surface and deep crustal processes.