The Qiman Tagh Orogen as a window to the crustal evolution in northern Qinghai-Tibet Plateau

Abstract

The Qiman Tagh Orogenic Belt (QTOB), located along the northern part of the Qinghai-Tibet plateau, was constructed through protracted accretion and collision of a collage of terranes during subduction and closure of the Qiman Tagh Ocean, a branch of Paleo-Tethys Ocean from the Neoproterozoic to Early Mesozoic. The orogen is located between the Qaidam Basin and Kumukuri Basin, and cut by the Altun Fault to the west. The early Neoproterozoic (ca. 1000–820Ma) ages from this orogen suggest a link with the formation of the supercontinent Rodinia.The QTOB is tectonically divided into the North Qiman Tagh Terrane (NQT) and the South Qiman Tagh Terrane (SQT). The NQT developed as an active continental margin, and preserves abundant Early Paleozoic granitoids, which possibly formed through the melting of old basement, and a series of mafic–ultramafic rocks considered as VA (volcanic arc) type ophiolites. In contrast, the SQT witnessed intra-oceanic subduction, where SSZ (supra-subduction zone) type ophiolites are documented together with island arc tholeiite (IAT) and calc-alkaline lavas, in a primary oceanic island arc environment during the Early Paleozoic. With continued subduction, the young island arc was transformed into a mature island arc with thickened crust. This region preserves typical evidence for sedimentation and volcanism in the initial stages of volcanic arc development. The collision between the SQT and NQT occurred probably in the Late Silurian (ca. 422Ma) and continued until ca. 398Ma, as evidenced from the ages of the abundant within-plate granitic magmatism in the NQT that formed after 398Ma. In the SQT, voluminous oceanic island arc granitoids formed during the Early-Middle Devonian (ca. 418–389Ma), with contrasting geochemical features as to those in the NQT. The SQT is interpreted as an exotic terrane that has been incorporated into the continental margin and contributed significantly to the continental growth in this orogenic belt. A trench jam might explain the large gap (ca. 357–251Ma) of granitoid magmatism.The final closure of the Paleo Tethyan Qiman Tagh Ocean might have occurred in the Late Permian, and resulted in the accretion of the Kumukuri microcontinent; which formed in response to the orocline formation of western Qiman Tagh Orogen and the rotation of the western South Qiman Tagh Terranes. A series of Y-depleted granitoids formed during Early-Middle Triassic (before 237Ma), which might be associated to the partial melting of thickened lower crust induced by the oceanic lithosphere delamination. Subsequently, a series of calc-alkaline and alkaline granitoids generated through melting of older crustal material which were emplaced in the SQT, and their formation is interpreted to be linked with the transition from post-collision to within-plate settings. Our model is not only suitable to trace the tectonic evolution of the Qiman Tagh orogen, but also valid for the plate tectonic setting orogens in the modern earth.