Tectonic evolution of the West Junggar Region, Xinjiang, China

Abstract

The West Junggar region of northwestern China consists of a Hercynian‐age fold belt occupying a late Paleozoic zone of convergence between three major Eurasian plates: Siberian, Tarim, and Kazakhstan. The northern part of the West Junggar area includes part of the Irtysh‐Zaysan fold belt that extends to the northwest into the USSR, where it marks a broad boundary between the Siberian and Kazakhstan plates. The central part of the West Junggar area includes the southeastern extension of the Chingiz‐Tarbgatay early Paleozoic island arc sequence. The southern part of the West Junggar area is truncated by the deep fundamental Junggar‐Alakol fault that has brought the Yili microplate into juxtaposiition with the West Junggar region in the vicinity of Ebin Nur Lake. To the east the Junggar area is covered by sediments of the Junggar Basin. Paleogeographic reconstructions and geologic evidence indicates that these plates began converging in the Middle Carboniferous, and by Late Carboniferous alkali granites began invading the imbricated leading edges of the coalescing plates. Paleomagnetic data reveals that the plates continued to move along strike‐slip faults, bringing the Tarim plate and Yili microplate into their present positions sometime in the early Mesozoic. Six separate ophiolite occurrences in the West Junggar region are strongly tectonized and dismembered. The Tangbale ophiolite is considered to be Late Cambrian (508 ± 20 Ma.) and represents the earliest known oceanic crust in Western China. Petrologic and chemical data indicate that the Tangbale ophiolite represents a possible back arc or forearc tectonic setting situated close to a developing island arc. The Darbut ophiolite has been tectonized to a melange and is overlain by Late Devonian flysch. Ages of the radiolaria in the cherts associated with the pillow lavas indicate that it formed in the Middle Devonian. Other occurrences of ophiolite melange in the northern part of the West Junggar region may be as young as Late Devonian. These ophiolites record ocean crust formation in the Early to Middle Paleozoic within the Junggarian ocean, and their present tectonic setting indicates continental accretion and imbrication shortly after their formation. Associated blueschist blocks in the ophiolite melange further supports the concept of imbrication brought about by B‐type subduction and associated obduction. Postcollisional alkali granites intrude the oceanic sequences of the West Junggar Region and are considered to represent melted lower crustal material. Lead isotopic systematics on feldspars from six separate granite plutons indicate ratios that could only be considered oceanic as far as the magmas are concerned. The implications of this finding is that the Junggar Basin basement rocks in the north and west are probably oceanic. Evidence presented in this study shows a complex history for the development of the West Junggar region. Major movements of consolidating plates in the Late Paleozoic followed by rejuvenation of these same zones of closure during the Himalayan tectonic event of the Late Tertiary demonstrates that convergence zones and fold belts may respond to vertical and horizontal stress even after many million years of inactivity.