Accretionary growth and crust formation in the Central Asian Orogenic Belt and comparison with the Arabian-Nubian shield

The Central Asian Orogenic Belt is one of the largest accretionary terrains on Earth and records a ca. 800 Ma history of arc and microcontinent accretion, from south to north, during evolution and closure of the southwest Pacific-type Paleo-Asian ocean in the period ca. 1020 to ca. 325 Ma. We contest the evolutionary model for the belt proposed by previous authors in terms of a single, long island arc. Accretion of ophiolites, arcs, and Precambrian microcontinents took place in southern Siberia in late Neoproterozoic to Cambrian times. Ultrahigh-pressure subduction and metamorphism occurred in the Cambrian at Kokchetav, Kazakhstan, and high-pressure metamorphism took place in the Gorny Altai, together with arc-ward accretion of a seamount. In the Chinese Altai, Precambrian microcontinents and island arcs collided into the accreting margin. Overall the Central Asian Orogenic Belt records the formation of small forearc and backarc ocean basins that probably evolved between island arcs and microcontinents and were closed during continuous accretion between the Neoproterozoic and Paleozoic. During this time the southward-growing southern margin of the Siberian craton always faced an open ocean. Final closure of the Paleo-Asian ocean probably occurred in the late Permian when the North China craton was attached to the orogenic belt. Large volumes of felsic volcanic rocks and the presence of Precambrian zircon xenocrysts as well as ancient detrital zircons in arc-derived sediments suggest substantial reworking of old crust despite seemingly primitive Nd isotopic characteristics. Similar characteristics in arc terranes of the Arabian-Nubian shield in Saudi Arabia suggest that previously proposed anomalously high crust-formation rates in both the Central Asian Orogenic Belt and Arabian-Nubian shield require revision.