Geochemical and geochronological studies of granitoid rocks from the Western Tianshan Orogen: Implications for continental growth in the southwestern Central Asian Orogenic Belt

Abstract

The Western Tianshan Orogen in northwestern China is part of the southwestern margin of the Central Asian Orogenic Belt (CAOB), which represents the largest tectonic assembly of accretionary complexes and the most significant area of Phanerozoic continental growth in the world. Granitoid rocks have a widespread occurrence and occupy about 30% of the whole exposure of the orogen. Ages obtained for the granitoids vary from 896 Ma to 247 Ma, while initial 87Sr/ 86Sr ratios vary from 0.70329 to 0.72070 and ε Nd values from − 14.1 to + 7.3. The occurrence of Neoproterozoic granitic gneisses implies the presence of Precambrian basement in this area. During oceanic subduction, continental growth occurred as a result of several combined processes, i.e. by addition of oceanic crustal melts (adakites), the intrusion of basaltic magmas derived by partial melting of the metasomatized depleted mantle wedge and the upwelling of granitic magma derived from a mixed source of basaltic magmas and old continental basement. However, during the post-collisional period, vertical accretion of underplated juvenile mantle material may have been accomplished during ‘slab breakoff’ delamination. This study suggests a two stage model of continental growth of ‘syn-subduction lateral accretion of arc complexes’ and ‘post-collisional vertical accretion of underplated mantle material’, which may also be relevant for the western segment of the CAOB in the Phanerozoic. The reworking and involvement of old crust are more obvious in the Western Tianshan Orogen compared to other parts of the CAOB comprising the Western Junggar, Eastern Junggar, Alataw and Altay terranes.