Maturation of an early Paleozoic continental arc in the Central Asian Orogenic Belt: Petrogenesis of Silurian granitoids in northern West Junggar, NW China

Transformation of basaltic intra-oceanic arcs to mature continental arcs is responsible for considerable Phanerozoic juvenile crustal growth in the Central Asian Orogenic Belt (CAOB), which is the largest accretionary orogenic belt in the world. Controversy has long surrounded the crustal maturation process of oceanic arc settings. West Junggar is an important component of the CAOB that developed multiple Paleozoic intra-oceanic arcs and subduction-accretion complexes, with complete and systematic records of crustal differentiation and maturation, making it an excellent area for studying the crustal growth and tectonic evolution of the CAOB. In this paper, we present a systematic study of the petrology, whole-rock geochemistry, and zircon U-Pb-Hf and plagioclase Sr isotopes of the granitoids from the Boshchekul-Chingiz arc in northern West Junggar (NW Junggar). These granitoids are mainly monzonites, quartz monzonites, and K-feldspar granites, with zircon U-Pb ages of 434–418 Ma, depleted mantle-like high positive εHf(t) values, and low plagioclase 87Sr/86Sr ratios. They belong to the high-K calc-alkaline to shoshonitic and metaluminous to weakly peraluminous series and are characterized by alkali-enrichment with high SiO2 and low MgO contents. Their homogeneous trace element compositions involving large ion lithophile element– and light rare earth element–enrichment and high field strength element–depletion are consistent with contemporaneous, widespread I-type granitoids in the Boshchekul-Chingiz arc, reflecting common petrogenesis and magma source. These Silurian granitoids were generated by partial melting of the juvenile mafic lower crust in an active continental arc, followed by variable degrees of fractional crystallization of predominant plagioclase and K-feldspar. The initial magmas were calculated to have formed at depths of 38–49 km (average [avg.] 44 km), and temperatures of 698–851 °C (avg. 763 °C). Combining our results with a compilation of existing granitoid data throughout the Boshchekul-Chingiz arc, we identify a ca. 508–481 Ma nascent intra-oceanic arc and a ca. 452–380 Ma mature continental arc during the northward subduction of the Junggar-Balkhash Ocean, resulting in the formation of the Boshchekul-Chingiz arc. Continuous subduction led to extensive melting of the basaltic crust and subsequent crustal differentiation in the early intra-oceanic arc, resulting in the widespread Paleozoic granitoids in NW Junggar that are now a record of the crustal growth and evolution from a nascent basaltic intra-oceanic arc to a mature continental arc in the CAOB.