Syn- and post-collisional granitoids in the Central Tianshan orogen: Geochemistry, geochronology and implications for tectonic evolution

Abstract The eastern part of the Central Tianshan Belt in China is characterized by abundant granitoids, which are not only indicators of the interaction between crust and mantle but also can be used for tracking the tectonic evolutionary history of the orogen. Four granitic plutons, muscovite granites (MG), biotite monzonitic granites (BMG), biotite granites (BG), and alkali granites (AG), have been recognized to have intruded in the Mishigou area, Central Tianshan Belt. The MG has pronounced a S-type affinity, which is strongly peraluminous with a high aluminum saturation index, and displays positive Rb, Th, U and LREE anomalies with a strong negative Ba, Nb, Sr, P, Ti and Eu anomalies. Therefore, the magma of the MG is considered to have been derived from melting of thickened continental crust, which is composed mainly of the sediments eroded from a pre-Silurian continental island-arc. The U–Pb zircon age of 424.5 ± 2.6 Ma constrains the age of MG and crust thickening. The BG and BMG granitoids display I-type geochemical features and have affinities to subduction arcs. Both of them are characterized by calc-alkaline peraluminous granites with a significant LREE enrichment and a negative Eu anomaly, as well as a depletion in Ba, Nb, Ta, Sr, P and Ti and an enrichment in Rb, Th, U, Nd, Zr and Sm. These characteristics indicate that they were derived from a mixed magma source between lower continental crust and the input of components derived from the mantle wedge above the subduction zone. The LA-ICPMS zircon U–Pb ages of 411 ± 4.7 Ma and 402 ± 3.4 Ma represent the formation ages of the BG and BMG granitoids, respectively, which also constrain that the subduction of the South Tianshan oceanic crust also occurred during Early Devonian time. The AG shows a typical A-type granite affinity with depletion of Nb, Ta, Sr, P, Ti and Y. It is interpreted that AG was formed in a within-plate tectonic setting, and related to continental up-doming and rifting zones as a consequence of the extensional collapse after the collision of the Tianshan orogen. The U–Pb zircon age of 290 ± 5.1 Ma represents the formation age of the AG, and the time of the extensional collapse and rifting.