DISCOVERY OF NEOPROTEROZOIC ORTHOGNEISSES IN BEISHAN OROGEN: NEW CONSTRAINTS ON THE EVOLUTION OF THE CENTRAL ASIAN OROGENIC BELT

Located in the middle segment of the southern margin of the Central Asian Orogenic Belt (CAOB), the Beishan belt in northwestern China plays a pivotal role in understanding the tectonic evolution of the CAOB. Despite numerous studies primarily emphasizing on Phanerozoic events, the Precambrian evolution of the Beishan belt has been poorly constrained, and whether there is any Precambrian basement in the belt still remains controversial. Here we report new geochemical and zircon U-Pb-Hf isotopic data from the orthogneisses in the Beishan belt and adjacent Central Tianshan Block to shed a light on above problems.

LA-ICPMS U-Pb dating of zircons reveals the protoliths of the Gubaoquan orthogneisses in the Beishan belt crystallized at ~900 Ma and the Xingxingxia orthogneisses in the Central Tianshan Orogen formed at ~878 Ma. These orthogneisses show similar zircon Hf-isotope compositions with ε_Hf(t) values from -0.91 to +5.29 and two-stage Hf-isotope model ages of 1.44 to 1.83 Ga. However, they possess different geochemical compositions, suggesting their different tectonic environments. The Gubaoquan orthogneisses show low Na₂O and large ion lithophile elements (LILEs, e.g., Sr and Ba) contents with low A/CNK values (<1), and high contents of Fe₂O₃^t, Sc, Cr, Y and high field strength elements (HFSEs, e.g., Zr, Hf, Nb and Ta) with high Ga/Al ratios, akin to highly fractionated I-type to A-type granitoids. These features indicate that the Gubaoquan orthogneisses were probably generated in a post-collisional extensional setting. In contrast, the Xingxingxia orthogneisses exhibit relatively high SiO₂, LILEs contents, a depletion in HFSEs, and low ratios of K₂O/Na₂O, Rb/Sr (0.74-1.45) and Rb/Ba (0.30-0.36), which suggest that they are typical arc-related I-type granitoids formed in a subduction zone, similar to the coeval Neoproterozoic granitoids in the adjacent Tianshan Orogenic Collage and Tarim Craton.

Our study confirms the occurrence of Precambrian basement in the Beishan belt and neither the Tianshan Orogenic Collage nor the Tarim Craton could be linked with the Beishan belt during Neoproterozoic time. Compared with Phanerozoic events in the region, the Neoproterozoic orthogneisses might be reworked during Phanerozoic time and provide the sources for the voluminous Phanerozoic igneous-sedimentary rocks.