Early Neoproterozoic gneissic granitoids in the southern Yili Block (NW China): Constraints on microcontinent provenance and assembly in the SW Central Asian Orogenic Belt

Abstract

The Central Asian Orogenic Belt (CAOB) was built by amalgamation of diverse geologic units including several microcontinents. The nature and geodynamic evolution of the Precambrian basement of these units are essential to constrain their origin, and thus the geodynamic and paleogeographic reconstruction of the accretionary system. The Yili Block, located in the west of the Chinese Tianshan Orogen, is one of these Precambrian-based continental constituents. Field investigations, zircon U-Pb dating and Lu-Hf isotopic analysis, whole-rock geochemical and Sr-Nd isotopic studies were conducted on the gneissic granitoids from the Giergelang area, southern Yili Block. These rocks yielded early Neoproterozoic zircon U-Pb ages (962–934 Ma), containing substantial number of Paleo- to Mesoproterozoic inherited zircons (2403–1268 Ma); their protoliths belong to peraluminous high-K granite series displaying right-declining REE patterns and Eu negative anomalies together with LILE (K, Rb, U, Th) enrichment and depletions in Ba, Nb, Sr, P, Ti; they have negative εNd(t) values ranging from −2.5 to −4.7, and old Nd model ages varying from 1650 Ma to 1910 Ma; they also exhibit highly variable zircon εHf(t) values (−12.1 to 4.4), and Hf-in-zircon model ages (1558–2574 Ma). Integrating all these results, it is suggested that the primary magma of gneissic granitoids was generated by partial melting of supracrustal rocks with involvement of old crustal materials.A comparison of data from the Yili Block and its neighboring continental units reveals that the Neoproterozoic magmatic event is an important episode of crustal reworking for the major continental blocks within the CAOB. Similar Neoproterozoic magmatic events and comparable upper Neoproterozoic to lower Paleozoic stratigraphic sequences of the Yili, Chinese Central Tianshan and Tarim blocks, suggest that they had a similar crustal evolution and close tectonic and paleogeographic relation during Neoproterozoic, and were likely derived from one single continental plate.