Evidence of Crustal Evolution in the Early NeoProterozoic of China

Early Neoproterozoic crustal evolution in northern Yili Block: Insights from migmatite, orthogneiss and leucogranite of the Wenquan metamorphic complex in the NW Chinese Tianshan

Authors:

Wang et al

Abstract:

The northern part of the Yili Block, located in the northwest of the Chinese Tianshan, extends westward to joint with the Aktau-Junggar domain in Kazakhstan, and is one of the major continental constituents of the Central Asian Orogenic Belt (CAOB). The nature and tectonic significance of the basement of the continental domains are important for understanding the continental evolution and geodynamic processes of the CAOB. We investigated the Wenquan metamorphic complex (WMC) that represents the basement and metamorphosed sedimentary cover of the northern Yili Block. The WMC is mainly composed of gneissic S-type granite, migmatite associated with amphibolite, paragneiss, micaschist, quartzite, marble, and intruding leucogranitic dykes. The migmatites display banded, schlieren and ptygmatic structures containing rootless, lensoid or layered leucosome and leucocratic sills, which are indicative of in situ partial melting of the country rocks and variable degrees of melt migration. Geochemical data suggest that the protoliths of the orthogneiss and gneissic K-granite belong to peraluminous S-type granites characterized by wide range of I(Sr) values (0.68324–0.72365), low ɛNd(t) values (−1.9 to −4.4) and two-stage Nd model ages (tMD-2: 1.56–1.82 Ga). The S-type granites might have derived from both clay-poor and clay-rich source rocks that are probably meta-sedimentary and meta-volcanic rocks in the WMC. The leucogranites show relatively narrow range of I(Sr) values (0.70273–0.70419), more negative ɛNd(t) values (−6.9 to −9.1) and more consistent tMD-2 ages (1.51–1.55 Ga). The leucosome and leucocratic sills show variable Sr and Nd isotopic compositions probably due to localized and different degree of partial melting. Our new zircon SHRIMP and LA-ICPMS U–Pb ages, together with previously published data indicate that the migmatization occurred at 926–909 Ma, the gneissic S-type granites emplaced during 919–862 Ma, and the leucogranites emplaced in the period of 909–845 Ma. Inherited old zircons were found in all dated samples and may have come from their source rocks. Association of geochronologically overlapped and genetically linked migmatites, gneissic S-type granites and leucogranitic rocks is interpreted as the results of anatexis of the upper crustal rocks during an early Neoproterozoic tectonic/metamorphic event that needs to be further studied in the northern Yili Block. Such early Neoproterozoic migmatization and S-type granitic magmatism are considered as an important episode of reworking and cratonization of the continental crust of the Yili and adjacent continental blocks in Central Asia.