Precambrian evolution of the Chinese Central Tianshan Block: Constraints on its tectonic affinity to the Tarim Craton and responses to supercontinental cycles

Abstract

As the southernmost continental fragment in the Central Asian Orogenic Belt (CAOB), the Chinese Central Tianshan Block (CTB) is essential for understanding the evolution of the CAOB. However, its tectonic affinity with the Tarim Craton and links with supercontinent cycles are not clear. Here, we present whole-rock geochemistry, zircon ages and Hf-in-zircon isotopic data for augen- and mylonitic granitic gneisses in the eastern Chinese Central Tianshan Block (ECTB). Zircon U-Pb dating reveals that the augen- and mylonitic gneisses formed at ca. 918Ma and 896Ma, respectively. The gneisses have REE and HFSEs patterns comparable to the upper continental crust. Their Cr and Ni contents are similar to those of the typical S-type granites in the Lachlan belt. These rocks exhibit evolved zircon εHf(t) values (−9.0 to +1.6), which are consistent with those values of coeval crustal-derived rocks within the CTB. Together with the occurrence of muscovite and the existences of Paleo- to Mesoproterozoic inherited zircons (2.21–1.25Ga), the geochemical data indicate that protoliths of these gneisses are S-type granites. These results, compiled with published geochronological data, suggest that an Archean basement was most likely absent in the CTB. The basement rocks of the CTB were dominantly produced by crustal growth in the early Mesoproterozoic and then reworked at Neoproterozoic. We suggest that the Mesoproterozoic crustal growth and the early Neoproterozoic crustal reworking were likely related to the breakup of the Nuna (ca. 1.40Ga) and the assembly of the Rodinia (1.00–0.88Ga), respectively. Because the CTB displays different crustal evolution from the Tarim Craton, we conclude that these two blocks have no close tectonic affinity in the Precambrian.