Record of assembly and breakup of Rodinia in the Southwestern Altaids: Evidence from Neoproterozoic magmatism in the Chinese Western Tianshan Orogen

Abstract

Neoproterozoic gneissic granitoid rocks and a bimodal intrusive complex exposed in the Chinese Western Tianshan Orogen were geochemically and geochronologically studied in order to evaluate the assembly and breakup of Rodinia in the Southwestern Altaids. Zircon U–Pb ages of 903.5±2.2Ma and 933.6±1.2Ma were obtained for the Huoshaoqiao and the Wulasutanwutuaiken gneissic granitoid plutons, respectively. Both intrusions have a calcic to calci-alkalic composition and display an enrichment of LREE and LILE and a depletion of HREE and HFSE resembling the geochemical characteristics of Andean arc granites. Their in-situ zircon εHf(t) values of −1.7 to +1.9 and −2.2 to +4.5 further suggest a Late Paleoproterozoic crustal source with limited input of juvenile materials. The Wuwamen bimodal intrusive complex consists of meta-gabbro host rocks and crosscutting granitic dykes. Zircon U–Pb ages of 733±5Ma and 730±5Ma were determined for the meta-gabbros and the granitic dykes, respectively. The relative depletion of Nb, Ta and Ti, the Nd isotopic value (εNd(t)=−4.3) of the meta-gabbros, and the Hf isotopic values of gabbroic zircon grains (εHf(t)=+0.4 to +2.8) suggest that the protolithic mafic magma originated from a sub-continental fertilized lithospheric mantle source. Furthermore, the Nd isotopic value (εNd(t)=−11.2) of the granitic dykes and the Hf isotopic values of granitic zircon grains (εHf(t)=−14.3 to −5.6) indicate that the acidic magma originated from lower crustal melting. The dataset presented here in conjunction with previously published data suggest that the ∼900Ma plutons originated from remelting of older crust in an Andean-type active continental arc setting associated with the assembly of Rodinia, whereas the development of the ∼730Ma bimodal intrusive magmatism is interpreted as a result of the Rodinia breakup. Thus, the Neoproterozoic magmatism is believed to be a direct consequence of the geodynamic process related to the assembly and breakup of Rodinia in the Southwestern Altaids.