The Mojiawan I-type granite of the Kangding Complex in the western Yangtze Block: new constraint on the Neoproterozoic magmatism and tectonic evolution of South China

Abstract

ABSTRACT Studies of Neoproterozoic granites near the western margin of the Yangtze Block provide new evidence on the mechanism leading to the Neoproterozoic break-up of the Rodinia Supercontinent. This contribution focusses on the structural geology, geochemistry, zircon U-Pb geochronology, and Hf-O isotope systematics of the poorly documented Mojiawan Monzogranite in the Kangding Complex, which gives us better constraints on the break-up of the supercontinent. Zircon U-Pb data show that two samples of the Mojiawan Monzogranite were emplaced during 797 ± 13 Ma and 785 ± 11 Ma (ca. 791 Ma). The granitic samples are Na-rich, calc-alkaline, peraluminous with A/CNK values of 1.13 and 1.25, and their zircons have positive εHf(t) values between +4.28 and +9.79 with crustal model ages TDM2 of 1424 to 1079 Ma, indicative of a juvenile source. The granitic samples also have low Rb/Sr and Rb/Ba ratios and high δ18O values of 5.42‰–8.97‰, consistent with derivation from the partial melting of the lower to middle crust. Combined with the geochemistry of other granitic plutons along the western margin of the Yangtze Block, we propose that the region records rifting at ca. 800 Ma due to the action of a mantle super-plume leading to the break-up of the Rodinia Supercontinent.