Neoproterozoic slab window in the western Yangtze Block, South China: Evidence from adakitic granodiorites, gabbro-diorites and high-K granites in the Panxi arc belt

Abstract

Neoproterozoic igneous rocks are widespread along the margins of the Yangtze Block and record key information on the tectonic evolution of the South China Craton. In this paper, we present a comprehensive study of zircon U-Pb ages, whole-rock geochemistry and Sr-Nd-Hf isotopes on the Neoproterozoic granodiorites, gabbro-diorites, and high-K granites in the western Yangtze Block. The granodiorites, gabbro-diorites and high-K granites were dated at 848 ± 5.3 Ma, 824 ± 5.1 Ma and 819 ± 1.5 Ma, respectively. The granodiorites have high Sr (567 ∼ 607 ppm) contents and Sr/Y (84.4 ∼ 88.0) ratios and low alkalis (Na2O + K2O = 4.3 ∼ 4.9 wt%) and Y (6.6 ∼ 7.1 ppm) and Yb (0.7 ∼ 0.9 ppm) contents, similar to typical adakitic rocks. They show low MgO (0.9 ∼ 1.3 wt%), Cr (15.2 ∼ 26.7 ppm) and Ni (3.6 ∼ 4.4 ppm) contents and positive εNd(t) (+1.7 to + 4.2) and εHf(t) (+3.0 to + 8.2) values. The parental magmas for granodiorites were generated by partial melting of thickened juvenile mafic lower crust. The gabbro-diorites are characterized by the enrichment in LREEs, significantly negative Nb, Ta and Ti anomalies, and positive εNd(t) (+2.2 to + 4.2) and εHf(t) (+5.8 to + 10.4) values. They are inferred to be derived from partial melting of lithospheric mantle modified by subduction-related fluids. The high-K granites display high Zr + Nb + Ce + Y (208 ∼ 366 ppm) and alkali (Na2O + K2O = 7.6 ∼ 8.4 wt%) contents, high 10000*Ga/Al (2.31 ∼ 2.64) ratios and zircon saturation temperatures, similar to A-type granites. We suggest that these granites were formed by the partial melting of felsic crustal materials under low pressure and high temperature conditions. The association of granodiorites, gabbro-diorites and high-K granites indicates Neoproterozoic oceanic slab subduction in the western Yangtze Block. We suggest that a subducting oceanic slab window can account for Neoproterozoic magmatism in the western Yangtze Block.