Ca. 1.18-1.14 Ga A-type granites in the SW Yangtze Block, South China: New evidence for late Mesoproterozoic continental rifting

Abstract:

The late Mesoproterozoic history of the Yangtze Block, South China in the context of supercontinent cycles remains poorly understood due to scarce preservation of geological records. In this study, we report an integrated dataset of petrology, whole-rock geochemistry, zircon U-Pb ages and Hf-Nd isotopes for newly identified late Mesoproterozoic monzogranites in the southwestern Yangtze Block. SHRIMP and LA-ICP-MS zircon U-Pb dating reveals that these monzogranites crystallized at 1.18-1.14 Ga, which contributes robust evidence for the occurrence of granitic magmatism in the southwestern Yangtze Block prior to the Rodinia assembly. All samples possess typical characteristics of A₁-type granites, such as relatively high SiO₂ (73.31-77.50 wt.%), Na₂O + K₂O (7.14-8.09 wt.%) and Zr + Nb + Ce + Y (362-445 ppm) contents and zircon saturation temperatures (825-851 ºC), as well as low Y/Nb ratios (0.78-1.04). They have negative whole-rock ε_Nd(t) values of -6.9 to -5.7 and zircon ε_Hf(t) values of -7.4 to -2.5, with consistent two-stage model ages of 2.4-2.1 Ga. These results indicate that the magmas were generated by partial melting of early Paleoproterozoic crustal materials at a condition of low pressure and high temperature. Taking into account the presence of coeval intra-plate mafic magmatism, we suggest that these 1.18-1.14 Ga A₁-type monzogranites were most likely emplaced within a continental rift setting. Available data indicate that continental rifting may have occurred throughout the Yangtze Block at 1.2-1.1 Ga. This was followed by subduction- or arc-related magmatism initiated at 1.10-0.97 Ga. We suggest that the Yangtze Block was separate from the Rodinia supercontinent at 1.2-1.1 Ga and started to drift toward it at ca. 1.1 Ga.