Zircon Geochronology, Geochemistry and Petrogenesis of the Taibao Pluton in Northwest Guangdong Province

The tectono-magmatic evolution of the South China block during Early Mesozoic remains controversial. In this paper, it conducts the comprehensive study of petrology, zircon U-Pb geochronology, whole-rock element and Sr-Nd-Hf isotopic geochemistry for Late Triassic Taibao pluton with mafic microgranular enclaves (MMEs) in the Lianshan area, located in the northwest Guangdong Province. LA-ICP-MS U-Pb zircon analysis yielded ²⁰⁶Pb/²³⁸U weighted mean ages of 219.8±1.8 Ma, 220.5±1.0 Ma, 221.5±1.7 Ma and 220.2±1.1 Ma for four granite samples respectively, indicating that the pluton was formed during the Late Indosinian period. The host rocks are high K calc-alkaline granite, with lower SiO₂, higher total alkalis and A/CNK values ranging in 0.79-1.16. The mafic microgranular enclaves show lower SiO₂, higher FeO, MgO with contents of 55.13%-62.56%, 2.31%-4.60% and 5.53%-6.63%, respectively. Both of them show enrichment in light rare earth elements, and depletion in Ba, Sr, and Ti. They have relatively low ε_Nd(t) values (-3.42 to -2.60 and -0.90 to -0.52)and younger Nd model ages (T_DM2=1.30-1.35 Ga and 1.10-1.13 Ga, respectively). Zircon ε_Hf(t) values of the host rocks are scattered (-2.6-+4.3, with average value of +0.7) and the corresponding two-stage model ages are 0.95-1.39 Ga. The results suggest that the Taibao host granodioritic rocks and the MMEs were likely produced by mixing of depleted mantle and lower crust-derived magmas, under the post-orogenic (Indosinian) extensional tectonic setting. These results, considered with the coetaneous basic magmatic activities in the adjacent area, indicate that the transition from the Paleo-Tethys to the Paleo-Pacific tectonic regimes of the South China block occurred during the Late Triassic (233 Ma).