Whole-rock geochemical and zircon U-Pb-Hf isotopic compositions of the Yunlougang and Wucun granitic plutons in the Hetai area of South China: Implications for petrogenesis and geodynamic setting

In the Nanling tectono-magmatic belt of South China, the geodynamic settings and tectonic style of Indosinian and Yanshanian magmatism remain debatable. Hereby, an integrated study of zircon U-Pb ages and Lu-Hf isotopes as well as whole-rock elemental and Sr-Nd isotopic compositions was carried out on the Yunlougang (Late Permian) and the Wucun (Late Jurassic) granitic plutons in the Hetai area, located to southwest of the Nanling region. Zircon U-Pb dating yielded emplacement ages of 255 Ma for the Yunlougang biotite monzogranite, 240 Ma for the Yunlougang granodiorite as dyke crosscutting the former, and 155 Ma for the Wucun two-mica granite. The biotite monzogranite and the two-mica granite are strongly peraluminous S-type granites (A/CNK ratios of 1.06–1.21) with a relatively low zircon saturation temperature (631–741 °C). The high initial ⁸⁷Sr/⁸⁶Sr ratios (0.71733 to 0.73383) and low ε_Nd(t) values (−14.6 to −8.69) suggest a crustal protolith. However, the Wucun biotite granite is highly fractionated I-type one (most samples are metaluminous with A/CNK < 1.1) with a relatively high zircon saturation temperature (762–770 °C). Additionally, they exhibit less radiogenic ⁸⁷Sr/⁸⁶Sr_i ratios (0.70724–0.70786) and higher ε_Nd(t) values (−5.34 to −4.94) as well as zircon ε_Hf(t) values of −5.67 to −2.44, suggesting an origin of a crustal source with extra input from mantle. Based on the tectonic history of South China, we infer that the Yunlougang granite occurred in a compressive setting and might be produced by partial melting of a thickened ancient crust during the collision between the South China and the Indochina blocks, while the Wucun complex granites generated in an extensional environment that was influenced by the paleo-Pacific tectonic regime. Combined with previous studies, a NE-oriented I-type granite belt probably existed along the juncture belt of the Yangtze and Cathaysia blocks, which was respond to the foundering rather than rollback of the paleo-Pacific plate that led to the rapid change of the temperature of granite precursor and the involvement of mantle components into the Hetai region.