Petrogenesis and Tectonic Setting of the Xishan Pluton in Nanling Range: Lithogeochemistry, Zircon U–Pb Chronology and Hf Isotope Evidence

Abstract

In this article, a systematic LA-ICP-MS zircon U–Pb geochronology study was carried out on the Xishan pluton in Nanling Range. The results show that the formation ages of granitic porphyroclastic lava, medium-fine-grained porphyritic granite, and dacite are 154.0 ± 1.2–156.1 ± 2.0, 153.2 ± 0.9–157.3 ± 3.2, and 151.2 ± 3.1 Ma, which are determined to be the early Yanshanian. Geochemical data show that the Xishan pluton was characterised by rich silicone-alkali and poor calcium–magnesium with K₂O/Na₂O ratios of 1.65–2.22, Al₂O₃ of 1.00–1.14, and FeO*/MgO ratios of 8.33–19.0 (with an average of 12.94). The rare earth content of granites ranges from 322.92 to 441.54 ppm, which is significantly higher than the world average. The rare earth element distribution curve is right-leaning light rare earth enrichment type, with obvious negative europium anomaly, and the δEu values range from 0.06 to 0.24; These rocks are enriched in high-field-strength elements (HFSEs, e.g., Ga, Y, Nb, Zr, and Hf) and depleted elements such as Ba, Nd, Sr, P, Ti, etc, with Ga/Al ratios of 2.95 × 10⁻⁴ to 4.59 × 10⁻⁴ (with an average of 3.50 × 10⁻⁴); Zr + Nb + Ce + Y of 367.1–652.5 ppm (with an average of 441.95 ppm) and zircon saturation temperatures Tzr is 755.04°C–892.50°C (with an average of 810.04°C), similar to geochemical features of A-type granites. The Hf isotope study shows that the ε _Hf(t) of the Xishan pluton ranges from 2.11 to −8.77, and the petrographic, geochemical and available data confirm that it originates from the mixing of crustal and mantle-derived magma, and the contribution of the latter is < 10%. The hafnium II model age of the Xishan pluton is 1251–1766 Ma, which confirms that its source rocks were detached from the mantle reservoir at the time of the Middle Proterozoic. Combined with the evolutionary history of the Nanling Range area, it is assumed that the Jiangnan Orogenic Zone was partially reactivated by the continuous lateral subduction of the ancient Pacific Plate in the Late Mesozoic, which triggered partial melting of the crust to form the A₂-type granite type, which is the origin of the Xishan pluton.