关键词: 岩石成因, U-Pb定年, 锆石Hf同位素, 江南造山带, 九岭复式岩基

Abstract: Jiuling composite batholith records the Neoproterozoic plate convergence and collision orogeny event in South China. This paper conducts geochemical studies, LA-ICP-MS zircon and monazite U-Pb dating, and zircon Hf isotope analysis on the Yajishan pluton in its northern part. The Yajishan pluton consists of middle- to fine-grained two-mica monzogranite and fine grained muscovite monzogranite, formed at 828 to 817 Ma and 821 to 811 Ma, respectively, belonging to the high-K calc-alkaline granite peraluminous series. The Cr, Co, and Ni contents of the Yajishan pluton are much lower than those of the upper crust, with an A/CNK value greater than 1.1, high Rb/Sr and Rb/Nb values, low Nb/Ta, Zr/Hf, total rare earth element (REE) content, LREE/HREE ratio, and strong Eu negative anomalies. Its positive zircons εHf(t) values (+3.2~+6.4; -0.6~+3.7) are consistent with the surrounding Shuangqiaoshan Group strata (-17.7~+15.8), accompanied by lower CaO/Na2O ratio and Mg# value. The study suggests that the Yajishan pluton was formed through partial melting of the Shuangqiaoshan Group after large-scale tectonic napping and crustal thickening caused by the collision between the Cathaysian and Yangtze blocks. The Neoproterozoic granitic magma in the Jiuling area from its early to mid-to-late stages roughly underwent three stages, from the first stage (Jiuling and Meiling pluton) and the second stage (Shihuajian, Jiuxiantang and Xiyaowu pluton)of tonalite- biotite granodiorites-biotite granite to the third stage (Yajishan pluton) of two-mica monzogranite-muscovite monzogranite, reflecting the tectono-magmatic evolution law of multiple magma pulses creating a composite batholith.

Key words: petrogenesis, U-Pb dating, zircon Hf isotope, Jiangnan Orogen, Jiuling composite batholith