湘中包金山矿区花岗闪长斑岩的锆石U-Pb年龄、Hf-O同位素组成及其地质意义

Zircon U-Pb ages and Hf-O isotopes of granodiorite-porphyry in Baojinshan mining area and their geological significance

包金山金矿床是近年在湘中地区探获的以金为主的多金属矿床,其成矿与印支晚期岩浆活动关系密切。利用高精度SIMS锆石U-Pb定年方法得到矿区两条花岗闪长斑岩年龄分别为(225.1±1.5)Ma和(223.3±1.4)Ma,在误差范围内一致,属印支晚期大规模岩浆活动的产物;矿区两条花岗闪长斑岩的^(176)Hf/^(177)Hf值均集中分布在0.2825附近,对应的ε_(Hf)(t)值介于-5.9~3之间,平均为-3.98,两阶段模式年龄T_(DM2)为0.96~1.45 Ga,平均为1.35 Ga;δ^(18)O值为0.824%~0.973%;Hf-O同位素研究表明花岗闪长斑岩主要为中元古代下地壳岩石重熔形成,且有部分幔源物质参与成岩作用;与紫云山花岗岩体对比分析,表明两者为同源岩浆演化的产物,且间接指示包金山金矿成矿时间为227~223 Ma;花岗闪长斑岩形成于多板块汇聚的动力学背景,为印支地块向北挤压和太平洋板块向西俯冲共同作用的结果。

Baojinshan gold deposit is Au-dominated polymetallic deposit discovered recently in central Hunan, its mineralization is considered to be closely related to the magmatic activities in late Indosinian. High accuracy SIMS zircon U-Pb age determination was used in this study to obtain the ages of two granodiorite-porphyry dykes at （225.1±1.5） Ma and （223.3±1.4） Ma, indicating that this deposit was the product of extensive magma activity in late Indosinian. The ^176Hf/^177Hf values of two granodiorite-porphyry dykes intensively distributed around 0.2825, with the corresponding erie（t） ranged from -5.9 to 3, averaging -3.98. The TDM2 model age was estimated from 0.96 GPa to 1.45 Ga, averaging 1.35 Ga; the 3180 value of granodiorite-porphyry ranged from 0.824% to 0.973%. Hf-O isotope analysis shows that the granodiorite-porphyry is mainly formed by the rock remelting of Mesoproterozoic lower crust, and mantle-sourced materials are considered to participate the diagenesis. Contrastive analysis on the granodiorite-porphyry in Baojinshan with Ziyunshan granite suggests that both of them are the products of comagrnatic evolution, which indirectly indicates that the metallogenetic time of Baojinshan gold deposit ranges in 227-223 Ma. The granodiorite-porphyry is formed under the multi-plates convergent dynamics background, and affected by the northward squeeze of Indochina Block and westward subduction of Pacific Plate.