江南造山带中段大湖塘同构造花岗斑岩的成因--锆石U-Pb年代学、地球化学和Nd-Hf同位素制约

Origin of the Dahutang syn-collisional granite-porphyry in the middle segment of the Jiangnan orogen:Zircon U-Pb geochronologic,geochemical and Nd-Hf isotopic constraints

大湖塘钨多金属矿床位于江南造山带之九岭-鄣公山隆起西段。本文对大湖塘与成矿有关的燕山期花岗斑岩开展了锆石U-Pb年代学、地球化学、Nd-Hf同位素研究。结果表明,较早期(成矿期)大湖塘花岗斑岩LA-ICP-MS锆石U-Pb年龄为146.01±0.78Ma。花岗斑岩具有高硅富碱的特征和较高的分异指数DI,结合主、微量元素特征,该花岗斑岩应属于高分异的S型花岗岩。Rb/Sr、Rb/Nb比值都明显高于上地壳所对应的均值0.32和4.5,也都大于中国东部上地壳所对应的均值0.31和6.83,表明其源自成熟度较高的地壳物质。Nb/Ta和Zr/Hf比值明显低于正常花岗的Nb/Ta比值11和Zr/Hf比值33~40,而且存在REE四分组效应,说明在岩浆演化晚期存在流体-熔体的强烈作用。不均匀的Hf同位素组成[εHf(t)=-19.9^-5.3;tDM2=1.54~2.46Ga]显示该花岗斑岩主要是由年龄为1.9~1.6Ga的前寒武古老地壳(可能是双桥山群)和部分基性岩熔融形成的,并且混入了少量扬子克拉通晚太古代的基底岩石(2.5Ga)。较早期花岗斑岩A/CNK值均大于1.15,暗示其不应为火山弧花岗岩,应属于同构造(同碰撞)花岗岩,而较晚期花岗斑岩在构造判别图解上则显示出后碰撞特点。较早期和较晚期花岗斑岩形成的构造背景演化过程,表明本区经历的从挤压向伸展转变的"挤压-伸展旋回"过渡期持续了约11Ma。正是长达11Ma的"挤压-伸展旋回"为本区多期成岩作用所必需的动力持续供给提供了充分的时间,而这也是本区能够发生大规模W-Cu成矿作用的一个重要的地质前提。

The Dahutang tungsten polymetallic deposit is located in the western segment of the Jiuling-zhang- gongshan uplift in the Jiangnan orogen. The zircon U-Pb geochronologic, geochemical and Nd-Hf isotopic studies of the Yanshanian Dahutang ore-forming granite-porphyry were carried out in this paper. The LA-ICP-MS zircon U-Pb age of the Dahutang granite-porphyry is 146.01±0.78 Ma. The granite-porphyry is characterized by high content of silicon and alkalis and high differentiation index, which, along with the characteristics of the major and trace element, suggests that the granite might be highly differentiated S-type granite. The Rb/Sr and Rb/Nb ratios of the granites are significantly higher than those of the upper crust and eastern China＇ s upper crust, indicating that the granite-porphyry originated from melting of high-maturity crustal components. The ratios of Nb/Ta and Zr/Hf are much lower than those of normal granite, and there is REE tetrad effect, sug- gesting a strong melt-fluid interaction at the late stage of magma evolution. Heterogeneous Hf isotopic composi- tion [ eHf（ t ） = - 19.9 -- - 5.3 ; tDM2 = 1.54 -- 2.46 Ga ] demonstrates that the granite-porphyry was derived dominantly from a Precambrian crustal source of 1.9--1.6 Ga and part of basic rocks, with the involvement of small amounts of late Archean basement rocks of Yangtze craton （2.5 Ga）. The A/CNK ratios of the granite- porphyry are all higher than 1.15, suggesting that the granite-porphyry is a syn-eollisional （syn-orogenic） granite rather than a volcanic arc granite; the young granite-porphyry has the post-collisional （post-orogenic） charac- teristics in the tectonic environment discrimination diagrams. The tectonic setting evolution of the old granite- porphyry and the young granite-porphyry indicates that ＂the extrusion-extension cycle＂ lasted about liMa with the transition from extrusion to extension in this area. This extrusion-extension cycl of l lMa provided sufficient required dynamic duration for the multiple rock-forming and ore-forming activities in this area, and also served as an important geological prerequisite for extensive mineralization.