湘西大神山印支期花岗岩的岩石学和地球化学特征

湘西大溶溪钨矿床在空间、成因上与大神山花岗岩关系密切,但目前大神山花岗岩的研究程度较低,这严重制约了对其成岩机理、形成地质背景、以及对大溶溪钨矿矿床成因与成矿机理的认识。本文对大神山花岗岩的岩石学和地球化学进行了研究,并揭示了该花岗岩的成因及其成岩的构造背景。研究表明,大神山花岗岩呈岩株产出,主要为黑云母二长花岗岩,其锆石LA-ICP-MSU-Pb年龄为(224.3±1.0)Ma。相比于华南地区其它印支期花岗岩,大神山花岗岩具有酸度、碱度偏低,而富含MgO、CaO的特点,属准铝质-弱过铝质、高钾钙碱性岩系。该花岗岩稀土总量较低,表现出富轻稀土、弱负铕异常、轻重稀土分馏明显的特征;成矿元素W含量极高,Ba/Rb、Ni/Co、Rb/Sr值普遍较低,显示其岩浆结晶分异不充分。从成因上看,大神山花岗岩为I型花岗岩,形成于扬子地块与华夏地块强烈会聚之后的后碰撞晚造山阶段,可能是幔源岩浆底侵与下地壳局部熔融所形成的幔、壳混熔岩浆不断演化的结果。

Geochemistry and Petrogenesis of the Indosinian Dashenshan Granite,Western Hunan, South China

The Darongxi tungsten deposit in western Hunan is spatially and genetically related to the Dashenshan granite, which consists mainly of biotite monzonitic granite. LA-ICP-MS zircon U-Pb dating reveals that the granite emplaced at (224.3±1.0) Ma. Compared with other Indosinian granites in South China, the studied granite is relatively depleted in SiO2 (67.70%～70.46%) and K2O+Na2O (6.68%～7.33%), lower A/CNK (0.85～1.05), but enriched in MgO and CaO. And it belongs to the metaluminous or slightly peraluminous high-K cal-alkaline granite series. The Dashenshan granite is characterized by low REE concentration (94.3×10-6～122.7×10-6), relative LREE-enrichment (LREE/HREE 12.5～16.4), mild negative Eu anomalies (δEu 0.44～0.80) and obvious LREE/HREE fractionation ((La/Yb)N 17.4～24.6), and significantly enriched in tungsten. The ratios of Ba/Rb, Ni/Co and Rb/Sr for the Dashenshan granite are relatively low, suggesting a low degree of fractional crystallization. Our geochemical data reveal that the Dashenshan granite emplaced during the late stage of post-collision between the Yangtze and the Cathysia blocks, and belongs to I-type granite. Due to the NWW-trending compression by the Pacific Plate, the intra-continental orogeny took place within the South China block during late Middle Triassic (T2). The collision between Yangtze and Cathysia blocks and subsequently intra-continental subduction resulted in the over thickened lithosphere, and thus caused the lithospheric delamination and upper mantle upwelling in the Xuefengshan region, western Hunan. The underplating mantle magma led to the melt of the lower crust in the studied area, and produced a mantle-crust mixing magma which seems to be mainly derived from the partial melting of lower crustal materials rather than upper mantle. Thus, the mantle-crust mixing magma gradually evolved and finally formed the Dashenshan granite.