宁芜盆地浅成侵入岩的锆石U-Pb年代学和Hf同位素研究及其地质意义  

Zircon U-Pb Dating and Hf Isotopic Compositions of Porphyrites from the Ningwu Basin and Their Geological Implications

作　　者：胡劲平[1] 蒋少涌[1] 

机构地区：[1]内生金属矿床成矿机制研究国家重点实验室(南京大学),南京大学地球科学与工程学院,南京210093

出　　处：《高校地质学报》2010年第3期294-308,共15页Geological Journal of China Universities

基　　金：江苏省自然科学基金重点课题(BK2008026);国家自然科学基金(40221301)联合资助

摘　　要：宁芜火山岩盆地是长江中下游成矿带中最重要的矿集区之一,以产出玢岩型铁矿而著称。盆地内的岩浆活动产物可分为龙王山组,大王山组,姑山组和娘娘山组4个旋回火山岩及相关浅成侵入岩。本文对该盆地中凹山铁矿的成矿母岩——辉长闪长玢岩进行了精确的锆石U-Pb定年和Hf同位素组成测定。3个样品的锆石U-Pb年龄分别为(123±1)Ma,(127±3)Ma和(125±2)Ma。前人已对该盆地中火山岩开展过较多的定年工作,获得大王山组的锆石U-Pb年龄为127~131Ma。因此,与成矿密切相关的辉长闪长玢岩与第二旋回大王山组火山岩同期,形成年龄相近或略晚于火山岩。辉长闪长玢岩中锆石εHf(t)值为-6.3^-8.6,结合前人的地球化学数据,该岩石的形成可能与软流圈地幔上涌、富集岩石圈地幔熔融并与地壳物质混合有关,这一过程发生于华南约125Ma时岩石圈强烈伸展时期。同位素组成特征表明,与盆地内火山岩相比,宁芜浅成侵入岩形成过程中有更多的地壳物质加入。Ningwu volcanic basin is one of the most important basins related to mineral deposits in middle-lower reaches of the Yangtze River belt. Four volcanic units occur in the basin, in an ascending order, the Longwangshan Formation, the Dawangshan Formation, the Gushan Foramtion and the Niangniangshan Formation, with some high-level sub-volcanic igneous rocks intruding them. Detailed zircon U-Pb dating and Hf isotopic composition measurement were performed on the host rocks （gabbro-diorite porphyrites） of the Washan iron deposit. The zircon U-Pb ages of three samples are （123±1） Ma, （127±3） Ma and （125±2） Ma, respectively.Together with previously published precise zircon U-Pb dating results for volcanic rocks （127 to 131Ma）, these results have placed a better constraint on the magmatic and mineralization activity duration of the Ningwu volcanic basin. The zircons from the Ningwu porphyrites have εHf（t） values from -6.3 to -8.6. Together with other available geochemical data, the Hf isotope data indicate that the Ningwu porphyrites may have derived from upwelling depleted asthenospheric mantle mixed with enriched mantle and crust, which was controlled by the extensive lithosphere extension at around 125 Ma. The lower εHf（t） values and slightly younger ages of the gabbro-diorite porphyrites than their hosting volcanic rocks may indicate more crust material involvement in the porphyrite petrogenesis.

关 键 词：宁芜火山岩盆地 辉长闪长玢岩 锆石U-PB年龄 HF同位素 

分 类 号：P597[天文地球—地球化学] P588.143[天文地球—地质学]