赣南银坑晚白垩世早期安山玢岩年代学及地球化学特征-《华东地质》

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Title:: Chronology and geochemistry of the early stage of Late Cretaceous andesitic porphyrite in Yinkeng area, Southern Jiangxi Province

Author(s):: YU Chang-qi; HE Gen-wen; ZHOU Xing-hua; LI Wei; ZENG Zai-lin; LU Guo-an; (Gannan Geological Survey Party, Bureau of Geology and Exploration and Development of Mineral Resources of Jiangxi Province, Ganzhou 341000, China)

Keywords:: Key words:Late Cretaceous andesitic porphyrite; zircon U-Pb chronology; geochemical characteristics; extension; Southern Jiangxi Province

摘要:: 摘要: 银坑安山玢岩位于赣南雩山成矿带中部中生代断陷盆地边部。为研究岩浆岩侵位时代、岩石成因及构造背景,该文进行了岩相学、锆石U-Pb年代学、地球化学等测试分析。研究表明:赣南银坑地区安山玢岩为高钾钙碱性系列岩石,岩石明显富集大离子亲石元素(K、Ba、Rb)、轻稀土元素和不相容元素(Th、U),相对亏损高场强元素(Nb、Ta、Ti、P); 稀土元素配分曲线为右倾型,(La/Yb)_N为7.90～8.56,无明显Eu异常。Th含量为(6.01～6.36)×10^-6,Th/Ta、Zr/Hf和Rb/Sr值分别为10.7～11.7、33.15～35.33和0.198～0.216,均介于地壳与地幔值之间,指示岩浆源岩主要为地幔物质的部分熔融,并受到了陆壳混染作用影响。LA-ICP-MS锆石U-Pb年龄为(99.89 ± 0.30)Ma,表明岩浆活动时代为晚白垩世早期,形成大地构造背景与太平洋板块向西俯冲有关,为活动大陆边缘弧后伸展作用下的产物。通过对红层盆地沉积序列进行对比分析,认为该安山玢岩活动年龄可大致代表这些断陷盆地的形成时代。

Abstract:: Abstract:The Yinkeng andesitic porphyrite is located in the middle of Yushan metallogenic belt and at the edge of Mesozoic fault basin in Southern Jiangxi Province. To study the emplacement age, petrogenesis and tectonic setting of magmatite, the paper analyzes the petrography, zircon U-Pb chronology and geochemical characteristics. The results show that the Yinkeng andesitic porphyrite belongs to high-K calc-alkaline series, characterized by obvious enrichment of LILEs(K, Ba and Rb), LREEs and active incompatible elements(Th and U), and depleted HSFEs(Nb,Ta,Ti and P), with right-dipping REE distributin curves,(La/Yb)_N of 7.90～8.56 and no obvious Eu anomaly. The Th content of(6.01～6.36)×10^-6, Th/Ta of 10.7～11.7, Zr/Hf of 33.15～35.33 and Rb/Sr of 0.198～0.216, all between crustal and mantle values, indicates the magmatic source rock is mainly from the partial melting of mantle materials and affected by crustal contamination. The LA-ICP-MS zircon U-Pb dating result of(99.89 ± 0.30)Ma show that the magma activity occurred in the early stage of Late Cretaceous, and the geotectonic setting was related to the westward subduction of Pacific Plate, which was the product of back-arc extension of active continental margin. Through the comparative analysis of sedimentary sequence of red-bed basins, it is concluded that the activity age of andesitic porphyrite can roughly represent the formation age of the fault basins.