西藏雄村铜金矿区侏罗系砂岩地球化学特征:对构造背景的约束

摘要

Xiongcun copper-gold deposit is located in the middle of the southern margin ofthe Gangdise orogenic belt.Its southern margin is near to the Yarlung Zangbo suture zone (IYS) and Xigaze forearc basin, and the north is Xietongmen Eocene biotite granodiorite batholith.Based on the comprehensive microscopical identification and the analytical contents of major elements and trace elements (including REE) from the Xiongcun sandstones, the tectonic settings and provenance were studied.The result of study indicates that the content of SiO2 varies from 53.27％ to 66.13％ (average 58.27％) and is similar to that (66.1％) of Paleozoic greywackes.The high content of Al2O3 varies from 14.53％ to 24.04％ (average 20.12％), this may suggest that there are some feldspar in sandstones and with strong clayization.Combined with the microscopic characteristics, the Xiongcun sandstone may belong to feldspathic lithic sandstone.The special major element assemblages Fe2O3 + MgO (7.3 ％ ～ 17.6 ％), TiO2 (0.68 ％ ～ 1.25 ％) and Al2O3/SiO2(0.22％～0.45％) of the Xiongcun sandstones are similar to those of Oceanic Island Arc.It can be seen from the spider diagram that the Xiongcun sandstones are depleted in HFSE elements such as Ta, Nb, P,Ti and HREE, and enriched with LILE elements such as K, Rb, U and REE.The chondrite standard curve of REEs has rightward trend which reveals an enrichment of LREEs and an indifference of HREEs.The contents of standard trace elementsTh (2.5×10-6), Zr (114×10-6), Hf (3.6×10-6), Ce (27.95×10-6), Ti (0.54×10-6) and the ratios of trace elements Th/U (2.9), Zr/Th (46), La/Sc (0.72), Th/Sc(0.11) show that the genesis of the Xiongcun sandstones is Oceanic Island Arc.The discriminatory diagrams of La-Th-Sc, Th-Sc-Zr/10 and Th-Co Zr/10 were the same result.There are weak negative Ce anomalies and positive Eu anomalies in the PAAS-normalized rare element plots, which show the same conclusion with these from major elements and trace elements, indicating that the sandstones are influenced by both seawater and the fluids from deep.Diagrams of Co/Th-La/Sc and La/Th-Hf show that the provenance of Xiongcun sandstones is mixed with intermediate acidic felsic volcanic rocks and andesitic volcanic rocks.Combined with the detrital zircon ages of the sandstones (studied by other researcher), it is considered that the detrital zircon ages are not early in Late Triassic, and the provenance is of island arc volcanics features.%西藏雄村铜金矿区位于冈底斯造山带中段南缘,其南紧邻雅鲁藏布缝合带(IYS)和日喀则弧前盆地,北为谢通门始新世大岩基.通过对雄村矿区砂岩进行系统的镜下鉴定,主量、微量元素分析,揭示雄村组砂岩构造背景和物源属性.研究结果显示,雄村砂岩样品SiO2含量53.27％～66.13％,平均含量58.27％,与古生代杂砂岩(66.1％)相近.Al2O3含量较高(14.53％～24.04％),平均20.12％,指示砂岩长石矿物较多且黏土化较强.综合微观特征,雄村砂岩为长石岩屑砂岩.主量元素中特征氧化物组合Fe2O3+MgO(7.3％～17.6％)、TiO2(0.68％～1.25％)、Al2O3/SiO2(0.22％～0.45％)与大洋岛弧成因砂岩值相似;微量元素蛛网图上雄村砂岩相对亏损Ta、Nb、P、Ti及HREE等高场强元素而富集K、Rb、U、REE等大离子亲石元素,REE球粒陨石标准化后具有轻稀土富集而重稀土元素较平坦的右倾分布模式.特征微量元素Th(2.5×10-6)、Zr(114×10-6)、Hf(3.6×10-6)、Ce(27.95×i0-6)、Ti(0.54×10-6)含量及Th/U(2.9)、Zr/Th (46)、La/Sc (0.72)、Th/Sc (0.11)比值表明雄村砂岩具有大洋岛弧成因性质,La-Th-Sc、Th Sc-Zr/10、Th-Co-Zr/10等判别图解显示砂岩形成源区可能位于大洋岛弧环境.稀土元素页岩标准化后具有弱负Ce异常及正Eu异常,指示砂岩受海水及深部热液双重影响,同样与大洋岛弧环境页岩标准化后特征相似.Co/Th-La/Sc和La/Th-Hf图解表明雄村砂岩物源区为长英质中酸性火山岩源区和安山质火山弧源区,结合他人对砂岩中碎屑锆石的年龄研究,认为雄村矿区砂岩没有早于晚三叠世的碎屑锆石,具有岛弧火山岩源区特点.