扬子西缘登相营群基性岩墙锆石U-Pb年代学及岩石地球化学特征

Zircon U-Pb geochronology and geochemistry of mafic dyke swarms in Dengxiangying Group on west margin of Yangtze Block,China

登相营群变质基底岩系中发育大量基性岩墙群,是扬子陆块西缘新元古代一次重要的岩浆-构造事件的标志。作者利用高精度SHRIMP锆石U-Th-Pb方法对基性岩墙进行了单颗粒锆石微区测龄,以及对全岩地球化学进行分析研究。结果表明,登相营群中基性岩墙锆石U-Pb加权平均年龄为(774±10)Ma(早南华纪),并获得主变质期锆石U-Pb表面年龄为(141±3)Ma(侏罗纪晚期至早白垩世),代表中生代区域上一次构造-岩浆热事件。岩石地球化学表明,基性岩墙的Al2O3质量分数为14.40%～23.61%,具有高TiO2(质量分数为1.73%～3.23%)、低P2O5(质量分数为0.13%～0.44%)的特征,显示亚碱性玄武质岩石类型;稀土元素总体含量较低,∑REE质量分数为(61.0～169.8)×10-6,稀土配分曲线较平缓或略右倾,(wLa/wYb)N为0.74～2.28,(wCe/wYb)N为0.69～2.21,稀土不具明显的Ce和Eu异常;痕量元素显示大离子亲石元素(LILE)和不相容元素Rb,K,Ba不同程度富集;Pb,Zr,Hf相对富集;Nb,Ta,Ti弱亏损;wZr/wY(3.09～8.70)和wZr/wSm(33.5～74.83)比值较高,具有板内玄武岩的地球化学特征。结合区域地质资料,认为扬子陆块西缘新元古代中期(～770 MaB.P.)基性岩墙形成于晋宁运动晚期造山后板内拉张环境,亏损的软流圈地幔物质上涌,并在侵位过程中受到地壳不同程度的混染,与Rodinia超大陆裂解事件同步。

In the Dengxiangying Group, there are a large number of marie dyke swarms, indicating an important tectonic-magmatic event during the Neoproterozoic period on the west margin of Yangtze Block in China. Based on the SHRIMP U-Th-Pb dating for the single-grain zircon and the whole-rock geochemical analysis and test, the mafic dyke swarms in the Dengxiangying Group have the intrusion age of 744±10 Ma （corresponding to the early stage of Nanhua）. And people also obtain one metamorphic zircon with the metamorphic age of 141 ± 3 Ma（corresponding to the Late Jurassic to Early Cretaceous）which indicates one tectonic-magmatic thermal event in the Mesozoic. The geochemical characteristics show that the mafic dykes belong to the subalkaline basaltic series, whose mass fraction of A1203 is 14.40-23.61%, high mass fraction of TiO2 is 1.73%-3.23%, and low mass fraction of P2O5 is 0. 13%-0. 44%. The REE results show that all of the samples have lower total REE（61.0×10^-6 - 169.8×10^-6） with the REE lines gentle or slightly to right, the （La/Yb）N value of 0.74-2.28 and （Ce/Yb）N of 0.69-2.21, and a weak or no Eu and Ce anomaly. The trace elements show that in the mafic dyke, the LILE and incompatible elements Rb, K, and Ba are concentrated with various degrees, Pb, Zr, and Hf do relatively, Nb, Ta, and Ti are weak loss, and Zr/Y （3.09-8.70） and Zr/Sm （33.5-74.83） ratios are high, characterized by the geochemistry of intra-plate basalt. Combined with the regional geological data, the authors consider that the Neoproterozoic （-770 Ma B. P. ） mafic dyke formed in the intra-plate extensional environment after Jinning orogeny on the west margin of Yangtze Block, sourced from the depleted asthenosphere mantle （DM）, and contaminated by the crustal materials when the mantle upwell. This process is synchronous with the breakup of Rodinia supercontinent.