南秦岭勉略构造带黑沟峡火山岩年代学及地球化学特征——新元古代勉略洋盆存在及俯冲的记录

Ages and Geochemical Features of the Heigouxia Volcanic Rocks in the Mianxian—Lüeyang Area of South Qinling Orogen:Evidence for Existence and Subduction of Neoproterozoic Mianxian—Lüeyang Ocean

黑沟峡火山岩位于南秦岭勉(县)略(阳)构造带略阳地区,由拉斑玄武岩及少量钙碱性安山岩组成。对黑沟峡火山岩进行了LA-ICP-MS锆石U-Pb年代学和岩石地球化学研究。结果显示,黑沟峡安山岩LA-ICP-MS U-Pb测年结果为807±13Ma(MSWD=3.6,n=14),限定其形成时代为新元古代中期。玄武岩SiO_2含量为43.54%~54.51%,富Na贫P,稀土配分曲线整体左倾,整体表现出N型洋脊玄武岩地球化学特征,其源区受到再循环物质的改造。安山岩SiO_2含量较高(57.92%~60.71%),Na_2O和K_2O含量变化大,Nb、Ta、P、Ti等负异常明显,La/Yb、Sc/Ni、Th/Yb等微量元素特征表现出大陆边缘弧火山岩地球化学特征;较高的Mg~#值(40.40~60.72)以及变化较大的Cr、Ni含量、A/CNK值(0.58~1.62),表明安山岩为壳幔混合成因。本文研究表明,黑沟峡火山岩并非前人认为的晚古生代"双峰式火山岩",而是新元古代勉略洋盆存在及俯冲作用的产物。结合区域资料,认为勉(县)略(阳)地区存在新元古代早中期弧—盆体系,且洋盆北向俯冲作用持续到800 Ma左右,该过程很可能是对Rodinia超大陆聚合事件的响应。

Heigouxia volcanic rocks, located in Ltieyang area, Mianltie tectonic zone, South Qinling Orogen, are a combination of tholeiite basalt and little calc-alkaline andesite, and their LA-ICP-MS zircon U-Pb geochronological and petrogeochemical studies are determined in this paper. The results show that Zircon U-Pb age determined using laser ablation-inductively coupled plasma-mass spectrometry from Heigouxia andesitie sample is 807 ± 13Ma （MSWD = 3.6, n = 14） , indicating the crystallized-age of the Heigouxia volcanic rocks in Middle Neoproterozoic. Heigouxia basaltic rocks possess the features of N-MORB on the whole, such as left-leaning REE patterns, Na-rich, and P-poor. The region of basaltic magma was reformed by recycled material. Heigouxia andesitic rocks have higher SiO2 concentrations （43.54% -54.51% ） and instable varieties of Na2O and K2O. On account of the geochemical characteristics such as negative anomaly of Nb, Ta, P, Ti and La/Yb, Sc/Ni, Th/Yb ratios, Heigouxia andesitic rocks are deemed as the remnants of ancient continental marginal arc. Relatively high Mg# and large varieties of Cr, Ni, and A/CNK （0.58 - 1.62） indicate the crust--mantle mixing model of the andesitic rocks. Above all, Heigouxia volcanic rocks are considered as the product of Neoproterozoie Mianltie Oecan and its subduetion rather than Late Paleozoic bimodal volcanic rocks. Combined with regional geological data, the authors consider that there existed Neoproterozoic arc--basin system in Mianltie area, and the north trend subduction did not quit until about - 800Ma. This process was likely to be response to the amalgamation of Rodinia supercontinent.