西秦岭上三叠统华日组火山岩埃达克岩特征及其地质意义

The characteristics and geological significance of the Adakite rocks of the upper Triassic stata Huari formation volcanics in west Qinling

西秦岭德乌鲁一带华日组为一套中酸性火山岩,分布于夏河-合作大断裂和力士山-围当山断裂之间。通过对这套火山岩总体特征研究,其具有SiO2含量58.1%70.27%(>52%);Al2O3含量14.59%17.98%>(15%);MgO含量0.21%4.66%<(5%);Mg^#为0.120.47(<0.5);里特曼指数δ为0.231.92(<4),为钙碱性系列;Sr含量103×10^-6~388×10^-6<400×10^-6为喜马拉雅型花岗岩(Sr<400×10^-6);Y含量1.76×10^-6~15.9×10^-6(<18×10-6),与经典C型埃达克岩定义一致。华日组火山岩一段安山岩和二段杏仁状安山岩中获得锆石U-Pb年龄分别为226.3±1.1Ma和231.7±2.1Ma,表明火山岩形成于晚三叠世。结合西秦岭造山带的构造演化特征,认为华日组火山岩形成于秦岭造山带碰撞后的陆相环境。因此,晚三叠世早期,秦岭造山带的陆陆碰撞,扬子板块俯冲至华北板块之下使得地壳增厚、地热梯度增高。下地壳中变质的玄武岩由于温度升高,发生部分熔融,形成埃达克质熔体。而埃达克岩的高温和高压萃取了大量Au、Cu元素,从而形成富金含矿热液,在岩浆侵入后期,含矿热液充填于华日组火山岩和德乌鲁岩株中形成岗岔金矿、以地南铜金矿等矿床。

The huazhi formation in the Dewule area of West Qinling is a set of intermediate-acidic volcanic rocks which is distribute between Xiahe-Hezuo fault and Lishishan-Weidangshan fault.Studing the general characteristics of this set of volcanic rocks show that SiO2 content is 58.1%~70.27%>52%,Al2O3 content is 14.59%~17.98%(>15%),MgO content is 0.21%~4.66%(<5%),Mg^#content is 0.12~0.47<(0.5).The rittman indexδis 0.23~1.92(<4)show it is calc-alkaline series.Sr content is 103×10^-6~388×10^-6(<400×10^-6)which show it is Himalayan type granite(Sr<400×10^-6).Y content is 15.86×10^-6~17.07×10^-6(<18×10-6)and They have the same characteristics with the C-type adakite ones.The U-Pb isotope age were respectively obtained from the andesite of the first section and almond-shaped andesit were 226.3+1.1 Ma and 231.7+2.1 Ma,indicating that the volcanic rocks were formed in the late Triassic.Therefore,in the early of the later Triassic,the Yangtze plate subducted and the North China plate happened to the continent-continent collision in the Qinling orogenic,the Yangtze plate subducted under the North China plate and this resulted in thickening of the earthcrust and increasing of the geothermic gradient.The metamorphic basalts in the lower crust were partially melted due to the temperature rising and formed adakite melt.A lot of Au and Cu elements were extracted from high temperature and high pressure of the adakite rock,so that they becomed the gold-rich hydrothermal fluid and in the later stage of magmatic invasion they were filled in the volcanic rocks of huazhi formation and the Dewulu rock and formed Gangcha gold deposit and Yidinan Coppergold deposit.