内蒙古协力地区次火山岩地球化学特征及其成矿潜力

Geochemical characteristics and metallogenic potential of the subvolcanic rocks in Xieli area,Inner Mongolia

突泉协力地区晚侏罗—早白垩世的次火山岩主要由石英二长斑岩、花岗斑岩、闪长玢岩等组成。通过岩相学和地球化学分析,石英二长斑岩和闪长玢岩的里特曼指数δ介于2.55~3.84之间,为钙碱性岩系,铝饱和指数ACNK为1.00~1.18,为弱过铝质I型花岗岩;稀土总量较低(∑REE为113.41×10^(-6)~170.27×10^(-6)),轻、重稀土分异较强(La_N/Yb_N为8.94~17.98),Eu具弱负异常,δCe为0.97~1.09,Ce异常不明显;Nb、Ta、P、Ti等相对亏损,Rb、Ba、K、Hf等相对富集,Nb/Ta比值(10.03~11.96,平均为10.87)小于16.2,岩浆来源于上地幔或下地壳,形成过程中同化混染了壳源物质。岩浆的形成与晚侏罗—早白垩世造山晚期华北板块北缘软流圈上隆过程中、在火山型被动陆缘处发生强烈的构造—岩浆改造有关,其岩石地球化学及含矿性参数显示,研究区具有寻找次火山岩—斑岩型Au、Ag、Cu矿的潜力。

In this paper, petrologic and element geochemical studies were carried out for the Late Jurassic-Early Cretaceous subvolcanic rocks in Xieli area, Tuquan county. The subvolcanic rocks include quartz-monzonite porphyry, granite porphyry and diorite porphyrite. Major element geochemistry shows that the rocks are cal-alkaline and late-orogenic type Ⅰ granites, which are characterized with enrichment of alkali and relatively high potassium. The Rittman index δ ranges from 2.55 to 3.84. The alumina saturation index ACNK ranges from 1.00 to 1.18. Total REE concentration is low （∑REE = 113.41×10^-6- 170.27×10^-6） and indicates obvious differentiation between LREE and HREE （ LaN/YbN = 8.94 - 17.98 ） with weak Eu negative anomalies （ δEu = 0.75 - 0.90 ） and the abnormality of δCe （SCe =0.97-1.09） is not apparent. In addition, the rock mass is relatively poor in such elements as Nb, Ta, P and Ti, and rich in Rb, Ba, K and Hr. Trace element of Nb/Ta is less than 16.2, which indicates that the magma were formed by partial melting of the ernst. Rising mantle asthenosphere and thinning of crust cause volcanic eruption and tectonic evolution on the volcanic passive continental margin of the northern margin of the North China Plate during late orogenic stage. Geochemical characteristics and ore-bearing parameters indicate that there is great prospecting potential for subvolcanic-type and porphyry-type Au-Ag-Cu deposit in this area.