Genesis and tectonic magmatic evolution of Carboniferous granites in the Hongliuhecao-Yuejinshan area of Beishan orogenic belt

Abstract

北山造山带位于中亚造山带南缘，区内广泛发育古亚洲洋-陆演化的岩浆记录，是研究北山造山带构造-岩浆-成矿作用及其动力学背景的重要窗口。红柳河槽-跃进山地区是北山造山带中北部的重要组成部分，区内与成矿有关花岗岩的形成时代、成因及构造背景尚不明确，由此制约了对北山造山带内成岩成矿动力学背景的探讨。本次在系统、详细的野外地质调查、岩石（相）学观察的基础上，结合岩浆岩地球化学、锆石U-Pb年代学和Lu-Hf同位素等证据，深入剖析了北山造山带内红柳河槽-跃进山地区的花岗岩形成时代、类型、源区和动力学背景，取得了以下认识：（1）通过锆石LA-ICP-MS U-Pb定年测得与成矿有关的狼娃山二长花岗岩、红柳河槽正长花岗岩、跃进山北正长花岗岩、跃进山二长花岗岩形成时代分别为327.1±2.4Ma、326.7±2.4Ma、321.3±2.4Ma和309.7±2.2Ma，表明红柳河槽-跃进山地区铜、钨、钼、铅、锌等元素成矿与石炭纪（327.1~309.7Ma）岩浆活动有关；（2）通过花岗岩地球化学研究表明，红柳河槽-跃进山地区的花岗岩均属于钙碱性-高钾钙碱性、准铝质-弱过铝质系列，强烈富集LREE、LILE，亏损HREE、HSFE，具有典型弧岩浆的地球化学特征，花岗岩成因类型均为I型；（3）本次获得狼娃山二长花岗岩和红柳河槽正长花岗岩的ε_Hf（t）值分别为5.90~9.92（平均值7.93）和5.87~10.28（平均值8.11），t_DM2分别为0.703~0.959Ga和0.93~1.03Ga，且具有较低的Nb/Ta比值（5.1~14.2，平均值8.5），与典型新生下地壳来源岩浆（8.3）相似，表明岩浆物质可能源于新生下地壳岩石部分熔融；（4）根据锆石Ti温度计查明北山造山带红柳河槽-跃进山地区狼娃山二长花岗岩、红柳河槽正长花岗岩、跃进山北正长花岗岩、跃进山二长花岗岩的平均结晶温度分别为765℃、765℃、816.4℃、771.5℃，锆石Ce^（Ⅳ）/Ce^（Ⅲ）平均值分别为85.27、108.4、103.6、135.7，表明从石炭纪早期到晚期，岩浆的氧逸度呈逐渐升高的趋势。根据以上研究，结合前人研究成果，初步建立了北山造山带构造-岩浆演化模型如下：石炭纪北山造山带北部红石山洋向南侧公婆泉-明水-旱山复合构造单元（弧）之下发生俯冲，诱发新生下地壳部分熔融形成的壳源岩浆；其侵入到地壳浅部，并最终形成红柳河槽-跃进山花岗岩体。;Beishan orogenic belt is located in the southern margin of the Central Asian Orogenic Belt. Magmatic records of paleo-Asian ocean-continent evolution widely existed in this area, and as a result, it is an important window to study the tectonic magmatic mineralization and dynamic background of Beishan orogenic belt. Hongliuhecao-Yuejinshan area is an important component of the middle-northern part of Beishan orogenic belt, however, the formation age, genesis and tectonic background of granites related to mineralization in the area are not clear, which seriously restricts the discussion of diagenetic and metallogenic dynamic backgrounds in Beishan orogenic belt. Based on systematic and detailed field geological survey and petrographic (facies) observation, combined with the evidence of magmatic rock geochemistry, zircon U-Pb chronology and Lu-Hf isotope, we analyzes the formation age, type, source area and dynamic background of granite in Hongliuhecao-Yuejinshan area in Beishan orogenic belt, from which the following conclusions can be obtained:(1) According to zircon LA-ICP-MS U-Pb dating, the formation ages of Langwashan monzogranite, Hongliuhecao syenogranite, Yuejinshanbei syenogranite and Yuejinshan monzogranite related to mineralization are 327.1±2.4Ma, 326.7±2.4Ma, 321.3±2.4Ma and 309.7±2.2Ma, indicating that the mineralization of copper, tungsten, molybdenum, lead, zinc and other elements is related to the Carboniferous magmatism of 327.1~309.7Ma. (2) The geochemical study of granites shows that the granites in Hongliuhecao-Yuejinshan area are strongly enriched in LREE and LILE, while deficient in HREE and HSFE, which demonstrated that they have the geochemical characteristics of typical arc magma and the I-type granites, and belong to calc alkaline, high-K calc alkaline, quasi aluminous-weak peraluminous series. (3) The ε_Hf(t) values of Langwashan monzogranite and Hongliuhecao syenogranite are 5.90~9.92 (average 7.93) and 5.87~10.28 (average 8.11), respectively; and the t_DM2 values are 0.703~0.959Ga and 0.93~1.03Ga, respectively; furthermore, they have a low Nb/Ta ratio (5.1~14.2, average 8.5), which is similar to the typical magma from the newborn lower crust (8.3). All these charateristics indicate that the magma material may have originated from the partial melting of the newborn lower crust rocks. (4) According to the zircon Ti thermometer, the average crystallization temperatures of Langwashan monzogranite, Hongliuhecao syenogranite, Yuejinshanbei syenogranite and Yuejinshan monzogranite in the studying area are 765℃, 765℃, 816.4℃ and 771.5℃, respectively; while the zircon Ce^(Ⅳ)/Ce^(Ⅲ)average values are 85.27, 108.4, 103.6, 135.7, respectively, indicating that the oxygen fugacity of magma increased gradually from the early to the late Carboniferous. Based on the above analyses, together with previous research results, the tectonic magmatic evolution model of Beishan orogenic belt is preliminarily established:the Hongshishan ocean in the north of the Beishan orogenic belt subducted under the Gongpoquan-Mingshui-Hanshan composite tectonic unit (arc) in Carboniferous, which induced partial melting of the newborn lower crust and formed the magma; this crustal magam intruded into the shallow crust, and finally formed the Hongliuhecao-Yuejinshan granite.