北山风雷山地区白山组流纹岩LA-ICP-MS锆石U-Pb年龄及构造环境

摘要

Located at the junction of North China Plate intersection, Tarim plate and Kazakhstan plate, Beishan area is characterized by nearly EW-trending tectonic-magmatic activity. The rocks mainly include rhyolite, rhyolitic breccias-containing debris crystal tuff, dacitic crystal tuff, andesitic crystal tuff and a small amount of dacite. They can be divided into two sections based on features and rock eruption spatial superposition relationship. Geochemistry shows that rhyolites have high SiO2 content (77.05%~77.52%) and low TiO2 (0.06%-0.19%), CaO (0.38%-1.04%), MgO (0.09%-0.17%) content with K2O/ Na2O higher than 1. They are relatively enriched in large ion lithophile elements (LILE) Rb and K, while depleted in high field strength elements (HFSE) Nb, Ta, P and Ti. The total amount of light rare earth elements (ΣLREE average value being 127.55×10-6) is significantly higher than that of the heavy rare earth elements (ΣHREE average value being 10.92 × 10-6) , and LREE fractionation obviously ((La/Yb)N varies from 6.37 to 7.88, with an average of 7.05, whereas (La/Sm) N varies from 4.51 to 4.61, with an average of 4.55. Eu has obvious negative anomaly (δEu average value being only 0.62). Petrological and geochemical characteristics indicate that the tectonic setting was a continental-margin-arc. Zircon U-Pb age is 318.5 ± 1.2Ma for rhyolite by LA-IC-MS method. Combined with regional geological features, the authors believe that Baishan Formation in Fengleishan formed in the Late Carboniferous.%提北山地区位于华北板块、塔里木板块和哈萨克斯坦板块的交汇部位，总体上为一近东西向展布的构造-岩浆活动带。风雷山白山组岩性主要为紫红色流纹岩，流纹质含角砾岩屑晶屑凝灰岩、英安质、安山质晶屑凝灰岩夹少量灰色英安岩等，依据喷发特征和岩石空间叠置关系将其划分为一段、二段。岩石地球化学特征显示，流纹岩SiO2含量高（77.05%~77.52%）；K2O/Na2O＞1（1.03~1.07）；TiO2（0.06%~0.19%）、CaO（0.38%~1.04%）、MgO(0.09%~0.17%)含量低，岩石的里特曼指数平均为1.74，相对富集大离子亲石元素（LILE）Rb、K，亏损高场强元素（HFSE） Nb、Ta、P、Ti，稀土总量平均为138.47×10-6，轻稀土总量（ΣLREE平均为127.55×10-6）明显高于重稀土（ΣHREE平均为10.92×10-6），轻重稀土分馏明显（(La/Yb)N变化于6.37~7.88，平均为7.05；(La/Sm)N变化于4.51~4.61，平均为4.55， Eu具较为明显的负异常（δEu平均仅为0.62）。岩石学、地球化学特征表明构造背景为陆缘弧。流纹岩LA-MC-ICP-MS锆石U-Pb年龄为(318.5±1.2)Ma，结合区域地质特征，白山组应形成于晚石炭世。