青藏高原东北缘红墙玄武岩的年代学、地球化学特征及成因初探  

Geochronology,geochemistry and petrogenesis of the Hongqiang basalts from Northeast Qinghai-Tibetan Plateau

作　　者：丁一[1,2,3] 喻学惠[1,2] 莫宣学[1,2] 李小伟[1,2] 黄雄飞[1,2] 韦萍[1,2] 和文言[1,2] 

机构地区：[1]地质过程与矿产资源国家重点实验室 [2]中国地质大学(北京)地球科学与资源学院,北京100083 [3]江苏省地质矿产局第四地质大队,江苏苏州215129

出　　处：《地学前缘》2013年第3期180-191,共12页Earth Science Frontiers

基　　金：中国地质调查局重要基础研究项目"青藏高原重大关键地质问题研究"下属子课题"青藏高原重要构造岩浆事件与深部过程研究"(1212011121260)

摘　　要：西秦岭地处青藏高原东北缘,是古亚洲构造域、特提斯构造域和滨太平洋构造域的交接转换带,也是分野中国大陆东、西部地理与地质构造的关键部位。该区广泛分布了大量中生代火山岩。由于该区基础地质研究程度很低,特别是缺少可靠的年代学和地球化学资料,对该区中生代火山岩的成因及地球动力学背景一直存在争议,影响了人们对西秦岭大地构造属性及发展演化历史的全面认识。文中提供了甘肃西秦岭夏河县麻当乡红墙村中生代玄武岩的岩相学、地球化学和同位素年代学研究资料和数据,对火山岩的成因与动力学条件进行了初步讨论。红墙玄武岩中分离出的锆石LA-ICP-MS U-Pb定年结果表明,该玄武岩形成于(104.8±0.99)Ma(MSWD=0.63),与野外观察结果一致,证明红墙玄武岩是早白垩世火山作用产物。岩相学和地球化学的观察和研究显示,该套岩石以出现斜长石斑晶、基质中出现大量斜长石微晶或微斑晶为特征,岩石具有较高的SiO2、Al2O3、Na2O和较低的CaO,属于钠质碱性玄武岩,而不同于该研究区东部礼县—宕昌一带新生代超钾质火山岩。红墙玄武岩在地球化学方面具有富集轻稀土和部分高场强元素(如Nb、Ta、Zr、Hf、Ti等),亏损大离子亲石元素(Rb、K等),正的εNd(t)=5.9～7.5和低的87Sr/86Sr(t)=0.703 3～0.705 6以及206Pb/204Pb(t)=17.707～18.319、207Pb/204Pb(t)=15.398～15.626和208Pb/204Pb(t)=37.266～38.454等特征。所有这些特征一致表明,红墙早白垩世玄武岩具有与大陆OIB相似的地球化学特征,其源区可能具有DMM和PREMA端员混合的特点,推测其主要来自亏损的软流圈地幔。结合对火山岩产出大地构造背景的综合分析及与礼县—宕昌新生代超钾质火山岩的对比,提出红墙早白垩世钠质碱性玄武岩是大陆裂谷岩浆作用的产物,其成因和动力学背景与自中生代以来南北构造带的裂谷性质及其发展�West Qinling orogenic belt located at the northeastern margin of Tibetan Plateau is a transform junc- tion belt among the Paleo-Asian tectonic domain, the Tethyan tectonic domain and the Pacific tectonic domain, and is a key region in the division of the geographical and geological structures between the eastern and western China. A large amount of Mesozoic volcanic rocks are widely distributed in West Qinling. However, there are some debatable issues on petrogenesis and geodynamic settings of the Mesozoic volcanic rocks in this region, because of poor basic geological research in the past, especially lacking reliable geochronological and geochemi- cal data of the volcanic rocks, which had affected, the comprehensive understanding of the tectonic setting and evolutionary history of West Qinling. In this paper, we present petrographic, geochemical and isotopic chrono logical data of Mesozoic basahs in Hongqiang Area of Xiahe County in West Qinling, Gansu Province, and dis cuss its petrogenesis and geodynamic setting. The LA-ICP-MS zircon U-Pb age of the basalt is （104. 8--q-_ 0.99）Ma （MSWD=0. 63） which is consistent with its field relationships, and indicates that the magma activi- ty of the Hongqiang basalts occurred mainly in the Early Cretaceous. The petrographic study shows that the basalts are characteristic of occurring plagioclase phenocrysts with large amount of plagioclase microlites and micro-phenocrysts in the groundmass. In petrochemistry the volcanic rocks are characteristic of higher SiO2, AlcOa, Na^O and lower CaO contents demonstrating that the basahs belong to a sodic-alkaline magma series which is different from the Cenozoic ultrapotassic volcanic rocks from Lixian-Dangchang in the eastern part of the study area. As to geochemistry, the Hongqiang basalts are rich in REE and HREE （Nb, Ta, Zr, Hf, Ti） and slightly depleted in LILE （Rb, K）, and have positive eNa （t） values （5. 9 -- 7. 5）, lOW initial STSr/86Sr ratios （0. 703 3 - 0. 705 6） and Pb isotopic ratios are 2~6Pb/

关 键 词：西秦岭 早白垩世 钠质 OIB 软流圈地幔 

分 类 号：P588.145[天文地球—岩石学]