| 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 |
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| 引用本文： | 丁一,喻学惠,莫宣学,李小伟,黄雄飞,韦萍,和文言.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[J].地学前缘,2013,20(3). |
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| 作者姓名： | 丁一 喻学惠 莫宣学 李小伟 黄雄飞 韦萍 和文言 |
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| 作者单位： | 1. 地质过程与矿产资源国家重点实验室;中国地质大学(北京)地球科学与资源学院,北京100083;江苏省地质矿产局第四地质大队,江苏苏州215129
2. 地质过程与矿产资源国家重点实验室;中国地质大学(北京)地球科学与资源学院,北京100083 |
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| 基金项目： | 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" |
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| 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演化有关。西秦岭东、西两段新生代与早白垩世火山岩在岩相学、全岩化学以及岩石系列和类型方面的差异,可能与不同时期软流圈地幔源区的演化、岩浆起源深度和部分熔融程度等有关。
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| 关 键 词： | 西秦岭 早白垩世 钠质 OIB 软流圈地幔 |
Geochronology, geochemistry and petrogenesis of the Hongqiang basalts from Northeast Qinghai-Tibetan Plateau |
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| Abstract: | West Qinling orogenic belt located at the northeastern margin of Tibetan Plateau is a transform junction 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 geochemical 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 chronological data of Mesozoic basalts in Hongqiang Area of Xiahe County in West Qinling,Gansu Province,and discuss its petrogenesis and geodynamic setting.The LA-ICP-MS zircon U-Pb age of the basalt is(104.8±0.99)Ma(MSWD=0.63) which is consistent with its field relationships,and indicates that the magma activity 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,Al2O3,Na2O and lower CaO contents demonstrating that the basalts 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 εNd(t) values(5.9-7.5),low initial 87Sr/86Sr ratios(0.703 3-0.705 6) and Pb isotopic ratios are 206Pb/204Pb(t)=17.707-18.319,207Pb/204Pb(t)=15.398-15.626 and 208Pb/204Pb(t)=37.266-38.454 3 respectively.All of these mentioned above indicate that the Early Cretaceous basalts of Hongqiang have the geochemical characteristics similar to continental OIB,and their source may be a depleted asthenosphere mantle with mixing characteristics of DMM and PREMU.Combined with a comprehensive analysis of the geotectonic setting of the Hongqiang basalts and contrastive research between Hongqiang Early Cretaceous basalts and Lixian-Dangchang Cenozoic ultrapotassic volcanic rocks,we suggest that the Early Cretaceous Hongqiang basalts may be a product of continental rift magmatism,its petrogenesis and dynamic background are strongly associated with the nature and evolution of North-South Tectonic Belt since Mesozoic.The differences on the petrography,whole rock geochemistry,magma series and types of the volcanic rocks between Early Cretaceous sodic-alkaline basalts and Cenozoic ultrapotassic volcanic rocks in the E-W segment of West Qinling may be related to the evolution of the asthenospheric mantle source during different periods and the depth of magmagenesis as well as the degree of partial melting. |
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| Keywords: | West Qinling Early Cretaceous sodic OIB asthenospheric mantle |
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