西秦岭凤太矿集区花岗闪长斑岩脉的成因类型和年龄及其地质意义
投稿时间:2017-04-09  修订日期:2017-07-27  点此下载全文
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作者单位E-mail
陈绍聪) 中国地质科学院矿产资源研究所 shaocong0211@163.com 
王义天) 中国地质科学院矿产资源研究所  
胡乔青) 中国地质科学院矿产资源研究所  
张娟) 中国地质科学院矿产资源研究所  
王瑞廷) 西北有色地质勘查局  
王长安) 西北有色地质勘查局总队;西北有色地质勘查局总队  
刘协鲁) 北京探矿工程研究所  
魏然) 中国地质科学院矿产资源研究所  
温深文) 西北有色地质勘查局717总队  
刘伟) 陕西凤县四方金矿有限责任公司  
欧阳升) 陕西省潼关县国土资源局  
基金项目:国家自然科学基金项目(面上项目,重点项目,重大项目)
中文摘要:凤太矿集区是西秦岭最重要的成矿区之一,金、铅锌等金属矿产资源丰富。区内中酸性岩脉广泛发育,本次工作对产于矿集区中北部NWW向铜铃沟-夹山沟断裂内的孔棺和金铜沟花岗闪长斑岩脉进行了LA-ICP-MS锆石U-Pb定年和元素地球化学分析,分别获得年龄数据230.7±1.8Ma和230.4±1.8Ma,表明其为印支晚期岩浆活动的产物,形成于中三叠世。花岗闪长斑岩脉具高硅富铝、贫镁特征;稀土元素配分呈右倾型,轻、重稀土强烈分馏,无明显Eu异常,富集大离子亲石元素,亏损高场强元素,Nb/Ta比值接近大陆地壳;87Sr/86Sr同位素比值分别为0.712833±0.000017和0.712365±0.000016,略大于扬子板块北缘新元古代碧口群Sr同位素比值。地球化学特征表明成岩物质以壳源为主,岩浆可能主要来自扬子板块北缘碧口群的部分熔融,源区有石榴子石等矿物残留,无斜长石残留。两岩脉均为I花岗岩,形成于同碰撞初始阶段。结合矿集区及周边岩浆作用特征,认为凤太矿集区经历了早期(250Ma左右)洋壳俯冲阶段、中期(230Ma左右)同碰撞阶段以及晚期(220-200Ma)后碰撞阶段的花岗质岩浆作用。230Ma的孔棺和金铜沟花岗闪长岩限定了矿集区NWW向断裂带形成的上限。区内NWW向褶皱和断裂形成较早(约中三叠世),NE向构造发育略晚(约晚三叠世)。近年来越来越多的同位素年龄数据显示,区域构造、岩浆以及金、铅锌多金属成矿时代都集中于190-230Ma,暗示了三者的时空耦合性,都是印支期秦岭造山作用过程的产物。
中文关键词:岩石地球化学  LA-ICP-MS锆石U-Pb年龄  花岗闪长斑岩脉  凤太矿集区  西秦岭
 
Genetic type and geochronology of the granodiorite porphyry dikes in Fengtai ore cluster, West Qinling Orogen and its geological significance
Abstract:The Fengtai ore cluster in Shaanxi Province is one of the most important ploymetallic ore cluster in the West Qinling Orogen, where the deposits of lead-zinc and gold are well developed. Plenty of dikes like diorite porphyrite, granite porphyry and lamprophyre, are developed in the whole Fengtai ore cluster, besides the Xiba, and Huahongshuping granitoid plutons occur in the east and north parts. In this paper, two samples are collected from the Kongguan and Jintonggou granodiorite porphyry dikes, which are filled in the NWW-trending Tonglinggou-Jiashangou fault in the north central part of the cluster. LA-ICP-MS dating yielded zircon ages of 230.7±1.8Ma(MWSD=0.68) and 230.4±1.8Ma(MSWD=0.27), respectively, which show that both of the dikes formed in Middle Triassic. Geochemical features of the two dikes are as follows: high enrichment of SiO2, Al2O3 and large ion lithophile elements (LILE), like Rb, U, K; strong depletion of high field strength elements (HFSE), such as Th, Nb, Ta, P and Ti, and Nb/Ta ratios are similar to that of the continental crust; strongly fractionated REE patterns ((La/Yb)N=250.69 and 134.94) and no notable Eu anomalies; The 87Sr/86Sr ratios of the two samples are 0.712833±0.000017 and 0.712365±0.000016, respectively, slightly larger than that of the Neoproterozoic Bikou group. All the features mentioned above indicate that the dikes belong to I-type granitoids and the magma might derive from partial melting of the Neoproterozoic Bikou group, and there were garnet and no plagioclase left in the source. We suggest that the dikes were formed at the beginning of the syn-collision during the Mesozoic collision between the Yangtze and the North China blocks. By integrating the regional tectonic and magmatic characteristics, we propose that the granitoid intrusions of early stage (±250Ma) are most likely developed during subduction, that of middle stage (±230Ma) are formed during the continental collision, and the post-collisional granitoid rocks commonly constitute the late stage granitoid (220-200Ma). The age of 230Ma from these two dikes gives an upper time limit of the NWW faults in the cluster. The NWW-trending structures in the cluster were formed during the middle Triassic, and the NE-trending faults and joints were formed during the late Triassic. More and more age data show that the time of deformation, metamorphism (tectonic metamorphism), magmatism and mineralization in the Fengtai ore cluster are all focused on 190-230Ma, implying they are space-time coupling with each other, and are the products of the Qinling orogenic process in Indosinian.
keywords:Petro-geochemistry  LA-ICP-MS zircon U-Pb ages  granitoid porphyry dikes  Fengtai ore cluster  West Qinling Orogen
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