藏南洛扎地区基性岩墙群锆石U-Pb定年、地球化学特征及构造意义  

Zircon U-Pb dating and geochemistry of mafic dike swarms in the Lhozag area,southern Tibet,China,and their tectonic implications.

作　　者：童劲松[1] 刘俊[2] 钟华明[2] 夏军[2] 鲁如魁[3] 李运怀[2] 

机构地区：[1]中国地质大学地球科学与资源学院,北京100083 [2]安徽省地质调查院,安徽合肥230001 [3]西北大学,陕西西安710069

出　　处：《地质通报》2007年第12期1654-1664,共11页Geological Bulletin of China

基　　金：中国地质调查局地质调查项目(编号:19991300009101)资助

摘　　要：藏南洛扎地区广泛分布有近东西向产出的基性岩墙群,主要侵位于晚侏罗世—早白垩世及其以前的地层中,锆石U-Pb同位素定年获得成岩年龄138.0Ma±3.5Ma。基性岩墙可以划分为2种主要类型:一类低K(K2O=0.06%～0.54%),稀土元素总量和轻、重稀土元素分异程度低[∑REE=58.95×10-6～115.5×10-6,(La/Yb)N=0.23～2.94],亏损LILE;另一类富K(K2O=1.22%～1.67%),稀土元素总量和轻、重稀土元素分异程度较高[∑REE=199.97×10-6～381.47×10-6,(La/Yb)N=6.57～11.5],富集LILE而亏损HFSE。地球化学研究结果表明,基性岩墙为具大陆拉斑玄武岩特征的大陆板内岩浆侵入体,可能分别源于亏损的软流圈地幔和富集的岩石圈地幔。基性岩墙群是新特提斯洋晚期大规模扩张的产物,标志着喜马拉雅被动大陆边缘在晚侏罗世—早白垩世处于强烈的拉张、裂离和岩石圈伸展减薄、软流圈上涌的构造环境与动力学背景。Distributed widely in the Lhozag area, southern Tibet, the nearly E-W-trending mafic dike swarms are mainly intruded into Late Jurassic-Early Cretaceous and earlier strata. The U-Pb dating of zircons from dike swarms yielded an age of 138.0±3.5 Ma. The dike swarms are mainly classified into two types. One type is low in K （K2O=0.06-0.54%） and total REE content （∑REE= 58.95-115.5×10^-6） with weak ffactionation of LREE and HREE [（La/Yb）N=0.23-2.94] and depletion of LREE. By contrast, the other type is rich in K （K2O= 1.22-1.67%） and total REE content （∑ REE=199.97-381.47 ×10^-6） and has strong fractionation of LREE and HREE [（La/Yb）tq=6.57-11.5], exhibidng LILE enrichment and HFSE depletion. Geochemical study shows that these dike swarms were intrusions of continental within-plate magmas with the characteristics of continental tholeiific basalt and may have been derived from the depleted asthenospheric mantle and enriched lithospheric mantle. They are the product of large-scale spreading of the Neo- Tethys ocean in its late stage. The dike swarms indicate that the passive margin of the Himalaya was in a tectonic environment of strong extension and splitting and a dynamic setting of lithospheric extension and thinning and asthenospheric upwelling during the Late Jurassic-Early Cretaceous.

关 键 词：基性岩墙群 锆石U—Pb年龄 地球化学特征 藏南洛扎地区 

分 类 号：P588.124[天文地球—岩石学] P597.3[天文地球—地质学]