天水东岔地区宽坪岩群碎屑岩和花岗岩中的锆石U-Pb定年及其地质意义  

Zircon U-Pb dating of clastic rocks and granites of Kuanping Group in Dongcha areas of Tianshui,and its geological implications

作　　者：高胜[1,2] 陈丹玲[1,2] 宫相宽[1,2] 任云飞[1,2] 李海平 

机构地区：[1]大陆动力学国家重点实验室 [2]西北大学地质学系,陕西西安710069 [3]陕西省区域地质调查院,陕西西安712000

出　　处：《地学前缘》2015年第4期255-264,共10页Earth Science Frontiers

基　　金：国家自然科学基金项目(41090374 41072051);西北大学大陆动力学国家重点实验室科学技术部专项项目

摘　　要：利用LA-ICP-MS锆石原位定年技术和阴极发光图像(CL)分析,对北秦岭西段天水东岔地区宽坪岩群黑云母斜长片麻岩及其中的片麻状花岗岩进行了系统的锆石U-Pb年代学研究。其中,黑云母斜长片麻岩中的锆石多呈浑圆形态,且具有岩浆锆石的振荡环带结构,部分颗粒中可看到原岩锆石残核。定年结果获得667 Ma和1 055 Ma两组明显的年龄峰值,其中以1 055 Ma的峰值最明显和集中。同时获得最小的岩浆锆石年龄为(530±4)Ma,对应Th/U比值为0.38,代表本区宽坪岩群的形成年龄上限。该年龄与中、东部宽坪岩群目前得到的最新年龄(577±6)Ma一致,共同表明宽坪岩群整体应形成在新元古代末期或古生代早期。片麻状花岗岩锆石多为自形柱状形态并具有明显的振荡环带结构,具有高的Th、U含量(分别为(12~658)×10-6和(17~703)×10-6)和Th/U比值(变化于0.42~1.21),以及轻稀土亏损,重稀土明显富集的左倾稀土配分曲线型式,为典型的岩浆成因锆石。定年结果207Pb/206Pb集中变化于(1 746±51)^(1 844±33)Ma,平均为(1 806±18)Ma(MSWD=2.3,1σ),为花岗岩的结晶年龄。该年龄明显老于宽坪岩群的形成年龄,而与华北板块西、南缘分布的17~18亿年的岩浆事件一致,因而,该片麻状花岗岩与宽坪岩群碎屑岩应是构造关系,花岗岩可能是卷入到秦岭造山带中的华北西南缘的基底物质。西秦岭宽坪岩群碎屑岩定年最明显的特征是年龄谱主峰集中于667~1 055Ma,没有出现指示华北板块物源区的18亿年和23~25亿年的年龄峰值,而与北秦岭微板块和华南板块年龄谱系具有一定的相似性,也与扬子板块北缘广泛发育的新元古代构造岩浆事件一致,表明北秦岭微板块和华南板块、特别是扬子板块北缘可能是宽坪岩群的主要物源区。Cathodol analysis and LA-ICP-MS U-Pb dating were conducted on zircon grains from biotite plagioclase gneiss of the Kuanping Group and the gneissic granite in Dongcha, Tianshui of the western Qinling. As shown by CL images, most zircons from the biotite plagioclase gneiss are characterized by oval or rounded shapes, typical magmatic oscillatory zoning, and residual cores can be seen in some zircon grains. The dating results yielded two age groups with the peak values of 667 Ma and 1055 Ma. The 1055 Ma group is most obvious and concentrated. The youngest magmatic zircon age is 530 ± 4 Ma （Th/U= 0. 38）, which represents the upper limited sedimentary age of Kuangping Group in Dongcha area and is well consistent with the newest age 577±6 Ma of the Kuanping Group in the mid-east region, indicating that the whole Kuanping Group should have been formed in the late Neoproterozoic and the early Paleozoic. The zircons from the gneissic granite are idiomorphic, and the magmatic oscillatory structure, high Th/U ratios （0. 42 - 1.21） and left-dipping REE patterns indicate that they are magmatic zircons. The analyzed spots yielded concordant ^207Pb/^206Pb ages from 1746±51 Ma to 1844±33 Ma, with a weighted mean age of 1804±16 Ma （MSWD= 2.3, 1σ）, which represents the crystallization age of the gneissic granite; this age is obvious older than the formation age of Kuanping Group, but similar to the time of magma events （1.7 - 1.8 Ga） distributed in western and southern margin of North China Block. Therefore, the gneissic granite and the clastic rocks of Kuanping group should have the structural contact relationship; its materials came from southwest margin of North China Block and were involved into the Qinling Orogenic Belt. The clastic rocks of Kuanping group in this area is mainly marked by the age of 667 - 1055 Ma, and there are no peak ages of 1.8 Ga and 2.3 -2. 5 Ga that marked the north China plate provenance; however, the age has obvious similarities with south China plate and North Qinling microplate

关 键 词：片麻岩 锆石U-PB定年 宽坪岩群 东岔 西秦岭 

分 类 号：P588.345[天文地球—岩石学]