摘要
虽然前人对澜沧岩群做了大量的研究,但缺乏同位素年代学方面的研究。此次研究针对澜沧江南段菖蒲塘-大田山地区澜沧岩群绢白云母石英片岩进行碎屑锆石U-Pb定年,为澜沧岩群的形成时代增添年代学证据。极大多数(129粒)锆石阴极发光(CL)图像显示其有明显的结晶振荡环带,指示其为岩浆成因的碎屑锆石,129粒碎屑锆石具有多组峰值年龄,最年轻一组年龄加权平均值为452±26Ma,表明澜沧岩群最早沉积时限不早于452±26Ma;极少数(1粒)锆石具弱阴极发光,缺乏内部结构特点,认为其为变质重结晶锆石,变质结晶锆石U-Pb年龄为255±3Ma,与前人研究所得澜沧岩群变质时代基本吻合。综合分析,澜沧岩群的沉积时限不早于452±26Ma,在二叠纪末期可能发生变质作用。
Although numerous previous studies,no direct evidences of geological ages are recorded for the Lancang Group Complex in western Yunnan. The U-Pb age data of the detrital zircons in sericite quartz schist from the Lancang Group Complex in the Changputang-Datianshan zone have provided convincing evidence for the formation age of the Lancang Group Complex. The cathodoluminescence( CL) images show that these zircon crystals generally display the well-defined oscillatory zonation and suggest a magmatic origin of the zircons. The age data for the detrital zircons in the sericite quartz schist sample PM08-B1 range between 452 and 3016 Ma,with the youngest age group of 452 ± 26 Ma,implying that the maximum depositional age of the Lancang Group Complex is less than452 ± 26 Ma. One zircon grain with a U-Pb age of 255 ± 3 Ma exhibits a slight cathodoluminescence image and has no internal structures,which is characteristic of a type of metamorphic recrystallization zircons and in general aggrement with the previous data. In general,the Lancang Group Complex is dated at a depositional age of less than 452 ± 26 Ma,The U-Pb age( 255 ± 3 Ma) of the metamorphic crystalline zircons reveals that the metamorphic events once took place during the Late Permian.
引文
[1]冯本智,卢民杰.滇西惠明组前寒武纪铁矿床的形成条件[J].长春地质学院学报,1982,(2):11-30.
[2]翟明国,从柏林,张儒媛.滇西澜沧群两类火山岩系的识别及其地质意义[J].中国科学,1990,(1):77-85.
[3]柏万灵.滇西南澜沧群中氧化锰矿成矿规律及找矿前景[J].云南地质,1994,13(4):333-340.
[4]彭兴阶,罗万林.澜沧江南段双变质带的初步确定[A].青藏高原地质文集[C].北京:地质出版社,1983.21-30.
[5]卫管一,罗再文.试论滇西澜沧群蛇绿岩套形成的构造环境[J].成都地质学院学报,1982,(3):41-46.
[6]1∶25万福贡县幅、丽江市幅、腾冲县幅、大理市幅、潞西市幅、凤庆县幅地质调查成果与进展[J].沉积与特提斯地质,2005,25(Z1):145-147.
[7]王泽传,赵茂春,严城民,朱延浙.滇西北大地构造单元的划分与特征[J].沉积与特提斯地质,2015,35(2):66-75.
[8]Hu Z C,Gao S,Liu Y S,et al.Signal enhancement in laser ablation ICP-MS by addition of nitrogen in the central channel gas[J].Journal of Analytical Atomic Spectrometry,2008,23:1093-1101.
[9]Hu Z C,Liu Y S,Gao S,et al.A“wire”signal smoothing device for laser ablation inductively coupled plasma mass spectrometry analysis[J].Spectrochimica Acta Part B:Atomic Spectroscopy.2012,78(0):50-57.
[10]Liu Y S,Hu Z C,Gao S,et al.In situ analysis of major and trace elements of anhydrous minerals by LA-ICP-MS with applying an internal standards[J].Chemical Geology,2008,257(1-2):34-43.
[11]Liu Y S,Gao S,Hu Z C,et al.Continental and oceanic crust recycling-induced melt-peridotite interactions in the Trans-North China Orogen:U-Pb dating,Hf isotopes and trace elements in zircons of mantle xenoliths[J].Journal of Petrology,2010,51(1-2):537-571.
[12]Wiedenbeck M,Alle P,Corfu F,et al.Three natural zircon standards for U-Th-Pb Lu-Hf trace element and REE analyses[J].Geostandards and Geoanalytical Research,1995,19(1):1-23.
[13]Ludwing K R.ISOPLOT3.00:A Geochronological Toolkit for Microsoft Excel[M].Berkelery:Berkeley Geochronology Center,California,2003.
[14]李长民.锆石成因矿物学与锆石微区定年综述[J].地质调查与研究,2009,33(3):161-174.
[15]王冬兵,尹福光,罗亮,等.昌宁-孟连构造带牛井山地区大猛龙岩群碎屑锆石U-Pb定年及其地质意义[J].矿物岩石,2015,35(3):72-80.
[16]卫管一,冯国荣,罗再文,等.滇西澜沧群、崇山群地层层序及其火山作用和变质作用[J].成都地质学院学报,1984,(2):12-20.
[17]翟明国,从柏林,乔广生,等.中国滇西南造山带变质岩的SmNd和Rb-Sr同位素年代学[J].岩石学报,1990,16(4):1-11.
(1)云南省1∶25×104凤庆县幅区域地质调查报告
