南盘江盆地八渡辉绿岩斜锆石和锆石U-Pb年龄及其地质意义?
投稿时间:2018-06-18  修订日期:2018-07-06  点此下载全文
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夏文静 中国科学院大学地球与行星科学学院 xiawenjing89@163.com 
闫全人 中国科学院大学地球与行星科学学院 qryan@ucas.edu.cn 
向忠金 中国地质科学院地质研究所  
夏磊 中国科学院大学地球与行星科学学院  
江文 中国科学院大学地球与行星科学学院  
李晓剑 中国科学院大学地球与行星科学学院  
周斌 中国科学院大学地球与行星科学学院  
邓莉 中国科学院大学地球与行星科学学院  
中文摘要:南盘江盆地是华南板块西南缘最大的沉积盆地之一。它的构造属性、形成时间及机制长期存在争议,影响着对华南印支造山过程的准确理解。南盘江盆地内除三叠系填充物外,断续出露着古生代碳酸盐岩。这些古生代碳酸盐岩实为三叠纪盆地下伏地层在盆地伸展张开过程中形成的断隆,在这些断隆周边发育有似层状或脉状辉绿岩,记录了南盘江盆地发育初期或早期的地质演化信息。本文用Cameca IMS 1280二次离子质谱仪对南盘江盆地东南部八渡辉绿岩中的斜锆石和锆石进行了U-Pb同位素测试,分别获得206Pb/238U年龄加权平均值269.3±4.2Ma(95%置信度,N=15,MSWD=0.27)和265.0±2.9Ma(95%置信度,N=8,MSWD=0.02),表明八渡辉绿岩侵位于中二叠世沃德期(Wordian;265.1~268.8 Ma),指示了南盘江盆地在此时处于伸展构造体制。这与南盘江盆地内广泛发育晚二叠世—早三叠世(258~248 Ma)基性岩浆活动,以及南盘江盆地晚二叠世—早三叠世时期沉积环境由碳酸盐岩台地相沉积快速转变为以浊积扇相为主的深水沉积等地质事实,共同表明南盘江盆地在中二叠世—早三叠世处于伸展构造体制。
中文关键词:南盘江盆地  八渡辉绿岩  斜锆石  锆石  SIMS  U-Pb年龄  动力学背景
 
Baddeleyite and zircon U-Pb dating of Badu diabase in the Nanpanjiang basin and its tectonic significance
Abstract:The Nanpanjiang basin is the biggest terrigenous clasitic basin developed on the southwest margin of the South China block. The tectonic setting of the Nanpanjiang Basin during its forming, the onset time and the mechanism of the basin''s opening have been long debated. These debates obstruct our understanding on the evolution of the Indochina orogen. Inside the Nanpanjiang basin, excepted the Triassic deposition, some Paleozoic carbonatites also outcropped, which developed into faulted blocks of the basement during the basin stretching. Around the Paleozoic carbonate faulted blocks, somestratoid or veined diabase outcropped. As the initial record of the basin evolution, the diabase is a perfect research target for studying the age and geodynamics of the basin''s opening. In this study, the diabase samples were acquired near to the Badu town, Tianlin county, Guangxi Province, which is in the southeastern part of the Nanpanjiang Basin. The U-Pb dating was conducted with Cameca IMS 1280 on the baddeleyite and zircon concentrated from the diabase samples. The results give the weighted mean 206Pb/238U ages of 269.3±4.2 Ma (n=15,MSWD=0.27) and 265.0±2.9 Ma (n=8,MSWD=0.02), respectively. This result demonstrates that the Badu diabase emplaced during the Wordian, Middle Permian (265.1~268.8 Ma) and indicates that the Nanpanjiang Basin was stretching during this period. Besides this magmatic event dated by this study, a later mafic magmatic event continued widely in the Nanpanjiang basin during the Late Permian to the Early Triassic (258~248 Ma). Sedimentology study suggests that from the Late Permian to the Early Triassic the sedimentary environment of the Nanpanjiang basin changed from calci-clastic platform to turbidite fan. All these facts suggest that the Nanpanjiang Basin experienced extension during the Middle Permian to Early Triassic.
keywords:the Nanpanjiang Basin  Badu diabase  baddeleyite  zircon  SIMS  U-Pb  geodynamic
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