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西藏雅鲁藏布缝合带三叠系朗杰学群沉积岩古生物、地球化学特征及其物源区和构造背景分析
投稿时间:2019-03-19  修订日期:2019-07-23  点此下载全文
引用本文:
DOI:10.19762/j.cnki.dizhixuebao.2020008
摘要点击次数: 51
全文下载次数: 83
作者单位E-mail
王金贵 河北省区域地质调查院 airstarry@163.com 
张鑫全 河北省区域地质调查院  
魏文通 河北省区域地质调查院  
专少鹏 河北省区域地质调查院  
陈超 河北省区域地质调查院  
张振利 河北地质大学  
常立海 黑龙江省生态地质调查研究总院  
申宗义 河北省区域地质调查院  
中文摘要:随着近年来对朗杰学群的解体,发现前人对于朗杰学群、涅如组、修康群及洛林岩群的研究存在混淆,研究对象是否为朗杰学群应重新审慎,同时,几者是否为同一构造背景产物仍存在较大争议。本文对朗杰学群命名地(贡嘎县姐德秀镇-朗杰学乡一带)的朗杰学群重矿物特征、地球化学特征及古生物化石进行了研究。朗杰学群主量元素和微量元素化学成分低于地壳克拉克值,SiO2含量平均为74.14%,K2O/Na2O值平均为0.70,反映砂岩成熟度较低,没有经历了明显的成岩后生改造;化学蚀变指数(CIA)较集中,平均为71.55,属低级-中等风化程度,指示气候和构造环境都处于相对稳定状态;100Mg/Al2O3值平均为7.85,CaO/Fe+CaO值平均为0.314,古环境分析显示,朗杰学群形成于中等盐度的深水厌氧环境,且由北向南沉积水体逐渐加深;重矿物以锆石为主,其次为金红石,分选磨圆较差,ATi指数为93.3~98.97,ZTR指数为48.01~85.75,反映朗杰学岩群的物源区火山岩较为丰富,且搬运距离较近;REE-La/Yb图解、Hf-La/Th图解、F1-F2图解、Th/Sc-Zr/Sc图解及重矿物特征均显示,朗杰学物源区为中酸性火成岩及陆缘碎屑岩,结合前人研究的古水流方向证据,并对比直孔-松多特提斯洋的演化及南冈底斯中-晚三叠世岩浆岩与朗杰学群碎屑锆石峰值年龄(240Ma),确认了朗杰学群物源来自南冈底斯;构造判别图解显示朗杰学群为大陆岛弧与活动大陆边缘环境之间特定构造背景下的产物,形成环境更靠近活动大陆边缘一侧;古生物化石以凸海燕蛤(Halobia convexa Chen, 1964)为代表,显示朗杰学群形成于晚三叠世卡尼期-诺利期;综合风化程度、物源区、构造背景及形成时代分析,朗杰学群的形成与雅鲁藏布特提斯洋的演化相吻合。
中文关键词:雅鲁藏布缝合带  朗杰学  碎屑沉积岩  地球化学  重砂矿物  古生物  物源分析  构造背景
 
Paleontology, Geochemistry and provenance of sedimentary rocks from the Triassic Langjiexue Group in Yarlung Zangbo suture zone ,Tibet and their tectonic setting
Author NameAffiliationE-mail
WANG Jingui Regional Geology Survey Institute of Hebei Province airstarry@163.com 
ZHANG Xinquan Regional Geology Survey Institute of Hebei Province  
WEI Wentong Regional Geology Survey Institute of Hebei Province  
ZHUAN Shaopeng Regional Geology Survey Institute of Hebei Province  
CHEN Chao Regional Geology Survey Institute of Hebei Province  
ZHANG Zhenli Hebei GEO University  
CHANG Lihai Heilongjiang General Institute of Ecological Geological Survey and Research  
SHEN Zongyi Regional Geology Survey Institute of Hebei Province  
Abstract:With the decomposition of the Langjiexue Group in recent years, it has been found that the previous studies on the Langjiexue Group, Nieru Group, Xiukang Group and Luolin Group are confused.The study of Langjiexue Group should be reconsidered.Moreover, it is controversial whether the above strata were formed in the same tectonic environments.In this paper, we study the heavy mineral characteristics, geochemical characteristics and paleontological fossils of the Langjiexue Group in Jiedexiu Town-Langjiexue country, Gongga County, China.The chemical compositions of major and trace elements in the Langjiexue Group are lower than the Clarke value of the crust. The average content of SiO2 is 74.14%, and the average value of K2O/Na2O is 0.70. The above characteristics indicates that the maturity of sandstone is low, and it has not undergone obvious epigenetic reworking.The chemical alteration index (CIA) is relatively concentrated, averaging 71.55, which belongs to low-medium weathering degree, indicating that the climate and tectonic environment are in a relatively stable state.The average values of 100Mg/Al2O3 and CaO/Fe+CaO are 7.85 and 0.314 respectively, which indicates that the Langjiexue Group formed in the deep-water anaerobic environment with moderate salinity and gradually deepened from north to south.Heavy minerals are mainly zircon, followed by rutile, with poor separation and grinding circle. ATi index is 93.3-98.97, ZTR index is 48.01-85.75, reflecting that volcanic rocks in the source area of the Langjiexue Group are abundant, and their transportation distance is relatively close.REE-La/Yb diagrams, Hf-La/Th diagrams, F1-F2 diagrams, Th/Sc-Zr/Sc diagrams and heavy mineral characteristics all show that the source area of Longjiexue Group is intermediate-acid igneous rocks and continental margin clastic rocks. Based on the evidence of paleocurrent direction, the evolution of the Zhikong-Songdo Tethys Ocean, the Middle-Late Triassic magmatic rocks in South Gangdise and the peak age (240 Ma) of clastic zircons of the Langjiexue Group, the provenance of the Langjiexue Group is South Gangdise.Tectonic discriminant diagrams show that the Langjiexue Group is a product of the specific tectonic background between the continental island arc and the active continental margin environment, and the formation environment is closer to the active continental margin side.Paleontological fossils, represented by Halobia convexa, indicate that the Langjiexue Group was formed during the Carnian-Norian period of the Late Triassic.Based on the analysis of weathering degree, provenance area, tectonic background and formation age, the formation of the Langjiexue Group coincides with the evolution of the Yarlung Zangbo Tethys Ocean.
keywords:Yarlung Zangbo suture zone  Langjiexue Group  clastic sedimentary rocks  geochemistry  placer mineral  paleontology  source analysis  tectonic background
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