摘要
前人通过滇西三江带内岩浆记录精细刻画了古特提斯洋分支洋--哀牢山洋的形成及演化历史,然而区域沉积记录是如何响应哀牢山洋俯冲及闭合过程的,目前仍未得到深入研究。我们通过对哀牢山构造带北段点苍山变质杂岩内三叠纪至侏罗纪3件沉积岩样品中255颗碎屑锆石进行U-Pb定年、微量元素及Hf同位素分析,结合区域已有研究,恢复哀牢山洋末期演化的区域沉积响应。结果表明,变质砂岩DC1801、片岩DC1703和碳酸盐岩DC1702中最年轻岩浆锆石群加权平均年龄分别为~180 Ma、~247 Ma和~254 Ma,结合其接触关系以及区域地层对比,约束其最大沉积年龄分别为侏罗纪早期(DC1801)和三叠纪早期(DC1702和DC1703)。不同时期碎屑沉积岩物源分析表明三叠纪碎屑沉积岩(DC1703)主要源自哀牢山构造带内部近源的多期岩浆物质(~0.8 Ga、~0.45 Ga和~0.25 Ga)、冈瓦纳裂解前印度大陆内部多期再循环的格林威尔晚期(~0.95 Ga)和泛非期(~0.6 Ga)岩浆物质贡献。侏罗纪碎屑沉积岩(DC1801)物质主要源自思茅地块内部印支期岩浆活动,而~0.45 Ga、~0.8 Ga和~0.95 Ga等时期的碎屑物质贡献比例明显降低。同一沉积盆地同时代碳酸盐岩与陆源碎屑岩中碎屑锆石结构大体相似,但碳酸盐岩中代表多期循环的远源物质的年龄所占比例却降低。点苍山碎屑沉积岩锆石Hf同位素组成随时间变化证实了哀牢山洋闭合时间为~247 Ma,而在点苍山变质地体南段发育的海相碳酸盐岩与陆源碎屑岩(片岩)的沉积组合记录了哀牢山洋末期演化至闭合过程。碎屑锆石Eu/Eu*异常揭示了区域地壳自~247 Ma哀牢山洋闭合至~235 Ma的加厚过程,三叠纪至侏罗纪碎屑沉积物源的转化可能与哀牢山洋闭合引发的沉积区抬升有关。
The formation and evolution of the Ailaoshan ocean(AO), a branch of the Paleo-Tethyan ocean, has been precisely depicted via deciphering the magmatic records in the Sanjiang area, western Yunnan. However, regional sedimentary records that responded to the subduction and closure of the AO have still not been fully understood. Therefore, an integrated study has been carried out on 255 detrital zircon grains including U-Pb dating and analyses of trace element concentrations and Lu-Hf isotopic compositions from three sedimentary samples in the Diancangshan metamorphic terrane(DMT) in the Ailaoshan tectonic belt(ATB). Together with previously study, regional sedimentary records during the last stage evolution of the AO has been delineated. The weighted mean ages of the youngest igneous zircon groups of metamorphic sandstone DC1801, schist DC1703, and carbonate DC1702 are ~180 Ma, ~247 Ma, and ~254 Ma, respectively. Together with their field contact relationships and regional sedimentary correlation, the oldest depositional ages of these sedimentary rocks are restricted to the Early Jurassic(DC1801) and Early Triassic(DC1702 and DC1703). Provenance analyses show that different igneous rocks with ages of ~0.8 Ga, ~0.45 Ga, and ~0.25 Ga in the ATB and the late Grevillian(~0.95 Ga) and Pan-African(~0.6 Ga) igneous materials in the Indian continent before dispersion of the Indochina block from Gondwana were the main inputs for the Triassic clastic sedimentary rock(DC1703). In contrast, the Indosinian igneous rocks in the Simao block were the main inputs for the Jurassic clastic sedimentary rock(DC1801), while contributions of the ~0.45 Ga, ~0.8 Ga and ~0.95 Ga materials were largely depressed. Carbonates and coeval clastic sediments in the same depositional basin show similar detrital zircon U-Pb age distribution patterns, but signals that represent for far sourced materials notably decreased in carbonates. Evolution of the Hf isotopic compositions through time verified that the closure of the AO was at ~247 Ma. The sedime
作者
陈静
李大鹏
康欢
耿建珍
张菁菁
CHEN Jing;LI Dapeng;KANG Huan;GENG Jianzhen;ZHANG Jingjing(School of Earth Sciences and Resources,China University of Geosciences,Beijing 100083,China;Institute of Geology,China Earthquake Administration,Beijing 100029,China;Tianjin Institule of Geology and Mineral Resources,China Geological Survey,Tianjin 300170,China)
出处
《现代地质》
CAS
CSCD
北大核心
2021年第4期883-913,共31页
Geoscience
基金
国家自然科学基金项目(41502212)
中央高校基本科研业务费拔尖青年教师创新能力培养项目(2652019056)。
关键词
青藏高原
哀牢山构造带
碎屑锆石
古特提斯洋
地球化学
Tibetan Plateau
Ailaoshan tectonic belt
detrital zircons
Paleo-Tethyan ocean
geochemistry
