Geochemistry and detrital zircon U-Pb geochronology of the Jurassic Ridang Formation in the southern Tibet and its tectonic implications
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摘要:
喜马拉雅东段印度被动大陆边缘沉积的中—新生界地层是研究新特提斯大洋演化的重要载体。本文对西藏山南市错那县曲卓木乡的下侏罗统日当组粉砂岩和页岩进行碎屑锆石U-Pb定年和全岩地球化学分析。其碎屑锆石年龄有两个峰值区间，分别是546~496 Ma和952~853 Ma，峰值年龄分别为516 Ma和926 Ma，最年轻年龄为292.5±5.5 Ma。日当组的Al2O3含量较高，平均值为20.8%，而CaO、Na2O和K2O含量均较低（0.4%~4.0%），表明日当组富含粘土矿物，其物源区成分成熟度高，风化强，代表沉积物的再循环以及构造静止期或者克拉通环境。日当页岩轻稀土元素富集，重稀土元素亏损，Eu负异常明显。本文综合古地理环境，认为日当组物质来源主要为冈瓦纳大陆北缘的印度板块和澳大利亚板块，构造背景为稳定的被动大陆边缘。
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关键词:
- 特提斯喜马拉雅 /
- 日当组 /
- 碎屑锆石U-Pb年代学 /
- 砂岩地球化学 /
- 大地构造背景
Abstract:The Mesozoic-Cenozoic strata deposited in the passive continental margin of India in the eastern Himalayas are important carriers for the study of the evolution of the Neo-Tethyan Ocean. In this paper, detrital zircon U-Pb dating and whole-rock geochemical analysis of the Lower Jurassic Ridang Formation siltstone and shale in Quzhuomu Township, Cuona County, Shannan City, Tibet Autonomous Region have been carried out. The detrital zircons have two peak ages, of 496-546 Ma and 853-952 Ma, with the peak ages of 516 Ma and 926 Ma, respectively, and the youngest age of 292.5±5.5 Ma. The content of Al2O3 in the Ridang Formation is high, with an average of 20.8%, while the contents of CaO, Na2O and K2O are low (0.4%-4.0%), indicating that the Ridang Formation is rich in clay minerals. The source area has high compositional maturity and strong weathering, which represents the recycling of sediments and the tectonic quietness or cratonic environment. The Ridang shale is enriched in light rare earth elements (LREEs) and depleted in heavy rare earth elements (HREEs), and the negative Eu anomaly is obvious. Based on the paleogeographic environment, it is considered that the material of Ridang Formation is mainly derived from the Indian plate and the Australian plate on the northern margin of Gondwana continent, and the tectonic background is a stable passive continental margin.
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图 1 喜马拉雅研究区地质构造简图（据Cao et al., 2021修改）
Figure 1.
图 5 错那县下侏罗统日当组岩石类型和风化程度图解（底图据Herron, 1988, McLennan et al., 1993, Nesbitt and Young, 1982）
Figure 5.
图 6 错那县下侏罗统日当组稀土和微量元素标准化蛛网图 （球粒陨石和原始地幔值引用自Sun and McDonough, 1989）
Figure 6.
图 7 错那县日当组源岩类型和构造背景判别图解 （底图引用自Bhatia and Crook, 1986, Verma and Armstrong-Altrin, 2013, Wang et al., 2012）
Figure 7.
图 8 研究区周缘板块和地体碎屑锆石U-Pb年龄直方图 （据Cao et al., 2018修改）
Figure 8.
图 9 藏南早侏罗世古地理位置复原图 （据Li et al., 2016修改）
Figure 9.
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