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Geochemistry of Triassic Siliceous Rocks of the Muyinhe Formation in the changning-menglian belt of Southwest China

Journal of Earth Science 2016年第3期27卷 403-411页

作者： Tsuyoshi Ito Xin Qian Qinglai Feng State Key Laboratory of Geological Processes and Mineral Resources China University of Geosciences Stratigraphy and Tectonics Research Group Institute of Geology and Geoinformation Geological Survey of Japan School of Earth Sciences China University of Geosciences

The changning-menglian belt, distributed over southwestern Yunnan Province in Southwest China, contains oceanic rocks that are considered to be remnants of the Paleotethys. This study observed Triassic siliceous rocks of the Muyinhe Formation in the changning-menglian belt and analyzed their geochemistry. The samples have high concentrations of SiO2（81.65 wt.%–88.38 wt.%; average： 84.99 wt.%±2.14 wt.%）. Most of the samples were plotted in the non-hydrothermal field on the Al-Fe-Mn diagram. Most of the samples were plotted in the continental margin field on the Fe2O3/TiO2-Al2O3/（Al2O3＋Fe2O3） and（La/Ce）N-Al2O3/（Al2O3＋Fe2O3） diagrams. Moreover, the samples show a flat REE（rare earth element） pattern normalized to NASC（North America shale composite）. These geochemical results, in addition to the lack of rhythmical bedding of the siliceous rocks, strongly suggest that the siliceous rocks are unlikely to represent pelagic deposits. Although previous studies have suggested that the siliceous rocks are pelagic deposits, the present results indicate that the extent of the pelagic ocean basins in the Paleotethys during the Triassic is probably less than previously believed. These non-pelagic deposits may represent the closure stage of the Paleotethys.

关键词： geochemistry Triassic siliceous rock Paleotethys Muyinhe Formation changning-menglian belt

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Petrogenesis of Early Carboniferous Ultramafic-Mafic Volcanic Rocks in the Southern changning-menglian belt,Southeastern Tibetan Plateau:Implications for the Evolution of the Paleo-Tethyan Ocean

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Acta Geologica Sinica(English Edition) 2022年第3期96卷 858-874页

作者： XIE Zhipeng WU Xuelu WANG Tengfei XUE Chuandong YANG Tiannan WANG Wei SUI Binxi XIN Di Department of Earth Sciences Kunming University of Science and TechnologyKunming 650093China Institute of Geology Chinese Academy of Geological SciencesBeijing 100037China

The changning-menglian belt represents the main Paleo-Tethyan Suture in the southeastern Tibetan Plateau,which divides Gondwana-and Eurasia-derived continental fragments from each *** belt contains ultramafic-mafic volcanic rocks that provide evidence of the tectonic processes which operated during the evolution of the Paleo-Tethyan *** geochemical data for Early Carboniferous volcanics in the southern changning-menglian belt show that wehrlites have cumulate and poikilitic textures,and contain low-Fo(84.2-87.2)olivine,clinopyroxene with low Mg#values(79.4-85.6),and spinel with high Cr#values(67.8-72.4).Estimated equilibrium temperatures obtained using olivine-spinel Fe-Mg exchange geothermometry range from 978℃ to 1373℃(mean=1205℃;n=3).These observations combined with a lack of reaction or melt impregnation textures indicate that these units represent magmatic cumulates rather than having formed as a result of mantle-melt *** wehrlites and basalts in the belt have subparallel rare earth element(REE)-and primitive-mantle-normalized multi-element patterns with slightly positive Nb-Ta anomalies,but negligible Eu and Zr-Hf *** volcanics have similar Sr-Nd-Pb isotopic compositions withεNd(t)values of 4.2-4.5(mean=4.3;n=3)and 4.0-4.4(mean 4.2;n=4),respectively,and also have similar immobile element ratios,such as Nb/La,Nb/U,Th/La,Zr/Nb,Th/Ta,La/Yb,Nb/Th,Nb/Y,and Zr/*** characteristics indicate both units have ocean island basalt(OIB)-like geochemical affinities,consistent with the fact that the clinopyroxene in the wehrlites is compositionally similar to OIB-related cumulus *** suggests that both the wehrlites and basalts were derived from similar parental magmas that underwent generally closed-system magmatic differentiation dominated by fractionation of the olivine and *** parental magma was likely generated in an oceanic seamount setting from an OIB-type mantle source(i.e.,asthenospheric mantle)contain

关键词： geochemistry petrogenesis wehrlite OIB changning-menglian belt Paleo-Tethys Tournaisian Manxin Tibetan plateau

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Devonian to Triassic Successions of the changning-menglian belt,Western Yunnan,China

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Acta Geologica Sinica(English Edition) 2003年第4期77卷 440-456页

作者： JINXiaochi WANGYizhao XIEGuanglian InstituteofGeology ChineseAcademyofGeologicalSciences26BaiwanzhuangRoadBeijing100037 GeologicalSurveyofYunnan 44YujiangRoadYuxiYunnan653100

Phanerozoic strata are distributed in several north-south trending zones in the central part of the changning-menglian belt. Four types of Devonian to Triassic stratigraphic successions can be identified: (1) elastics with limestone lenses in the mid-section, changing up-section into alternations of fine elastics and cherts; (2) elastics with chert intercalations and limestone lenses, and topped by Permian basic volcanics; (3) elastics-basic volcanics-carbonates-clastics; and (4) limestones, dolomitic limestones-dark gray thin-bedded limestones, argillaceous limestones, mudstones and siliceous mudstones. Devonian to Triassic cherts occur in different horizons and different zones from east to west. These cherts are usually transitional to their neighboring elastics. There is no continuous Devonian to Middle Triassic chert sequence in the central zone of the changning-menglian belt as Liu et al. (1991,1993) reported. Volcanics and the overlying carbonates described by some workers as 'seamount' sequences are more likely to have formed in a marine environment on continental crust. Succession (4) is newly recognized in the area from Menglai to Yong'an in Cangyuan County and further north to Padi of Gengma County. Basalts, cherts and elastics also appear in this area. Mid-Triassic (very probably Ladinian) radiolarians extracted from bedded cherts in the Ganzhejidi section indicate that they are in higher stratigraphic positions. The change from bedded cherts via siltstones to thick-bedded sandstones with thin-bedded fine intercalations in the Ganzhejidi section and (some other outcrops along the road from Cangyuan to Gengma) suggests a fundamental change of sedimentary environment caused by a rapid increase of a large quantity of detritus supply. These siliciclastic sediments are possibly syn-orogenic deposits.

关键词： stratigraphy Devonian Carboniferous Permian Triassic changning-menglian belt western Yunnan

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