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《Journal of Stratigraphy》 2017-03
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THE QINGBAIKOUAN TUMEN FORMATION-COMPLEX ISLAND ARC VOLCANIC-CLASTIC ROCKS ON THE NORTHERN MARGIN OF YANGTZE BLOCK AND ITS SIGNIFICANCE

HU Zheng-xiang;CHEN Chao;MAO Xin-wu;YANG Qing-xiong;DENG Qian-zhong;KONG Ling-yao;YANG Cheng;Hubei Geological Survey;  
During the revision of new regional geology of Hubei Province,we first defined Tumen Formation-complex as a suite of volcanic rocks with a small amount of clastic rock,which were previously regarded as part of the Huashan Group.The Tumen Formation-complex is distributed in the Tumen-XiaobuYuantan area and represents late Qingbaikouan island arc volcanic-sedimentary rocks in the northern margin of Yangtze block.The type section is located along the Lijiawan-Tumen road in Tumen County.The Tumen Formation-complex mainly consists of basic basalt,with some intermediate andesite and felsic dacite,rhyolite,and minor amount of pyroclastic rock and siltstone.Affected by the tectonic activities,the Tumen Formation-complex is moderately deformed and slightly metamorphosed.SiO_2 composition of the volcanic rocks in the Tumen Formation-complex ranges from 50.47% to 81.57%,consisting of mafic,intermediate,and felsic end members.Most volcanic rocks show features of calc-alkalic sequence and a few of them show features of tholeiite sequence.Rare earth elements in these volcanic rocks are LREE-enriched,with a right-inclined pattern,some degree of negative Eu anomalies,enrichment in large ion lithophile elements(such as Rb,Ba,K,and Pb),and depletion in high field-strength element(such as Nb,Ta,and Ti).These geochemical features are characteristic of island arc volcanic rocks.LA-ICP-MS zircon U-Pb ages of felsic volcanic rocks of the Tumen Formation-complex at Tumen and Yuantan are 841 Ma and 827.9±5.9 Ma(MSWD=0.57)respectively,suggesting that the Tumen Formation-complex is Qingbaikouan in age.These island arc volcanic rocks may have been the products of oceanic-continental collision in the northern margin of the Yangtze block during the Jinning Orogeny.Together with 858-876 Ma island arc intermediate-felsic intrusive rocks and 871 Ma mafic gabbros at Sanligang,volcanic rocks in the Tumen Formation-complex indicate that oceanic-continental collision occurred in the Dahongshan region,northern margin of Yangtze block,around 876-828 Ma.
【Key Words】: lithostratigraphy chemostratigraphy zircon U-Pb dating geochemistry island arc volcanicclastic rocks Tumen Formation-complex Qingbaikou Period Yangtze Block
【Fund】: 湖北省地质局科技项目(KJ2015-1);; 中国地质调查局项目(12120113012800、DD20160030-06)资助
【CateGory Index】: P587
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