The origin of bedding-parallel fibrous calcite veins in the Lower Permian Chihsia Formation in western Hubei Province,South China  

作　　者：Hao Yu Xiqiang Zhou Jianguo Wang Chuan Guo Hengye Wei Daizhao Chen 

机构地区：[1]Key Laboratory of Orogenic Belts and Crustal Evolution,Ministry of Education, Peking University [2]Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences [3]College of Earth Science, East China Institute of Technology

出　　处：《Science Bulletin》2015年第20期1778-1786,共9页科学通报（英文版）

基　　金：supported by the National Natural Science Foundation of China(40839907,41302021 and41203030)

摘　　要：Bedding-parallel fibrous veins occurring as lenticular to flattened intercalations were found in the organic-rich marlstone/calcareous shale of the upper Lower Permian Chihsia Formation in western Hubei Province, South China. They dominantly consist of fibrous calcite crystals with smooth and tight boundaries, forming fence- like inward, syntaxial growth clusters toward the vein center along which a median suture line generally occurs. Petrographic evidence indicates that these veins may form at relatively shallow burial depth, where fluid overpres- sures would have incrementally created the bed-parallel vein space, resulting in displacive growth of fibrous calcite. On the other hand, the C, O and S isotopic data across the vein reveal slightly depleted δ13Ccarb values （-3.32 ‰ to ＋0.19‰ VPDB） and moderately depleted δSOcarb values （--9.6 ‰ to --7.3 ‰ VPDB） with respect to those of coeval seawaters and slightly heavier δ34Spyrite values （--7.88 ‰ CDT） with respect to those of ambient rocks. Stable isotope evidence consistently suggests significant contribution of bacterial sulfate reduction （BSR） to the formation of the fibrous calcite cements in the vein. The BSR could have been intensive with the availabilities of residual sulfate and abundant organic matters in the Chihsia sediments during shallow burial, increasing the alkalinity of pore waters and further promoting carbonate precipitation. Thus, the bedding-parallel fibrous calcite vein in the upper Lower Permian Chihsia Formation is an important time-specific petrographic capsule, providing clues for understanding the diagenetic process in organic- rich sediments.Bedding-parallel fibrous veins occurring as lenticular to flattened intercalations were found in the organic-rich marlstone/calcareous shale of the upper Lower Permian Chihsia Formation in western Hubei Province,South China. They dominantly consist of fibrous calcite crystals with smooth and tight boundaries, forming fencelike inward, syntaxial growth clusters toward the vein center along which a median suture line generally occurs.Petrographic evidence indicates that these veins may form at relatively shallow burial depth, where fluid overpressures would have incrementally created the bed-parallel vein space, resulting in displacive growth of fibrous calcite.On the other hand, the C, O and S isotopic data across the vein reveal slightly depleted d13Ccarbvalues(-3.32 % to +0.19 % VPDB) and moderately depleted d18Ocarbvalues(-9.6 % to-7.3 % VPDB) with respect to those of coeval seawaters and slightly heavier d34Spyritevalues(-7.88 % CDT) with respect to those of ambient rocks.Stable isotope evidence consistently suggests significant contribution of bacterial sulfate reduction(BSR) to the formation of the fibrous calcite cements in the vein. The BSR could have been intensive with the availabilities of residual sulfate and abundant organic matters in theChihsia sediments during shallow burial, increasing the alkalinity of pore waters and further promoting carbonate precipitation. Thus, the bedding-parallel fibrous calcite vein in the upper Lower Permian Chihsia Formation is an important time-specific petrographic capsule, providing clues for understanding the diagenetic process in organicrich sediments.

关 键 词：Fibrous calcite vein C-O-S isotope DIAGENESIS Chihsia Formation 

分 类 号：P534.46[天文地球—古生物学与地层学] P578.61[天文地球—地质学]