胶-辽-吉造山带辽河群石墨矿碳同位素特征及成因分析

作为华北克拉通周缘三条石墨成矿带之一的东部带,胶-辽-吉古元古代石墨成矿带是研究早前寒武纪石墨成矿机制及地球早期气候特征的天然实验室.本文对胶-辽-吉造山带辽河群石墨矿(甜水乡马沟石墨矿)进行了详细的岩相学、地球化学、拉曼光谱学以及碳同位素等方面的研究.研究结果表明辽河群含石墨矿岩石主要为含石墨变质杂砂岩、含石墨黑云母...

Carbon isotope and genesis studies of graphite deposits in the Liaohe Group of the Jiao-Liao-Ji Orogenic Belt

As the eastern part of the three graphite metallogenic belts around the North China Craton,the Jiao-Liao-Ji Proterozoic graphite metallogenic belt is a natural laboratory for studying the mechanism of Early Precambrian graphite mineralization and the characteristics of early Earth climate.In this study,we performed detailed petrological,geochemical,Raman spectroscopy,and carbon isotope studies for the graphite deposits in the Liaohe Group(Magou graphite deposits in Tianshui Country)of the Jiao-Liao-Ji Orogenic Belt.The results show that the graphite-bearing rocks in the Liaohe Group are mainly graphite-bearing meta-greywacke,graphitebearing biotite feldspar gneisses,and graphite-bearing tremolite marble.These graphite-bearing rocks are located in the sandstone and shale regions in the log(Fe₂O₃/K₂O)-log(SiO₂/Al₂O₃)diagram,and have similar rare earth elements characteristics with PostArchean Australian Average Shales(PAAS).Geochemical features show that they have low maturity,and are the products of rapid accumulation without a long-distance transportation,which indicate that they are mainly deposited in the back-arc basin of an active continental margin.The carbon isotope study shows that the carbon isotope values of the graphite deposits in the Liaohe Group have a wide range of variation(δ^13 C_PDB=-16.49‰~25.93‰),which is caused by the demethanation of organic matter during metamorphism.Combined with the Raman spectrum of the graphite deposits,we propose that the graphite deposits in the Liaohe Group are formed by the metamorphism of organic matter,and its metamorphism can reach high amphibolite facies(551~627℃).Based on the comprehensive analysis of the previous geochronology data,we suggest that the protolith of the graphite deposits were deposited at2.13~2.17 Ga,the organic matter trapped within these sediments gradually metamorphosed into graphite and agglomerated into graphite deposits during the later arc-continent collision and post-collision stages(2000~1895 Ma;1875~1850 Ma).The sedimentary age(2.3~2.05 Ga)of a large number of graphite deposits around the North China Craton coincides with the time of the Great Oxidation Event(GOE),which may be the sedimentary response of the GOE in the early Earth.