Journal Article
Carbon isotope and genesis studies of graphite deposits in the Liaohe Group of the Jiao-Liao-Ji Orogenic Belt
Zhu, JJ; Liu, FL; Liu, FX; Shi, C; Wang, F; Xu, WT
Yanshi Xuebao / Acta Petrologica Sinica
ISSN: 1000-0569
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, graphite bearing 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 ( delta(13) C-PDE = -16.49 parts per thousand similar to - 25.93 parts per thousand), 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 similar to 627 degrees C).Based on the comprehensive analysis of the previous geochronology data, we suggest that the protolith of the graphite deposits were deposited at 2.13 similar to 2.17Ga, 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 similar to 1895Ma; 1875 similar to 1850Ma). The sedimentary age (2.3 similar to 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.
Physical Sciences - Other Topics; Jiao-Liao-Ji belt, Liaohe Group, Graphite deposits, Carbon isotope,; Great oxidation event; north china craton, isua supracrustal belt, u-pb geochronology,; meta-mafic rocks, eastern block, atmospheric oxygen, khondalite-series,; metamorphic rocks, metasedimentary rocks, raman-spectroscopy