胶北荆山群南墅石墨矿形成时代及成因分析OA

The Jiao-Liao-Ji Belt is the most representative Paleoproterozoic orogenic belt in the North China Craton.It not only hosts massive Paleoproterozoic continental crustal sediments but also contains abundant Paleoproterozoic graphite deposits,making it an ideal natural laboratory for investigating the genesis of Early Precambrian graphite and early Earth's climatic conditions.We conducted detailed petrographic,geochemical,Raman spectroscopic,carbon isotopic,and zircon geochronological studies on the Nanshu graphite deposit in the Jiaobei area,southern Jiao-Liao-Ji Orogenic Belt.The results show that the ore-bearing rocks in the Nanshu deposit are primarily graphite-bearing diopside quartz gneiss,graphite-bearing garnet felsic gneiss,and graphite-bearing two-pyroxene granulite.Their carbon isotopic compositions(δ13CVPDB)range from-13.42‰ to-10.17‰,-21.00‰ to-17.28‰,and-12.99‰ to-12.65‰,respectively.Graphite occurs as flaky or granular crystals uniformly distributed within these rocks.Geochemical characteristics indicate that the protoliths of the graphite-bearing rocks are mainly sand-stone and shale that did not undergo long-distance transportation,and are products of an active continental margin.Raman carbon thermometry yields peak metamorphic temperatures of 693～796℃,corresponding to the high-am-phibolite to granulite facies.We propose that the carbon source of the Nanshu graphite deposit was primarily organic matter mixed with minor inorganic carbon.Decarbonation(CH4)during burial metamorphosis contributed signifi-cantly to the positive shift in graphite carbon isotopes.Additionally,in the protolith of diopside-quartz gneiss,inor-ganic carbon released through the metamorphic reaction CaMg(CO3)2+SiO2=CaMgSi2O6+2 CO2,also contributed to the enrichment of heavy carbon isotopes.Zircon U-Pb dating of graphite-bearing diopside quartz gneiss yielded detrital zircon ages ranging from 3.617 to 2.054 Ga,with a dominant peak at 2.46 Ga,and metamorphic zircon ages of 1.82 Ga.Integrating previous data from the Jingshan Group,we constrain the depositional age of the depos-it to 2.1～1.9 Ga and the metamorphic-mineralization age to 1.82 Ga.We conclude that during 2.1～1.9 Ga,substantial organic matter was rapidly deposited together with terrigenous clastics in a back-arc basin along an active continental margin.This organic material was subsequently transformed into graphite and concentrated into econom-ic deposits during graphitization associated with the arc-continent collision orogeny.