新太古代-古元古代东海岩群双湖石墨矿成因:来自有机碳同位素的证据

ORIGIN OF THE SHUANGHU GRAPHITE ORES IN THE LATE ARCHEAN-PALEOPROTEROZOIC DONGHAI GROUP: EVIDENCE FROM ORGANIC CARBON ISOTOPE

中国北方古元古代孔达岩系内含有丰富的石墨矿,其地质年龄主要集中在20-23亿年之间;片麻岩系中石墨的碳同位素值主要分布在-22.8‰到-21.5‰之间,被认为来源于古代有机质沉积后的变质作用;而少量石墨的碳同位素数据在-10‰到-5‰之间,接近热液来源的碳同位素,被认为与无机来源有关。苏鲁高压变质带中的石墨矿勘查过程中,发现了出露赋存在新太古代-古元古代东海岩群班庄组中的双湖石墨矿。此前中国科研人员对新太古代-古元古代石墨矿的研究,主要集中在华北板块。本次研究对华北板块与华南板块之间的苏鲁造山带的东海岩群班庄组中的碳酸盐岩以及石墨,分别进行了无机碳同位素与有机碳的地球化学研究。结果发现东海岩群班庄组石墨的有机碳同位素大致分布在-22‰到-32‰之间,吻合此前古元古代的有机碳同位素数据,表明该套石墨沉积为生物成因。

Based on previous studies, the graphite deposits are highly developed in the Khondalite series in North China, and they are mainly distributed in the Paleoproterozoic strata between 2 000 and 2 300 Ma. The average values of δ13Corg in gneiss graphite are mainly distributed between-22.8‰ and-21.5‰. It is believed to be derived from metamorphism after deposition of ancient organic matter;while the carbon isotope data of a small amount of graphite is between-10‰ and-5‰, which is close to the carbon isotope of hydrothermal source, and is considered to be related to inorganic sources. During the exploration of the graphite ore in the Sulu high-pressure metamorphic belt, the graphite deposits called Shuanghu graphite deposits in the Banzhuang formation of the Donghai group aged from Neoarchean to Paleoproterozoic was discovered. The previous studies by Chinese researchers regarding the Paleoproterozoic graphite deposits were mainly focused in the North China block.In this study, the geochemical studies of inorganic carbon isotopes and organic carbon were carried out for carbonate rocks and graphite in the Banzhuang Formation of the Donghai Group in the Sulu orogenic belt between the North China block and the South China block. It is found that the organic carbon isotope of graphite in the Banzhuang Formation of the Donghai Group is roughly distributed between-22‰ and-32‰, which is consistent with the previous Paleoproterozoic organic carbon isotope, indicating that this set of graphite deposits is biological in origin.