南江坪河石墨矿变质岩原岩恢复及古沉积环境探讨

Protolith Recovery and Discussion on Paleosedimentary Environment of Pinghe Graphite Metamorphic Rocks in Nanjiang

南江坪河石墨矿是四川省典型的含钒晶质石墨矿,在对坪河石墨矿变质岩岩相学和岩石地球化学特征研究的基础上,分析了变质岩的原岩类型及其古沉积环境。变质岩样品的ωSiO 2/ωAl 2O 3,ωK 2O/ωNa 2O,ωEu/ωSm,ωRb/ωSr,ωSr/ωBa值均表明原岩物质来源为陆源碎屑沉积;(ωLa/ωYb)N,δEu,δCe均值分别为7.67,0.75,0.81,反映轻、重稀土分馏程度较高并呈轻微Eu,Ce负异常的特征。原岩恢复相关判别图解显示其原岩是一套以含炭质黏土细-粉砂岩为主,夹少量泥质砂岩、基性岩的岩石组合,具有明显的前寒武纪陆源碎屑河流与边缘海沉积的特点。古水体氧化-还原环境微量元素判别指标、ωBa-ωSr图解和不同气候带黏土岩的成分图解均揭示其形成于干燥气候带条件下缺氧静水低能的泻湖环境,古水体介质主要为陆相淡水。

Pinghe graphite mine is a typical vanadium-bearing crystalline graphite mine in Nanjiang county,Sichuan province.It is of great significance to study the genesis of graphite mine to restore the original types of metamorphic rocks and discuss the paleosedimentary environment.Based on the study of metamorphic petrography and petrogeochemistry of Pinghe graphite deposit,the original rock types and paleosedimentary environment of metamorphic rocks were analyzed.TheωSiO 2/ωAl 2O 3,ωK 2O/ωNa 2O,ωEu/ωSm,ωRb/ωSr,ωSr/ωBa values of the metamorphic rock samples all indicate that the source material of the original rock is terrigenous clastic deposition.The mean values of(ωLa/ωYb)N,δEu andδCe were 7.67,0.75 and 0.81,respectively,these results indicated that light and heavy rare earth fractionation degree was relatively high and presented slight negative anomaly characteristics of Eu and Ce.The correlation discriminant diagram of the original rock recovery shows that the original rock is a set of rock assemblage consisting mainly of carbonaceous clayey fine siltstone with a small amount of argillaceous sandstone and basic rock,it has obvious characteristics of precambrian terrigenous clastic river and marginal sea sediments.The determination index of trace elements in the oxidation-reduction environment of ancient water body,theωBa-ωSr diagram and the composition diagram of clay rocks in different climate zones all reveal that it were formed in the anoxic hydrostatic and low-energy lagoon environment in the condition of dry climate zone,and the ancient water body medium was mainly terrestrial fresh water.