辽宁北镇石墨矿含矿岩石地球化学及SHRIMP锆石U-Pb年代学研究

辽宁北镇杜屯石墨矿矿体赋存于中元古界高于庄组,通过对矿区含矿岩石地球化学特征及碎屑锆石的SHRIMP U-Pb年代学开展系统研究,探讨了石墨矿床的形成时代及构造环境。地球化学特征表明,含矿岩石稀土元素含量较高,并富集Rb、Ba、K、Sr等大离子亲石元素,反映出原岩(沉积物质)主要来源于陆源碎屑物质,并有少量海源物质混入,为缺氧沉积环境。锆石SHRIMP U-Pb测年结果显示:碎屑锆石表面平均年龄为(2 535.64±25.59) Ma,代表含矿层原岩沉积上限晚于早元古代早期;重结晶锆石表面年龄为(1 216.75±12.33)～(1 675.70±17.46) Ma,代表含矿层的沉积下限不晚于中元古代晚期;自生锆石表面平均年龄为(212.53±3.69) Ma,显示晚三叠世岩浆-构造热事件对晶质石墨的重结晶、富集成矿起到重要的控矿作用。因此杜屯石墨矿床成因归结为中元古代浅海环境沉积富含有机质的黑色岩系,在后期变质作用和晚三叠世岩浆活动影响下,黑色岩系碳质重结晶形成浅变质石墨矿床。

Geochemistry and SHRIMP Zircon U-Pb Chronology of Ore-bearing Rocks from Graphite Deposit in Beizhen County,Liaoning Province

Dutun graphite deposit in Beizhen County of Liaoning Province occurs in the Middle Proterozoic Gaoyuzhuang Formation.This study carried out systematic analyses on geochemical characteristics of ore-bearing rocks and SHRIMP U-Pb dating for detrital zircons to understand the forming age and tectonic setting of graphite deposit in this area.The geochemical results show that the ore-bearing rocks are characterized by high contents of REEs and enrichment of LILE(such as Rb,Ba,K and Sr),suggesting that the source rocks derived mainly from terrigenous clastic materials mixed with minor amount of marine materials and deposited in an anoxic environment.SHRIMP U-Pb dating yields an average surface age of(2535.64±25.59)Ma for detrital zircons,indicating that upper limit of deposition of ore-bearing rocks was later than the Early Proterozoic;the surface ages of(1216.75±12.33)to(1675.70±17.46)Ma for recrystallized zircons,representing that low limit of deposition of the ore-bearing sequence was not later than the late Mesoproterozoic;and an average surface age of(212.53±3.69)Ma for authigenic zircons,revealing that Late Triassic magmatic-tectonic thermal events played an important ore-controlling role in recrystallization,enrichment and mineralization of crystalline graphite.Therefore,it can be concluded that black organic-rich rock series deposited in the Mesoproterozoic neritic environment was transformed into epimetamorphic graphite deposit through carbonaceous recrystallization,driven by late-stage metamorphism and late Triassic magmatic activity.