Study On Geochemical Characteristics Of Volcanic Rocks From Dongjun Region In Ergun

Dongjun region in Ergun is located in middle-northern Great Xing’an Range which is characterized by the widespread Mesozoic volcanic rocks, but some important basic geological problems such as Mesozoic volcanic rocks’ chronology and lithogenesis, tectonic background of nonferrous metallic ore deposit, are still serious controversial. The existing research indicates that Mesozoic volcanic association and geochemical characteristic are complicated. By the research of petrography and petrogeochemistry in the study-area, this article determines volcanic petrographic and petrogeochemical characteristic, source property and evolution character, and then discusses tectonic background and dynamic mechanism, while the relationship between volcanic rocks and mineralization.It has been found that volcanic facies are very complete and that lithology is complicated by the petrographic study. In the study-area, volcanic rock types include intermediate-basic volcanic lava for effusive facies, pyroclastic rock for explosive facies, volcanic sedimentary rock for volcanogenic sedimentary facies, granite porphyry for subvolcanic facies. Petrogeochemical characteristic suggests volcanic rocks of the area are high in potassium calc-alkaline series with high total REE content which reveals enrichment of light rare earth and deficit of heavy one, and have a obvious fractionation between LREE and HREE, but no Eu negative abnormality. The trace elements are enriched in large-ion lithophile elements, but deficit of high field strength elements such as Nb, Ta and so on. Overall, the volcanic rocks of study-area have similar characteristic with the ones from subduction zone. Source of magma is the metasomatic enriched mantle influenced by oceanic crust subduction. During evolution, magma should have undergone fractional crystallization of olivine, garnet, picotite, but without contamination of crust materials. The volcanic rocks should be formed from tectonic background of the active continental margin, and belong to compressional tonic setting which comes from the closure effect of Mongolian-Okhotsk. The relationship between volcanic rocks and mineralization is very close. Rhyolitic pyroclastic rocks aren’t only geochemical barrier, but also channel of hydrothermal fluid migrating, and may provide part of ore-forming materials. Intermediate-basic volcanic lava as surrounding with active rock chemical character react with hydrotherm by metasomatism leading to bring-in of potassium and taking out of sodium that change the component of hydrotherm, which promote precipitation action of metallogenic material.