Genesis of giant plagioclase phenocrysts in Cenozoic basalts from Songhuajiang River,Northeast China:Implications for the magma plumbing system of the shield-forming stage beneath Changbaishan-Tianchi Volcano
The texture and composition of plagioclase in igneous rocks can record information of magmatic evolution during the crystal growth,since it is sensitive to documenting the history of magmatic processes in magma plumbing systems.The Cenozoic basalts in Northeast China generally develop olivine and clinopyroxene phenocrysts,but plagioclase phenocrysts are rare.However,plagioclase phenocrysts are common in the shield-forming basalts(5～1Ma)of Changbaishan-Tianchi Volcano.Recently,we report a new set of Pliocene basalts on both sides of the Songhuajiang River which contain giant plagioclase phenocrysts(1～2cm in diameter).These plagioclase-phyric basalts have high Ba/Th and K/U ratios,and show EM1-type Sr-Nd-Pb isotopic composition,which is similar to those of the shield-forming basalts of Changbaishan-Tianchi Volcano.The plagioclase-phyric basalts in Songhuajiang River are characterized by low magnesium(MgO=3.86％～4.23％)and high titanium(TiO2=3.15％～4.25％)contents,indicating that the magma has undergone a high degree of evolution,which is consistent with the plagioclase-phyric basalt in the shield-forming stage of Changbaishan-Tianchi Volcano.Their temporal,spatial and geochemical relationship suggests that the plagioclase-phyric basalt of Songhuajiang River is the product of the shield-forming magmatism of Changbaishan-Tianchi Volcano,which had flowed northwestward for at least 150 kilometers along the ancient Songhuajiang River.Backscattered electron images show that most of the plagioclase phenocrysts in plagioclase-phyric basalts of Songhuajiang River have a core-rim texture,some have no zoning texture,and a few have sieve texture.The chemical composition of plagioclase phenocrysts varies greatly,in which the core-mantle area is mainly labradorite(An48-65),and the rim is albite-labradorite(An4-61).Besides,the chemical composition of the matrix plagioclase is also heterogeneous,the core-mantle component is composed of andesine-labradorite(An41-53),and the rim belongs to albite-andesine(An4-46).The core and mantle of most plagioclase phenocrysts have a fine oscillation zone,their An values only show a small amplitude oscillation,suggesting that the physical and chemical conditions of the plagioclase phenocrysts were relatively stable during the early growth process.Therefore,the magma chamber(s)should have been continuously fed by homologous magma,and lasted for a long time,which are conducive to the growth and formation of giant plagioclase phenocrysts.In the process of magmatic convection,the crystals can migrate to the higher temperature areas,forming the resorption structures.The sudden drop of An value at the rim of plagioclase phenocrysts and the development of resorption structure indicate that the physical and chemical conditions have changed during the late stage of growth of plagioclase phenocrysts,corresponding to the decompression process of rapid magma ascent.Therefore,the giant plagioclase phenocryst in plagioclase-phyric basalts of Songhuajiang River records that the shield-forming basalts in Changbaishan-Tianchi Volcano have experienced a long-term of residence process in the magma chamber(s)before eruption.
Plagioclase-phyric basaltsPlagioclase phenocrystsMagma plumbing systemCenozoic basaltsNortheast China
万方数据
维普
