松花江新生代斜斑玄武岩中斜长石巨斑晶的成因及其对长白山天池造盾期岩浆储运系统的制约  

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

作　　者：张叶凡 陈立辉 王小均 刘建强 曾罡[2] 周中彪 ZHANG YeFan;CHEN LiHui;WANG XiaoJun;LIU JianQiang;ZENG Gang;ZHOU ZhongBiao(State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi an 710069,China;State Key Laboratory for Mineral Deposits Research,School of Earth Sciences and Engineering,Nanjing University,Nanjing 210023,China)

机构地区：[1]西北大学地质学系,大陆动力学国家重点实验室,西安710069 [2]南京大学地球科学与工程学院,内生金属矿床成矿机制研究国家重点实验室,南京210023

出　　处：《岩石学报》2024年第7期2019-2036,共18页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(42130310)资助.

摘　　要：岩浆岩中斜长石的结构和成分可以记录其晶体的生长过程中岩浆演化的丰富信息,从而可以灵敏反映岩浆储运系统的历史。东北地区新生代玄武岩一般多发育橄榄石和单斜辉石斑晶,斜长石斑晶少见,但在长白山天池造盾期(5~1Ma)玄武岩中常见有斜长石斑晶。最近我们在松花江两岸新发现一套上新世含斜长石巨斑晶(粒径为1~2cm)的玄武岩。这些沿松花江分布的斜斑玄武岩具有高Ba/Th比和K/U比的微量元素特征、EM1型的Sr-Nd-Pb同位素组成,与长白山天池造盾期玄武岩一致。它们在主量元素上具有低镁(MgO=3.86%~4.23%)、高钛(TiO_(2)=3.15%~4.25%)的特征,说明其与长白山天池造盾期玄武岩中的斜斑玄武岩一样都经历了较高程度的演化。时间、空间以及地球化学特征上的紧密联系说明松花江斜斑玄武岩也是长白山天池造盾期岩浆作用的产物,即长白山天池造盾期玄武质岩浆当时流入了松花江古河道,向西北方向流动的距离达到了150km左右。背散射电子图像显示松花江斜斑玄武岩中斜长石巨斑晶多数具有核-边结构,部分无环带结构,少数具熔蚀结构。斜长石巨斑晶的化学成分变化大,其中核-幔部主要为拉长石(An_(48-65)),边部为钠长石-拉长石(An_(4-61))。基质斜长石的化学成分也不均一,核-幔部的组成为中长石-拉长石(An_(41-53)),边部为钠长石-中长石(An 4-46)。多数斜长石巨斑晶的核-幔部具有细密振荡环带,An值仅呈较小幅度振荡变化,暗示其前期生长过程中物理化学条件较为稳定,地壳岩浆房持续受到同源岩浆的补给作用,岩浆房持续的时间较长,有利于斜长石巨斑晶的形成。在岩浆对流过程中晶体可被运移至温度更高的区域,形成熔蚀结构。斜长石巨斑晶的边部An值突降以及熔蚀结构的发育,表明斜长石巨斑晶生长后期所处的物理化学条件发生了突变,对应于岩浆快速上升的减压过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(TiO_(2)=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(An_(48-65)),and the rim is albite-labradorite(An 4-61).Besides,the chemical composition of the matrix plagioclase is also heterogeneous,the core-mantle component is composed of andesine-labradorite(An_(41-53)),and the rim belongs to albite-andesine(An 4-46).The core and

关 键 词：斜斑玄武岩 斜长石巨斑晶 岩浆储运系统 新生代玄武岩 中国东北 

分 类 号：P578.968[天文地球—矿物学] P588.11[天文地球—地质学]