摘要
斜斑玄武岩能较好地记录岩浆作用过程、动力学机制和构造环境信息。本文对华北克拉通东北缘长白山新生代斜斑玄武岩的基质和斜长石斑晶进行了元素地球化学和Sr同位素分析研究,以制约其岩浆作用过程和岩石成因机制。结果显示,长白山斜斑玄武岩是岩浆演化晚期的产物,具有较低的MgO、Cr、Ni,富集大离子亲石元素,亏损Nb、Ta,具轻微Ti正异常;基质的^(87)Sr/^(86)Sr比值较低且较均匀(0.705080~0.705248),Nb/U比值相对较高且几乎不变(44.62~49.65),表明补给岩浆未受到地壳物质的同化混染作用。斜长石斑晶普遍显示核-幔部振荡环带结构,其An变化幅度最高达8%,受岩浆房中温度和岩浆补给、混合作用影响。另外,斜长石斑晶在边缘很小范围内的An快速降低,斜长石斑晶与基质斜长石具有相似的An、TiO_(2)、MgO、TFeO范围,且斜长石斑晶边缘、基质斜长石微晶和玄武质岩浆均具有近似的结晶温度,表明它们都形成于很近地表的次火山过程。研究表明,长白山斜斑玄武岩位于板内拉斑玄武岩区域,其斜长石An值与大陆拉斑玄武岩相似,其形成环境应为板内拉张环境。
The plagioclase-phyric basalt might have preserved the detailed information of its magmatism processes,related geodynamic mechanism,and tectonic settings.In this paper,we present the details of elemental and Sr isotopic compositions of zoning plagioclase phenocrysts and matrixes of the Cenozoic plagioclase-phyric basalt samples from the Changbaishan area in the Northeastern margin of the North China Craton,in order to constrain the magmatism processes and petrogenetic mechanism of this Cenozoic mafic volcanic rock.The results show that the plagioclase-phyric basalt in the Changbaishan area was the product of evolved basaltic magma at late stage,with relatively low contents of MgO,Cr,and Ni,enriched large ion lithophile elements,depleted Nb-Ta,and slightly positive Ti anomaly.Its matrixes are characterized with relatively low and uniform^(87)Sr/^(86)Sr values(0.705080-0.705248),and relatively high and almost constant Nb/U ratios(44.62-49.65),indicating that its supplementary magma had not been assimilated and contaminated by the crustal materials.Plagioclase phenocrysts generally have a core-mantle oscillation zoned structure,with the maximum variation of An values of 8 mol%,which could be caused by the temperature variation,and magma recharge and mixing in the magma chamber.In addition,the rapid decrease of An values of micron-sized various zones in the margins of plagioclase phenocrysts from inner to outer zones,the similarities in An values and TiO_(2),MgO,TFeO contents of the outer margins of plagioclase phenocrysts and the matrix plagioclase microcrystals,and the closely similar crystallization temperatures of the margins of plagioclase phenocrysts,matrix plagioclase microcrystals,and the basaltic magma indicate that they were formed in a subvolcanic process extremely near the surface.It is believed that the plagioclase-phyric basalt in the Changbaishan area,whose samples composition are plotted in the field of intraplate tholeiitic basalt and whose matrix plagioclase microcrystals have similar An values to those of the continental tholeiitic basalt,should have been formed in the intraplate extensional environment.
作者
陈琦
于宋月
CHEN Qi;YU Song-yue(State Key Laboratory of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550081,China;University of Chinese Academy of Sciences,Beijing 100049,China)
出处
《矿物岩石地球化学通报》
CAS
CSCD
北大核心
2022年第2期332-347,共16页
Bulletin of Mineralogy, Petrology and Geochemistry
基金
南方海洋科学与工程广东省实验室重大专项团队项目(GML2019ZD0202)
国家自然科学基金资助项目(41573009)。
关键词
斜斑玄武岩
振荡环带
岩浆作用
新生代
长白山
华北
plagioclase-phyric basalt
oscillation zone
magmatism
Cenozoic
Changbaishan
North China
