长白山天池火山造锥阶段玄武质火山活动期次划分及成因探讨

在天池火山造锥阶段,长白山火山区玄武质火山活动频繁。文章在野外调查的基础上,通过年代学及地球化学研究,对其活动期次进行划分,并探讨其岩浆来源与演化。天池火山造锥阶段的玄武质火山岩主要呈火山渣锥或小型河谷玄武岩形式分布,其形成可划分为两期:一期为老房子小山期,形成时限为0.87~0.54 Ma,属碱性岩石系列;另一期为老虎洞期,形成时限为0.34~0.1 Ma,属碱性岩石系列和拉斑岩石系列。地球化学特征显示,碱性系列玄武岩具高Al、Ti、K、P和低Mg特征,拉斑系列玄武岩具高Mg、富Fe、Ca和低Na特征;二者稀土和微量特征较为一致,稀土元素配分曲线呈右倾型,略显正铕异常,并富集Ba、K、Pb、P、Ti,亏损Th、U、Sr,但拉斑系列玄武岩的稀土元素和微量元素含量及轻重稀土分馏强度均低于碱性系列。天池火山造锥阶段形成的玄武质火山岩均来源于进化岩浆,具有同源特征,经历了一致或相似的演化过程,岩浆房赋存位置相当于上地幔—下地壳的过渡部位,结晶分异岩浆作用显著、地壳混染作用微弱,其成分变化受控于多期次结晶分异作用和早期结晶再循环的岩浆作用过程。

Stage division and genesis discussion of basaltic volcanism during the coneforming stage of Tianchi volcano in Changbaishan region

During the cone-forming stage of Tianchi volcano, activity of basaltic volcanism was frequent. In this paper, based on field geological survey and analysis of chronology and geochemistry, the authors determined active stages of the basaltic volcanism,and investigated magmatic source and evolution. The results show that the activity of the basaltic volcanism forming scoria cones or mini valley basalt can be divided into 2 stages:Laofangzixiaoshan period (about 0.87-0.54 Ma) belonging to alkaline basaltic volcanism and Laohudong period (about 0.34-0.1 Ma) belonging to alkaline and tholeiitic volcanism. Alkaline basalt is rich in Al, Ti, K and P, and poor in Mg, and tholeiite is rich in Mg, Fe and Ca, and poor in Na. The two types of basalt have similar characteristics in REEs, trace elements and Eu anomalies, exhibiting obvious right-declined REE patterns, slightly positive Eu anomalies, enrichment of Ba, K, Pb, P and Ti, and depletion in Th, U and Sr. Nevertheless, in contrast to alkaline basalt, content of REEs and trace elements, and fractional degree of HREE and LREE are lower in tholeiite. The basalt formed during the coneforming stage of Tianchi volcano was derived from evolutional magma, and was identical with the transitional position of uppermantle and lower-crust, which had characteristics of common fountain, and had the same or similar evolution process with significant crystallization differentiation and faint crustal contamination. Compositions of the evolutional magma were controlled by multi-stages crystallization differentiation and recirculation during early crystallization.