闽中马面山群变质火山岩微量元素地球化学特征及其构造意义

Trace Element Geochemistry and Tectonic Setting for Proterozoic Metavolcanics of the Mamianshan Group in the Central Part of Fujian Province

闽中地区元古宙马面山群变质火山岩沿政和 -大埔断裂带分布 ,根据其主要化学成分特征 ,应为一套“双峰式”火山岩。稀土元素地球化学特征表明 :本区绿片岩及钠长变粒岩LREE富集 ,Eu含量基本正常 ,微量元素MORB标准化蛛网图和微量元素构造判别图显示样品具大陆拉斑玄武岩特征 ,并呈大陆裂谷性质 ;斜长角闪岩没有明显的Eu负异常 ,MORB标准化蛛网图曲线前部呈小的隆起 ,Nb Ta亏损较为明显 ,其岩石特征具有过渡性的钙碱性玄武岩及T MORB的地球化学属性 ,原始地幔及Nb标准化蛛网图显示其可能来源于亏损地幔或有陆壳物质的混染。说明本区由大陆裂谷演化发展成为初始洋盆 ,有岩浆喷发 ,并可能发生板内俯冲。因此 ,这些变质火山岩应形成于裂谷—初始洋盆—板内俯冲的构造环境。

The Proterozoic metavolcanics of the Mamianshan Group in the central part of Fujian province are distributed along the Zhenghe—Dapu fault belt. Based on major element geochemical signatures, the volcanic rocks in this area are 'bi-modal'. The chondrite-normalized REE patterns of greenschists and leptynite are enriched in LREE with no or small Eu anomaly. Their MORB normalized spiderdiagrams and trace element tectonic discrimination diagrams have the WPB (within-plate basalt) features, showing continental rifting character. Plagioclastic amphibolites have no obvious Eu negative anomaly. Their MORB normalized spiderdiagrams have a Rb-Ba enrichment and a Nb-Ta depletion. These are typical of transitional calc-alkaline basalts and T-type MORB. Their primary mantle normalized and Nb-normalized spiderdiagrams indicate their derivation from a depleted mantle or a contamination by the continental crusts. It is considered that this area was a failed rift. There was an evolution from a contineutal rifting background to an initial oceanic basin environment. Soon after, convergence and subduction took place.