闽中地区马面山群东岩组变质岩形成的古构造环境研究

Paleotectonic setting of the Dongyan Formation of the Mamianshan Group in Central Fujian Province,south-east China.

闽中地区马面山群东岩组地层主要为绿片岩为主的一套古火山沉积建造。其主要岩性类型包括各种成分的绿片岩、大理岩、石英片岩及变粒岩类。绿片岩显示海底火山喷发特征,变粒岩原岩为中酸性岩类。东岩组变质岩岩石化学研究表明,绿片岩的原岩应为玄武岩类。变粒岩类主要属于英安岩及流纹岩。这些特征反映东岩组具双峰式火山岩特征,形成于大陆内部张性环境。绿片岩稀土元素特征也显示和大陆拉张环境中的火山岩类稀土特征非常相似,属大陆拉斑玄武岩;微量元素分布显示出该组变质岩原岩类似于大洋岛和大陆裂谷的板内碱性玄武岩。因此闽中地区中元古代可能处于板内古裂谷环境。

The Dongyan Formation of the Mesoproterozoic Mamianshan Group, Central Fujian Province, comprises mainly greenschist, marble, quartz schist and felsic rocks. Results of geochemical analyses, indicate that most of the metamorphosed rocks of the Dongyan Formation are derived from mafic and felsic volcanic rocks and less commonly from sedimentary rocks. The greenschist rocks are typical of the middle and upper Dongyan Formation, and include albite-actinolite schist and epidote-albite-actinolite-chlorite schist. These rocks show features suggesting that they are metamorphosed submarine basaltic lavas. Geochemical data suggest that the protolith of the greenschist rocks was probably of tholeiitic composition and that the felsic rocks were derived from dacite and rhyolite volcanic rocks. REE patterns and trace element data of the mafic metavolcanic rocks (greenschist) support the idea that they are continental tholeiites. In addition, the overall bimodal character (tholeiite, calc-alkaline basalt, alkaline basalt, dacite and rhyolite) of the metamorphic rocks of the Dongyan Formation indicates that these volcanics were erupted in a continental rift setting.