湖北竹溪蔓荆沟碱性正长岩地球化学特征及其构造环境分析

Geochemical characteristics and tectonic setting of the alkaline synthesis in Zhuxi,Hubei Province

在南秦岭造山带中发现的竹溪蔓荆沟碱性正长岩,呈透镜状侵入于辉绿岩体内,个别呈独立岩体侵入于早古生代地层中。单个正长岩体规模较小。通过对其进行岩相学及岩石地球化学特征研究,表明其为全碱(Na_2O^+K_2O)含量较高的钾质碱性正长岩;岩石中Mg~#值较低,明显低于原始岩浆Mg~#值,说明其为原始岩浆经历较高程度演化的产物;岩体中稀土元素总量ΣREE较高,轻稀土元素LREE富集明显,重稀土元素HREE亏损;通过微量元素构造环境判别分析认为,研究区正长岩形成于大陆裂谷环境,且形成于大陆裂谷早期阶段,其形成时代为早志留世。

The alkali feldspar syenite in Zhuxi Manjinggou, which is located in the south Qinling orogenic belt, intruded into the disbase body in small lenticular form. Locally, it intruded into Early Paleozoic strata. A study of its petrological characteristics and geochemical characteristics reveals the followings characteristics：the alkali feldspar syenite in this area is potassic alkaline syenite with high total alkali （Na2O＋K2O）; the Mg# in the rock is obviously lower than that in the original magma, which indicates that the magma experienced a high degree evolution; the REE in the body is high, and exhibits LREE enrichment and HREE depletion. The trace element diagrams show that the syenite in this area originated from early continental rift environment in the Early Silurian.