阿尔泰可可托海3号伟晶岩脉中铌铁矿族矿物环带构造及其岩石学意义

The Complex Zonation of Columbite-group Minerals from the Koktokay No. 3 Granitic Pegmatite Dyke, Altay, NW China and Its Petrological Implications

本文利用电子探针和背散射电子成像技术系统地研究了新疆阿尔泰可可托海3号伟晶岩脉各结构带中的铝铁矿族矿物。研究表明,本区伟晶岩脉中铌铁矿族矿物主要为铌锰矿-钽锰矿。成分特征显示从岩脉边部带到核部,Ta/(Nb+Ta)比值呈近垂向演化,但Mn/(Fe+Mn)比值没有明显变化。另一方面,铌铁矿族矿物在早期正岩浆阶段(Ⅰ-Ⅳ带)的成分变化非常局限,而在晚期的岩浆-热液过渡阶段(Ⅴ-Ⅶ带)的成分变化非常大。背散射电子成像技术观察还表明本区铌铁矿族矿物在正岩浆阶段没有环带构造特征,而在岩浆-热液过渡阶段有明显的环带构造特征,而且随着伟晶岩固结过程的进行,环带构造主要沿着渐变环带构造→韵律环带构造→交代构造的趋势变化。研究认为,本区铌铁矿族矿物在不同结构带的成分变化和在单颗粒中的环带构造是热液流体参与铌铁矿族矿物结晶的反应,环带构造的类型主要是由热液流体参与结晶的程度所控制。

The columbite-group minerals from the Koktokay No. 3 granitic pegmatite, Altay, NW China were studied in the paper by using the electron microprobe analysis and back-scattered electron imaging. The results indicate that the columbite-group minerals in this district are mainly Mn-enriched, which are mainly manganocolumbite and manganotantalite. From the border zone to the core of the pegmatite, the ratio of Ta/ (Nb+Ta) varies almost vertically with nearly constant ratio of Mn/(Fe+Mn). The chemical compositions of the columbite-group minerals from the earlier magmatic stage vary limitedly > while those from the later magma-hydrothermal transitional stage vary over a wide range. The back-scattered electron images show that the compositional zonation is not clear in columbite-group minerals from the magmatic stage (zones Ⅰ ~Ⅳ), while those from the magma-hydrothermal transitional stage (zones Ⅴ ~ Ⅶ ) exhibit a complex compositional zonation. Indeed, the zonation patterns vary from the progressive, via the oscillatory to the replacement structure during the consolidation of the granitic pegmatite. The study indicates that the composition variations of the columbite-group minerals from different zones of pegmatite and their compositional zonation are due to hydrothermalism during the crystallization of columbite-group minerals, and the zonation patterns are mainly controlled by the degree of hydrothermalism.