法国中央高原Beauvoir花岗岩中铌钽矿的带状构造:类型、化学成分和形成条件  

ZONING IN COLUMBITE CRYSTALS FROM THE BEAUVOIR GRANITE, MASSIF CENTRAL, FRANCE: TYPES, COMPOSITION AND CONSTRAINTS ON THEIR FORMATION

作　　者：王汝成[1] Monchoux Pierre Fontan Francois 

机构地区：[1]南京大学地球科学系,南京210008 [2]法国图鲁兹大学矿物学和结晶学实验室

出　　处：《矿物学报》1991年第3期225-233,T001,共10页Acta Mineralogica Sinica

摘　　要：运用扫描电子显微镜和电子探针,对法国中央高原的Beauvoir花岗岩中铌钽矿进行的研究表明,这些矿物具有多种类型的带状构造。 大部分铌钽矿具有同心带状构造和平行带状构造,前者的每一个带的宽度可在2—60μm之间变化,而后者则在1—20μm。这两种带状构造主要是由于Nb和Ta的含量的变化。它们亦可以统称为韵律式带状构造。 铌钽矿的裂隙状带状构造只见于花岗岩岩体的顶部。不规则状的带沿铌钽矿晶体的边缘或微裂隙发育,常形成树枝状构造。从化学成分上来看,这种带状构造主要是由于Fe、Mn含量的变化。因此,它亦可被称为交代构造。 铌钽矿的韵律式带状构造由Beauvoir花岗岩的复杂的结晶过程引起,主要的影响因素可能有:岩浆的化学成分和岩浆结晶的物理化学条件的周期性变化。而来自围岩(云母片岩)的富铁的热液影响了铌钽矿的结晶,导致了铌钽矿的交代构造的形成。Scanning electron microscope and electron microprobe analyses of the columbite crystals from the Beauvoir granite (Massif central, France)indicate that the Nb-Ta minerals show different zoning patterns.Most of the columbite crystals show concentric zoning and parallel zoning. The zones in the concentrically zoned crystals are usually 2-60μm in width, and those in parallel zoned crystals are 1-20μm. The phenomenon of zoning is chemically related to variations in Ta and Nb contents. Such zoning is also referred to as rhythmically zoned structure.Fissure-zoned crystals of columbite are only observed at the top of the granite pluton. This type of zoning is characterized by an irregular zone resulting from corrosion of early crystals on the rims or along the microfissures, often giving rise to an arborescent strueture. Chemically, fissural zoning is due to variations in Fe and Mn concentrations. Therefore, this kind of zoning is also referred to as replacement texture(or patchy zoning).Rhythmical zoning in columbite crystals is attributable to the complex crystallization history of the Beauvoir granite. The major factors controlling rhythmical zoning are considered to be the periodic variations in chemical composition of the granitic magma and of the physico-chemical conditions for magmatic crystallization. Extraneous fluids from the country rocks(mica schist) enriched in Fe would exert great influence on the crystallization of Nb-Ta minerals, hence leading, to the formation of patchy zoning as observed in the columbite crystals.

关 键 词：铌钽矿 花岗岩 带状构造 法国 

分 类 号：P578.47[天文地球—矿物学]