西藏过铝花岗岩矿物化学特征及其岩石学意义

本文系统地讨论了西藏过铝花岗岩中斜长石、钾长石、黑云母、白云母和电气石的矿物化学特征。结果表明:岩石中的斜长石均以Ab为主,占78.73%~100%,An分子为0.13%~18.93%,而Or分子仅为0.19%~3.74%;在长石的成分分类图解中多分布于更长石区内,少量分布于钠长石区。钾长石的端元组分中Or含量最高,为81%~98.48%,Ab为1.52%~19.00%,基本不含An分子,在成分分类图解中集中分布在Ab—Or线的正长石区内。斜长石中Ab>An>Or,主要属更长石,极少量为中长石;钾长石具有高Or含量,显示Or> > Ab >> An,属钾透长石。白云母在以22个氧原子计算的分子式中,Si与Al的阳离子数均较高,其中Si为6.0575~6.6412,属多硅白云母;黑云母中Si阳离子数绝大多数小于6,值介于5.3227~6.1265之间,平均为5.5718;Al离子数为1.8735~2.6464,四面体配位为Si、Al^Ⅳ所占据,八面体配位的阳离子中Al^Ⅵ为0.2101~1.7388,大离子位中以K为主。过铝花岗岩中黑云母的Mg(Mg+Fe)比全岩的Mg(Mg+Fe)比值略高,但二者具有正相关关系,说明了其成因仍以岩浆成因为主。在云母类矿物的成因图解显示,研究区内大多数黑云母均位于C区,属壳源。

Mineral chemistry and petrography of the peraluminous granites in Xizang

The present paper deals,in detail,with the mineral chemistry of plagioclase,K-feldspar,biotite,muscovite and tourmaline from the peraluminous granites in Xizang.The results show that plagioclase in various rocks contains 78.73%-100%of Ab,0.13%-18.93% of An,and only 0.19%-3.74% of Or. In the classification diagram of feldspar,the projection plots of plagioclase mostly fall into the field of oligoclase,and rarely into the field of albite. In the end-members of K-feldspar,Or reaches up to 81%-98.48%,and Ab to 1.52%-19.00% in the absence of An. In the same diagram,the projection plots of K-feldspar are concentrated to the field of orthoclase along the Ab-Or line. Plagioclase contains Ab > An > Or,suggesting a kind of oligoclase,with rare andesine.K-feldspar is higher in Or content,and assigned to K-sanidine showing Or >> Ab >> An. In the formula of muscovite,the cation numbers of Si and Al are generally higher,with Si cation numbers of 6.0575-6.6412,suggesting a kind of lepidomorphite. In the formula of biotite,the cation numbers of Si are mostly less than 6,ranging between 5.3227 and 6.1265,and with an average of 5.5718.The cation numbers of Al range between 1.8735 and 2.6464.The tetrahedral coordination is occupied by Si and AlⅣ,the latter of which range between 0.2101 and 1.7388 in the octahedral coordination in which K is dominated in the macro-ionic sites. The Mg (Mg + Fe) ratios are slightly higher in biotite from the peraluminous granites than those in the whole rocks. The positive correlation between them indicates that the peraluminous granites are of magmatic origin. In the genetic diagram of mica minerals,biotite in the study area mostly falls into the field C,indicating a crustal origin.