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 Al^Ⅳ, 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.