Petrogenesis of extremely lithium-rich spodumene aplites of the Tastyg deposit, Sangilen Highland, Tyva Republic

Abstract

1262 The magmatic origin of Li rich spodumene gran ites and pegmatites is confirmed by geological data, finds of melt inclusions in the minerals of these rocks, and, additionally, for homogenous veins, by the simi larity of their average bulk compositions to that of the Li saturated granitic eutectoid melt [1, 2]. Being alu minous leucocratic granitoids in composition, spo dumene pegmatites are typically characterized by sig nificant predominance of normative albite over ortho clase and, therefore, are called albite–spodumene pegmatites. The high lithium contents and “albite” evolution trend make them similar to Li–F rare metal granites. This resemblance is debatable [3], since the shift of the minimum of a granite melt toward the albite apex for lithium–fluorine granites is supposedly caused by the increase in size of the quartz crystalliza tion field under the influence of fluorine. However, this model is inappropriate for spodumene pegmatites, which differ in the extremely low contents of fluorine and often other volatile components (B, P, H2O). The study of spodumene aplites and pegmatites of the Tastyg deposit, which were formed under lowered activity of H2O, F, and B, revealed for the first time an unusual trend of their differentiation toward petalite composition, which resulted in extremely high lithium contents in these granitoids [4, 5].