Compositional relations in Li-micas from S.W. England and France: an ion- and electron-microprobe study

Abstract

AbstractA combination of ion-microprobe (for Li) and electron-microprobe (for other major elements including F) methods has been used to analyse Li-rich micas from the S.W. England batholith (mainly the St Austell granite) and the Massif Central, France. Rocks showing various degrees of hydrothermal alteration were studied in order to separate the original compositional trends from alteration trends. The original compositional trend is essentially one of increasing Li with increasing degree of evolution. The main atomic substitution in the original micas is 3Li substituting for A1 and 2 vacancies in octahedral sites; substitution of Li for R2+ (Fe, Mn, Mg) in octahedral co-ordination is generally subordinate. Alteration trends involve a loss of Li, Fe, F, Rb and Cs, and a gain in A1. The effects of volatile elements on phase relations of granites are reviewed and it is concluded that the original Li-micas were primary, i.e. crystallized from the melt. It is suggested that the late-magmatic stage passed transitionally into the hydrothermal stage leading inevitably to subsolidus recrystallization (autometasomatism) of the primary minerals, so introducing further textural and mineralogical complexities to the rocks.