The geochemical differentiation of S-type pegmatites: constraints from major-trace element and Li-B isotopic composition of muscovite and tourmaline

Pegmatites often exhibit regional as well as internal zonation. In this study, we use major and trace element and Li-B isotopic composition of muscovite and tourmaline from internally zoned and un-zoned S-type pegmatites and their host granites to characterize the geochemical and isotopic fractionation associated with their formation. The internally zoned pegmatites comprise three distinct textural zones, namely wall, intermediate and core. Muscovite and tourmaline occur in all the zones of these pegmatites. The trace element concentrations/ratios of muscovite from the three zones form well-defined differentiation trends marked by enrichment of incompatible elements such as Rb, Cs, Sr, B, Zn, Nb, Ta, P and the depletion of Ni, Co, V, Sc, Ti, Ba from the wall zone, through the intermediate zone to the core zone. This is suggestive of a strong role of fractional crystallization in producing the compositional diversity in the internally zoned pegmatites. Alkali element ratios such K/Rb and K/Cs in muscovite exhibit near exponential decline from the wall to the core zone which is suggestive of Rayleigh-type fractional crystallization. Fractional crystallization modelling reveals that the formation of the wall zone requires < 69% crystallization, the intermediate zone 85-95% and the core zone ca. 99% crystallization, leading to extreme enrichment of Rb and Cs and other incompatible elements. Muscovites and tourmaline from the un-zoned pegmatites display similar compositional trends as the internally zoned ones, but with a significant compositional gap between the host granite and the pegmatite. Lithium isotopic composition of muscovites and B-isotopic composition of tourmalines become progressively lighter from wall to core zone of the internally zoned pegmatite and from the granite to the pegmatite for the un-zoned pegmatites. This is suggestive of an important role of vapour exsolution in the formation of the pegmatites. Taken together, the geochemical and isotopic trends in the pegmatites can be explained by Rayleigh fractional crystallization operating in tandem with vapour exsolution.