Mica-liquid trace elements partitioning and the granite-pegmatite connection: The St-Sylvestre complex (Western French Massif Central)

Abstract

We constrain the genetic relation between granite and pegmatite parental melts from the Variscan Saint Sylvestre leucogranite complex and associated pegmatite bodies (Massif Central, France) through compositions of micas. Using mica trace element concentrations and available partition coefficients for Li, Rb, Ba, Cs and F, we calculated the trace element contents of granitic and pegmatitic melts at equilibrium with micas. Biotite in leucogranites and pegmatites ranges mostly from Fe-biotite to siderophyllite. More evolved facies contain protolithionite and zinnwaldite and lepidolite occurs in the most fractionated pegmatite. White micas have homogeneous compositions, from muscovite to Li-phengite. In granites, biotite and muscovite trace element distributions are clustered. In pegmatites, mica trace element contents globally follow differentiation from the least to the most evolved body. The reconstructed pegmatitic and granitic melts present strong compositional similarities such as enrichments in Li, Rb, Cs and depletion in Ba that suggest a similar origin. However, micas are shown to have selectively equilibrated with the last melt (or fluid) in contact and so trace element concentrations of early magmatic liquids are rarely preserved. Inversion of the mica data constrains the composition of parental melts and the trace element evolutions during crystallization. Leucogranite and pegmatite melts show mutual incompatible element evolutions inconsistent with a parent-daughter genetic relation. The data and the modelling suggest that they represent non-cogenetic melts generated by discrete episodes of partial melting. Differentiation is thus inherited from source processes rather than being the consequence of fractional crystallization of a common parental melt/magma. We suggest that both the leucogranites and the pegmatites originate from partial melting of a heterogeneous source rather than pegmatites being the product of granite crystallization.