Petrogenesis of highly fractionated I-type peraluminous granites: La Pedriza pluton (Spanish Central System)

Abstract

The La Pedriza pluton stands out as the most extensively fractionated granite (Rb < 629; Sr < 2 and Ba < 2 ppm)
of the Spanish Central System Batholith. These granites show a strong enrichment in some rare metal contents
(Nb = 44, Y = 136, Yb = 10.7, U = 17, Ta = 7, Sc = 15 ppm). The petrography and geochemistry (including Sr-
Nd isotopes) reveal that the pluton is composed of at least four units. These are classified as I-type peraluminous
leucogranites (A/CNK=1.03-1.17), P-poor (P2O5<0.2 wt%) and Na2O-rich (< 4.24 wt%) exhibiting differences in
their HFSE and REE contents and εNd compositions. Moreover, the units of the La Pedriza granite display different
trends of fractional crystallization. REE spectra of the two most fractionated units suggest the involvement of a
fluorine-rich melt in the last stages favouring the crystallization of xenotime and niobotantalates. Intermediate
meta-igneous granulite protoliths are proposed as source rocks. The most evolved units of the La Pedriza pluton
show chemical features convergent to A-type granites; these are explained by extensive fractional crystallization
of a P-poor, I-type granite magma.