Geochemistry of mafic phenocrysts from alkaline lamprophyres of theudSpanish Central System: implications on crystal fractionation, magmaudmixing and xenoliths entrapment within deep magma chambers

摘要

The Permian alkaline lamprophyres from the Spanish Central System (SCS) are highly porphyritic rocks which carryuda heterogeneous population of clinopyroxene and kaersutite zoned phernocrysts. Clinopyroxene phenocrysts may show 1) normaludzoning (Cpx-I), 2) reverse zoning with Fe-rich green cores (Cpx-II), and 3) reverse zoning with colourless Al-poor, silica-rich coresud(Cpx-III). Kaersutite phenocrysts also show a slight reverse zoning. Major and trace element composition of Cpx-I suggests thatudtheir compositional variation is related to a crystal fractionation process from melts similar to the host lamprophyres. The Cpx-IIudcores represent crystallization from highly evolved melts (low Mg-Cr contents and incompatible element enrichment), geneticallyudrelated with the SCS alkaline magmatism, and the growth or surrounding Mg-rich inner rims points to a magma mixing process.udThe major and trace element composition of Cpx-III cores supports derivation from a magma which has fractionated plagioclase.udThis characteristic, together with their similarities when compared to clinopyroxenes from charnockite xenoliths, suggests thatudthey might be xenocrysts from deep calc-alkaline cumulates. The composition of melts in equilibrium with clinopyroxene andudamphibole phenocrysts supports a model in which Cpx-II and Cpx-III cores would have been incorporated into a more primitiveudlamprophyric magma stagnated at lower crustal levels. The low pressure composition of all phenocryst outer rims indicates thatudthey crystallised directly from the host alkaline magma at their subvolcanic emplacement levels.