ORIGIN AND TIMING OF THE POST-VARISCAN GABBRO–GRANITE COMPLEX OF PORTO (WESTERN CORSICA)

Abstract

The post-Variscan complex of Porto consists of metaluminous to slightly peraluminous A-type biotite granites mingled with gabbro-dioritic rocks, and late dykes with basaltic to trachyandesitic composition. U-Pb zircon dating by LA-ICP-MS on two mafic intrusive samples constrains the time of the gabbro–granite crystallisation at 281 ± 3 Ma and 283 ± 2 Ma. Hornblende 40Ar-39Ar ages from a late trachyandesite dyke date the dyking event at 280 ± 2 Ma, which is within error the U-Pb zircon ages of the intrusives. Biotite granites show variable major and trace element compositions and similar initial εNd (−0.3 to +0.9). Whole rock chemistry variations and trace element compositions of plagioclase and allanite indicate that the granites are genetically linked, essentially through fractional crystallisation of feldspars and minor allanite. On the basis of whole-rock chemistry e.g. initial εNd +4.9 to +1.7 and trace element clinopyroxene compositions, we have ascertained that the mafic intrusives and basic dykes formed from isotopically depleted mantle source-derived melts with similar trace element signature. These basic melts experienced slightly different evolutionary histories, controlled by fractional crystallisation and crustal contamination, mainly by the acid magma that gave rise to the associated biotite granites, but also by the enclosing older Variscan granitoids. U-Pb zircon data suggest that the Porto complex was affected by hydrothermal fluid circulation at 259 ± 9 Ma.