Palaeozoic continental-type gabbros in the Gran Paradiso nappe (Western Alps, Italy): Early-Alpine eclogitization and geochemistry

Abstract

Palaeozoic gabbros intruded by late-Variscan granitoids occur in the southern part of the Gran Paradiso nappe (Western Alps). They are the first gabbroic rocks described in the Variscan basement of the internal Pennine nappes. Both gabbros and granitoids sustained the early-Alpine high-pressure metamorphism (at T around 500°C and P between 12 and 20 kbar) and garnet- Ca Na amphiboles-omphacite-talc-paragonite assemblages developed in the gabbros. In little-deformed metagabbros, the complete eclogitic recrystallization and the relatively advanced compositional homogenization of some eclogitic minerals were probably favoured by fluid infiltration; however, the microstructural re-organization failed and the pre-alpine magmatic domains survived in response to the lack of pervasive deformation. The little-deformed metagabbros are bounded by glaucophane-bearing ductile shear zones formed during the early post-eclogitic path. Major- and trace-element abundances and REE patterns of the gabbros indicate derivation from a basaltic sub-alkaline melt with a continental affinity. Although a hiatus exists between the gabbros and the associated Gran Paradiso granitoids, the ratios between some hygromagmatophile elements and the normalized patterns for some elements suggest a few affinities between the two lithologies; these rocks probably formed during the same magmatic cycle and the granitoids originated either from anatexis of continental crust during gabbro intrusion or, more likely, by fractionation and interaction between the early basaltic melt and the sialic crust.