Trace Element and Isotope Geochemistry of Gabbro-Derived and Felsic Granulites from the Northern Apennines (Italy): Evidence for Assimilation/Fractional Crystallization in the Lower Crust

Abstract

The Late Cretaceous sedimentary melanges from the External Liguride Units of the Northern Apennines include large slide blocks of mantle peridotites, MOR-basalts and lower and upper continental crust rocks representative of a continent-ocean transition between the Internal Liguride oceanic domain (Late Jurassic Western Tethys) and the thinned continental margin of the Adria plate (Marroni et al., 1998). The slide-blocks of lower continental crust consist of gabbro-derived and felsic granulites, which locally preserve primary contacts. The gabbro-derived granulites show variable degree of recrystallisation related to the granulite-facies re-equilibration. However, rocks which preserve a gabbroic fabric are found locally. The most frequent rock-types are olivine-bearing gabbronorites and Fe-Ti-oxidebearing gabbronorites; troctolites and anorthosites are also present. The felsic granulites include quartzo-feldspathic granulites and rare quartz-poor to quartz-free rocks. The gabbroic protoliths of the granulites were emplaced at about 290 Ma at deep crustal levels, where they underwent slow cooling and recrystallization under granulite-facies conditions (P = 0.7-0.8 GPa, T = 800-900~ They were exhumed to upper levels, in association with the felsic granulites, in late Triassic-middle Jurassic times (Marroni et al., 1998). Permian gabbroic rocks with granulite-facies metamorphic overprint are widespread in the Western Alps, in the remnants of the Adria plate margin. These gabbroic complexes are ascribed to the extensional tectonics which postdate the Variscan orogeny. In particular, striking similarities between the gabbro-derived granulites from Northern Apennines and the Ivrea Mafic Body (Pin, 1990) can be shown, i.e. similar ages of emplacement, close paragenetic and compositional resemblances and comparable retrograde metamorphic evolution. This work shows a geochemical investigation of gabbro-derived and felsic granulites from the Northern Apennines, in order to unravel (i) the affinity of the parental liquids of the gabbroic protoliths, (ii) the role of fractional crystallization and crustal assimilation in the igneous differentiation process, (iii) the nature of the mantle source. Trace element and Sr-Nd isotopic analyses have been performed on selected granulite samples, and clinopyroxene and plagioclase cores from undeformed gabbro-derived granulites were analysed for trace elements by ion microprobe. Relics of igneous textures, mineral and bulk-rock compositional variations indicate a comagmatic origin for the gabbro-derived granulites. They can be generally recognized as cumulitic rocks with negligible amounts of residual trapped liquid, on the basis of low SiO2/A1203 ratios (Kempton and Harmon, 1992) and overall low contents of incompatible trace elements. The mg# value ranges from 80 to 52, and points to negative correlations with TiO2 (up to 5 %) and MnO, thus indicating a tholeiitic differentiation trend. Major element variations are consistent with a process of fractional crystallization of plagioclase + olivine as liquidus phases, followed by plagioclase + clinopyroxene + orthopyroxene. In addition, petrographic relations in undeformed gabbroic rocks suggest the growth of AIspinel and Fe-Ti-oxides as intercumulus igneous minerals. The range of incompatible trace element ratios (Zr/ Y = 0.7-2.9, Y/Nb <~ 9, Zr/Nb ~< 10) encompasses that of subalkaline basaltic liquids. The R E E distribution (Fig. 1) is characterized by: (i) slightly LREE-enr iched patterns showing decreasing Eu positive anomaly with increasing absolute R E E