Interaction between basic and acid magmas during the latest stages of the post-collisional Variscan evolution: Clues from the gabbro–granite association of Ota (Corsica–Sardinia batholith)

Abstract

The intrusive association of Ota (post-Variscan Corsica–Sardinia batholith) consists of mafic rocks and granites displaying mingling relations. The mafic rocks are olivine gabbros to quartz-diorites, and the granites are subsolvus and characterised by biotite as the only mafic phase. Minor amounts of amphibole-bearing granodiorites to granites develop along the contacts between mafic rocks and granites. The rather high initial ε Nd (+ 4.5) of the most primitive mafic sample indicates that parental basic melts had an extensive contribution from a depleted mantle source. In the mafic rocks, the decrease of initial ε Nd (to + 2.1) is associated with an increase of Th and U concentrations in clinopyroxene, thus indicating that the Ota basic melts underwent a process of crustal contamination. Associated biotite granites have low initial ε Nd (+ 0.4) and high Th and U concentrations. The chemical evolution of Ota basic melts most likely occurred through fractional crystallisation and concomitant contamination with the acid magma that gave rise to associated biotite granites. The incompatible element signature of late-stage anhedral amphibole from variably evolved mafic rocks is similar, thus suggesting that the whole Ota mafic sequence recorded the contamination process, possibly by percolation within the gabbroic crystal mush of a melt contaminated by the acid magma. Hybridisation between basic and acid magmas is also documented by contact amphibole-bearing granitoids. In particular, major and trace element zoning of amphibole from granodiorites reveals that such a hybridisation was associated with reactions between crystals from the basic magma (amphibole and clinopyroxene) and the acid melt. The occurrence of chemically homogeneous amphiboles in associated granites suggests that “contact” hybridisation comprised also a process of gradient-induced chemical diffusion.