Petrogenesis of late Hercynian calc-alkaline dykes of mid-eastern Sardinia: petrographical and geochemical data constraining hybridization process

Abstract

During the final stages of the emplacement of the Hercynian batholith, a calc-alkaline dyke swarm intersected the crystalline basement of Sardinia. In mid-eastern Sardinia, these dykes range in composition from basaltic andesite to dacites-rhyolites with oligophyric and porphyritic textures, respectively. Plagioclase is the dominant phase among the phenocrysts, with subordinate brown and green amphibole; biotite, K-feldspar and quartz are abundant in more differentiated compositions. The biotite Rb-Sr age of this magmatism ranges between 291 and 271 Ma. Compositional data can be modelled using two separate trends: 1) AFC-type processes with fractionation of An-rich plagioclase, orthopyroxene and Cr-rich clinopyroxene are responsible for compositional variations amongst the most basic rocks; 2) andesitic to rhyolitic magmas evolve starting from basic magmas which are aligned along the first trend due to hybridization process. The complex and long-lived mafic magma and the crust interaction indicate a MASH-like scenario characterized by different magma chambers, as proposed for the late-Hercynian magmatism in other areas.