Petrologic study of the Post-Hercynian dyke magmatism in Calabria and Sicily

Abstract

During the late Paleozoic Mesozoic, huge volumes of plutonic and volcanic rocks, as well as widespread basic to acidic dyke swarms, were produced mostly related to extensional tectonics that took place soon after the final stages of the Hercynian continental collision, causing wrenching and faulting of the crust and subsequent continental break-up leading to the definitive Jurassic opening of the Tethys ocean. Products of the Hercynian post-collisional dyke magmatism occurring in the Calabria Peloritani Orogen (CPO) and in the western-central Sicily, have been studied. Calabrian dykes, outcropping in five different locations of Serre Massif, intrude the metamorphic basement as well as the late Hercynian granitoids of the CPO and show a composition ranging from basaltic andesite to rhyodacite, with a medium to high-K calcalkaline affinity. Petrographic, mineral chemical, whole-rock major and trace elements geochemistry and Sr-Nd isotopic data, are consisting with a post-collisional context and enriched lithospheric mantle sources, metasomatized by subduction-related fluids. Variable interactions with the still thickened crust are also suggested and anatexis of the lower-most crustal levels is envisaged for the most acidic compositions genesis. Sicilian dykes have been found outcropping in two different locations, in central Sicily (Leonforte area) where a basaltic sill, with alkaline affinity, intrudes the sedimentary sequences of the Ladinian Lercara Fm., and in western Sicily (Roccapalumba Margana Lercara area), where two different dyke outcrops intrude the middle - late Triassic sedimentary rocks of the Lercara Fm. They range in composition between basalt and basaltic andesite and show a tholeiitic affinity. Petrographic, mineral chemistry, major and trace elements geochemistry and Sr-Nd isotopic composition of Sicilian dykes are compatible with an alkaline tholeiitic transition testifying the Triassic evolution from the late- to post-Hercynian extensional stages to a purely anorogenic rifting context, starting with the early production of alkaline magmas, produced by low degrees of partial melting of a deep asthenospheric mantle source, and by later generation of tholeiitic magmas, related to major extensional rates. Performed AFC calculations suggested interaction of tholeiitic magmas with lower crustal rocks, thus indicating that the lithospheric extension was not still complete at the time of this magmatic activity. In conclusion, studied magmatic rocks testify the late Palaeozoic - Mesozoic transition, well documented in other regions of SW Europe, from a post-collisional context associated to the collapse of the Hercynian Belt to an anorogenic scenario related to sequential Pangea continent breakup and Tethes ocean opening.