Convective mixing and geochemical differentiation of S-type plutons (Asinara Island, Italy)

Abstract

The redistribution of magma from deep, partially molten, source zones to emplacement sites usually localized in the upper crust plays an essential role for the 4D chemo-thermo-mechanical evolution of the lithosphere. In this paper, we investigate the incremental formation of a megacrystic S-type tabular intrusion emplaced in the Variscan crust of the Asinara Island, N Sardinia (Italy). LA-ICP-MS UPb zircon dating indicates that the pluton solidified at the end of Carboniferous, at about 312 Ma. Contact metamorphic assemblages and P-T pseudosection modelling are consistent with emplacement at shallow crustal level in the cordierite-andalusite stability field, at pressure about 0.25–0.3 GPa. The temperature of melt is estimated between 700 and 850 °C by combining Ti-in-zircon, Ti in biotite thermometry and thermodynamic modelling. The petrochronological constraints are complemented by combining field structural analysis, microstructural observation, geochemical mapping and rheological modelling. The proposed emplacement mechanism is finally discussed in relation to the evolution of incremental magmatic systems and the overall geochemical diversity of the continental crust. The results support an initial stage involving formation of K-feldspar-rich cumulates. Subsequent incremental injections of new, hotter and crystal-poor, magma induced incipient Rayleigh-Taylor instability, which stopped soon thereafter likely because of fast cooling of the convective layer. The proposed emplacement mechanism is finally discussed in relation to the geochemical variability, or lack thereof, of S-type granitic plutons.