Rapid incremental assembly of the Monte Capanne pluton (Elba Island, Tuscany) by downward stacking of magma sheets

The late Miocene Monte Capanne pluton (Elba Island, Italy) is characterized by the widespread occurrence of euhedral K-feldspar megacrysts, for which variations in size and abundance have been determined at 392 stations. The variability of megacryst distribution defines three main facies characterized by low (San Piero facies), high (Sant'Andrea facies), and intermediate (San Francesco facies) megacryst abundance. The three facies show minor yet systematic differences in whole-rock major- and trace-element contents, isotopic composition, and biotite mineral chemistry, with no detectable link between chemical variabilities and megacryst abundance. These results, along with the re-construction of the crystallization sequence, suggest that the facies formed at depth as distinct magma batches with their own peculiar geochemical features, which were preserved after ascent and emplacement. The new geological map based on K-feldspar megacryst distribution thus reveals the composite structure of the pluton, which was built up incrementally by downward stacking of three slightly different magma batches, resulting in a sheeted pluton in the intermediate-shallow crust. The three magma batches were emplaced in a short time sequence hampering geochronological efforts to unravel age differences between internal facies.