COMPETING PROCESSES OF HYBRIDIZATION AND FRACTIONATION IN THE MONTE CAPANNE MAGMATIC SYSTEM, ELBA ISLAND, ITALY

The late Miocene Monte Capanne monzogranite, produced during post-collisional extension of the internal part of the Apennine orogenic belt, provides an excellent opportunity to evaluate the contributing factors that produced a wide variety of products within a single magmatic system. These products derived from competing processes, the first being hybridization by magma mixing/mingling between crust-derived, anatectic peraluminous melt and mantle-derived mafic magma, and the second being crystal fractionation.

Magmatism in the Monte Capanne system started with shallow emplacement of the hybridized San Martino laccolith, locally up to 700 m thick. This was followed by emplacement of three facies of the Monte Capanne pluton, all with higher degrees of hybridization. Emplacement of the least hybridized representative, the Cotoncello dike, occurred last and within the main pluton. These relationships argue that hybridization occurred at depths below the emplacement level, as well as within the chamber. Early magma interaction is further supported by a great abundance of microgranular mafic enclaves in the main pluton, lesser amounts in the San Martino laccolith, and only a minimal amount in the Cotoncello dike. These enclaves have intermediate compositions, isotopic ratios equal their host, amphibole clots after pyroxene, plagioclase with calcic cores, and abundant xenocrysts derived from their host, all pointing to a hybrid origin. Numerous large enclaves are multiple in character, having captured less evolved enclaves. Microgranite dikes, aplites and associated pegmatites represent fractionation products of the Monte Capanne system.

Additional evidence bearing on the evolution of the Mt. Capanne system comes from reaction microtextures of REE-Y-Th-U accessory minerals, and trace element and isotopic zoning of K-feldspar megacrysts. Taken all together, the petrographic, geochemical and isotopic evidence supports a scenario of interaction between the felsic crustal melts and mafic mantle melts, with their modification of each other starting in the crustal source area and continuing both during transport and within the emplacement chamber.