• emplacement
• facies
• granite
• megacryst
• mixing
• radiogenic isotopes

Granitic plutons frequently show cryptic facies variations characterized by diffuse contacts that are often interpreted to be the result of local differentiation processes at the emplacement level. An integrated field, petrographical and geochemical approach has been proposed for the study of the Monte Capanne pluton (Elba Island, Italy). This integrated approach indicates that facies differences, contrary to previous studies, reflect the original characters of distinct magma batches formed at depth and sequentially emplaced to produce a sheeted pluton. In addition, the reconstruction of the crystallization sequence of the rock associated with cutting-edge techniques for the determination of trace element and isotope variations within magmatic minerals (K-feldspar and biotites) have been used to investigate the evolution of the Monte Capanne pluton magmatic system. These new mineral-scale geochemical data allow to re-evaluate the role of magma mixing and wall-rock assimilation in the petrogenesis of the pluton, leading to a comprehensive interpretation of its evolutionary history.
The late Miocene Monte Capanne pluton was emplaced during the post-collisional extensional evolution of the Northern Apennine orogen. The pluton has a mainly monzogranitic composition and is characterized by the widespread occurrence of euhedral K-feldspar megacrysts, whose variations of size and abundance have been determined at 350 stations across the pluton. This megacryst distribution analysis defines three main facies: the San Piero facies (low to very low megacryst concentration), Sant'Andrea facies (high to very high megacryst concentration) and San Francesco facies, with intermediate megacryst concentration. The three facies show minor yet systematic differences in major and trace element contents, isotopic composition and biotites mineral chemistry. These variations are independent from the megacryst abundance. Overall, the San Piero facies displays lower SiO2 coupled with higher CaO, MgO, Fe2O3 and Al2O3 abundances with respect to Sant'Andrea facies, as well as higher Sr, V, Cr and Ba contents. Isotopically, Sant’Andrea rocks are distinctly richer in radiogenic Sr than are the samples from the San Piero facies. Biotite composition is characterized by the lowest Fe# values (average 0.49 ± 0.01) for San Piero facies while in Sant’Andrea exhibits the highest values (average 0.54 ± 0.01). The San Francesco facies defines intermediate fields partly overlapping those of the two extreme facies.
The association of geochemical data on whole rocks, K-feldspar megacrysts and biotites together with the reconstruction of crystallization sequence suggests that the facies formed at depth as distinct magma batches, acquiring peculiar geochemical features that were preserved after ascent and emplacement. The new geological map based on K-feldspar megacryst distribution reveals the composite structure of the pluton, characterized by an external shell constituted by the Sant’Andrea facies, and a core formed by the San Piero facies. In this view, the Monte Capanne pluton results to be assembled incrementally by downward stacking of three slightly different magma batches, building up a sheeted pluton in the intermediate-shallow crust. The three magma batches emplaced in a short time sequence preventing the development of sharp contacts, so that each batch emplaced with no enough time for magma to completely solidify, before the emplacement of the following batch. Consequently, any geochronological effort to unravel age difference between internal facies is hampered.
Isotopic mineral-scale data highlight that the facies/batches, formed at depth, followed a same geochemical evolution characterized by a progressive decrease in the 87Sr/86Sr ratio associated to minor variations in the Pb isotope ratios and an homogeneous 143Nd/144Nd. However, textural observations suggest that each batch attained different P-T conditions along its history, resulting in the different K-feldspar distribution and, for San Piero and San Francesco facies, in megacrysts partial resorption.
The geochemical evolution of magma batches is largely related to a complex mixing involving a crust-derived anatectic magma and two mantle-derived magmas similar to the high-K calc-alkaline rocks from Capraia Island and the Tuscan lamproites. The nature of the anatectic magma is largely unconstrained and the role of Cotoncello body as anatectic end-member is strongly criticized on the basis of geochemical and field evidences.