Petrographic, geochemical and isotopic constraints on magma dynamics and mixing in the Miocene Monte Capanne monzogranite (Elba Island, Italy)

The Monte Capanne pluton (Tuscan Magmatic Province, Elba Island, Italy) displays a great variety of magmatic products. Mafic microgranular enclaves (MME) are widespread and display variable size, texture and composition. Petrographic and mineralogical features highlight the importance of magma mixing in their genesis. In particular, patchy-zoned phenocrysts and corroded An-rich plagioclase are common in the MME, which, together with their abundance in the monzogranitic host, suggest that mixing between mantle and crustal-derived magmas occurred relatively early in the crystallisation history of the pluton. A gabbroic enclave is characterised by the abundance of An-rich plagioclase and amphibole, the latter possibly replacing former clinopyroxene. Differences in biotite composition between MME reflect the heterogeneous nature of the mafic magmas prior to the enclave formation. The importance of magma mixing is also evident in K-feldspar megacrysts, which commonly exhibit resorption surfaces followed rimward by a high-Ba zone. Hybrid Orano dykes, which are the latest magmas injected into the Monte Capanne pluton, display specific disequilibrium textures (i.e. sieve-textured plagioclase phenocrysts) and mineral content (i.e. abundance of amphibole) compared to the MME. The genesis of most MME, which display a wide spectrum of major and trace element compositions compared to the host monzogranite, involves a complex interplay between mixing and crystallisation following the input of mantle-derived magma into a silicic magma chamber. Possible loss of the liquid phase from the evolving basic magma may have produced MME distinctively depleted in LREE and enriched in MgO. Fingerprinting the chemical and isotopic signature of the basic magma involved in the genesis of the MME is further complicated by subsequent (but limited) exchange between MME and host. The basic magma is inferred to be similar to coeval high-K calc-alkaline volcanic rocks occurring on Capraia Island. Isotopically, the monzogranite and its MME display distinctive trends pointing towards a low-var epsilonNd end-member, represented by a garnet mica schist xenolith. We propose that these trends, together with the abundance of variably digested metasedimentary xenoliths dispersed throughout the pluton, reflect assimilation of metapelitic crust early in the evolution of the pluton during storage of the magma within the Tuscan basement, i.e. prior to the final emplacement of the pluton at upper crustal levels. The hybrid Orano dykes, which were injected at the end of the Monte Capanne crystallisation history, display distinctive trace element compositions, similar to REE-Sr-Ba-rich andesites on Capraia. The Orano dykes originated from a deeper level of magma storage and interacted with a crystal-rich enclave-bearing monzogranitic mixture. The proposed scenario involving periodic recharge by mantle-derived magmas and crustal assimilation is also in agreement with isotopic variations observed within a zoned K-feldspar megacryst, which exhibits a core with high 87Sr/86Sr and a rimward decrease of 87Sr/86Sr, arguing for a high 87Sr/86Sr silicic system replenished by lower 87Sr/86Sr mantle-derived magmas.