The Orano Porphyry dyke swarm (Elba Island, Italy): the last "beat" of the Monte Capanne pluton system

Recent advances in granite research established the many simi- larities in transfer and emplacement mechanisms between volca- nic and plutonic systems (magma transfer via dykes, high transfer rates, multiple injections, etc.) with the major difference defined by the role of magma traps in hampering the ascent of granitic magmas (Rocchi et al. 2002; Petford et al. 2000). In volcanic complexes, sequentially emplaced magma batches are quenched in distinct lithostratigraphic units at the surface. On the con- trary, the relatively long crystallization history of plutonic sys- tems (in the order of 104-105 years) may obliterate the record of such multiple events. Understanding the real history of a plu- tonic system cannot be addressed by a ''simple'' petrological approach aimed at identifying sources involved in the magma- tism since the time variable also has to be constrained. The reconstruction of the sequence of injection events is a pre- condition, however difficult, to build-up a realistic scenario in which the sequential arrival of distinct pulses of magmas at the emplacement level can be linked to significant changes occurring in both the source level as well as in tectonic setting controlling the magmatism (regional vs. local). The Miocene intrusive activity of Elba Island (Italy) covers a significant time span (from about 8.5 to 5.9 Ma), thus offering the opportunity to investigate temporal variations of deep-seated processes and materials during the post-collisional evolution of the Apennine orogenic belt. Ten intrusive units of various geometries, sizes and chemistries are exposed on Elba Island (Dini et al. 2002; Westerman et al. 2004) occupying almost half of its surface (two plutons, four laccoliths, and four dyke systems): seven of them build up the older central-western Elba complex (8.5-6.8 Ma), while the last three define the younger, and much smaller, eastern Elba complex (5.9 Ma). The magma formation processes recorded between ca. 8.5 and 6.8 Ma in central-western Elba, changed from crust-, to hybrid-, to mantle-dominated, as the Apennine fold belt was progressively thinned, heated, and intruded by mafic magmas during late Miocene time. Very unu- sual melts emplaced at the beginning and at the end of the igneous activity did not contribute to the generation of main hybrid magmas of Monte Capanne pluton. They do, however, emphasize the highly variable nature of crustal and mantle sources that can be involved, during a short time span, in post-collisional, exten- sion magmatism. After the reconstruction of this first-order history, new de- tailed mapping carried out on Monte Capanne pluton provided data to unravel a second-order history in which multiple injec- tions coalesced in a single, composite, and geochronologically indistinguishable pluton (?6.9 Ma). Several facies can be detected in the pluton, the most relevant being (i) the monzogranitic Sant'Andrea Facies, characterized by numerous large K-feldspar megacrysts and mafic enclaves, and (ii) the granodioritic-mon- zogranitic San Piero Facies, quarried for its homogeneous texture almost devoid of large megacrysts and mafic enclaves. The patchy distribution of the Sant'Andrea facies, dominantly around the margin of the pluton, suggests that its arrival is followed by stirring during emplacement of the San Piero facies. Both the facies are crust-mantle hybrid products. Finally, the last ''beat'' of the Monte Capanne magmatic system produced a relatively more mafic dyke swarm: the Orano Porphyries. The Orano dyke swarm was emplaced at 6.85 Ma, cutting the Monte Capanne pluton. Orano porphyry is typically dark and contains an assemblage of olivine, clinopyroxene, phlogopite, plagioclase. These petrographic features, coupled with geochemical and iso- topic data sug