The Upper Miocene magmatism of the Island of Elba (Central Italy): compositional characteristics, petrogenesis and implications for the origin of the Tuscany Magmatic Province

Abstract

Late Miocene intrusive magmatism of the Island of Elba, Tuscany (central Italy), consists of stocks, laccoliths, sills, and dikes showing dominant monzogranite and granodiorite compositions, with minor leucogranitic dike-sill complexes, aplites and pegmatites. A few mafic rocks occur as dikes, and as microgranular enclaves hosted inside the main intrusions. The Elba magmatism belongs to the Tuscan Magmatic Province, an 8.5 to 0.3 Ma old association of mafic to felsic rocks, of mantle and crustal origin, cropping out in Tuscany and northern Latium. Major and trace element abundances of Elba rocks are extremely variable, testifying to complex origin and evolutionary history for magmas. 87Sr/86Sr (~ 0.708-0.723) and 143Nd/144Nd (~0.5121-0.5124) are close or within the field of upper continental crust, with mafic dikes showing the lowest Sr- and the highest Nd-isotope ratios. Petrological, geochemical and textural data of Elba igneous rocks are better explained by invoking a leading role for multiple mixing processes between crust-derived felsic magmas and mafic-intermediate melts of ultimate mantle origin, accompanied by fractional crystallisation. Proxies of crustal anatectic melts are represented by some highly radiogenic-Sr rocks from northern Monte Capanne pluton. Crustal magmas were formed by melting of sedimentary rocks, likely metagreywakes, at pressures exceeding 0.3 GPa. Mafic-intermediate magmas have calcalkaline to shoshonitic compositions and originated in an anomalous mantle, moderately contaminated by siliceous sediments. Selective enrichments in Sr, Ba and LREE are shown by some intermediate rocks (Orano dikes), revealing the occurrence of a distinct magma type at Elba. Similar compositions are also observed at Capraia island, San Vincenzo and Campiglia (southern Tuscany), suggesting a regional relevance for this magma type. Sr-Ba-LREE-rich rocks do not show obvious genetic relationships with other Tuscany magmas and may represent a distinct end-member whose origin deserves further studies to be understood. Petrogenetic processes occurred at Elba allow a better understanding of magma genesis at the regional scale. Some of the first-order petrological characteristics of the Tuscany Magmatic Province are comprised inside the space defined by three main end-member compositions, represented by acidic peraluminous rocks, calcalkaline-shoshonitic basalts, and mafic ultrapotassic lamproites. Mixing of various proportions of end-members generated a wide spectrum of hybrid compositions. These, in turn, were affected by evolution processes, such as fractional crystallisation and crustal assimilation, further complicating the petrological puzzle of the Tuscany Magmatic Province. The lamproitic and calcalkaline magmas were respectively generated by melting of phlogopite-bearing harzburgitic and lherzolitic mantle rocks, which were contaminated by subducted upper crustal siliceous materials. Lithospheric mantle contamination occurred during the Eocene, forming harzburgite-sediment melange bodies. These were dismembered by ascent of lherzolitic asthenosphere behind the eastward migrating Northern Apennine subduction front, during Miocene to Quaternary backarc extension. Asthenospheric upwelling generated an increase in temperature, dehydration and melting of dispersed harburgite-sediment melange megaliths, with release of fluids that contaminated the surrounding newly emplaced lherzolites. Melting of such a heterogeneous mantle gave the wide range of mafic magmas observed in Tuscany. Backarc mantle doming and uprise of hot mafic magmas prompted anatexis within the continental crust of the overriding plate.