Petrological and Geochemical data of Porphyritic Dikes from the Capo Arco Area (Eastern Elba Island, Northern Tyrrhenian Sea).

New geological surveying at a 1:10.000 scale (CARG Project) allowed to refine the stratigraphic, structural and magmatic setting of the Elba Island. This paper aims at characterizing two dikes of likely Late Miocene age (Casa Carpini dikes), previously defined as lamprophyres (i.e. kersantite), outcropping in eastern Elba, on the eastern and southern slopes of the Monte Arco, close to Porto Azzurro. The grey to light-grey Casa Carpini dikes, the phyllites and metasandstones of the Ligurian-Piedmontese Acquadolce Unit, are quartz-diorites in composition with a maximum of 23% of mafic minerals (biotite). They show a microcrystalline porphyritic texture consisting of plagioclase, biotite, quartz, K-feldspar, and rare muscovite with scattered xenocrysts of quartz, plagioclase and K-feldspar. Plagioclase phenocrysts display normal zoning with An% = 9-23; the variability of An in the plagioclase microcrysts is wider (5-34%). The REE patterns are characterised by fractionated LREE and MREE and negative anomaly of Eu. The mantle-normalized trace element patterns show high content of the most incompatible elements with a positive spike of Th and Pb, and high LILE/HFSE ratios. Negative spikes of Nb, P and Ti are also observable. No negative anomalies of Sr are present, and Ce/Sr ratios are much lower than other magmatic rocks from Elba. A depletion in K, Rb and Cs is also observed in the studies rocks, possibly a result of secondary processes. Sr-Nd isotopic ratios for the two outcrops of the studied dikes show significant differences, with Sr-87/Sr-86 = 0.711845 and 0.711769 and Nd-143/Nd-144 = 0.512223 and 0.512246. The Casa Carpini dikes are petrographically different from most of the dike rocks associated to the granitoid plutons and laccoliths of the Elba Island. Instead, analogies can be found with the granodiorites to quartz-monzodiorite Orano porphyries, the last magmatic products of western Elba. The petrological and geochemical characteristics of the studied dikes are typical of the intermediate-acidic rocks of the Elba Island. As other rocks from Elba, they show petrographic and geochemical evidence of mixing between a calcalkaline mafic-intermediate magma similar to that of Capraia and a crustal anatectic melt. The geochemical signatures of the studied dikes suggest a more important role of mafic magmas with respect to the bulk of the Elba magmatism.