A lamproitic component in the high-K calc-alkaline volcanic rocks of the Capraia Island, Tuscan Magmatic Province: evidence from clinopyroxene crystal chemical data

The Island of Capraia (Tuscan Archipelago) is the westernmost volcanic centre of the Tuscan Magmatic Province. It is made by the overlap of two volcanoes: the Capraia and the Zenobito ones. The Capraia Volcano is a large composite apparatus dominated by lava flows and lava domes with a high-K calc-alkalic affinity built up over the period between 7.5 and 6.9 Ma at its top. The Zenobito Volcano is a small monogenetic apparatus that produced a cinder cone associated to a very small plateau-like lava structure in a short time spanning at 4.6 Ma, overlapping the south-westernmost edge of the Capraia Volcano. Outcropping rocks of the older volcano are high-K calc-alkalic (HKCA) rocks ranging in composition from dominant andesite and dacite to rare rhyolite, straddling latite and trachyte fields. Volcanic rocks from the Zenobito Volcano are much more uniform in composition ranging from potassic trachybasalt to shoshonite, but some subalkalic samples falling within the andesite basaltic field have been also found. To explore the relationships between clinopyroxene structural and chemical parameters and the physicochemical conditions of their crystallisation, selected clinopyroxene crystals from the older high-K calcalkalic rocks (10 crystals) and from the younger shoshonitic rocks (2 crystals) of the Capraia and Zenobito Volcanoes, respectively, have been studied. Single-crystal X-ray diffraction measurements and microprobe analyses have been carried out. The clinopyroxene from Capraia Volcano has augitic composition whereas clinopyroxene from Zenobito Volcano is enriched in the diopsidic component. Structural parameters show significant variations, in particular in the VM1 and Vcell. These volumes are larger for clinopyroxene crystals from Capraia volcanic rocks than those of clinopyroxene crystals from shoshonitic rocks of the Zenobito Volcano. It is unlikely that the increase of VM1 and Vcell observed in the clinopyroxene crystals from HKCA rocks can be simply referred to a different chemical composition of the host magmas. More likely, it could reflect different pressure of crystallisation suggesting that the magma chamber of the Capraia Volcano HKCA volcanic rocks was sited at shallower depth than that of shoshonitic volcanic rocks of the Zenobito Volcano. On the other hand, comparison between clinopyroxene diopsidic crystals from Zenobito Volcano and those from Radicofani volcanoes highlights a close structural and chemical similarity. By contrast, clinopyroxene crystals from HKCA rocks show significant differences with respect to those of other calc-alkaline volcanic rocks from volcanic arcs, such as the Aeolian Arc. In particular, the deficiency in Al found in the clinopyroxene crystals from Capraia Volcano recalls a similar characteristic observed in clinopyroxene from lamproites. This possibly suggests that the Capraia high-K calc-alkalic magmas might be related in some ways to lamproitic magmas and that clinopyroxene crystals record this petrologic characteristic, which is not anymore observed in the bulk rock compositions. The occurrence of lamproitic component in the genesis of HKCA magmas of the Capraia Volcano may also explain the large variation in the K2O contents of the Capraia volcanic rocks, which is not easily explained by any common evolution process. It also agrees with the hypothesis that calc-alkalic, shoshonitic and lamproitic magmas in the Tuscany Magmatic Province are cogenetic, and that the lower incompatible element contents of calc-alkalic and shoshonitic rocks reflect a dilution effect generated by a larger degrees of partial melting in a veined mantle source. Finally, Capraia Volcano and Zenobito Volcano magmas are considered to represent different episodes of partial melting of different mantle sources.