Petrogenesis of the High-Alumina Basalt-Andesite suite from Sant'Antioco Island, SW Sardinia, Italy

Present-day Sant'Antioco Island (SW-Sardinia, Italy) hosted one of the southernmost, and youngest, subduction-related, Cenozoic magmatic events of western Sardinia. A high-alumina basalt-andesite rock association, the focus of this paper, crops out in the southern portion of the island and represents the mafic end-member of volcanism developed during Miocene. Basaltic andesites and andesites are the dominant rock-types, while basalts and dacites are occasional. Minero-petrographic and geochemical characteristics reveal:i)a clear calc-alkaline signature for these rocks, in spite of high FeO*/MgO ratios that mimic a tholeiitic affinity; ii) a magma evolution mainly controlled by fractional crystallization and iii) some degree of crustal assimilation. Phase relationships and compositions, mass balance calculations and T, P, X, estimates consistently indicate that fractional crystallization occurred at low pressure (P=100-400 MPa), under different P(H2O) conditions, which explain the observed wide shifts in major oxides (e.g., Al(2)O(3) and MgO), for comparable SiO(2) contents, and the ambiguous tholeiitic character as well. Phase relationships in the least evolved lithotypes point out that higher H(2)O concentrations (estimated at up to 6 wt%) in the melts reduced the crystallization of plagioclase and favoured that of olivine, whereas lower H,0 contents (up to 3 wt%) promoted plagioclase fractionation and earlier crystallization of orthopyroxene at the expense of olivine. This X(H2O) imprint was recorded up to the most evolved compositions. Finally, the study rock suite can be derived from a parental (near-primary) magma matching in composition high-MgO basalts from the Montresta calc-alkaline district in NW Sardinia, in the light of the close compositional similarity between the high-Al(2)O(3) basaltic andesites in the two districts. Thus, starting from a common parental composition, here we reconstruct liquid lines of descent, where the relative proportions of fractionating plagioclase vs mafic phases, as controlled by X, in the melts, may account for the compositional differences of Sant'Antioco rock-types ab initio.