Alkaline gabbroids of the Chirii outcrop (North Minusa depression): mineralogy and melt evolution

We studied the chemical composition of rock-forming minerals in gabbroids from the Chirii outcrop and the evolutionary features of parental basic melt during the crystallization of these rocks. Results were compared with data for basanites from pipes of the North Minusa depression. The mineralogical composition and thermobarogeochemical data of the gabbroids were examined in detail, and chemical analyses of rock-forming minerals (clinopyroxene, plagioclase, amphibole, biotite, titanomagnetite, and apatite) were carried out. Based on the homogenization temperatures of primary melt inclusions, we established the minimum temperatures and sequence of mineral crystallization in the gabbroids: clinopyroxene (>1160 °C), plagioclase, magnetite → amphibole (>950 °C) → biotite. The rock crystallization proceeded at shallow depths. Thermometric data are confirmed by results of modeling of equilibrium gabbroid crystallization. The crystallization of parental basic melt was accompanied by the accumulation of SiO2, Al2O3, alkalies, and Cl and depletion in femic components. The melt evolved to granodiorite and alkali-syenite compositions. Compared with basanites from pipes, the parental melt had a longer evolution. The geochemical features of the gabbroids indicate that they, like basanites, crystallized from intraplate alkali-basaltoid magmas. But in petrochemistry and mineralogy the Chirii gabbroids differ considerably from the pipe basanites.