Potassic primary melts of vulsini (Roman Province): evidence from mineralogy and melt inclusions

The origin and the relationships between the high potassic (HKS) and potassic (KS) suites of the Roman Comagmatic Province and the nature of their primary magmas have been intensively debated over the past 35 years. We have addressed these problems by a study of mineralogy (olivine Fo92-87, Cr-spinel and diopside) and melt inclusions in olivine phenocrysts from a scoria sample of Montefiascone (Vulsini area). This rock is considered as one of the most primitive (MgO = 13.5 wt%, NiO = 340 ppm; Cr = 1275 ppm) in the northern part of the Roman Comagmatic Prov- ince. The compositions of both the olivine and their melt inclusions are controlled by two main processes. In the case of the olivine Fo < 90.5, fractional crystalli- zation (olivine + diopside + minor spinel) was the principal mechanism of the magma evolution. The ol- ivine (Fo92 90.5) and the Cr-spinel (Cr# = chain. The northern part of this province offers a large 100.Cr/(Cr + A1)--63 73) represent a near-primary liquidus assemblage and indicate the mantle origin of their parental magmas. The compositions of melt inclusions in these olivine phenocrysts correspond to those of poorly fractionated H20-rich ( ~ 1 wt%) pri- mary melts (MgO = 8.4-9.7 wt%, FeO t~ 6 7.5 wt%). They evidence a wide compositional range (in wt%: SiO2 = 46.5 50, K20 = 5.3-2.8, P205 = 0.4-0.2, S=0.26 0.12; C1=0.05 0.03, and CaO/A1203-- 0.8-1.15), with negative correlations between SiO2 and K20, A1203 and CaO, as well as positive correlations between K20, and P2Os, S, C1, with nearly constant ratios between these elements.