Crystallization trends of pyroxenes from alkaline subvolcanites of Punta delle Pietre Nere (Gargano, Southern Italy)

Abstract

The results of microprobe analyses of clinopyroxenes from alkaline melasyenites and layered melagabbros, produced by intra-plate magmatism of Paleocene age at Punta delle Pietre Nere, are here given and discussed. The analysed pyroxenes range from diopsidic to acmite-rich compositions. The first crystallized pyroxenes (diopside) show AlVI contents suggesting shallow depths of crystallization. In addition pyroxenes from melasyenite and those from melagabbro display different Cr contents, Al/Ti and Mg/(Mg+Fe2++Fe3+) ratios confirming their crystallization from melts produced by different parental liquids. Diopsides and salites show an overall trend towards high Al, Ti and Fe3+, suggesting that the crystallization occurred under decreasing SiO2/Al2O3 ratios and under relatively high pH2O-pO2 conditions. Pyroxenes from the Pietre Nere melasyenite show a progressive variation towards acmite rich compositions at Mg/(Mg+Fe2++Fe3+) lower than 0.5; those from the layered melagabbro, instead, show a continuous enrichment in Ca Fe3+ AlSiO6. This different behaviour is due to the co-crystallization, with the latest pyroxenes, of phases with different K/Na and Si/Al ratios. © 1982 Springer-Verlag.