Crystal Chemistry of Clinopyroxene from Alkaline undersaturated rocks of the Monte Vulture Volcano, Italy

The Monte Vulture is a Late Pleistocene stratovolcano, composed of highly undersaturated alkaline potassic to ultrapotassic rocks belonging to the Roman Magmatic Province. These rocks are notably richer in Na2O if compared to similar rocks of the Roman Province. Two distinct magmatic Series have been recognized: (1) feldspar-bearing series, ranging from basanite to phonolite, and a volumetrically subordinate (2) feldspar-free series, consisting of melilitite, melafoidite, and haüynophyre. The clinopyroxene compositions of the feldspar-bearing series ranges from diopside to ferro-salite (hedenbergite), and shows, from basanite to phonotephrite, increasing FeOtot, Al2O3, and TiO2 and decreasing of MgO contents. Clinopyroxene in basanites and tephrites has generally high Fe3+ contents, which is typical for clinopyroxene from the Roman Province. Clinopyroxene of the feldspar-free series shows a more restricted variation in MgO, and has often very high Al2O3, FeOtot, and TiO2 contents. In all the crystals examined the Al3+ content is high and is present mostly on the T site and for a minor part on the M1 site. The M1–O2 distance shows a good correlation with the RM13+ content; clinopyroxene from basanites (feldspar-bearing series) has the lowest RM13+ and that from melilitite and haüynophyre (feldspar-free series) the highest RM13+ contents. Clinopyroxene crystals from feldspar-free rocks have smaller M1 and larger T polyhedral volumes when compared to those in olivine–melilitites and melilitites (kamafugites) from Umbria, but they show similar polyhedral volumes as clinopyroxene crystals from leucite-bearing rocks. Although clinopyroxene from feldspar-free rocks has small M1 volumes due to the high R3+ contents, M1 volumes of clinopyroxene from melilitites are larger than expected because of the higher (Fe3+/Al3+)M1 values. This larger M1 volume of clinopyroxene in melilitites causes a shortening of <M2–O1,O2> and a lengthening of the M2–O3 distance. Strong compositional and structural similarities between clinopyroxene of Monte Vulture and Leucite-bearing rocks of the Roman Province (plagioclase-bearing High Potassium Series=HKS) indicate a common petrogenetic affinity. On the other hand, differences between clinopyroxene in feldspar-free rocks from Monte Vulture and that in kamafugites (i.e, olivine melilitites, kalsilitites) from Central Italy, suggest significant magma dissimilarities between these two groups.