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 u ndersaturated alkaline potassic to ultrapotassic rocks belonging to the Rom an Magmatic Province. These rocks are notably richer in Na2O if compared to similar rocks of the Roman Province. Two distinct magmatic Series have bee n recognized: (1) feldspar-bearing series, ranging from basanite to phonoli te, and a volumetrically subordinate (2) feldspar-free series, consisting o f melilitite, melafoidite, and hauynophyre. The clinopyroxene compositions of the feldspar-bearing series ranges from diopside to ferro-salite (hedenb ergite), and shows, from basanite to phonotephrite, increasing FeOtot, Al2O 3, and TiO2 and decreasing of MgO contents. Clinopyroxene in basanites and tephrites has generally high Fe3+ contents, which is typical for clinopyrox ene from the Roman Province. Clinopyroxene of the feldspar-free series show s 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. Th e M1-O2 distance shows a good correlation with the R-M1(3+) content; clinop yroxene from basanites (feldspar-bearing series) has the lowest R-M1(3+) an d that from melilitite and hauynophyre (feldspar-free series) the highest R -M1(3+) contents. Clinopyroxene crystals from feldspar-free rocks have smal ler M1 and larger T polyhedral volumes when compared to those in olivine-me lilitites and melilitites (kamafugites) from Umbria, but they show similar polyhedral volumes as clinopyroxene crystals from leucite-bearing rocks. Al though clinopyroxene from feldspar-free rocks has small M1 volumes due to t he high R3+ contents, M1 volumes of clinopyroxene from melililites are larg er 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 struct ural similarities between clinopyroxene of Monte Vulture and Leucite-bearin g rocks of the Roman Province (plagioclase-bearing High Potassium Series = HKS) indicate a common petrogenetic affinity. On the other hand, difference s 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. (C) 19 99 Elsevier Science B.V. All rights reserved.