Geochemistry of mafic cainozoic volcanic rocks from Sardinia (western mediterranean)

The Cainozoic volcanism of Sardinia (Italy) can be divided into two main cycles with different magmatic and geodynamic significance. The early cycle — Oligo-Miocenic in age (29-13 My ago) — shows the calc-alkaline character typical of converging plate areas. The later activity, ranging from Lower Pliocene (about 5 My ago) to recent Pleistocene, produced mostly basic lavas extruded onto a continental plate («within plate basalts»). It was related to a period of tensional tectonics which had affected the western Mediterranean area prior to, and during, the volcanic activity. Intermediate and acid volcanic products were associated with the mafic rocks of the latest magmatic episodes. The main groups of rocks — the basic ones, already classified from their petrographic features and geological setting — can be characterized very well when a statistical elaboration of their chemical analyses is used. In fact, from chemical data it is possible to distinguish the Oligo-Miocene volcanic products from those of Plio-Pleistocene age. Moreover, within this latter group basanites, alkalic and subalkalic basaltic rocks can clearly be distinguished. Samples that had not clearly been defined on the basis of their petrographic characteristics (anonymous samples) have been attributed to one or the other of the main groups by means of discriminant functions. Chemical variations in the Plio-Pleistocenic rocks are due to fractionation episodes at shallow depths superimposed on primary magmatic variations. A model of partial fusion of the mantle accounts for many but not all the observed original chemical variations. Different physical melting conditions, the effects of minor mineral phases in the mantle and, probably, crustal contamination were also effective in creating the observed chemical variations.