PETROGENESIS OF MAFIC LAVAS FROM THE NORTHERNMOST SECTOR OF THE IBLEAN DISTRICT (SICILY)

A wide spectrum of mafic volcanic rocks, ranging from Qz-tholeiites to strongly undersaturated alkaline lavas (ankaratrites), were erupted d uring Upper Miocene and Plio-Pleistocene in the Iblean area. The rathe r primitive nature of these products indicates that the differentiatio n processes occurred from the source region to the surface affected th ese magmas only in subordinate way. For this reason, on the basis of t he presented chemical and mineralogical data, the petrogenetic study o f the Iblean volcanic suite is mainly aimed at constraining and evalua ting the partial melting processes and the nature of the mantle source s. The modelling of the available chemical data (both major and trace elements) indicates that these near-primary melts cannot have been pro duced by different degrees of melting from the same source; this would suggest that their differences reflect, at least in part, significant variations in source composition. The calculations suggest that suita ble mantle sources are represented by variously ''enriched'' spinel-lh erzolites, containing accessory phases such as amphibole for tholeiiti c and transitional basalts (A-Th, B-Th and TB), amphibole + phlogopite for alkaline lavas (AB, Bn) and amphibole + phlogopite +/- apatite +/ - carbonate for highly alkaline rocks (A-Ank, B-Ank). In this light, t he wide range of mafic lavas collected in the Iblean area appear to ha ve been produced by different degrees of partial melting, ranging from 2-3% for the highly alkaline rocks to 22 % for the Qz-tholeiites, of a lithospheric mantle characterized by small-scale heterogeneity. This process has been favoured by relatively high thermal conditions in th e mantle beneath the studied area and induced by the Miocene and Plio- Pleistocene transcurrent-extensional tectonic regime. The chemical com positions of the various lavas depend on the relative proportions of t he differently enriched mantle domains involved. Small degrees of part ial melting sampled preferentially low-solidus enriched domains, produ cing the alkaline magmas. At higher partial melting degrees, larger do mains of less enriched mantle portions contributed to the magmatogenet ic processes, thus generating the subalkaline magmas.