Petrogenesis of mafic lavas from the northernmost sector of the Iblean district (Sicily)
Bianchini, Gianluca; Clocchiatti, Roberto; Coltorti, Massimo; Joron, Jean Louis; Vaccaro, Carmela
European Journal of Mineralogy Volume 10 Number 2 (1998), p. 301 - 316
published: Mar 31, 1998
manuscript accepted: Oct 21, 1997
manuscript received: Jul 3, 1996
ArtNo. ESP147051002010, Price: 29.00 €
Abstract A wide spectrum of mafic volcanic rocks, ranging from Qz-tholeiites to strongly undersaturated alkaline lavas (ankaratrites), were erupted during Upper Miocene and Plio-Pleistocene in the Iblean area. The rather primitive nature of these products indicates that the differentiation processes occurred from the source region to the surface affected these magmas only in a subordinate way. For this reason, on the basis of the presented chemical and mineralogical data, the petrogenetic study of the Iblean volcanic suite is mainly aimed at constraining and evaluating the partial melting processes and the nature of the mantle sources. The modelling of the available chemical data (both major and trace elements) indicates that these near-primary melts cannot have been produced 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 suitable mantle sources are represented by variously "enriched', spinellherzolites, containing accessory phases such as amphibole for tholeiitic 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, the wide range of mafic lavas collected in the Iblean area appear to have 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 the mantle beneath the studied area and induced by the Miocene and Plio-Pleistocene transcurrent-extensional tectonic regime. The chemical compositions of the various lavas depend on the relative proportions of the differently enriched mantle domains involved. Small degrees of partial melting sampled preferentially low-solidus enriched domains, producing the alkaline magmas. At higher partial melting degrees, larger domains of less enriched mantle portions contributed to the magmatogenetic processes, thus generating the subalkaline magmas.