Chemical and isotopic composition (Os, Pb, Nd, and Sr) of Neogene to Quaternary calc-alkalic, shoshonitic, and ultrapotassic mafic rocks from the Italian peninsula: Inferences on the nature of their mantle sources

Abstract

During the Neogene and Quaternary, potassic and ultrapotassic magmas erupted in association with shoshonitic and calc-alkalic magmas across much of the Italian penin sula. On the basis of the temporal and spatial distribution of this volcanism, and its mineralogical and compositional characteristics, three different magmatic provinces have been defi ned. The northwesternmost province, the Tuscan Magmatic Province, is domi nated by leucite-free ultrapotassic rocks (lamproite), shoshonite, and minor calcalkalic rocks. The Roman Magmatic Province is dominated by leucite-bearing rocks with variable degrees of silica saturation, from undersaturated (leucitites and plagioleucitites) to strongly undersaturated (kamafugites), but minor amounts of shoshonitic to high-potassium calc-alkalic rocks are still present. The Lucanian Magmatic Province, located at the southeasternmost edge of the volcanic belt, is dominated by foiditic (hauynites and leucitites) and kamafugitic (melilitites) members, all strongly undersaturated in silica. In spite of these petrologic differences, the Neogene Italian potassic and ultrapotassic rocks display similar trace-element patterns. Depletion in high fi eld strength elements with respect to large ion lithophile elements is a common feature. Sr, Nd, and Pb iso topic compositions of mafi c high-MgO rocks range widely, relating mainly to geographic location of eruption and to enrichment in alkalies. The Os isotopic composition of these samples, however, does not clearly correlate with eruption location, but is dependent on the amount of “continental crust component” added to the magmas. Some of the studied samples are compatible with crustal contamination