PUMA
Istituto di Geoscienze e Georisorse     
Perini G., Conticelli S., Francalanci L., Davison J. The relationship between potassic and calc-alkaline post-orogenic magmatism at Vico volcano, central Italy. In: Journal of Volcanology and Geothermal Research, vol. 95 pp. 247 - 272. Elsevier Science, 2000.
 
 
Abstract
(English)
Post-orogenic magmatism in Italy took place during the Plio-Pleistocene. Alkaline ultrapotassic magmas and sub-alkaline magmas were erupted from closely-spaced volcanoes throughout the central part of Italy. Therefore, central Italy is one of the best site to study the relationship between calc-alkaline and alkaline magmatism in complex tectonic regimes. Activity at Vico volcano developed in three periods between 0.419 Ma and 0.095 Ma. Vico products overlie leucite-free volcanic rocks of Monte Cimino volcano that have a variable affinity from high-potassium calc-alkaline to ultrapotassic. Most of the Vico rocks are leucite-bearing Roman-type (ultrapotassic). However, during the first period, leucite-free silicic rocks are interbedded with leucite-bearing products. The early-erupted Vico magmas are isotopically distinct from the youngest magmas. Age-corrected Sr (Sri=0.71110-0.71149) and Nd (Ndi=0.51210-0.51211) isotope ratios fall within the gap between Roman-type (including younger Vico) and Monte Cimino rocks. Some leucite-free Vico rocks contain orthopyroxene crystals and Al contents in clinopyroxenes are lower than those in clinopyroxenes from Roman-type rocks. These characteristics are, however, also found in Monte Cimino rocks. At the beginning of the first period, leucite-free latites to trachytes and rhyolites with high Sri (0.71127-0.71149) were erupted. Trachytes and leucite-bearing latites with lower Sri (0.71110-0.71138) were erupted later. The younger trachytes have higher Rb, and LRE element contents than older leucite-free rocks with similar SiO2 contents. Furthermore, the onset of leucite crystallization is marked by a change in 87Sr/86Sr and SiO2. Sr-isotope ratios and their time-related variations, as well as the presence of two different trends in some trace element variation diagrams, define two distinct modes of magma differentiation. Magmas evolved along a stratigraphically "lower trend" at the beginning of the activity whereas they evolved differently along a stratigraphically "upper trend" later in the volcanic history. Modelling of geochemical and Sr-isotope variations indicates that, along the lower trend, fractional crystallisation was the main evolutionary process, whereas along the upper trend mixing between a leucite-free magma and a leucite-bearing magma occurred during crystallisation. The Sr-isotope compositions of rocks from the early stage of activity at Vico suggest mixing between magmas having different petrological affinities: a leucite-bearing Roman-type magma and a high-potassium calc-alkaline magma.
URL: http://https://www.journals.elsevier.com/journal-of-volcanology-and-geothermal-research/
Subject petrology
mineralogy
Sr-isotope
magma mixing
genesis of potassic rocks
Vico volcano
Roman province
central Italy


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