Petrogenesis and evolution of Mt. Vulture alkaline volcanism (Southern Italy)

摘要

The Late Pleistocene Mt. Vulture strato-volcano developed at the intersection of NESW and NW-SE lithospheric fault systems, on the easternmost border of the Apennine compressional front overthrust onto the Apulian foreland. The initial phase of the volcanic activity is represented by pyroclastic deposits, including lava blocks, and subordinate eccentric domes, mostly phonolitic in composition. The later stages of activity formed the bulk of the strato-volcano (pyroclastic products and subordinate lavas), mostly tephritic in composition, with minor intercalations of basanite, melafoidite and melilitite lavas and dikes.Variations in rock and mineral composition suggest that the volumetrically predominant basanite-tephrite (foidite)-phonotephrite-phonolite series can be accounted for by fractional crystallization processes starting from basanitic parental magmas, in agreement with the remarkably constant ~(87)Sr/~(86)Sr isotopes (0.70586-0.70581). Mass-balance calculations indicate that the variably differentiated magmas may have been produced by removal of wehrlite, clinopyroxenite and syenite cumulates, some of which are occasionally found as cognate xenoliths in the volcanics. Fractionation processes probably developed I multiple-zoned magma chambers, at depths of 3-5 km, corresponding to the tectonic discontinuity between the allochthonous Apennine formations and the underlying Apulian platform. Highly differentiated phonolitic magmas capping the magma chambers and their conduits thus appear to have fed the initial volcanic activity, whereas dominantly tephritic products wee erupted in later stages.The least evolved mafic magmas, namely basanites, mela-foidites and melilitites, are characterized by diverse Na/K ratios and critical SiO_2-undersaturation, which indicate their derivation as independent melts generated from distinct, heterogeneously enriched mantle sources and by variable partial melting degrees.Primitive mantle-normalized incompatible element patterns of Vulture mafic lavas invariably share analogies with both orogenic subduction-related magmas (High Low Field Strength Elements/High Field Strength Elements ratios, K, Rb and Th contents and marked Ti and Nb negative anomalies) and alkaline lavas from within-plate and rift settings (high Light Rare Earth Elements, P, Zr, Nb and Na). These geochemical features may be accounted for by magma generation from deep lithospheric mantle sources, enriched in Na-alkali silicate/carbonatites anorogenic components, subsequently affected by orogenic subduction-related K-metasomatism, analogous to that which modified magma sources of the Roman Magmatic Province along the internal Apennine Chain.