Source contamination and mantle heterogeneity in the genesis of Italian potassic and ultrapotassic volcanic rocks: Sr-Nd-Pb isotope data from Roman Province and Southern Tuscany

Abstract

The Tyrrhenian border of the Italian peninsula has been the site of intense magmatism from Pliocene to recent times. Although calc-alkaline, potassic and ultrapotassic volcanism overlaps in space and time, a decrease of alkaline character in time and space (southward) is observed. Alkaline ultrapotassic and potassic volcanic rocks are characterised by variable enrichment in K and incompatible elements, coupled with consistently high LILE/HFSE values, similar to those of calc-alkaline volcanic rocks from the nearby Aeolian arc. On the basis of mineralogy and major and trace element chemistry two different arrays can be recognised among primitive rocks; a silica saturated trend, which resulted in formation of leucite-free mafic rocks, and a silica undersaturated trend, charactrerised by leucite-bearing rocks. Initial 87Sr/86Sr and 143Nd/144Nd values of Italian ultrapotassic and potassic mafic rocks range from 0.70506 to 0.71672 and from 0.51173 to 0.51273, respectively. 206Pb/204Pb values range between 18.50 and 19.15, 207Pb/204Pb values range between 15.63 and 15.70, and 208Pb/204Pb values range between 38.35 and 39.20. The general εSr vs. εNd array, along with crustal lead isotopic values, clearly indicates that a continental crustal component has played an important role in the genesis of these magmas. The main question is where this continental crustal component has been acquired by the magmas. Volcanological and petrologic data indicate continental crustal contamination to be a leading process along with fractional crystallisation and magma mixing. Considering, however, only the samples thought to represent primary magmas, which have been in equilibrium with their mantle source, a clearer picture emerges. A large variation of εSr vs. εNd is still observed, with εSr from −2 to +180 and εNd from + 2 to −12. A bifurcation of this array is observed in the samples that plot in the lower right quadrant, with mafic leucite-bearing Roman Province rocks buffered at εSr = + 100 whereas the mafic leucite-free potassic and ultrapotassic rocks point to strongly radiogenic Sr compositions. We may argue that mafic leucite-bearing Roman Province rocks point to εSr and εNd values similar to those of Miocene carbonate sediments whereas mafic leucite-free potassic and ultrapotassic rocks point to a silicate upper crust end-member. Lead isotopes plot well inside the field of island arcs, overlapping the values of pelagic sediments as well, but bifurcation between the samples north and south of Rome is observed. The main characteristic for the mantle source of Italian potassic and ultrapotassic magmas is the clear upper crustal signature acquired prior to partial melting through metasomatic agents released by the subducted slab. In addition, one lithospheric mantle source in the north and an asthenospheric mantle source, pointing to an HIMU reservoir, in the south were recognised. The chemical and isotopic differences observed between the northern and southern sectors of the magmatic region were possibly due to the presence of a...

Faunal migration in late‐glacial central Italy: implications for human resource exploitation

“…68 The dentine of the same tooth lies, however, within the range of the other samples. Given these isotopic patterns, and considering that the teeth are not significantly worn, we estimate that mineralisation took place over about 9 months, between winter and late summer.…”

Magmatism, mantle evolution and geodynamics at the converging plate margins of Italy

“…4) requires extensive element transfer to the mantle wedge, which can be accomplished by large degrees of sediment melting (Peccerillo & Martinotti 2006). Such a contamination process is strikingly distinct from that envisaged for the nearby Roman and Intra-Apennine provinces, a conclusion corroborated by recent geochemical and isotopic investigations (Conticelli et al 2002Avanzinelli et al 2009;Prelevic et al 2008Prelevic et al , 2010. Moreover, the highly 'primitive' characters of some rocks, especially lamproites, and their moderate or low concentrations of Na 2 O and CaO suggest a residual harzburgite as the pre-metasomatic mantle rock, probably representing the local lithosphere (e.g.…”

“…As discussed above, magma contamination processes were ubiquitous, and had strong effects on oxygen isotope composition but not on Sr-Nd-Pb radiogenic isotope and incompatible element ratios (e.g. Rogers et al 1985;Conticelli et al 2002;Peccerillo 2005, and references therein).…”

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