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

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 ultrapota...

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Bibliographic Details
Published in:Mineralogy and petrology 2002-01, Vol.74 (2-4), p.189-222
Main Authors: Conticelli, S., D'Antonio, M., Pinarelli, L., Civetta, L.
Format: Article
Language:English
Subjects:
Magma
Mineralogy
Petrology
Potassium
Volcanoes
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Summary: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 kSr vs. kNd 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 kSr vs. kNd is still observed, with kSr from m2 to +180 and kNd from + 2 to m12. 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 kSr = + 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 kSr and kNd 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 sampl
ISSN:0930-0708
1438-1168
DOI:10.1007/s007100200004