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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

By
Sandro Conticelli
Sandro Conticelli
Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via Giorgio La Pira, 4, I 50121, Firenze, Italy
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Richard W. Carlson
Richard W. Carlson
Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, DC 20015, USA
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Elisabeth Widom
Elisabeth Widom
Department of Geology, Miami University, 114 Shideler Hall, Oxford, Ohio 45056, USA
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Giancarlo Serri
Giancarlo Serri
Dipartimento di Scienze della Terra, Università degli Studi di Parma, Parco Area delle Scienze, 157, I 43100, Parma, Italy
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Published:
January 01, 2007

During the Neogene and Quaternary, potassic and ultrapotassic magmas erupted in association with shoshonitic and calc-alkalic magmas across much of the Italian peninsula. On the basis of the temporal and spatial distribution of this volcanism, and its mineralogical and compositional characteristics, three different magmatic provinces have been defined. The northwesternmost province, the Tuscan Magmatic Province, is domi nated by leucite-free ultrapotassic rocks (lamproite), shoshonite, and minor calc-alkalic rocks. The Roman Magmatic Province is dominated by leucite-bearing rocks with variable degrees of silica saturation, from undersaturated (leucitites and plagio-leucitites) 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 (haüynites and leucitites) and kamafugitic (melilitites) members, all strongly under-saturated in silica. In spite of these petrologic differences, the Neogene Italian potassic and ultrapotassic rocks display similar trace-element patterns. Depletion in high field strength elements with respect to large ion lithophile elements is a common feature. Sr, Nd, and Pb isotopic compositions of mafic 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 en route to the surface. In most cases, however, there are several lines of evidence suggesting the possibility that crustal components were added directly to the mantle source prior to partial melting. Large amounts (many tens of weight percent) of “crustal component” must be added to the peridotitic mantle in order to obtain the 187Os/188Os of the lamproites in Tuscany. These large amounts of crustal components have been recycled into the mantle in the form of either melts or fluids. The recycling can be reconciled with a veined mantle in which the crustal component is concentrated. Partial melting of veins would then produce the high-silica and high-magnesium lamproitic magmas from Tuscany. Dilution of the crustal components by increasing partial melting of surrounding mantle peridotite, or alternatively, a reduction of metasomatic veins, could then produce shoshonitic and high-K calc-alkalic mafic magmas.

Southeastward geochemical and isotopic variations are reconciled with decreasing direct contributions from crustal components introduced into the mantle by sub-duction, but an increasing role of subducted fluids from dehydration of CO2-rich sediments. The coupled isotopic and chemical characteristics of Italian magmas cannot be reconciled with an ocean-island basalt (OIB)–like primary magma composition due to the substantial overprinting by crustal- and or subduction-related components.

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GSA Special Papers

Cenozoic Volcanism in the Mediterranean Area

Luigi Beccaluva
Luigi Beccaluva
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Gianluca Bianchini
Gianluca Bianchini
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Marjorie Wilson
Marjorie Wilson
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Geological Society of America
Volume
418
ISBN print:
9780813724188
Publication date:
January 01, 2007

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