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Conticelli S., Avanzinelli R., Poli G., Braschi E., Giordano G. Shift from lamproite-like to leucititic rocks: Sr-Nd-Pb isotope data from the Monte Cimino volcanic complex vs. the Vico stratovolcano, Central Italy. In: Chemical Geology, vol. 353 pp. 246 - 266. Elsevier, 2013.
 
 
Abstract
(English)
In the Italian peninsula leucite-free and -bearing ultrapotassic rocks occur intimately associated but well separated in time. Silica-saturated ultrapotassic and associated shoshonitic magmas erupted during the Pliocene to Lower Pleistocene. Silica under-saturated leucite-bearing ultrapotassic rocks, mainly leucitites, were emplaced sometime later, during the Middle-Upper Pleistocene. The transition from leucite-free to -bearing rocks is diachronous and in some cases is complicated by the occurrence of crustal-derived magmas coeval with early leucite-free magma.The Monte Cimino-Vico area is a key locality for studying the transition from leucite-free to -bearing ultrapotassic rocks. There a volcanic complex is overlain by some 500 ka younger leucite-bearing rocks of Vico stratovolcano. Most of the potassic and ultrapotassic rocks of the Monte Cimino volcanic complex are characterised by high Mg-# (66.4-77.8), and compatible element abundances in spite of their high silica contents (52.9-58.0 wt.%). Ultrapotassic leucite-free rocks transitional to lamproites are olivine-bearing orthopyroxene- and plagioclase-free latites and K-rich basaltic trachyandesite and they are confined in the final mafic activity of the Monte Cimino volcanic complex. Two-pyroxene parageneses are observed in the early Monte Cimino volcanic complex giving equilibration temperature as high as 1050 C and suggesting a sub-crustal derivation of their parental magmas. A two-pyroxene high-K calc-alkaline magma is then inferred for the genesis of the early Monte Cimino activity (e.g. dome complex and silicic lava flows and ignimbrites), which was modelled through crystal-fractionation from a high-K basaltic andesitic magma plus mixing with crustal anatectic rhyolitic magma. Final mafic lavas fill the isotopic and chemical gap between shoshonite-like and lamproite-like rocks, and each lava flow could be considered as a mantle-derived term from a heterogeneous sub-continental lithospheric mantle. Final mafic lavas are high-silica and -MgO ultrapotassic rocks due to crystallisation of primitive magmas generated by partial melting of a vein metasomatised network within a lithospheric upper mantle source, at low pressure and high PH2O. The shift to the younger Vico volcano, in which leucite-bearing magmas prevail, is thought to be due to the arrival in the mantle source of newly formed metasomatic agent from the undergoing slab characterised by the increasing of a sedimentary carbonate recycled component. This new component appears in the Middle Pleistocene and it is responsible for the composition of the magmatism of the Roman region. The recycling of carbonate-bearing pelitic sediments within the mantle wedge produces a carbonate-rich metasomatic agent. The latter is responsible for stabilisation of phlogopite-bearing wehrlitic veins within the mantle wedge. Partial melting of this newly formed vein network under high XCO2 generated silica under-saturated, and then leucite-bearing, ultrapotassic primary melts. A further shift is observed among the Vico post-caldera magmas, in which the appearance of a deep asthenospheric component is argued to be channelled late through slab-tears. Final mafic lavas fill the isotopic and chemical gap between shoshonite-like and lamproite-like rocks, and each lava flow could be considered as a mantle-derived term from a heterogeneous sub-continental lithospheric mantle. Final mafic lavas are high-silica and -MgO ultrapotassic rocks due to crystallisation of primitive magmas generated by partial melting of a vein metasomatised network within a lithospheric upper mantle source, at low pressure and high PH2O. The shift to the younger Vico volcano, in which leucite-bearing magmas prevail, is thought to be due to the arrival in the mantle source of newly formed metasomatic agent from the undergoing slab characterised by the increasing of a sedimentary carbonate recycled component. This new component appears in the Middle Pleistocene and it is responsible for the composition of the magmatism of the Roman region. The recycling of carbonate-bearing pelitic sediments within the mantle wedge produces a carbonate-rich metasomatic agent. The latter is responsible for stabilisation of phlogopite-bearing wehrlitic veins within the mantle wedge. Partial melting of this newly formed vein network under high XCO2 generated silica under-saturated, and then leucite-bearing, ultrapotassic primary melts. A further shift is observed among the Vico post-caldera magmas, in which the appearance of a deep asthenospheric component is argued to be channelled late through slab-tears.
URL: http://www.journals.elsevier.com/chemical-geology/
Subject sediment recycling
leucite-free/leucite-bearing ultrapotassic
rocks
lamproite
Sr-Nd-Pb isotopes
Tuscan and Roman magmatic provinces
central Italy


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