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Constraints on the Genesis of Potassium-rich Italian Volcanic Rocks from U/Th Disequilibrium |
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| Authors: | Avanzinelli Riccardo; Elliott Tim; Tommasini Simone; Conticelli Sandro |
| Institution: | 1Dipartimento Di Scienze Della Terra, Università Degli Studi Di Firenze, Via Giorgio La Pira, 4, I-50121, Firenze, Italy 2Bristol Isotope Group, Department Of Earth Sciences, University Of Bristol, Wills Memorial Building, Queen'S Road, Bristol Bs8 1Rj, Uk 3Istituto Di Geoscienze E Georisorse, Cnr, Via Giorgio La Pira, 4, I-50121, Firenze, Italy |
| Abstract: | We present new U-series isotope, 87Sr/ 86Sr, 143Nd/ 144Nd andtrace element data for a set of mafic, K-rich rocks from volcanoesin Central–Southern Italy. These shoshonitic to ultrapotassiclavas display strongly depleted high field strength element(HSFE) abundances with respect to other incompatible trace elementstogether with high but variable 87Sr/ 86Sr and low but variable143Nd/ 144Nd values. Such characteristics are thought to bedue to addition of subducted crust of variable amount and compositionto their mantle sources prior to magma genesis. Rocks from thenorthernmost region (i.e. Tuscan Magmatic Province and NorthernRoman Magmatic Province) display (230Th/ 238U) activity ratiosclose to radioactive equilibrium, suggesting that metasomatismof their sources occurred before 400 ka and recent melting tookplace at shallow depths, in the absence of garnet. A 238U excessof up to 27% has been measured in rocks from the NeapolitanDistrict. The occurrence of significant U excesses is a featureof arc magmas, but is typically seen in depleted lavas ratherthan in highly enriched rocks such as these ( 20 ppm Th). Thissignature requires a recent addition of a U-rich component tothe already strongly enriched mantle wedge beneath this regionof Italy. We suggest that a supercritical liquid, from deeplysubducted carbonate-rich sediments of the still-active Ionianslab, is responsible for generating a high-U, low-Th component,which produces the observed disequilibria. A 30% 230Th excessmeasured in a single unaltered sample from the Lucanian MagmaticProvince, along with a less marked negative HFSE anomaly, suggeststhe contribution of a deeper, garnet-bearing component in thegenesis of these magmas, plausibly related to the upwellingof asthenospheric mantle around the corner of the Ionian slab. KEY WORDS: U/Th disequilibria; potassic and ultrapotassic rocks; subduction: metasomatism; mantle melting; Central and Southern Italy |
| Keywords: | : U/Th disequilibria potassic and ultrapotassic rocks subduction: metasomatism mantle melting Central and Southern Italy |
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