HR: 0800h
AN: V51C-0711    [Abstracts]
TI: Insights From in-situ U-Pb, Trace Elements and Hf Isotopic Analyses in Zircon From a Young, Incrementally-Filled, Monzogranitic Pluton
AU: * Gagnevin, D
EM: damien.gagnevin@ucd.ie
AF: UCD School of Geological Sciences, Belfield, Dublin, 4, Ireland
AU: Daly, J S
EM: stephen.daly@ucd.ie
AF: UCD School of Geological Sciences, Belfield, Dublin, 4, Ireland
AU: Horstwood, M
EM: msah@bgs.ac.uk
AF: NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham, NG12 5GG, United Kingdom
AU: Whitehouse, M J
EM: martin.whitehouse@nrm.se
AF: Laboratory for Isotope Geology, Swedish Museum of Natural History, PO 50007, Stockolm, SE-104 05, Sweden
AU: Kirkland, C L
EM: chris.kirkland@nrm.se
AF: Laboratory for Isotope Geology, Swedish Museum of Natural History, PO 50007, Stockolm, SE-104 05, Sweden
AB: Ion microprobe U-Pb zircon analyses of the Miocene Monte Capanne pluton (Tuscan Magmatic Province, Elba, Italy) - one of the youngest granitic plutons exposed on Earth - reveal ages consistent with incremental intrusion of the three petrographically defined facies of the pluton (Dini et al. 2002). A positive correlation between U-Pb age and high uranium contents (up to 3%) compromises the apparently large age range (7-10 Ma) for the duration of magmatism and is attributed to an unconstrained matrix effect during sputtering. Selecting only zircon data with normal U concentrations, suggests timescales in the order of ca. 1 Ma (from 8.08 ± 0.09 Ma to 7.04 ± 0.29 Ma), i.e., of similar range to magmatic activity in the coeval Capraia volcano (from 7.80 ± 0.08 Ma to 7.10 ± 0.09 Ma), North of Elba. Importantly, zircon ages for the components of the pluton (mafic enclaves, dykes, granite porphyries) partially overlap, suggesting that zircon was transferred and/or recycled throughout the lifetime of the whole plutonic system. Much older inherited zircons also occur (from Archaean to Mesozoic) and are interpreted as being derived mostly from the wall rocks. Detailed BSE imaging has revealed a variety of textures (including patchy-zoning, small and large scale oscillatory zoning etc), associated with an extremely large range of trace to minor elements compositions (e.g., Hf: 7159-21284 ppm, U: 67-65319 ppm and Th: 0-46225 ppm). Intra-grain variations (zoning) in most elements (P, Th, U, Y and HREE) are interpreted to reflect changes in melt chemistry due to iterative mixing and recharge, where growth/resorption of zircon was also associated with reactions between other accessory minerals (particularly monazite, allanite and apatite; Dini et al., 2004). εHf(t) ranges from -14.95 to -4.25 and two populations are found in almost every component of the pluton, which concur with the magma mixing model. A positive correlation between Th and εHf(t) may be related to some wall-rock contamination (in agreement with the occurrence of xenocrystic zircon in most of magma products) and/or derivation from a strongly metasomatised mantle source during mantle-crust mixing (in agreement with previous works on the Tuscan Magmatic Province). Such a model involving mixing, crustal contamination and crystal fractionation is consistent with both chemical and isotopic zoning in K-feldspar megacrysts (Gagnevin et al., 2005a, b) and whole-rock geochemistry (Gagnevin et al., 2004). References: Dini et al. (2002), Geological Magazine 139, 257-279. Dini A. et al. (2004), Lithos 78, 101-118. Gagnevin et al. (2004), Lithos 78, 157-195. Gagnevin et al. (2005a), Journal of Petrology 46, 1689-1724. Gagnevin et al. (2005b), Geochimica Cosmochimica Acta 69, 1899-1915.
DE: 3618 Magma chamber processes (1036)
DE: 3625 Petrography, microstructures, and textures
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting