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U–Pb isotopic ages and Hf isotope composition of zircons in Variscan gabbros from central Spain: evidence of variable crustal contamination

Villaseca González, Carlos and Orejana García, David and Belousova, Elena and Armstrong, Richard A. and Pérez-Soba Aguilar, Cecilia and Jeffries, Teresa E. (2011) U–Pb isotopic ages and Hf isotope composition of zircons in Variscan gabbros from central Spain: evidence of variable crustal contamination. Mineralogy and petrology, 101 . pp. 151-167. ISSN 0930-0708

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Abstract

Ion microprobe U–Pb analyses of zircons from
three gabbroic intrusions from the Spanish Central System
(SCS) (Talavera, La Solanilla and Navahermosa) yield
Variscan ages (300 to 305 Ma) in agreement with recent
studies. Only two zircon crystals from La Solanilla massif
gave slightly discordant Paleoproterozoic ages (1,848 and
2,010 Ma). Hf isotope data show a relatively large variation
with the juvenile end-members showing ɛHfi values as high
as +3.6 to +6.9 and +1.5 to +2.9 in the Navahermosa and
Talavera gabbros, respectively. These positive ɛHfi values up
to +6.9 might represent the composition of the subcontinental
mantle which generates these SCS gabbros. This ɛHfi
range is clearly below depleted mantle values suggesting the
involvement of enriched mantle components on the origin of
these Variscan gabbros, and is consistent with previous
whole-rock studies. The presence of zircons with negative
ɛHfi values suggest variable, but significant, crustal contamination
of the gabbros, mainly by mixing with coeval granite
magmas. Inherited Paleoproterozoic zircons of La Solanilla
gabbros have similar trace element composition (e.g. Th/U
ratios), but more evolved Hf-isotope signatures than associated
Variscan zircons. Similar inherited ages have been
recorded in zircons from coeval Variscan granitoids from the
Central Iberian Zone. Granitic rocks have Nd model ages
(TDM) predominantly in the range of 1.4 to 1.6 Ga,
suggesting a juvenile addition during the Proterozoic.
However, Hf crustal model ages of xenocrystic Proterozoic
zircons in La Solanilla gabbro indicate the presence of
reworked Archean protoliths (TDM2 model ages of 3.0 to
3.2 Ga) incorporated into the hybridized mafic magma.

Item Type:Article
Uncontrolled Keywords:U–Pb isotopic; Hf isotope; Zircons; Variscan; Central Spain; Crustal contamination
Subjects:Sciences > Geology > Petrology
ID Code:17208
Deposited On:26 Nov 2012 12:54
Last Modified:26 Nov 2012 12:54

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