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Ion microprobe U–Pb zircon geochronology of a late tectonic granitic–gabbroic rock complex within the Hercynian Iberian belt

Published online by Cambridge University Press:  09 November 2006

H. P. ZECK ,
M. T. D. WINGATE  and
G. POOLEY
H. P. ZECK
Affiliation:
Tectonics Special Research Centre, University of Western Australia, Crawley, WA 6009, Australia Present address: Geological Museum, 1350K Copenhagen, Denmark.
M. T. D. WINGATE
Affiliation:
Tectonics Special Research Centre, University of Western Australia, Crawley, WA 6009, Australia
G. POOLEY
Affiliation:
Centre for Microscopy and Microanalysis, University of Western Australia, Crawley, WA 6009, Australia
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Abstract

Ion microprobe (SIMS) dating shows that three plutonic rock bodies representative of the major, ∼15 000 km2, late tectonic (late-D3), plutonic rock series in the Central Iberian Zone of the Hercynian belt in west-central Iberia have indistinguishable zircon crystallization ages. Ledrada biotite granite and Colmenar cordierite-bearing biotite granite show gradual transitions in field appearance, petrography and chemistry and have statistically indistinguishable weighted average ages of 306.8 ± 1.9(2σ) Ma and 306.5 ± 1.5(2σ) Ma, respectively, which indicates that they originated during a single event involving a heterogeneous magma which notably varied in Al-content. The third rock body, Navahermosa meta-gabbronorite, has a weighted average zircon crystallization age of 305.6 ± 1.4(2σ) Ma, statistically indistinguishable (variance analysis, F-test, α = 0.05) from the granites. Zircon crystals in the gabbronoritic rock are anhedral, skeletal, millimetre-sized and partake in main magmatic textures, whereas the zircon grains in the granites are of more common appearance, much smaller, usually euhedral and enclosed in main magmatic crystals. As gestation times of granitic zircon, the time between zircon crystallization and magmatic intrusion, may be up to 5 Myr, the crystallization age of the gabbronoritic zircon may be the best estimate of the time of emplacement of the magmatic complex. Our study indicates co-existence of basic and silicic magmas in the Hercynian crustal section at c. 306 Ma, suggesting common genetic control. The two granitic rocks carry inherited zircon ranging from c. 1300 to 330 Ma, indicating that pre-Hercynian basement rocks of Proterozoic to Palaeozoic age contributed to the granitic magma.

Keywords

Hercynian Iberian beltSIMSU–Pb zircon ageslate tectonic plutonicsgranitic–gabbroic complexes
Type
Original Article
Information
Geological Magazine , Volume 144 , Issue 1 , January 2007 , pp. 157 - 177
Copyright
© 2006 Cambridge University Press

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