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Published online 23 March 2009
Mineralogical Magazine; October 2008; v. 72; no. 5; p. 1103-1118; DOI: 10.1180/minmag.2008.072.5.1103
© 2009 Mineralogical Society of Great Britain and Ireland
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U-Pb geochronology of felsic volcanic rocks hosted in the Gafo Formation, South Portuguese Zone: the relationship with Iberian Pyrite Belt magmatism

D. R. N. Rosa1,{dagger},*, A. A. Finch2, T. Andersen3 and C. M. C Inverno1,{ddagger}

1 INETI-Geological Survey, Estrada da Portela-Zambujal, Alfragide, 2720-866 Amadora, Portugal
2 School of Geography and Geosciences, Irvine Building, North Street, St Andrews, Fife, KY16 9AL, UK
3 Department of Geology, University of Oslo, PO Box 1047 Blindern, N-0316 Oslo, Norway

* E-mail: diogo.rosa{at}ineti.pt

Felsic volcanic rocks exposed in the Frasnian Gafo Formation, in the Azinhalinho area of Portugal, display very similar geochemical signatures to volcanic rocks from the Iberian Pyrite Belt (IPB), located immediately to the south. The similarities include anomalously low high field-strength elements (HFSE) concentrations, possibly caused by low-temperature crustal melting, which translate into classification problems.

A geochronological study, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of zircon grains from these rocks, has provided concordia ages of 356±1.5 Ma and 355±2.5 Ma for two samples of rhyodacite porphyry, and 356±1.4 Ma for a granular rhyodacite. These results show that volcanism at Azinhalinho was broadly contemporaneous with IPB volcanism, widely interpreted as being of Famennian to Visean age. Considering that the host rocks of the Azinhalinho volcanic rocks are Frasnian, and therefore deposited synchronously with the Upper Devonian Phyllite-Quartzite Group sedimentation in the IPB basin, the radiometric ages imply that the Azinhalinho felsic rocks are intrusive and likely represent conduits or feeders to the volcanism of the IPB.

KEYWORDS: U-Pb, zircon, geochronology, LA-ICP-MS, Iberian Pyrite Belt






JOURNAL HOME HELP CONTACT PUBLISHER SUBSCRIBE ARCHIVE SEARCH TABLE OF CONTENTS
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