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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

Published online by Cambridge University Press:  05 July 2018

D. R. N. Rosa ,
A. A. Finch ,
T. Andersen  and
C. M. C Inverno
D. R. N. Rosa*
Affiliation:
INETI-Geological Survey, Estrada da Portela-Zambujal, Alfragide, 2720-866 Amadora, Portugal
A. A. Finch
Affiliation:
School of Geography and Geosciences, Irvine Building, North Street, St Andrews, Fife, KY16 9AL, UK
T. Andersen
Affiliation:
Department of Geology, University of Oslo, PO Box 1047 Blindern, N-0316 Oslo, Norway
C. M. C Inverno
Affiliation:
INETI-Geological Survey, Estrada da Portela-Zambujal, Alfragide, 2720-866 Amadora, Portugal
*
*E-mail: [email protected]
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Abstract

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-PbzircongeochronologyLA-ICP-MSIberian Pyrite Belt
Type
Research Article
Information
Mineralogical Magazine , Volume 72 , Issue 5 , October 2008 , pp. 1103 - 1118
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2008

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Footnotes

†

also at: Geology Department/CREMINER-University of Lisbon, Edifício C6, Piso 2, 1749-016 Lisboa, Portugal

‡

also at: CREMINER-University of Lisbon, Edifício C6, Piso 4, 1749-016 Lisboa, Portugal

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