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Podiform chromitite-bearing ultrabasic rocks from the Bragança Massif, northern Portugal: fragments of island arc mantle?

Published online by Cambridge University Press:  01 May 2009

J. C. Bridges ,
H. M. Prichard  and
C. A. Meireles
J. C. Bridges
Affiliation:
Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK
H. M. Prichard
Affiliation:
Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK
C. A. Meireles
Affiliation:
Servicos Geológicos de Portugal, Rua da Amieira, 4466 S. Mamede de Infesta, Portugal
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Abstract

The Upper Allochthonous Thrust Complex (UATC) of the Bragança massif in northern Portugal contains a set of ultrabasic rocks interthrust with granulites. The ultrabasic rocks have refractory silicate mineral and whole rock compositions which indicate an origin as depleted mantle. Phase relationships of harzburgite samples suggest that they formed in equilibrium with high-Mg picritic melts created through a high degree of mantle partial melt extraction. Chromite in small podiform deposits has 100 Cr/(Cr + Al) ratios of 62–85, which are consistent with crystallization from such melts. Most of the chromite composition parameters are similar to those of ophiolite deposits except for the high ferric iron contents (2.77–8.95 wt% Fe2O3). Such enrichment is a feature of chromite from island arc magmas. It is suggested that the extensive partial melt extraction and chromite mineralization in the ultrabasic rocks occurred in the upper few kilometres of island arc mantle. The ultrabasic rocks were tectonically emplaced into a granulite and eclogite-bearing arc-continent collision complex during the Early Ordovician and subsequently, in the mid-Devonian emplaced over the Central-Iberian terrane.

Type
Articles
Information
Geological Magazine , Volume 132 , Issue 1 , January 1995 , pp. 39 - 49
Copyright
Copyright © Cambridge University Press 1995

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