Abstract

A. Ribeiro, F. Pereira & F Marques write: In this comment we present an alternative interpretation of new zircon data obtained by Dallmeyer & Tucker for the Lagoa orthogneisses. The Lagoa orthogneiss and micaschist (the latter of probable Upper Cambrian to Lower Ordovician age) both display a schistosity and a NNW-SSE stretching lineation. In the orthogneiss, S/C (S, schistosity; C, shearing) structures indicate top-to-the-SSE sense of shear for this deformation event. Structural correlation and 40Ar/39Ar plateau ages on muscovites of the Lagoa orthogneiss suggest that this event is the first Variscan deformation phase. The micaschists, the orthogneisses and the contact between both units were intruded by mafic dykes and sills and were later metamorphosed in amphibolite fades conditions. The dykes become subparallel to the flat-lying foliation of the host rocks during deformation. A critical outcrop occurs NW of Cabeço do Gricho, on the well exposed left bank of the Sabor river. Within the orthogneiss, about 100 m above its basal contact with the ophiolite of Morais, there is a less deformed domain where dykes cut across an earlier foliation of the orthogneiss. This is the location of the samples dated by Dallmeyer & Tucker. In this outcrop, on the flat-lying foliation of the orthogneiss, an E-W stretching lineation is preserved and S/C structures define a westward sense of shear. Less penetrative, more discrete ductile shear zones can also be observed cutting the foliation of the orthogneiss and an eastward sense of shear can be deduced from the sigmoidal foliation developed

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