Anisotropy of magnetic susceptibility constraints on Variscan obduction processes in the Braganca Massif (NE Portugal)

In this paper, we present an anisotropy of magnetic susceptibility (AMS) an d structural study of high-grade metamorphic and ophiolitic rocks from the Braganca Massif, trying to contribute to the debate concerning the identifi cation of transport directions during the closure of the Variscan Ocean. Th e study area comprises high-grade metamorphic rocks of the Continental Allo chthonous Terrane (CAT) and the Northern Ophiolite Terrane (NOT) in the Bra ganca Massif. Mineral parageneses and fabrics were characterized through co nventional petrographic and microstructural studies, electron microprobe, a nd high field and thermomagnetic analysis. With very few exceptions, the ma gnetic lineations and foliations were coincident, within limited error, wit h the observed mesoscopic mylonitic and metamorphic foliations, and with th e mineral and stretching lineations. In places of the studied major tectoni c contacts where the mineral and stretching lineations were obliterated by post-kinematic recrystallization. AMS data revealed a magnetic lineation. A MS results confirmed the NNW-SSE to N-S amphibolite facies lineation common to CAT and NOT, and revealed an E-W greenschist facies lineation in NOT. A MS results combined with detailed structural and metamorphic data from prev ious work showed that there are two E-W lineations separated in time: (1) a n E-W mesoscopic and magnetic lineation in competent granulites that is old er than the N-S magnetic lineation in more ductile gneisses of a major shea r zone within the CAT; (2) an E-W magnetic lineation in incompetent greensc hists of the NOT, younger than the NNW-SSE to N-S mesoscopic and magnetic l ineations found in more competent amphibolites of CAT and NOT. The NNW-SSE to N-S lineations can be interpreted as the result of the uppermost allocht honous terranes transport to the NNW over the Iberian Terrane, dated With A r-40/Ar-39 at ca. 390 Ma. (C) 2001 Elsevier Science B.V. All rights reserve d.