Eurico Pereira

Tectonic evolution of the deep crust: Variscan reactivation by extension and thrusting of Precambrian basement in the Bragança and Morais massifs (Tras-os-Montes, NE Portugal)

AbstractThe Braganca amd Morais Massifs (NE Portugal) comprise a pile of four nappes on lop of the Autochthon of the Central-Iberian Zone : a Parautochthon (PTC), a Lower Allochthon (LATC), an Ophiolitic Complex (OTC) and an Upper Allochton (UATC). This article focuses on the tectonic evolution of the prc-Variscan basement preserved in the Upper Allochthonous Thrust Complex. (1) In the Morais Massif, the UATC is mainly composed of orthogneisscs, micaschists and high-grade melamorphic rocks restricted to a small duplex between the orthogneisses and the ophiolitic complex. The orthogneisses are pervasively deformed by D6 (Variscan D1). characterized by NNW-SSE stretching lineation, C-S structures, and sense of shear to the SSE. The high-grade melamorphic rocks show at least three ductile deformation phases older than the gneisses deformation. The micaschists and the orthogneisses are cut by mafic sills and dykes transformed into amphibolites by the Variscan tec-tonometamorphic evolution. In a restricted dom...

Reply to comment by M. Francisco Pereira et al. on ''Geodynamic evolution of the SW Europe Variscides''

] The interpretation reported by Ribeiro et al. [2007]favors the presence of a Cadomian basement within the SWEurope Variscides, eventually including older relics,considering as well the role of this basement in theevolution of the Variscan cycle; this particular issue wasdeveloped in a subsequent paper [Ribeiro et al., 2009]. Thepossibility of Grenville inliers is not definitely ruled out bythe arguments of Pereira et al. [2009]. Indeed, a few (nearlyconcordant) U/Pb zircon ages of 0.94 to 1.2 Ga are reportedby Linnemann et al. [2008]; several other U/Pb zircongeochronological data from Na¨gler et al. [1995], de la Rosaet al. [2002], Sola´etal.[2008], and Cordani et al. [2006](upper intercept date for sample 43B-5.1 assuming lead lossduring Variscan metamorphism at 360 Ma), although notdefinitive, also suggest the inheritance of Grenville ages inOssa-Morena Zone (OMZ) rocks. Furthermore, detritalzircons in Me´rtola Formation (South Portuguese Zone)proximal greywackes(includinghigh-grademetamorphiclith-oclastssimilar to those rocks found at the OMZ E´vora Massif)reveal a frequency age distribution peak at 0.94 Ga (R. Jorge,personal communication, 2009), consistent with derivationfrom inliers of a Greenville orogen, which is well datedbetween 1.2 and 0.95 Ga in the North American craton.Therefore, the possibility of pre-Cadomian cycles remainsdebatable (as signed by a question mark in the text of Ribeiroet al. [2007]), as well as the provenance of detrital zircons andtheconnectionsbetweenIberiaandWestAfrica,AmazoniaandLaurentia cratons prior to the assemblage of Pannotia Super-continent. Further investigations are needed to solve thisproblem within an acceptable range of uncertainty.[