Polyphase Variscan emplacement of exotic terranes (Morais and Bragança Massifs) onto Iberian successions: Evidence from 40Ar/ 39Ar mineral ages

Abstract

The Bragança and Morais Nappe Complexes comprise internally imbricated continental tectonic units (including schists, deformed metagranite, granulite and local eclogite) and a structurally underlying ophiolite nappe. High-pressure assemblages in some of the continental sequences were variably retrogressed during an amphibolite facies metamorphic overprint. By contrast, rocks within the ophiolite nappe display prograde amphibolite facies assemblages. Hornblende concentrates from both tectonic units exposed in the Bragança Nappe Complex record 40Ar/ 39Ar plateau and isotope correlation ages of c. 385–390 Ma. A similar date is recorded by hornblende within mylonitic amphibolite (ophiolite protolith) along a ductile thrust which separates the two major nappe units. Muscovite within blastomylonitic metagranite (Lagoa augen gneiss) in upper structural levels of the Morais Massif records 370–375 Ma 40Ar/ 39Ar plateau ages. Muscovite within phyllinite developed along a ductile thrust separating the gneiss from the structurally underlying ophiolite nappe records a 375 Ma age. Together the 40Ar/ 39Ar data suggest that a regionally significant, early Variscan amphibolite facies metamorphism occurred prior to c. 385–390 Ma (Early Devonian). This was associated with ductile imbrication of continental and ophiolite structural units with resultant formation of a composite nappe complex. The composite nappe complex must have been maintained at relatively high structural levels throughout the remaining Variscan evolution because there is no evidence for post-370 Ma rejuvenation of intracrystalline argon systems in any of crystalline structural units examined. This is in marked contrast to a significant Late Devonian (c. 330 Ma), locally high pressure metamorphism which is recorded in structurally underlying elements of the Lower Allochthon (part of a foreland thrust complex composed of metasedimentary units with Iberian protolith affinities). Together available chronologic constraints indicate emplacement of the previously assembled composite nappe complex into its present relative structural position at c. 330 Ma with resultant deformation and metamorphism of structurally underlying components of the Iberian thrust complex.