The basement complex in the Médio Coreaú Domain is comprised of the Granja Massif, a high-grade complex composed mainly of orthogneisses with tonalite-trondhjemite-granodiorite (TTG) affinities, as well as amphibolite gneisses, amphibolites, leucogranites, mafic granulites and enderbites, leptinites, kinzigites, and migmatites (Figure 5; Santos et al., 2001). U-Pb and Sm-Nd studies of these rocks (Fetter et al., 1995, 2000; Santos, 1999) show that samples from all main gneisses are early Paleoproterozoic in age (2.36-2.30 Ga) and yield Nd crustal residence (TDM) ages between 2.61 and 2.38 Ga. As most of the εNd (t = crystalization age) values of the basement gneisses are positive, they represent juvenile crustal growth, probably generated in arc-type setting. The 2.36-2.30 Ga age for the MCD is not only distinctly older than the ca. 2.2-2.0 Ga Transamazonian/Eburnean crust that is predominant in the rest of the Borborema Province, but it also represents a period where very little crustal growth has been recognized worldwide (e.g. Condie, 1989).
In addition to being the oldest part of the domain, the gneisses also represent the base of the structural pile in the MCD, and middle amphibolite to granulite facies, consisting of both orthopyroxene and clinopyroxene, the highest metamorphic grade of all the rocks present. During uplift and exhumation, however, some retrograde mineral assemblages developed, with principal mineral association composed of feldspar-muscovite-quartz and garnet-biotite.
Figure 5. Field photographs of basement rocks
Field photographs of basement rocks. a) Kinzigites (garnet, biotite, quartz, feldspar gneiss) from north of Granja town; b) migmatized granulites refolded during Neoproterozoic deformation; c) migmatite intensely refolded from north of Granja; d) mylonitic migmatite with concentration of partial melt along shear zone cutting photograph diagonally from lower right to upper left-hand-side.