Petrographical and geochemical signatures of the Granja paragneisses (NW Ceará, Brasil)

Abstract

The Granja Granulitic Complex (GCC) constitutes a NE–SW trending high-grade metamorphic belt occupying the western sector of the Médio Coreaú Domain (NW Ceará, Brasil) (Figs. 1 and 2). According to the published data, the GCC represents a segment of the Paleoproterozoic basement intensely reworked during the Brasiliano Orogeny. Lithologically, the GCC is mainly composed of garnet-sillimanite migmatitic paragneisses, enclosing discontinuous lenses of mafic granulites and enderbites. At outcrop scale, the GCC gneisses are well-foliated fineto medium-grained grey rocks showing a stromatic layering parallel to the main regional fabric (Fig. 3). The occurrence of lenses of leucocratic quartzfeldspathic material interlayered with the host paragneisses indicates that these rocks reached partial melting conditions during the Brasiliano orogenic event (≈ 600 Ma). Petrographically, the gneisses are characterized by the presence of mineral assemblages dominated by garnet, sillimanite and biotite with moderate amounts of quartz, plagioclase, cordierite and K-feldspar (Figs. 4a-b). Twelve whole-rock samples were analyzed for major and trace elements. As shown in Graph 1, all the analyzed samples plot in the sedimentary field. Their REE patterns closely resemble those of PAAS (PostArchean average Australian Shale) confirming the sedimentary origin of the paragneiss protoliths (Graph 2). Geochemical data also suggest that these sediments derived from source areas of felsic to intermediate composition and were deposited in a tectonically active continental margin (Graphs 3 and 4).