Petrografia das Rochas do Complexo Cajazeiras, Vila Cruzeiro do Sul, Domínio Bacajá, Cráton Amazônico

Abstract

The Cajazeiras Complex is one of the most extensive units of Bacajá Domain, the southern portion of the Transamazonic Province, Amazonian Craton. This work is focused on the petrographic characterization of the rocks of this unit, outcropping in the area of Vila Cruzeiro do Sul, the southern portion of the Bacajá Domain. The orthogneisses are widely dominant in this unit, being mainly composed of monzogranites, granodiorites, and medium-gray equigranular tonalites. Based on the mafic minerals content and the plagioclase/alkali feldspar ratio, four petrographic facies of the orthogneisses were identified: a) Amphibole–biotite monzogranite; b) Biotite monzogranite; c) Leucomonzogranite and; d) Biotite granodiorite/tonalite. The essential mineral paragenesis of orthogneisses is represented by quartz, microcline and plagioclase, the varietals are composed of biotite, amphibole and subordinately by clinopyroxene, the accessories are composed of titanite, allanite, apatite, opaque, zircon, and the secondary ones by chlorite, epitope, and muscovite. The kyanite, sillimanite, and garnet occur in biotite granodiorite facies. Based on the degree of deformation, it is possible to identify three rocks groups in the Cajazeiras Complex that appear in relatively well-defined domains in the mapped area: 1) Granitoids with magmatic flow texture - occur in the southern portion of the area, in a low temperature of dynamic recrystallization regimes, where the texture of the protolith is still preserved; 2) Protomylonitic granitoids - are characterized by the homogeneous deformation of the original crystals, formation of subgrains, new grains and quartz ribbons, indicating a moderate temperature regime and; 3) Mylonite/Ultramylonite granitoids - consist of recrystallized grains that define the planar rock fabric and recrystallization conditions at high temperature regimes. The magmatic flow texture, magma mixing features and the mineralogical composition of the gneiss suggest an igneous origin for the Cajazeiras complex protoliths. The presence of garnet and sillimanite suggests that these rocks reached granulite facies (T> 650°C), and the corona texture characterized by pyroxene crystals encircled by biotite or amphibole indicates that they were submitted to hydration conditions.