Redefinição geológica, petrográfica e o estudo susceptibilidade magnética em rochas do Complexo Cajazeiras e Granulito Novolândia, Vila Cruzeiro do Sul, porção oriental do domínio Bacajá

Abstract

This research consist in geological and petrographic study and of magnetic behavior of metamorphic rocks outcropping in Cruzeiro do Sul village, southern portion of the Bacajá Domain from Transamazonas Province. The metagranitoids from Cajazeiras Complex were grouped in biotite gneiss, garnet-biotite gneiss facies, which show gray to pink color with coarse to medium to even grained texture of monzogranitic and granodioritic compositions. In southern portion of the area, protomylonites are abundant, whereas in the northern portion shear zones are very common and the mylonitized to ultramylonitized rocks are dominant. The facies of the Novolândia Granulite are represented by garnet gneisses, mafic to intermediate granulite and garnet bearing gneisses. The variety garnet gneiss is dominant and occurs intercalated with lenses of mafic granulites. The presence of rounded mafic enclaves and compositional banding is a typical feature of this unit. The granulites have dark gray color, medium inequigranular, granoblastic texture and are composed of plagioclase, quartz, orthopyroxene, clinopyroxene, biotite and K-feldspar and sometimes garnet. The garnet-bearing gneisses show gray color and medium inequigranular texture. These rocks are mylonites and display compositional banding, it have granodioritic compositions and mineralogically composed by quartz, plagioclase, K-feldspar, biotite, orthopyroxene, clinopyroxene and epidote. The magnetic susceptibility (MS) of the metagranitoids from Cajazeiras Complex show values between 0.01 to 38.50 (x10-3 SI). The A population correspond to highest SM and are constituted mainly of biotite gneisses, whereas B population, show intermediate values of SM and composed by biotite gneiss and garnet-biotite gneiss facies. The C population consists of lowest values of MS and is represented by two samples of biotite gneiss and garnet-biotite gneiss facies. The low and intermediate values of MS noted in samples from populations B and C can be explained by the low content of magnetite as a function of its own leucocratic character and/or intense oxidation their magnetite crystals. The rocks belonging to A population probably shown high magnetite contents and wide preservation of the magnetite crystals related to rocks from B and C population. The MS values of rocks Novolândia granulite range from 0.01 to 47.07 (x10-3 SI). The A population is formed by rocks with high values of MS, being composed by gneisses and granulites with garnet. The B population has intermediate values of SM, encompassing mainly garnet gneiss. The C population has low levels of MS and consists exclusively of gneisses. The high values of MS related to granulites can be justified by the strong presence of magnetite in these rocks. The garnet gneisses facies have low MS values when compared to garnet bearing gneiss.