Petrografia e geoquímica do Granito Pojuca, Serra dos Carajás-PA

Abstract

The Pojuca Granite outcrops in the northern portion of the Serra dos Carajás, showing intrusive contact with the Archean rocks of the Itacaiúnas Supergroup. The petrographic studies, based on drilling cores, revealed that in the studied portion (sub-outcrop) the body is constituted by monzogranites (MzG), dominant, with variations for monzo and syenogranitic (MzSG) transitional terms, occurring subordinately amphibole-granitoids, alkali -feldspar-granites, pegmatoid rocks and hydrothermalized leucogranites. The rocks are, in general, isotropic, equigranular medium to coarse, pink, very poor in mafics (chlorite±amphibole±tourmaline <7%), constituting leucocratic to hololeucocratic varieties, with very localized brittle deformation. The monzogranites and the monzo to syenogranites are very similar petrographically, however they present different geochemical behavior, marked by the enrichment in K2O and depletion in Na2O in the rocks of facies of the MzSG in relation to the MzG. The amphibole-granitoids show petrographic and chemical characteristics that indicate a hybrid origin, while the alkali-feldspar-granites are related to hydrothermalized portions and occur very locally. The leucogranites (LG) are restricted to the southeast end of the granite and have a well differentiated mineralogical, petrographic and geochemical behavior in relation to the other studied facies. They represent the granite dome, in sharp lower contact with MzG and MzSG, with evidence of being precocious in relation to the other facies, having been fragmented during their placement. LG are medium and fine-grained rocks, pink to cream in color, locally gray or greenish, hololeucocratic (M<2%), being hydrothermalized to varying degrees. Petrographically, two subfacies are distinguished, individualized based on the intensity of the transformations: leucogranites (LG) and intensely hydrothermalized leucogranites (LGIH). Despite the difficulties in distinguishing between primary (magmatic) albite and decalcified plagioclase, which also have an albitic composition, it was concluded, based on petrographic and geochemical evidence, that leucogranites are effectively albite-granites, assuming the hypothesis of their magmatic origin. From the remarkable coincidence of ages between the Pojuca Granite and several anorogenic granites from the Eastern Amazon and the other existing similarities between them, the Pojuca Granite was compared with the Jamon, Musa, Cigano and Serra dos Carajás granites. In terms of modal composition it does not differ from the other granites, however the leucogranites (albite-granites) present in the Pojuca Granite represent a particular variety, which was not registered in the other bodies. The Pojuca Granite exhibits intraplate type A geochemical characteristics and similarities with several anorogenic granites from the Eastern Amazon. Among those chosen for comparison, their greater affinity with the Serra dos Carajás and Cigano granites and the existence of clear differences in relation to the Jamon and Musa granites are clear. Therefore, the existence of geochemical contrasts between the anorogenic granites of the Rio Maria Granite-Greenstone Terrain (TGGRM) and those of the Carajás Block (Pojuca, Serra dos Carajás and Cigano) are confirmed. The albite-granite identified in the Pojuca Granite differs in mineralogical, petrographic and geochemical terms from the other albite-granites described in the Amazon, but this comparison should be deepened in view of the economic and petrological importance that these rocks have.