Petrologia magnética e parâmetros de cristalização do Charnoquito Rio Seco: Implicações para evolução de granitos neoarqueanos da Província Carajás.

Abstract

This work presents new magnetic petrological data and crystallization conditions (T, fO2 and xH2O) of the Rio Seco Charnockite from a detailed study of the textural evolution of silicates and Fe-Ti oxide. The body is intrusive in Mesoarchean granitoids (2.88 Ga) and was subdivided into four petrographic types: (i) gabbronorite; (ii) ortho-clinopyroxene granodiorite; (iii) clinopyroxene-hornblende monzogranite; (iv) hornblende-biotite monzogranite. The magnetic values range from 0.231x10-3 to 22.1x10-3 Slv and shows bimodal character. Pyroxene-bearing rocks are concentrated in magnetic population A, whose magnetic susceptibility values range from 3.2x10-3 to 22.1x10-3, and are compatible with the SM values defined for oxidized neoarchean granites of the Carajás Province. Pyroxene-deprived rocks define magnetic population B, with lower values, between 0.231x10-3 and 0.309x10-3. They are included in the range of values defined for reduced granites. The amphibole crystallization temperature ranges from 713 to 809°C and the emplacement pressure ranges from 81 to 300 MPa. The fO2 (ΔNNO) ranges from -0.4 to +2.4 and the log fO2 from -12.0 to -16.0. The low Fe/(Fe+Mg) ratio in hornblende and biotite confirms the oxidizing conditions of these rocks, which belong to the magnetite series. The hygrometer used indicated water content in the magma ranging from 4.3 to 6.1% wt. The corona-type textures record the physical-chemical disequilibrium between silicates and favor the formation of magnetic minerals in the rocks with pyroxene. The rocks of population A present modal content of opaque minerals ranging from 0.2 to 1.7% with different degrees of exsolution of ilmenite in the magmatic stage (magnetite-ulvospinel series). The magnetic population B shows lower contents of opaque minerals (<0.3%) and is characterized by the presence of zoned goethite formed by oxyhydration processes in the late to post magmatic stage. The study of textural and mineralogical evolution reinforces the fractional crystallization model proposed in a previous work and explains the abundance of Fe-Ti oxide minerals in the variety with orthopyroxene, giving these rocks an oxidized character. The Charnockite Rio Seco has its emplacement controlled by a system of lineaments with ductile ruptile behavior, which is favorable to the interpretation of a pluton emplaced at shallow crustal levels. In deeper crustal conditions, the assimilation of crustal components may give rise to ferrous magmatism.