Petrologia magnética das associações magmáticas arqueanas de Canaã dos Carajás-PA

Abstract

The studied magmatic associations are located in the Serra Dourada area between the localities of Vila Treze and Vila Planalto, or to the east of Canaã dos Carajás city. The studied areas are situated within the so called Transition Domain which lies between the Rio Maria Granite Greenstone Terrane and the Carajás Basin near the extreme southeast of the sigmoidal structure of the Carajás Fault. Methodologic tests carried out on low magnetic suscetibility (MS < 1x10-3 SIv) samples consisted in comparisons between values acquired with flat and SMS cylindrical coils of SI-1 magnetic susceptibility meter. These tests revealed significant differences between the MS values acquired with the flat and cylindrical coils. In the samples with very low MS values, larger differences between the results were registered. Moreover, the obtained MS values did not show a regular pattern. This precluded the calculation of a correction factor in order to compensate for the different results. For samples with relatively high MS (>1x10-3 SIv) values, the tests showed that, in contrast with the observed in low MS samples, the values obtained with the two mentioned coils were similar although systematically lower in those derived from the flat coil. This demonstrated the need of a correction factor, defined as 1.7, in order to compensate for the much lower values derived from the flat coil in comparison to the cylindrical coil measurements. From the results obtained in these tests, only the MS values obtained from the flat coil above 1x10-3 SIv multiplied by 1.7 were considered in this work. Those lower than 1x10-3 SIv were ignored as imprecise. This was compensated by employing the cylindrical coil for measurements in samples with low MS. The studied rocks were subdivided into two principal groups: The first has predominance of relatively high MS values and is composed of biotitegranite/leucogranite (MS average 8.72 x 10-3 SIv), foliated granite (MS average 1.38 x 10-2 SIv), microgranite/dacite porphyry (MS average 9.28 x 10-3 SIv) and gabbro (MS average 2.69 x 10-2 SIv); the second has dominance of low MS values and is represented by trondhjemite (representative MS value of 7,54 x 10-5 SIv MS average 7.54 x 10-5 SIv) and tonalite-trondhjemite association (representative MS value of 5,11 x 10-5 SIv). The tonalite/quartz-diorite and the Planalto Granite display distinct characteristics from the other groups as they present strong variation in MS. In the Tonalite-quartz diorite there are strong evidence for neoformation of magnetite crystals under subsolidus conditions, these are responsible for the high MS values shown for some samples of this group. In the case of the Planalto Granite the reasons for the large MS variations are not yet clear understood and require additional studies. The opaque minerals identified in the studied magmatic associations are magnetite, ilmenite, hematite (martite), and rarely, pyrite, and chalcopyrite. The rocks of the high MS group, biotite granite/leucomonzogranite, foliated granite, dacite porphyry and gabbro are, compared to low MS group, also enriched in magnetite which is generally preserved or sometimes incipiently altered. These rocks were formed under oxidizing conditions, probably near the NNO buffer. The low MS group includes the trondhjemite and the tonalite-trondhjemite association which contain low modal or are devoid of magnetite. Ilmenite is the predominant Fe-Ti oxide mineral in this group of rocks. The magnetic characteristics reveal that these rocks probably formed in reducing conditions below the FMQ buffer. In the oxidized granitoids, the identified stages of magnetite and ilmenite evolution comprise: (1) the titanomagnetite and individual and composite ilmenite were formed in equilibrium during the magmatic stage; (2) Oxiexsolution processes transformed the titanomagnetite in intergrowths of pure magnetite with trellis and patch ilmenite; (3) in more intensely altered samples, the magnetite is homogeneous and devoid of trellis ilmenite and composite and individual ilmenite were reequilibrated; (4) in a later stage, ilmenite was intensely altered to complex oxide associations, indicating the presence of strongly oxidizing solutions, that do not intensely affected the magnetite. Comparisons between the studied rocks and similar granitoids found in the Carajás Metallogenic Province indicate that, in general, their magnetic characteristics are not distinct.