The magnetic susceptibility (SM) study of Fe-Ti oxide minerals in TTG’s rocks
(Tonalite-Trondhjemite-Granodiorite), Sanukitoids affinity rocks, and sodium
Leucogranodiorites in the Bannach (PA) area, allowed to establish the relations between the
magnetic behavior and the oxygen fugacity (fO2) conditions prevailing during the formation
and evolution of these rocks, as well as to establish parameters of comparison with similar
rocks of the Carajás Province. Three granitoides units in the Rio Maria Domain (DRM) were
defined: (i) Mogno Trondhjemite, which is characterized as a batholith to the south of the study
area, with plagioclase strongly saussuritized. In terms of magnetic susceptibility (SM), this unit
plots on the population with the lowest values of SM (population A) with a range between
2,2x10-5 to 3,9x10-4 Slv; (ii) Sodium Leucogranodiorite is characterized as a batholith to the
northeast of Bannach, characterized by being slightly more enriched in potassium feldspar than
the previous unit, belonging to the population with higher values of SM, between the intervals
of 6,88x10-4 to 9,03x10-3 Slv (population B); and finally the (iii) Sanukitoids affinity rocks,
located to the central-north of the study region, are distributed predominantly in the low SM
population (population A). Such variation of SM in these rocks can be attributed to the degree
of alteration of Fe-Ti oxide minerals, crystal granulation, mafic (biotite and epidote) and
opaque (magnetite and ilmenite) content. The Mogno Trondhjemite presents frequently
martites as inclusion and fine-grained magnetite, ilmenite transformed to titanite, in addition to
sulfides such as pyrite and Fe-Ti oxide (goethite). In the Sodium Leucogranodiorite is observed
that the magnetite is better preserved, with rare occurrences of sulphides. While the Sanukitoids
are composed of well-developed pyrite crystals, with partial or total transformation to goethite,
and magnetite with fine martites as inclusion. It is estimated that these rocks crystallized under
different fO2 conditions, it being believed that Mogno Trondhjemite and Sanukitoides probably
formed under low fO2 conditions, more reducing conditions than Sodium Leucogranodiorites,
possibly formed under higher conditions of fO2 (between the HM and FMQ buffers). Such
conditions can represent important continental crust participation in the origin of those rocks
(as source and/or important assimilation).
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