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Marcelo Oliveira, Federal University of Para (Brazil)
Roberto Dall'Agnol, Federal University of Para (Brazil)
Fernando Althoff, Federal University of Para (Brazil)
Albano Leite, Companhia Vale do Rio Doce (Brazil)
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2.87 Ga old Archean sanukitoid rocks are exposed in large domains of the Rio Maria Granite-Greenstone Terrane, located in the Carajás province, southeastern Amazonian craton. They are intrusive in ca. 2.98 old greenstone belts and in the oldest TTGs of the RMGGT with ages of 2.98 to 2.93 Ga. Archean Younger TTGs and potassic leucogranites and Paleoproterozoic anorogenic granites are intrusive in the Rio Maria Granodiorite.
The sanukitoid rocks vary from mafic to felsic. The dominant ones have granodioritic composition (Rio Maria Granodiorite) and display medium- to coarse- even-grained textures. They show generally greensish gray color, display a weak or locally striking WNW-ESE foliation and include decimetric mafic enclaves, probably due to mingling processes involving the granodiorite magma and mafic to intermediate sanukitoid magmas. There are significant geochemical contrasts between the occurrences of Rio Maria Granodiorite in different areas of the RMGGT, suggesting that this unit corresponds in fact to a granodioritic suite derived from similar but distinct magmas. The intermediate sanukitoid rocks occur mainly near Bannach town and are composed mostly of mesocratic quartz diorites and quartz monzodiorites, with dark-green color and fine-to coarse- even-grained texture.
The Rio Maria Granodiorite and associated intermediate rocks show similar textural and mineralogical aspects. They follow the calc-alkaline series trend in some geochemical diagrams. However, they have high Mg#, Cr, and Ni, conjugate with high contents of large ion lithophile elements (LILE), typical of sanukitoids series. The patterns of rare earth elements of different rocks are similar, with pronounced enrichment in light rare earth elements (LREE) and strong to moderate fractionation of heavy rare earth elements (HREE).
Field aspects and petrographic and geochemical characteristics demonstrate that the granodiorite and intermediate rocks are rocks of the sanukitoid series. However, geochemical data suggest that the intermediate rocks and the granodiorites are not related by a fractional cristallization process. It is concluded that the intermediate magmas derived from similar sources and processes than the granodiorite magmas, but the former probably result of a higher degree of melting, implying that these rocks are cogenetic, but not comagmatic. Associated with experimental evidence, the presence of hornblende and the absence of clinopyroxene and orthopyroxene at the liquidus suggest that the Rio Maria granodiorite magma was relatively water-enriched (>4 wt. %). Admitting emplacement of the Rio Maria Granodiorite at shallow crustal levels, the occurrence in it of well-preserved magmatic epidote crystals point to a rapid ascent of the magma.
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