Oxidized, magnetite-series, rapakivi-type granites of Carajás, Brazil: Implications for classification and petrogenesis of A-type granites |
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| Authors: | Roberto Dall'Agnol Davis Carvalho de Oliveira |
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| Institution: | aGroup of Research on Granite Petrology, Centro de Geociências, Universidade Federal do Pará, Caixa Postal 8608, 66075-100 Belém, PA, Brazil |
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| Abstract: | The varying geochemical and petrogenetic nature of A-type granites is a controversial issue. The oxidized, magnetite-series A-type granites, defined by Anderson and Bender Anderson, J.L., Bender, E.E., 1989. Nature and origin of Proterozoic A-type granitic magmatism in the southwestern United States of America. Lithos 23, 19–52.], are the most problematic as they do not strictly follow the original definition of A-type granites, and approach calc-alkaline and I-type granites in some aspects. The oxidized Jamon suite A-type granites of the Carajás province of the Amazonian craton are compared with the magnetite-series granites of Laurentia, and other representative A-type granites, including Finnish rapakivi and Lachlan Fold Belt A-type granites, as well as with calc-alkaline, I-type orogenic granites. The geochemistry and petrogenesis of different groups of A-types granites are discussed with an emphasis on oxidized A-type granites in order to define their geochemical signatures and to clarify the processes involved in their petrogenesis. Oxidized A-type granites are clearly distinguished from calc-alkaline Cordilleran granites not only regarding trace element composition, as previously demonstrated, but also in their major element geochemistry. Oxidized A-type granites have high whole-rock FeOt/(FeOt + MgO), TiO2/MgO, and K2O/Na2O and low Al2O3 and CaO compared to calc-alkaline granites. The contrast of Al2O3 contents in these two granite groups is remarkable. The CaO/(FeOt + MgO + TiO2) vs. CaO + Al2O3 and CaO/(FeOt + MgO + TiO2) vs. Al2O3 diagrams are proposed to distinguish A-type and calc-alkaline granites. Whole-rock FeOt/(FeOt + MgO) and the FeOt/(FeOt + MgO) vs. Al2O3 and FeOt/(FeOt + MgO) vs. Al2O3/(K2O/Na2O) diagrams are suggested for discrimination of oxidized and reduced A-type granites. Experimental data indicate that, besides pressure, the nature of A-type granites is dependent of ?O2 conditions and the water content of magma sources. Oxidized A-type magmas are considered to be derived from melts with appreciable water contents (≥ 4 wt.%), originating from lower crustal quartz-feldspathic igneous sources under oxidizing conditions, and which had clinopyroxene as an important residual phase. Reduced A-type granites may be derived from quartz-feldspathic igneous sources with a metasedimentary component or, alternatively, from differentiated tholeiitic sources. The imprint of the different magma sources is largely responsible for the geochemical and petrological contrasts between distinct A-type granite groups. Assuming conditions near the NNO buffer as a minimum for oxidized granites, magnetite-bearing granites formed near FMQ buffer conditions are not stricto sensu oxidized granites and a correspondence between oxidized and reduced A-type granites and, respectively, magnetite-series and ilmenite-series granites is not always observed. |
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| Keywords: | Carajás Amazonian craton Oxidized granites Reduced granites Magnetite-series Ilmenite-series Rapakivi A-type granite |
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