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MPI-03 Granite classification - a never-ending problem

Orosirian, A- and I-type acid volcanism in the Guyana Shield, northern Amazonian Craton

Lêda Maria Fraga, CPRM - Geological Survey of Brazil (Brazil)
Ana Maria Dreher, CPRM - Geological Survey of Brazil (Brazil)
Jean-Michel Lafon, Isotope Geology Laboratory of the Federal University of Pará - UFPA (Brazil)
 

In the central part of the Guyana shield, in Brazil, extensive Orosirian acid volcanic deposits are ascribed to the high-K calc-alkaline Surumu Group (SG). A recent 1:100,000 mapping program undertaken by the Geological Survey of Brazil (CPRM) led to the identification of A-type volcanic rocks in the area, which were grouped into the Cachoeira da Ilha Formation (CIF). The distinction between the SG and the CIF could be done mainly on the basis of geochemical criteria, as the volcanic rock-types of both units are petrographically similar. Radiometric anomalies also helped to delimitate the volcanic units.
The SG and CIF rocks are mainly dark gray ignimbrites including slightly to strongly welded types. A low temperature tectonic foliation has locally developed, and folds are present. Microgranitic dikes, rhyolitic lavas, and minor andesites and volcanogenic sedimentary rocks are also included in the SG. Subvolcanic rock varieties are subordinate in the CIF.
The CIF rocks exhibit higher SiO₂ contents (69.06-76.75wt.% CIF; 63.55-72.68wt.% SG) and plot as rhyolites, whereas the ignimbrites and rhyolitic lavas of the SG occupy the trachyte to rhyolite fields in the TAS diagram. Both groups are metaluminous to slightly peraluminous and have high alkalis contents (7.75-9.51% CIF; 6.97-10.05% SG). However, compared to the SG, the CIF shows lower Al₂O₃ (12,26-14.29% CIF; 14.16-16.22% SG) and CaO (0.05-1.11% CIF; 1.01-3.66% SG) values and higher FeO^t/FeO^t+MgO (0.81-0.96 CIF; 0.63-0.84 SG) and Ga/Al (2.51-3.30 CIF; 1.87-2.48 SG) ratios, reflecting its A-type signature. Negative Eu anomalies are discrete in the SG and pronounced in the CIF. In the Al₂O₃ versus FeO^t/FeO^t+MgO diagram the CIF and SG volcanic rocks plot into the reduced A-type and calc-alkaline fields, respectively. In the Y+Nb versus Rb diagram the SG plots into the Volcanic Arc Granite and Within Plate Granite fields and the CIF is confined to the latter field.
Owing to the extremely reduced size of the zircon crystals, a precise Pb-Pb single zircon evaporation age could not be obtained for the SG and CIF rocks. However, the results heavily suggest that they are contemporaneous. The SG yielded values between 1982±3 Ma and 2009±11 Ma (7 zircon crystals) and the CIF furnished values varying from 1983±15 Ma to 2019±05 Ma (6 zircon crystals). Considering the 1984±9 Ma U-Pb SHRIMP zircon age available for a SG volcanic from a neighboring area, it is suggested that the lower values represent more confidently the ages of the SG and CIF, and that the higher values reflect inheritance from Transamazonian source rocks. Moreover, coeval high-K calc-alkaline and A-type granites have recently been characterized in the studied area. A post-collisional setting is envisaged for the SG and CIF. Their chemical signatures may reflect differences in the composition and O₂ conditions in the source regions.