Magmatismo bimodal da área de Tucumã, Província Carajás: geocronologia U-Pb, classificação e processos

Abstract

In the geological context of the Carajás Province, dikes are an important mechanism for magma transport and represent the beginning of a rifting, which allow the emplacement through the crust significant amounts of magma. The geological mapping of the Tucumã area has enabled the identification of dike swarms, which are intruded in an Archean basement. The disposition of these dikes is consistent with the regional, NW-SE trending, and can reach up to 20 kilometric lengths. They were individualized in three main groups: (i) felsic dikes (70% of the dikes), composed exclusively of pink to dark purple porphyritic rhyolites with euhedral phenocrysts of quartz, K-feldspar and plagioclase immersed in a felsitic afiric matrix; (ii) mafic dikes, with restrict occurrence, composed primarily of basaltic andesites and subordinately by basalts, with a mineralogical assembly represented by plagioclase, cline- and orthopyroxene and olivine, forming an ofitic texture; and (iii) intermediate rocks, represented by gray to greenish gray andesites and dacites. Dacitic rocks are found outcropping associated to felsic dikes, showing different degrees of hybridization or mixing between mafic and felsic magmas. This is evidenced by large presence of mafic enclaves in the felsic dikes and the frequent presence of embayment textures. SHRIMP U-Pb zircon dating of felsic dikes yielded an age of 1888 ± 3.3Ma, which is interpreted as crystallization age. The felsic dikes are peraluminous to slightly metaluminous mainly due to the fractionation of K-feldspar and plagioclase with minor contribution of amphibole. They are characterized as akin to A2, ferrous and reduced granites. The intermediate and mafic dikes belong to tholeiitic series and are exclusively metaluminous with large fractionation of amphibole and plagioclase. It is noted a decrease in the amounts of CaO, FeOt, MgO, TiO2, Sr, Cr and P in the mafic rocks in the direction to the rhyolitic composition, which are more enriched in silica, while K2O/Na2O, Al2O3 and Na2O, Rb, Ba and Y increase in the same sense. In rhyolitic varieties, the REE patterns are characterized by LREE enrichment in relation to HREE (high ratios La/Yb), and a significant Eu anomaly. A similar behavior is also seen in the intermediate rocks. On the other hand, the mafic rocks presents flatter REE patterns (low ratio La/Yb) and little or no Eu anomaly. Geochemical modeling and the fractional crystallization vectors showed that the mafic rocks evolved by crystallization of pyroxene and plagioclase, while the K-feldspar and biotite are the fractionating phases in felsic magma. For a discussion about the origin of the intermediate rocks, it was utilized a simple binary mixture model, in which the rhyolites and basaltic andesite are the primary components. From this model was shown that by mixing of 60% of rhyolite and 40% of basaltic andesite melts is possible to generate the dacite composition, while the andesite liquid could be x originated by mixing of 60% and 40% of basaltic andesite and rhyolite melts, respectively. The model proposed suggested that mixing of basaltic and andesitic magmas occurred during the ascent and storage in the crust, where the andesitic dikes are probably generated by a more homogeneous mixture at high depths in the continental crust (mixing), while the dacite dikes can be generated in the upper crust at a lower temperature, thus providing a less efficient mixing process (migling). The petrographic, geochemical, and geochronological affinities observed between the felsic dikes studied and the A-type granites of the Rio Maria and São Felix do Xingu regions, demonstrate that the bimodal magmatism of the Tucumã area is a clear evidence that the Paleoproterozoic magmatism of the Carajás Province has been formed by processes involving thermal perturbations in the upper mantle, mafic underplating, and associated extension or transtension of the crust.