Navegando por Assunto "Rio Maria (PA)"

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Caracterização geológica, petrográfica e geoquímica de Granitos Arqueanos da Folha Marajoara, Terreno Granito-Greenstone de Rio Maria Sudeste do Pará
(Universidade Federal do Pará, 2009-07-08) DIAS, Samantha Barriga; DALL'AGNOL, Roberto; http://lattes.cnpq.br/2158196443144675
During geological mapping in the Marajoara Sheet, located in the south portion of Rio Maria Granite-greenstone Terrane, southeastern of Amazonian Craton, three groups of Archean granites were distinguished: The Guarantã Suite, the Rancho de Deus Granite and Leucogranitic stocks. The first group is composed of three plutons, named Guarantã Granite (Gg), Azulona Granodiorite (GDaz) and Trairão Granodiorite (GDt), which are intrusive in the Arco Verde Tonalite and were assembled in the Guarantã Suite. The Rancho de Deus Granite is an oval stock, in contact with the Rio Maria Granodiorite and the Guarantã Granite, the latter contact being controlled by a regional shear zone. The third group is formed by small leucogranite bodies, intrusive in the Arco Verde Tonalite and Rio Maria Granodiorite. The epidote-biotite granodiorites to monzogranites of the Guarantã Suite display EW to NW-SE foliation and a porphyritic texture, with coarse alkali feldspar phenocrysts in a fine- to medium-grained matrix. The Rancho de Deus Granite is composed of hornblende-biotite monzogranites with a discrete foliation and porphyritic texture, displaying coarse alkali feldspar phenocrysts in a medium, even-grained matrix. The leucogranites are equigranular, medium-grained rocks. Geochemical data show that the Guarantã Suite and the leucogranites are dominantly peraluminous rocks, whereas the Rancho de Deus rocks vary from metaluminous to peraluminous granites. The three granite groups show strong geochemical contrasts. The Guarantã Suite is enriched in Al2O3, CaO, Na2O, Ba, Sr and Ga and impoverished in Fe2O3, K2O, MgO, Rb and Th compared to the Rancho de Deus Granite. On the other hand, the leucogranites are enriched in SiO2 and K2O and impoverished in Fe2O3, MgO, Na2O, CaO, P2O5, Ba, Sr, Zr, compared to the other two groups. The three groups exhibit moderate to strong HREE fractionation; the Guarantã Suite granites show only discrete negative or positive europium anomalies or are devoid of it. The europium anomalies in the Rancho de Deus Granite are discrete but always negative and those of the leucogranites are also negative but more important. The Rancho de Deus Granite and the Guarantã Suite rocks are similar geochemically to the Archean CA2 calcalkaline granites. However, the geochemical contrasts between these two groups cannot be explained by magmatic differentiation processes, because their rocks have similar SiO2 contents and display distinct trends in geochemical plots. Hence, the hypothesis of a genetic linking between these two granites is discarded and it is assumed that their magmas derived from different sources or evolved by different magmatic processes or both. A comparison between the three studied granite groups and similar Archean rocks of the RMGGR has shown that the Guarantã Suite rocks are similar to the granodiorites and monzogranites found in small satellite stocks associated with the Xinguara Granite pluton and with the Guarantã Granite, as originally defined by Althoff (1996). The Rancho de Deus Granite is similar to the Rio Maria Granodiorite and other rocks of the sanukitoid series. Finally, the leucogranites display petrographic and geochemical affinity with the Mata Surrão Granite. The large area of exposition and volume of the Guarantã Suite demonstrated that this variety of leucogranites was relevant in the evolution of the RMGGT and that, besides the Xinguara and Mata Surrão potassic calc-alkaline leucogranites, they should be considered as an important event of granitic magmatism in the RMGGT. This suite is better exposed in the Marajoara area, but similar rocks, alike those of the satellite stocks of the Xinguara pluton, were previously described and other similar rocks were probably mixed with the mentioned leucogranites in the past.
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Geologia, petrografia e geoquímica dos diques da região de Rio Maria, SE-PA
(Universidade Federal do Pará, 1996-09-09) SILVA JÚNIOR, RENATO OLIVEIRA DA; DALL'AGNOL, Roberto; http://lattes.cnpq.br/2158196443144675
In the region of Rio Maria, SE of Pará, several dykes were mapped, mainly intrusive in the Granodiorite Rio Maria (GdRM) and, subordinately, in the anorogenic granite Musa (GM). Forming small elongated ridges that stand out in the morphology with a positive relief, arranged mainly according to NW-SE and E-W trends and, subordinately, N-NE. The maximum width of these bodies is 30 m, and they have an outcropping length of 1,500 to 2,000 m, reaching 3,000 m. Five groups of dykes were identified petrographically: diabases, diorites, quartz-diorites, dacites and rhyolites. The transverse and longitudinal profiles in the direction of these bodies show a decrease in rock granulation, in the center-edge direction of the body, usually culminating in aphanitic rocks, which represent, as a rule, the cooling edge of the dyke. Close to these edges, enclaves and tonsils filled with quartz-feldspathic material are frequently observed. The contacts with its host country are abrupt, sometimes marked by the presence of cooling edges, whose thickness varies from a few centimeters to, exceptionally, 2 m. The K-Ar ages of these dykes are: (1) 700 ± 8 Ma in qz-diorite (whole rock); (2) 883 ± 10 Ma in diorite (plagioclase); (3) 1,099 ± 39 Ma in diabase (plagioclase); (4) 1,802 ± 22 Ma in diabase with olivine (mafic concentrate). The first three ages are interpreted as minimum ages for these bodies. The age of 1,802 ± 22 Ma is compatible with the age of 1,707 ± 17 Ma (Rb-Sr in RT) obtained for dacites and porphyry granites. The geochronological data available for the dikes in the Rio Maria region allow placing them in the Proterozoic, and the most reliable ages suggest that at least part of these dikes is contemporary with the anorogenic granitic magmatism. Diabases have been divided into four subtypes: (1) diabase with olivine - exhibits subophytic texture. It consists of labradorite (An55-65), augite + pigeonite, olivine, opaques and hornblende; (2) porphyritic microdiabase - cuts diabase with olivine, formed by plagioclase phenocrysts immersed in a pilotaxitic matrix formed by plagioclase, augite, opaque and amphibole slats; (3)) amphibole-diabase- exhibits a granular texture with a subphytic tendency, is made up of labradorite (An54-64), opaque minerals augite, late amphibole (tremolite-actinolite) and, rarely, hypersthene; (4) auginite-diabase (RJ-18B) with ophitic texture, it is formed by labradorite (An56), augite, opaques and, secondary amphibole. Diorites and quartz-diorites show, in general, a granular texture tending to subphytic or porphyritic with a matrix rich in granophyric intergrowths. Diorites are formed by very saussuritized plagioclase, augite, quartz, hornblende and opaques. Quartz-diorites have similar mineralogy to diorites, differing only in the modal content of quartz and granophyric intergrowths. Some diorites and diabases present plagioclase phenocrysts with sieve texture, suggesting the action of mixing and/or mingling processes. The dacites are formed by porphyry dacites and porphyry dacites rich in mafics. Both have a porphyritic texture, locally glomeroporphyritic, formed by aggregates of plagioclase phenocrysts, quite saussuritized, hornblende locally involving augite, in addition to isolated quartz phenocrysts. The two subtypes have a predominantly granophyritic matrix, sometimes spherulitic. The presence of microcrystalline quartz was noted forming almond-shaped aggregates (spots), attributed to mixing processes. The rhyolites exhibit a porphyritic texture, locally glomeroporphyritic, formed by plagioclase and quartz phenocrysts, immersed in a microcrystalline matrix with a micrographic tendency. Microcrystalline aggregates consisting of chlorite, biotite and opaques also occur. The TAS diagram shows good correlation between the modal classification and the geochemical data. In this diagram the diabases and diorites plot within the field of their volcanic counterparts. Quartz-diorites generally fall into the field of low-silica dacites. The dacites themselves are a little richer in silica than the preceding group, although both focus on the same field. In the AFM diagram the samples are located in the tholeitic subalkaline field. The presence of compositional gaps between groups, mainly between diabases and diorites, and from these to quartz-diorites, reinforces the hypothesis that these rock groups studied do not present a continuous magmatic evolution. The RJ-18B sample, although presenting characteristics of diabase, shows geochemically greater affinities with the diorites. The geochemical data also reinforce the hypothesis that the dikes of Rio Maria, although showing a tholeitic affiliation, were probably generated from different liquids, since the compositional gaps between the various groups are very accentuated. The dacites, although they present, in some diagrams, an overlap with the quartz-diorites, they differ from them petrographically and geochemically, too, and, in terms of occurrence in the field. There is petrographic evidence that the crystallization of diabases was driven mainly by the fractionation of olivine, while in diorites, augite and plagioclase played a dominant role. The petrographic and geochemical data show that the diabases, with the exception of sample RJ-18B, are entirely different from the other groups in terms of magmatic evolution. Diorites and quartz-diorites, in turn, although petrographically similar, present a compositional gap that weakens and immediate hypothesis since the latter represents a more evolved term, derived from the former. Sample RJ-18B is interpreted as a mafic concentrate of these diorites. This hypothesis can be justified by the modal and chemical composition of this sample. Dacites and rhyolites are probably genetically linked to anorogenic granitic magmatism, with probable associated mixing processes, mainly in the case of dacites.