Navegando por Assunto "Rochas granitóides"

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Caracterização litoquímica e geocronologia Rb-Sr das rochas granitóides e ortognaisses da região de Santa Quitéria-Sobral, NW do Ceará
(Universidade Federal do Pará, 1992-12-21) TAVARES JÚNIOR, Stelio Soares.; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
In the SSW of the Sobral-Pedro II Lineament between the cities of Sobral, Forquilha and Santa Quitéria, in the northwest of the State of Ceará, there is a variety of isotropic and foliated granitic bodies, with a predominance of the former. The isotropic granites constitute batholithic masses and bodies of smaller dimensions, intrusive in the rocks of the gneissic-migmatitic complexes, in the set of supracrustal rocks, including those of the Ubajara and Jaibaras groups. The Forquilha orthogneisses, of tonalitic composition with subordinate trondhjemitic and granodiorite terms, constitute one of the individual units within the gneiss-migmatitic complexes. These rocks provided a total rock Rb-Sr age of 1981 ± 45 Ma, which is interpreted as a minimum age for gneissification processes. The similarity of this result with the ages obtained in the region of Granja evidences a significant performance of the Transamazonian Event in the NW of Ceará. The initial isotopic ratio (0.70138 ± 13) of these rocks suggests an evolution from a mantle source or from short-residence crustal protoliths. The Brasiliano granites have a predominantly monzogranitic modal composition, ranging from microclimate granites to granodiorites. They are bodies of alkaline composition, per to metaluminous, with geochemical characteristics that reveal a strong similarity with type A granites, which may represent alkaline granites of intraplate or post-tectonic tectonic environment. These characteristics, including the behavior of REEs, are also similar to those of the Mucambo and Meruoca granites. The Rb-Sr geochronological studies carried out on these granites provided an age of 524 ± 12 Ma and 482 ± 8 Ma for the Pajé and Serra da Barriga granites, respectively. As for the Morrinho Granite, conventional ages between 480 and 510 Ma were obtained. Adding these results to the existing data for the Meruoca and Mucambo granites, an epoch of intense granite genesis is evident in the entire NW region of Ceará, from the late Proterozoic to the Eo-Paleozoic. Foliated granites occur as elongated bodies in the gneissic terrains of the High Grade Belt of Cariré, in agreement with the regional structure. The geochemical characteristics obtained in these bodies show a lot of similarity with those of the isotropic granites set. However, the Rb-Sr age of 475 ± 15 Ma obtained in these granites is smaller than that of the isotropic granites and certainly corresponds to a rejuvenated age. This rejuvenation is probably related to the effects of Graben Jaibaras reactivations. It was not possible to determine the true age of the crystallization of these bodies, as well as the deformational processes that affect them, in a situation very similar to that found for the Chaval Granitoid. The arrangement of the initial ratios of the foliated granites (0.70521 ± 56) and the Pajé Granite (0.70488 ± 71) in relation to the evolution of the Rb/Sr ratios of the Forquilha orthogneisses is compatible with a derivation of the granites from these gneisses. This is not valid for Serra da Barriga Granite, which has a high initial ratio (0.70963 ± 160).
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Estudo da alteração hidrotermal,com ênfase no metamorfismo sódico, de rochas granitóides e máficas da região de Canaã de Carajás, Província Mineral de Carajás
(Universidade Federal do Pará, 2007-06-25) SOUZA, Francisca D'ávila Soares de; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983
The Sossego and Serra Dourada areas are located in the southern part of Carajas ridge, at the contact zone between the basement rocks (Xingu Complex; ~2.8 Ga) and the Grão Pará Group (2.76 Ga). Serra Dourada lies 15 km east of the Sossego deposit, at the transition strip that bounds the Itacaiúnas block and the Rio Maria granite-greenstone terrane. In the Sossego deposit granitoids, metavolcanic rocks, biotite-rich and mafic rocks were investigated, all of them were hydrothermalized and/or deformed to a greater or lesser extent. In Serra Dourada granitoids, their enclaves and mafic rocks were studied, the former being represented by unaltered, altered and milonitized varieties. The likely protoliths of the apogranitoids and metavolcanic rocks that occur in Sossego were syenogranites/granodiorites and porphyritic quartzodiorite/diorite, respectively, while the mafic altered rocks were derived from microgabros. The porphyroclastic texture of the biotite-rich rocks may be an evidence for their protoliths being, at least in part, the same as those of the metavolcanic rocks. Apogranitoids, metavolcanic and biotite-rich rocks have similar REE patterns, suggesting that they might be comagmatic, although some samples of the biotite-rich rocks show higher La/Yb ratios. Concerning the contents of less mobile elements (Nb and Zr), the metavolcanic and biotite-rich rocks are much more similar. Albitization, scapolitization, amphibolitization, chloritization, biotitization, epidotization and potassic feldspatization were the most important hydrothermal processes that affected those protoliths. Albitization is more common in the apogranitoids and acid metavolcanic rocks, whereas scapolitization is more typical of the biotite-rich and mafic rocks. In the apogranitoids two albite generations (I and II) were identified. The albite I represents crystals with chessboard texture produced by the replacement of potassium feldspar. Albite II is present in monomineralic veinlets which correspond to one of the latest stage of hydrothermal activity. Only albite I occurs in all granitoid varieties. In the mafic rocks two scapolite generations were recognized. One refers to crystals aggregates that replaced the primary plagioclase and the other to fine sinuous veinlets of scapolite+Mg-hornblende that, in these rocks, mark the latest stage of the hydrothermal alteration. In the biotite-rich rocks only scapolite I was identified. The scapolite that occurs in both the mafic and biotite-rich rocks have similar meionite (Me=27-28%) and Cl (3-4%) contents, and were most likely formed by a NaCl-rich fluid, with little influence of the rock composition. This fluid might have also been responsible for the production of albite in all analyzed samples. This albite is purer in the apogranitoids (Ab=98.5-99.3%) and the metavolcanic rocks (Ab=99-99.3%) than in the other rocks. In Serra Dourada area, the granitoids include syenogranites, granodiorite and tonalites as well as enclaves of tonalitic composition. Their mineralogical characteristics and calc-alkaline affinity allow them to be classified as I-type granitoids related to subduction with some influence of extensional regime. Most geochemical data indicate that they are cogenetic and have been produced by fractional crystallization. Albitization, scapolitization and minor biotitization were the most important types of alteration that affected those rocks, the first being restricted to the syenogranites, while the other two types are recorded only in the tonalites. The syenogranites present different degrees of alteration ranging from aposyenogranite without hydrothermal albite to albitite with ~75% of hydrothermal albite. In all granitoids, the hydrothermal albite is purer than the magmatic albite. Mass-balance calculations shows that, in relation to the average composition of syenogranites and for volume factors of 0.8-1.1, the albitites gained Na2O and Cu, and lost K2O, Ba, Rb, Sr and W. The scapolitized rocks resulted from the alteration of tonalites and are mainly characterized by scapolite-rich (up to ~70%) veins/veinlets that cross-cut the rocks. In the moderately altered varieties, the scapolite composition is richer in Na (Me=24%) and Cl (4%) in comparison to that present in veins and in the mylonitized granitoids. Massbalance calculations indicate that the formation of the scapolitized rocks was accompanied by losses of Al2O3, MgO, CaO, Sr and Zr and gains of Fe2O3(t), Na2O, K2O, volatiles, Rb, Ba and Cu. The fluids that caused albitization and scapolitization of the rocks were Na and Cl-rich and, apparently, the albitization of the syenogranites preceded the scapolitization of the tonalites. The scapolite composition seems to have been strongly controlled by the composition of primary plagioclase, as evidenced by its more calcic character in the mafic rocks than in the apotonalites. Estimates of Cl content (1-2%) of rocks from both areas were based on its concentrations in modal scapolite, biotite and amphiboles. The stability of scapolite requires high salinity fluids as those that have been reported in the Sossego deposit. No fluid inclusion data concerning the Na-rich fluids responsible for albitization and scapolitization in the Serra Dourada area are yet available, although the evidences, notably the abundance of scapolite, suggest an evaporitic or similar source.