Navegando por Autor "GALINDO, Antonio Carlos"

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Petrologia dos granitóides brasilianos da região de Caraúbas-Umarizal, oeste do Rio Grande do Norte
(Universidade Federal do Pará, 1993-05-07) GALINDO, Antonio Carlos; MCREATH, Ian; http://lattes.cnpq.br/5299851252167587; DALL'AGNOL, Roberto; http://lattes.cnpq.br/2158196443144675
Six major granitoid plutons were mapped in the Caraúbas-Umarizal area, in the West of Rio Grande do Norte State, Northeast Brazil. These are the Umarizal, Tourão, Caraúbas, Prado , Complexo Serra do Lima e Quixaba plutons. Excepting the Umarizal granitoid, all plutons were deformed during the Brasiliano Cycle, displaying a NE-trending foliation dipping mostly to SE. This fabric is related to the main Brasiliano tectonic episode. Brittle-ductile deformation is represented by fractures, faults and shear zones, which are common features of the area. The Portalegre shear zone (ZCP) is an prominent structure with more than 200 km long and up to 2 km wide. The emplacement of most of these granitoids was largely controlled by shear zones. The common occurrence of magmatic breccia structures in the Umarizal granitoid suggest its intrusion into relatively cool crust. Regarding the Tourão granitoid and similar plutons, it is envisaged an emplacement by diapirism followed by ballooning. The Umarizal granitoid is dominated by rocks with quartz monzonite and quartz syenite composition and coarse-grained textures, with biotite, amphibole, and clinopyroxene ± fayalite in variable proportions, and rare orthopyroxene. The Quixaba granitoid has a coarse to very coarse texture and quartz monzodiorite to quartz monzonite compositions. The Tourão, Caraúbas and Prado granitoids and the Serra do Lima complex closely resemble each other. They are dominantly represented by porphyritic monzogranites with subordinate occurrences of leuco-microgranites. The Prado granitoid has an associated diorite facies, which occurs mainly as enclaves in the granites. From the geochemical point of view these granites fali into four families: the Quixaba granitoid, the Prado diorite association, the Umarizal granitoid and the whole group of the Tourão, Caraúbas, Prado (grafite facies) and Serra do Lima complex granitoids. The first two present low silica and a geochemical signature of shoshonitic plutonic rocks, though they are more alkaline than is usual in this kind of association. The latter (Umarizal, Tourão, Caraúbas, Prado e Complexo Serra do Lima) display intermediate to high silica. The Umarizal granitoid has alkaline affnnities and several characteristics of A-type gravites. The remaining type display geochemical signature of subalkaline acid associations. Rb-Sr whole-rock dating show that the Caraúbas and Prado granitoid (630 23 Ma) are the oldest in the area, being followed by the Tourão granitoid and Serra do Lima complex (600 + 7 - 575 ± 15 Ma), and finally by the Umarizal granitoid (the youngest at 545 ± 7 Ma). Initial 87Sr/86Sr rations above 0,708 point to a dominantly crustal source for these magmas. The Quixaba granitoid and the Prado diorite association display geochemical features of a mantle source. Considering the presence of pyroxene and fayalite in the Umarizal granitoid and of clino-and orthopyroxene in the Quixaba granitoid, it is estimated that crystallization of these granitoids began at temperatures around 900 °C and pressures between 8 to 9 kbars. During crystallization, conditions of low oxygen fugacities prevailed in the near FMQ buffer. In the case of Umarizal granitoid the oxygen fugacites was controlled by the FMQ buffer and possibly also for the Quixaba one. Concerning the Umarizal granitoid, melting of rocks with mangeritic composition is assumed to generate its parent magma, while the magmas which formed the Tourão, Caraúbas, Prado (gravite facies) and Serra do Lima complex granitoids probably originated by melting of a monzonitic source. Fractional crystallization was the dominant process in the evolution of the magmas of these granitoids. The Meruoca gravite in Ceará State also displays a fayalite-bearing facies and is the only granitoid described up to now in the Borborema Province that lias analogies wíth the Umarizal grafite. Nevertheless they differ in some petrographical and geochemical aspects. Plutonic shoshonitic associations have been frequently described in this province, but they are compositionally distinct from the Quixaba granitoid. The Tourão, Caraúbas, Prado (gravite facies) and Serra do Lima complex granitoids are similar to a large number of plutons throughout the Borborema Province, representing the most common types of Brasiliano-age granitoids.