Navegando por Assunto "Granitoides Tipo I"

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Field and petrographic data of 1.90 to 1.88 Ga I- and A-type granitoids from the central region of the Amazonian Craton, NE Amazonas State, Brazil
(2012-12) VALERIO, Cristóvão da Silva; MACAMBIRA, Moacir José Buenano; SOUZA, Valmir da Silva
The SW Presidente Figueiredo district, which is located in the northeastern Amazonas State of the central Amazonian Craton, Brazil, consists of 1890 to 1898 Ma I-type granitoids (Terra Preta Granite, Água Branca Suite), A-type hornblende-bearing syenogranites (Canoas Syenogranite, Mapuera Suite), felsic to intermediate volcanic rocks (Iricoumé Group), and 1883 to 1889 Ma rapakivi granites (São Gabriel Granite, Mapuera Suite) and related rocks (quartz-gabbro-anorthosite and diorite), in addition to Castanhal quartz-monzonite, mylonites, and hornfels. The quartz-diorite facies of Terra Preta Granite were formed by mingling processes between a synplutonic quartz-gabbro dike and a hornblende granodiorite. Partially assimilated globules of Canoas hornblende-bearing syenogranite and their clear contacts with Terra Preta hornblende granodiorite suggest that Canoas Syenogranite is slightly younger than Terra Preta Granite. Canoas Syenogranite xenoliths inside São Gabriel Granite show that the granite is younger than the Canoas Syenogranite. New geologic and petrographic evidence improve the petrological understanding of these rocks and suggest that, in addition to fractional crystallization, assimilation and magma mingling played a role, at least at the local scale, in the evolution and compositional variation of the plutons. Such evidence is found in Terra Preta Granite mingled quartz-diorite, felsic material associated with the Canoas Syenogranite and in the intermediate microgranular enclaves, which exhibit primary biotite in hornblende-bearing rocks, plagioclase dissolution, corrosion of feldspars rims, alkali feldspar mantles, second apatite generation, and high xenocrystal contents in intermediate enclaves formed from the fragmentation of mafic intrusions. Petrographic analyses show that a deformational event recorded in the western part of the study area (with progressive deformation from E to W) is dated between the 1.90 Ga postcollisional magmatism and the intrusions of the São Gabriel Granite and related mafic/intermediate rocks (intra-plate). However, it is extremely necessary to obtain absolute ages for this metamorphic event.