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A associação anortosito-mangerito-granito rapakivi (AMG) do Cinturão Guiana Central, Roraima, e suas encaixantes paleoproterozóicas: evolução estrutural, geocronologia e petrologia
(Universidade Federal do Pará, 2002-12-19) FRAGA, Lêda Maria; COSTA, João Batista Sena; http://lattes.cnpq.br/0141806217745286
The geological mapping on a scale of 1:250,000, of an area of approximately 22,500 km2 in the central region of the state of Roraima, combined with the petrographic and microtectonic study and new geochronological, lithochemical and isotopic data, allowed the characterization of an anorthosite-mangerite association -rapakivi granite (AMG), Mesoproterozoic and its Paleoproterozoic host. Orthogneisses, foliated granitoids and charnockite rock bodies from the Serra da Prata Intrusive Suite show Pb-Pb (zircon evaporation) ages around 1.94 Ga, also inferred for the associated norites and gabbronorites. The Paleoproterozoic igneous suites were placed syn-kinematically, during Deformational Event D1, with the evolution of petroweaves indicative of high temperatures, from 600º-650ºC. These features include recrystallized feldspars by subgrain rotation, recrystallized perthitic alkali feldspars and quartz with checkerboard subgrains and have been observed in syn-plutonic dykes that cut the early-kinematic D1 fabric in the host country. The NE-NW arrangement of the Paleoproterozoic bodies was controlled by the previous structure of this sector of the Central Guiana Belt (CGC). The orthogneisses and foliated granitoids comprise two distinct suites, with lithochemical characteristics of type A granitoids, probably related to different oxidation conditions at the source. The charnockite rocks show chemical characteristics approaching those described for C-type magmatism. Sm-Nd TDM model ages between 2.19 Ga and 2.05 Ga, with ƐNd(T) values ranging from +0.68 to +2 ,47 suggested sources of limited crustal residence. The age of the orogenic events in Roraima has not yet been properly clarified, however, despite the limited data, a post-collisional positioning is proposed for the Paleoproterozoic suites studied after the accretion of transamazonian magmatic arcs. The Paleoproterozoic units constitute the basement of the Mesoproterozoic igneous suites, which comprise the anorthosites of the Repartimento unit and associated gabbronorites, the rapakivi granitoids of the Mucajaí Intrusive Suite (SIM), and the fine, porphyritic charnockites, of punctual occurrence and uncertain geochronological positioning. In SIM, three granite facies were identified (fayalite-pyroxene-quartz-mangerites to syenites; hornblende-biotite-granites; and biotite-porphyritic granites) geochemically and petrographically very similar to rapakivi granites from classical areas of Finland. The presence of fayalite in the most primitive rocks of the SIM indicates conditions of low oxygen fugacity, observed in several rapakivi granite complexes. Fine charnockites show no cpm to SIM chemical correlation. The Mesoproterozoic suites are part of an AMG (Anortosito-Mangerito-Granito rapakivi) association placed in an anorogenic environment between 1.54 and 1.53Ga. Model ages Sm-Nd, from 2.07 Ga to 2.01 Ga with ƐNd(T) values ranging from -2.37 to -1.27 suggest, for the granitoids in the association, crustal sources separate from the mantle in the Paleoproterozoic , probably during the Transamazônico. Mylonitic features related to the D2 Deformational Event, registering conditions of moderate to low temperatures (400º-450ºC), in a brittle-ductile environment, locally obliterate the igneous textures of the Mesoproterozoic units, as well as the high temperature D1 petrowebs in the Paleoproterozoic basement. These features are especially well developed in some shear zones that show dextral transpressive kinematics. The D2 event aged around 1.26 Ga relates to the K'Mudku Deformational Episode. The main D2 mylonitic zones were reactivated in the Mesozoic at shallow crustal levels and brittle conditions, during the evolution of Graben Tacutu.