Associações graníticas do oeste do estado de Roraima, Domínio Parima, escudo das Guianas, Brasil

Abstract

Preliminary mapping, supported by a recent aerogeophysical survey and a re-evaluation of earlier data (petrography and geochemistry), was carried out by CPRM (Geological Survey of Brazil) in western Roraima (Figs. 1 and 2). The results reveal that the Paleoproterozoic granite-greenstone basement (Urariqüera and Parima rocks; 2.2 to 1.9 Ga) of the Parima Domain (northern area of the Tapajós-Parima Belt) of the Guiana Shield is intruded by several granitic associations: Ericó (I-type, high K calc-alkaline), Rio Couto Magalhães (S-type), Auaris (aluminous A-type), Surucucus and Mucajaí (alkaline to aluminous A-type) and Tocobirém (C-type) granitoids. The Ericó Intrusive Suite (EIS) comprises granodiorite and monzogranite, with minor quartz monzonite and quartz monzodiorite. These have strongly zoned plagioclase (Fig. 4) and several mafic clots with hornblende (Fig. 5), biotite, apatite and a high content of opaques (around 2%). The EIS is peraluminous (Fig. 6) with a high-K calc-alkaline character (Fig. 7). It shows a chemical similarity to Australian I-type granite and, in the trace-element chondrite-normalised diagram (Fig. 9), shows negative Nb anomalies with moderate and low contents of certain HFSE (Zr, Y, Ta). The EIS probably correlates with the Pedra Pintada granite Suite (medium- to high-K calc-alkaline series) dated at ca. 1.96 to 2.00 Ga. The Rio Couto Magalhães Intrusive Suite (RCMIS) is represented mainly by leucogranite (Fig. 3), minor schorlite-bearing pegmatite and diatexite originating from anatexis of metasedimentary rocks. The peraluminous mineral assemblage is composed of muscovite, biotite and minor igneous garnet (Fig. 10) and cordierite. The RCMIS is peraluminous with normative corundum around 1.6%, and is composed of both fractionated S-type granite (muscovite-bearing leucogranite) and normal S-type granite (diatexite). The Auaris Intrusive Suíte (AIS) is represented mainly by syenogranite, monzogranite and minor quartz monzonite (Fig. 3), with a probable age of 1.52 to 1.55 Ga. It has peraluminous to metaluminous compositions (Fig. 6) and plots in the syn-collisional to late-orogenic granite fields (Fig. 8). However, other chemical parameters are similar to those of A-type granite, apart from Al2O3 which is much higher (around 15%). The Surucucus (SIS) and Mucajaí (MIS) intrusive suite vary from alkali-feldspar granite to monzonite (Fig. 3), locally with a rapakivi texture and ages of ca. 1.54 and 1.55 Ga. They define a extended alkaline series with a peralkaline to peraluminous character (Fig. 6). They are WPG-related (Figs. 8 and 11) and locally associated with mangeritic rocks. The major- and trace-element patterns (Figs. 9 and 11) are typical of A-type granite from an attenuated continental lithosphere (crustal source). The Tocobirém Intrusive Suite (TIS) comprises monzogranite, quartz monzonite, quartz monzodiorite, and rare quartz syenite (Fig. 3) with an anti-rapakivi texture and mafic minerals such as hornblende, and clino- and/or orthopyroxene. The TIS is a monzonitic series with a metaluminous to peraluminous character (Fig. 6). It plots in the late-orogenic granite field (Fig. 8), showing chemical analogies to C-type granite with a similar MgO content, although slightly high in P2O5, high in Al2O3 and low in FeO+Fe2O3, CaO and TiO2 (Fig. 12). It has also low SiO2, Nb, La and high Al2O3, TiO2, P2O5 in comparison to the EIS, RCMIS, SIS, MIS and AIS. Within the Parima Domain, the EIS probably corresponds to a mature calc-alkaline arc (late stage) with mantle-derived magma intrusive in an immature low-K calc-alkaline arc (Urarigöera magmatism), whereas the RCMIS represents the crustal thickening and anatexis events (collisional setting) reworking the older continental crust. The SIS, MIS, AIS and TIS were probably generated some 400 Ma later in a post-orogenic to anorogenic environment under extensional tectonic conditions.

Key words: Major-element analysis, Trace-element analysis, Rare earths, Brazil, Roraima, Guiana Shield