239 U-Th-Pb (EPMA) MONAZITE AND ZIRCON Pb-Pb GEOCHRONOLOGY OF THE TRANSAMAZONIAN HIGH-GRADE METAMORPHISM IN THE ARCHEAN AMAPÁ BLOCK (BRAZIL), SOUTHEASTERN GUIANA SHIELD Rosa-Costa, L.T. da1, Lafon, J.M.2, Cocherie, A.3, Delor, C.3 1 - CPRM – Geological Survey of Brazil, Superintendência de Belém, Brasil, ltravassos@amazon.com.br 2 - Pará-Iso, Centro de Geociências, Universidade Federal do Pará, Belém, Brasil, lafonjm@ufpa.br 3 – BRGM, Orléans, France, a.cocherie@brgm.fr, c.delor@brgm,fr Keywords: Monazite, Zircon, Geochronology, Transamazonian event, Southeastern Guiana Shield INTRODUCTION paraderived granulites; 4) amphibolite-facies grey gneisses The eastern portion of the Guiana Shield, in South (mainly tonalitic and granodioritic), which have igneous America, consists of an exceptionally large protoliths dated at about 2.65-2.60 Ga (Rosa-Costa et al., Paleoproterozoic belt, named Maroni-Itacaiúnas Province 2003, in press); and 5) several plutons of catazonal (Tassinari and Macambira, 2004), in which the evolution granitoids (charnockites, charnoenderbites and took place during the Transamazonian orogenic cycle mesopertite-granites) dated between 2.65-2.60 Ga (Ricci (2.26-1.95 Ga), and that matches with the Eburnean et al., 2002; Rosa-Costa et al., in press), which supposedly orogen in West African Craton (Feybesse and Milési, mark the time of an Archean granulitic event. 1994; Ledru et al., 1994). Despite the coherent Migmatization is widespread in the basement rocks, and Paleoproterozoic geochronological pattern of this belt, two the occurrence of charnockitic leucossomes indicates that major Archean domains have been documented in its it also occurred under granulite-facies conditions. These northwestern and southeastern portions, the Imataca Block, basement complexes host several orogenic granitic in Venezuela, and the Amapá Block, in the north of Brazil plutons, dated at about 2.22 Ga, 2.18 Ga, 2.15 Ga, 2.05 Ga (Figs. 1, 2A). and 2.03 Ga (Rosa-Costa et al., 2003, in press). Post- The basement assemblage from the southwestern portion orogenic plutonic magmatism is marked by the of the Amapá Block is constituted by several granulitic- emplacement of 1.75 m.y. old A-type granitic plutons gneissic metamorphic complexes, which present Paleo- to (Vasquez and Lafon, 2001). Neoarchean zircon ages (>3.0 to 2.60 Ga) for their igneous Along the southwestern border of the Amapá Block an precursors (Rosa-Costa et al., 2003, in press; Klein et al., expressive supracrustal belt marks the boundary of this 2003), and also includes Neoarchean charnockitic plutons Archean block with the Carecuru Domain, a granitoid- dated between 2.65 and 2.60 Ga (Ricci et al., 2002; Rosa- greenstone terrane, developed in a magmatic arc setting, Costa et al., in press). The Transamazonian overprinting in which was accreted to the southwestern border of the Archean rocks is testified by the emplacement of several Archean block during the Transamazonian orogenesis syn- to late-orogenic granitic plutons with ages ranging (Rosa-Costa et al., in press). This Paleoproterozoic domain from 2.22 Ga to 2.03 Ga, and Nd isotope signatures consists principally of calc-alkaline gneisses and pointing to an origin by reworking of Archean crust (Rosa- granitoids, dated at 2.19-2.18 Ga and at 2.15-2.14 Ga, and Costa et al., 2003, in press). of supracrustal sequences, constituted mainly by mafic The undoubted Paleoproterozoic reworking of the metavolcanics. Several granitic plutons mark episodes of Archean basement, point to a polymetamorphic evolution crustal reworking in the Carecuru Domain, being one of for the southwestern portion of the Amapá Block. The them dated at about 2.10 Ga. In addition, granulitic rocks purpose of this study is to constrain the age of the regional with Archean precursors are registered within that domain, high-grade event that affected the Archean rocks, through in an oval-shaped nucleus basically composed of granulitic the U-Th-Pb and Pb-Pb dating of monazite and zircon, gneisses dated at about 2.60 Ga (Rosa-Costa et al., 2003), respectively, and to determinate whether the high-grade which host 2.07 m.y. old charnockitic plutons (Rosa-Costa metamorphism is related to Transamazonian or to an oldest et al., in press). Archean event, or both. GECHRONOLOGICAL STUDY REGIONAL GEOLOGICAL BACKGROUND Three samples were selected for study and their The Archean basement assemblage of the location is shown in the figure 1. Monazite concentrates southwestern portion of the Amapá Block includes: 1) were extracted from two enderbitic gneisses (MV-27A and granulitic orthogneisses (mainly enderbitic and LT-214) and from a charnockitic leucossome (MV-27E). charnockitic banded gneisses) that have their igneous Zircon grains from this leucosome were also selected for protoliths dated at about 2.8 Ga (Rosa-Costa et al., 2003, dating. in press); 2) undated mesoperthite and/or clinopyroxene- The analyses on monazite grains were carried out on a bearing granitic orthogneisses, metamorphosed under CAMECA SX 50 electron probe micro-analyzer (EPMA), amphibolite-granulite transition facies; 3) undated at laboratory of BRGM, France. Age calculations were done using the Isoplot program (Ludwig, 2004) and a interpreted as growth ages and, consequently, provides a Microsoft Excel add-in program for determining U-Th-Pb reliable estimate of the age of the granulite-facies ages from EPMA measurements (Pommier et al., 2002), metamorphism, occurred at about 2.09 Ga, which possibly according to the procedure proposed by Cocherie and reached temperatures of at least 750°C. The zircon and Albarede (2001). All the calculations were done at 2σ monazite ages of the charnockitic leucosome reinforce this level. interpretation, since elucidative field data indicate that this The zircon geochronology was based on the Pb- leucosome is product of in situ melting of the enderbitic evaporation method (Kober, 1986, 1987). The isotopic gneiss MV-27A. Furthermore, the metamorphic mineral analyses were performed on a Finnigan MAT262 mass assemblage (orthopyroxene-bearing) of the leucosome spectrometer, at the Laboratory Pará-Iso of the UFPA, strongly suggests that the migmatization event marks the Brazil. The zircon ages and the 2σ errors on the ages were time when peak metamorphic conditions were reached. calculated following Gaudette et al. (1998). The Paleoproterozoic ages obtained in the investigated In the enderbitic gneiss LT-214, 138 U-Th-Pb EPMA samples indicate that, if basement rocks from the Amapá measurements were obtained on 8 monazite grains and Block were submitted to a high-grade event during yielded individual ages ranging between 2156 and 2010 Archean times, they were completely resetted during the Ma. These ages are considered as representing an Paleoproterozoic overprinting. Then, based on the current unimodal population, and an average weighted age of 2086 data, we can not establish any genetic or chronological ± 3 Ma (MSWD = 1.6) was calculated (Fig. 2B). relationship between the Neoarchean charnockitic Nine monazite grains from the enderbitic gneiss MV- magmatism and the granulitic rocks of the region, as 27A furnished individual ages between 2026 and 2182 Ma, previously suspected (Rosa-Costa et al., 2003) collected on 137 analytical points, providing an average The basement rocks from the southwestern part of the weighted age of 2096 ± 6 Ma (MSWD = 1.6). Amapá Block commonly exhibit pervasive NW-SE ductile In the charnockitic leucosome MV-27E, six monazite foliation, dipping systematically 20-50° SW, and mineral grains gave individual ages varying between 1988 and lineation with low plunge predominantly to SW. This 2179 Ma, yielding a weighted average age of 2087 ± 8 Ma pattern is strongly disturbed along NW-SE transcurrent (MSWD = 1.5). In addition, 7 zircon grains were analyzed zones defined by steeply dipping mylonitic foliation and and provided ages spreading between 2045 Ma and 2091 sub-horizontal lineation, marking the strike-slip Ma, at the higher steps of temperature of 1500 and 1550 movement. The structural features are coherent with a °C. A mean age of 2091 ± 5 Ma (USD = 2.6) was regime of oblique thrusting, with tectonic vergency from calculated from three oldest grains and is interpreted as the SW to NE. Apparently, the strike-slip zones development crystallization age of the charnockitic leucosome, which is coincides with the final stages of the thrusting movement. in agreement with the monazite age of the same sample Structural field features strongly indicate that the (Fig. 2B). granulitic metamorphism was contemporaneous to the development of the thrusting system. For instance, the DISCUSSION leucosome MV-27E, presenting monazite and zircon ages The monazite and zircon ages, which are similar about 2.09 Ga, occur in layers concordant to the foliation within the errors, make unambiguous the existence of a of the parental gneisses, MV-27A. tectono-thermal event at about 2.09 Ga affecting the Archean basement of the southwestern portion of the Amapá Block. The closure temperature (Tc) of Th-U-Pb system in monazite has been largely accepted to be at least about 700-750 °C (Suzuki et al., 1994), and even higher than 750 °C (Copeland et al., 1988; Spear and Parrish, 1996; Bingen and Bremen, 1998; Braun et al., 1998). It has also been demonstrated that, the U-Pb system of monazite may be resetted by secondary replacement of newly grown monazite rather than by volume diffusion of Pb (De Wolf et al., 1993; Zhu et al., 1997). Consequently, if diffusive Pb loss is not a common process in monazites, a record of the prograde path of metamorphism and even of peak Figura 1 . Location of the Amapá Block in southeastern Guiana metamorphic conditions should be preserved. Shield. Based on these considerations, we conclude that monazite ages from the studied granulites may be The structural pattern of the southwestern of Amapá structural framework coeval to regional granulite-facies Block, which registers an evolution from an early metamorphism about 2.09 Ga, is consistent with tangential tectonics to a dominantly transcurrent palinpastic reconstructions involving Man and Guiana tectonics, is classically described as evidence of collision shields, which also evoke a collisional stage at about 2.1 tectonics and testifies the oblique character of the Ga, resulting from the convergence of West African (Man collision (Shackleton, 1986). The development of this Shield) and Amazonian Archean plates (Feybesse and Milési, 1994; Ledru et al., 1994) that can be represented Ludwig, K.R. 2004. Users manual for ISOPLOT/EX a by the Amapá and Imataca blocks. geochronological toolkit for Microsoft Excel (version 3.1). Pommier A.; Cocherie A.; Legendre O. 2002 . EPMA Dating REFERENCES User’s manual: Age calculation from electron probe Bingen, B. and Van Breemen, O. 1998. U-Pb monazite ages in microanalyser measurements of U-Th-Pb. BRGM, 9 p. amphibolite- to granulite-facies orthogneisses reflect Ricci, P.S.F.; Carvalho, J.M.A.; Rosa Costa, L.T.; Klein, E.L.; hydrous mineral breakdown reactions: Sveoconorwegian Vasquez, M.L.; Vale, A.G.; Macambira, E.M.B.; Araújo, Province of SW Norway. Contrib. Mineral. Petrol., v.132, O.J.B. 2001. Geologia e recursos minerais do Projeto RENCA pp.336-353. – Fase I. Belém, CPRM. Braun, I.; Montel, J.M.; Nicollet, C. 1998. 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RESUMO O Bloco Amapá, sudeste do Escudo das Guianas, constitui um bloco arqueano envolvido em uma extensa faixa paleoproterozóica, cuja evolução principal ocorreu durante a Orogênese Transamazônica (2,26-1,95 Ga). Gnaisses granulíticos e um leucossoma charnoquítico da porção sudoeste do Bloco Amapá foram datados pelos métodos U-Th- Pb por microsonda eletrônica em monazita e Pb-Pb em zircão. As monazitas de um gnaisse enderbítico forneceram uma idade U-Th-Pb de 2086 ± 3 Ma MSWD = 1,6). Para um outro gnaisse enderbítico, as monazitas apresentaram uma idade de 2096 ± 6 Ma (MSWD = 1,6). As monazitas de um leucossoma charnoquítico forneceram uma idade U-Th-Pb de 2087 ± 8 Ma (MSWD = 1,5), enquanto que os zircões do mesmo leucossoma apresentaram uma idade Pb-Pb de 2091 ± 5 Ma (USD = 2,6). As idades obtidas nas monazitas podem ser interpretadas como idades de formação e, consequentemente, fornecem a idade do metamorfismo granulítico em torno de 2,09 Ga, o qual alcançou temperaturas de pelo menos 750°C. As idades em zircão e monazita do leucossoma charnoquítico, produto da fusão in situ do gnaisse enderbítico encaixante, reforçam essa interpretação e sugerem fortemente que o evento de fusão marca o momento em que o pico do metamorfismo foi alcançado. Os padrões estrutural e geocronológico da porção sudoeste do Bloco Amapá caracterizam um estágio colisional da orogênese Transamazônica, em torno de 2,10-2,09 Ga. Figure 2. – (A) Geological map of the investigated area, based on Ricci et al. (2001); (B) Th/Pb vs. U/Pb diagrams for the dated monazites and Pb- evaporation diagram for the dated zircons.