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Within the Amazonian Craton, Archean crust is restricted to the Carajás granite-greenstone terrain. The younger Maroni-Itacaiunas province, including supra-crustal sequences and associated calc-alkaline granitoids, is linked with the Birimian system in West Africa, making up a large Paleoproterozoic cratonic nucleus. Beginning at ca. 2.0 Ga, accretionary belts formed along the southwestern margin of this nucleus, giving rise to the Ventuari-Tapajós (2000–1800 Ma), Rio Negro–Juruena (1780–1550 Ma), and Rondonian–San Ignacio (1500–1300 Ma) tectonic provinces. Continued soft-collision/accretion processes driven by subduction produced a very large “basement” in which granitoid rocks predominate, many of them with juvenile-like Nd isotopic signatures. Felsic volcanics are also widespread; however, there is no evidence of Archean basement inliers, and regions with high-grade metamorphics are restricted.

The Sunsas-Aguapeí (1250–1000 Ma) orogenic belt, at the southwestern end of the craton, was originated in an extensional environment, later deformed during the Grenvillian collision between Amazonia and Laurentia. Over the cratonic area, a widespread anorogenic granitic magmatism (1000–970 Ma) is a reflection of this orogeny over the stable foreland. After the termination of the Sunsas orogeny, continental fragmentation affected the eastern margin of the Amazonian Craton. The intra-oceanic Goiás magmatic arc, closely associated with the Transbrasiliano megasuture, is the evidence of a large oceanic domain that started its consumption between 900 and 800 Ma, giving rise to juvenile material represented by calc-alkaline orthogneisses. Later, these units were deformed during the Brasiliano orogeny (700–500 Ma), in the process of amalgamation of Gondwana.

INTRODUCTION

Classical models of orogeny involve either Andean-type belts or Wilson cycle episodes of ocean opening and closing, culminating in continent-continent collision (Dewey, 1969; Wilson, 1966). In both cases, a preexisting basement is involved in the petrogenetic processes related to the production of granitoid rocks, whose isotopic signature usually indicates mixing of juvenile mantle-derived and reworked crustal components in the parental magmas.

A somewhat different model of accretionary orogens arises when intra-oceanic magmatic arcs are formed within large oceanic domains, in areas of long-lived plate convergence and B-subduction, when both convergent plates consist of oceanic lithosphere. The relicts of such tectonic setting are domains that are typically as broad as they are long (e.g., the Altaids, the Arabian-Nubian Shield, or the Svecofennian orogen), and comprise a large proportion of juvenile mafic to silicic calc-alkaline igneous rocks, as well as their sedimentary products. The main tectonic processes include the formation and collision of island arcs, oceanic plateaus, and microcontinents. “Soft” collision of these arc terranes during the accretion process will not produce extensive crustal thickening, as is seen in Himalayan-type orogens. In the absence of high mountain ranges, exhumation will be minimized, resulting in the exposure of upper to middle crustal levels exhibiting low to middle metamorphic grades. These domains will preserve such platetectonic signatures as island-arc assemblages, oceanic plateaus, accretionary complexes, ophiolite suites, and fold-and-thrust belts. (For a complete summary, see Kröner et al., 1987.)

In this work we will address some relevant concepts related to accretionary belts, indicating their main structural and deformational features, as well as their petrogenetic and isotopic characteristics. Next, we will examine the extensive regions of accretionary belts with large proportions of mantle-derived magmatic arcs of Proterozoic age that occur at the southwestern portion of the Amazonian Craton, or marginal to its southeastern border. Their tectonic evolution will also be reassessed, by means of the available geochronological control, with some emphasis on robust U-Pb determinations in zircon crystals. Finally, the overall geodynamic significance of these large domains will be considered, as well as their bearing for the paleogeographic reconstruction of the Proterozoic supercontinents, Rodinia and Gondwana.

STRUCTURAL FEATURES OF ACCRETIONARY BELTS

Extensive areas can be formed by stacking and lateral accretion of arc complexes produced in a collection of successive subduction zones, in processes of “soft collision and accretion”, characterized by the production of large amounts of granitoid magmas and associated felsic volcanics. These regions will not contain nappe complexes imbricating older continental basement, and high-grade collisional-type regional metamorphism (see for instance Kröner et al., 1987; Şengör and Natal'in, 1996; and Johnson, 2001, for the case of the Arabian-Nubian Shield). An “orogenic collage” is thereby produced, in which deformation, regional metamorphism, and crustal growth take place in a tectonic environment of ongoing plate convergence (Cordilleran, Pacific, Miyashiro-type, Turkic-type orogens). Deformational features of accretionary belts include extensional and compressive environments during steady-state convergence (arc/backarc versus accretionary prism), and they are normally overprinted by later compressive orogenic events (Kusky and Bradley, 1999).

At present, the Circum-Pacific system of orogenic zones is the paradigm for all kinds of accretionary belts. In the western Pacific, where the relative velocities of the convergent plates indicate a series of retreating orogens, island arcs grow oceanward, producing forearc accretion and backarc basin opening, for instance in the Japanese islands (Maruyama et al., 1997). In contrast, in the southeastern Pacific, the overriding plate advances toward the downgoing plate producing terrane accretion of previously rifted arc and microcontinental ribbons, and typical tectonic features on the overriding plate, such as retro-arc fold-and-thrust belts.

Accretionary orogens may also include some features of Himalayan-type collisional and Andean-type Cordilleran belts. In particular, the processes responsible for their amalgamation, cratonization, and incorporation into continental nuclei are not well understood, especially for pre-Mesozoic orogens. For instance, cratonization of orogenic belts may occur when there is a pause in lateral accretion. Renewed subduction may then result in a more conventional, Andean-type tectonic interface between an oceanic and a continental plate, marked by voluminous calc-alkaline magmatism as well as vertical crustal accretion at the margin of the now-cratonized block. In such cases volcano-sedimentary basins may cover large areas, subsequent to the respective orogenic pulses. Moreover, because of a long-term tectonomagmatic evolution, accretionary belts usually contain postaccretionary granitoid rocks, metamorphic rocks from various tectonic regimes, late-forming volcano-sedimentary basins, and anorogenic mafic to silicic bodies. However, the evidence for accretion in deeply exhumed crustal domains is difficult to recognize, since most of the high- and medium-level rocks are eroded away. In such cases, the “sea” of deformed amphibolite- to granulite-facies migmatitic gneisses remains as roots of the tectonic features summarized above.

PETROGENETIC AND ISOTOPIC CONSTRAINTS

Large portions of material from accretionary belts exhibit typical juvenile isotopic signatures, indicating essentially mantle-derived parental magmas. This indicates a significant growth of juvenile crust (Şengör and Natal'in, 1996), mainly within intra-oceanic island arcs, with subordinate proportions being added from accreted oceanic plateaus. The related rocks show nonradiogenic initial 87Sr/86Sr and positive ϵNd(t) isotopic parameters. More-over, their Sm-Nd model ages are usually slightly older than their U-Pb or Rb-Sr whole-rock isochron ages indicative of the original crystallization process. In addition, in such tectonic units there is little evidence of basement inliers of continental character.

Besides the ubiquitous differentiated volcanic rocks, such as rhyodacites and andesites, that are common in island arcs, granitoid rocks are always relevant, and in most cases they are the most common constituents of accretionary belts. Most of them belong to the calc-alkaline series, and rock types consist of a variety of differentiated I-type granitoids, such as tonalites, quartz diorites, diorites, and granodiorites, but also including highly differentiated types such as the K-rich granitoids, typical end members of the alkaline and per-alkaline series, as well as syenogranites and syenites. Regardless, granitoid rocks are the most representative materials of the continental crust and may be formed through many different petrogenetic processes, whose ultimate origin is the mantle.

Mantle–Continental Crust Differentiation

The amount of material formed in mantle-crust differentiation processes depends on the thermal regime of the planet, which has been losing heat throughout geologic time. Archean granitoids, formed at a time of high-heat regime, are typically associated with greenstone belts within granite-greenstone terrains. These granites are normally mantle-derived, as indicated by juvenile-like Nd isotopic signatures. On the other hand, purely juvenile material is not widespread in Middle to Late Proterozoic and Phanerozoic times. Crustal reworking is predominant in the orogenic belts of such ages, making it difficult to identify the proportion of mantle-derived material in the granitoid complexes. For instance, if we consider the whole of granitic-type rocks formed worldwide over the last 1600 m.y., those with crustal signatures (either those that assimilated significant crustal material or those formed directly by anatexis and crustal reworking) increasingly predominate with time over granitoids with juvenile signatures.

Although Sm-Nd fractionation may be associated with a variety of petrogenetic processes (e.g., Arndt and Goldstein, 1987), the principal REE (rare earth element) fractionation occurs when oceanic crust originates from asthenospheric melting. Because Nd will concentrate relative to Sm in the crust, a depleted mantle MORB (mid-oceanic-ridge basalt)–type component relative to CHUR (chondrite uniform reservoir) will dominate the asthenospheric sources.

Continental crustal material may originate either upon melting of the upper mantle to form mafic magmas, followed by differentiation, or through two-stage successive melting, or by melting plus fractionation to form silicic rocks (Ben Othman et al., 1984). Already within the continental crust, LREE (light rare earth element) enrichment from differentiation will decrease the Sm/Nd ratio and cause the slowing down of radiogenic Nd growth.

Over geologic time, the upper mantle has evolved in such a way that the asthenosphere has become depleted in LREE relative to the Bulk Earth, and its ϵNd(t) signature has become strongly positive relative to the chondrite uniform reservoir. Given this evolution, positive ϵNd(t) values in granitoid rocks indicate mantle-derived juvenile material (e.g., De Paolo, 1988). These rocks will also exhibit Sm-Nd model ages similar to their radiometric age, as well as low initial Sr isotopic ratios and Pb (µ1) mantle-like indicators. As a consequence, the accretionary belts within the major continents are easily identified as areas where large amounts of juvenile material are present, as indicated by positive values of ϵNd(t). Examples of accretionary belts with dominantly granitic rocks with juvenile signature, encompassing the entire Proterozoic and the Paleozoic, are the Birimian of West Africa, the Sve-cofennian of the Baltic Shield, the Mazatzal-Yavapai of North America, the Arabian-Nubian Shield in northeastern Africa, and the Altaids of central Asia. Nevertheless, mantle-derived material may be difficult to characterize solely by isotopic geochemistry. Hence, the use of ϵNd(t) to trace juvenile processes is not completely unequivocal, because the possible mantle sources may not be typically depleted, such as pristine mantle from plumes, mantle affected by metasomatism, or lithospheric mantle enriched by earlier recycling of crustal material.

Cratonization of Accretionary Belts

Extraction of continental crust from mantle during periodic accretion and jamming of the active subduction zones will occur over time spans of a magnitude equal to that of the major orogenic cycles, on the order of 200 m.y. From one cycle to the next, cessation of each stage of subduction would be accompanied by cratonization of the previously accreted material, and possibly by a change in the regional stresses within the lithosphere, along and inboard of the active margin. When cratonization occurs, Andean-type, “Cordilleran” magmatic arcs may be formed, and their granitoid intrusions will be aligned and emplaced in coherent belts along and over the marginal zone of the newly cratonized continental block. In addition, volcano-sedimentary basins, including differentiated volcanic rocks such as rhyodacites and andesites, may also form within extensional tectonic zones during the final collapse subsequent to the orogenic pulses of the accretionary belt.

Posttectonic (“anorogenic”) granitoids, formed within an intracratonic setting, may also be quite common in accretionary belts. Multiple origins are possible, but sources of material are predominantly mantle-derived. Voluminous anorogenic magmatism may originate in a multistage process, starting with under-plating of massive basaltic magmas, followed by their partial melting, producing granitic liquids, and by successive steps of fractional crystallization (Jahn et al., 2000). Mixing models, with dominance of the mantle component over crustal material, are common in the genesis of the anorogenic granites. The starting point of such a process could be lithospheric extension, causing decompressional melting in the lower mantle lithosphere producing intraplate magma. According to Windley (1993), this process may be regarded as a very late consequence of an orogeny, and not a truly anorogenic process.

A special type of anorogenic magmatism produces rapakivitype granites, formed by heat or magma transfer from the asthenosphere to the mantle lithosphere and to the base of the crust, leading to melting and assimilation (Rämö and Haapala, 1995). Ahäll et al. (2000), considering the tectonic setting of southern Sweden, suggested a process of “hybrid synorogenic response,” considering the temporal correlation of the anorogenic magmatism with subduction processes occurring at the evolving active margin. In this way, the stepwise orogenic accretionary growth would be contemporary with inboard anorogenic magmatism.

From the above, an accretionary belt could be the result of complex juxtaposition of tectonic units, including a great deal of intra-oceanic material with positive ϵNd(t) signatures, but also containing in places Cordilleran-type granites, collisional-type belts, microcontinents, volcano-sedimentary basins, and posttectonic to anorogenic-type complexes.

HISTORY OF THE AMAZONIAN CRATON BEFORE 2.0 Ga

Figure 1, adapted from Cordani et al. (2000), shows the main geotectonic provinces of South America as of the late Neoproterozoic. For the large Amazonian Craton, in addition to the Roraima and Xingu-Iricoumé Archean nuclei, five major Proterozoic tectonic provinces are displayed (partly covered by Proterozoic and Phanerozoic sedimentary sequences): the Maroni-Itacaiunas, Ventuari-Tapajós, Rio Negro–Juruena, Rondonian–San Ignacio, and Sunsas-Aguapeí. The boundaries between these provinces, indicated in Figure 1, correspond to important structural features, as well as to changes in the geochronological pattern of both basement rocks and sedimentary covers. In most places such boundaries must be considered only tentative, and we are aware that more geological mapping, supported by geochronological studies, is needed for a better definition.

Figure 1. Main geotectonic provinces of South America as of the late Neoproterozoic, with emphasis on the Amazonian Craton and its tectonic provinces. The São Francisco Craton, the small São Luís and Luiz Alves cratonic fragments, as well as the Neoproterozoic Brasiliano/Pan-African orogens are shown in the eastern part of the figure. These belts are the Borborema province (B) in northeastern Brazil, the Tocantins province (T) in central Brazil, and the Mantiqueira province (M) in eastern and southeastern Brazil. The Andean belt is indicated in the figure, with its main Precambrian inliers. Adapted from Cordani et al., 2000. Locations of Figures 3, 4, and 6 are shown.

Figure 1. Main geotectonic provinces of South America as of the late Neoproterozoic, with emphasis on the Amazonian Craton and its tectonic provinces. The São Francisco Craton, the small São Luís and Luiz Alves cratonic fragments, as well as the Neoproterozoic Brasiliano/Pan-African orogens are shown in the eastern part of the figure. These belts are the Borborema province (B) in northeastern Brazil, the Tocantins province (T) in central Brazil, and the Mantiqueira province (M) in eastern and southeastern Brazil. The Andean belt is indicated in the figure, with its main Precambrian inliers. Adapted from Cordani et al., 2000. Locations of Figures 3, 4, and 6 are shown.

The Amazonian Craton, with an area of ∼4,400,000 km2, is one of the largest cratons of the world. In this work we will follow the mobilistic approach initially proposed by Cordani et al. (1979) and adopted and modified by Teixeira et al. (1989), Tassinari and Macambira (1999), Tassinari et al. (2000), and recently by Tassinari and Macambira (2004). These reviews, although taking into account the regional correlations of geologic units and major structures, have been based essentially on evaluation of regional radio-metric age data, because the general geological knowledge of the region remains at reconnaissance scale. For many regions, the available outcrops are limited in number by the difficult access, intense alteration, and dense vegetation. As a consequence, the available petrological, geochemical, and structural data are insufficient to produce a detailed picture for most of the area.

Available radiometric age data are heterogeneously concentrated over an immense territory. The geochronological database consists of ∼2000 dated samples, and most of the ages are produced by the Rb-Sr and K-Ar methods. U-Pb (SHRIMP [sensitive high-resolution ion microprobe] and isotopic dilution on zircon), Pb-Pb (zircon evaporation), Sm-Nd (whole rock), and 40Ar/39Ar mineral ages are also available but are restricted to a few areas. Some regions, such as the Carajás mineral province, French Guyana, and parts of Rondônia and Mato Grosso States of Brazil, are relatively better known, and for these there is much better control of the tectonic evolution, as well as for the succession of magmatic pulses and metamorphic events.

We are aware that different views concerning tectonic units, or different positions for the tentative boundaries between provinces, have been reported (Santos et al., 2000; Santos, 2003). These two articles include a number of references that report a great deal of geochronological work performed in the last few years, based on U-Pb zircon SHRIMP and TIMS (thermal ionization mass spectrometry) determinations, as well as Pb-Pb evaporation. Such analyses were carried out, altogether, in a few hundred samples distributed throughout large areas of the Amazonian Craton. These results are obviously more robust and precise than the previously available Rb-Sr and K-Ar age determinations. The general conformity of these new results with ages expected from a given geochronological province is evidence of the general robustness of the earlier geochronological results. Thus, we adhere to previous conclusions regarding geochronological provinces in this contribution.

The Amazonian Craton is here subdivided into two Archean nuclei and five Proterozoic tectonic provinces, showing internally coherent structural and age patterns (Fig. 1). Tables 1 and 2 summarize the main geologic and geochronologic characteristics of these provinces.

TABLE 1. MAIN GEOLOGIC AND GEOCHRONOLOGIC CHARACTERISTICS IN THE CENTRAL AMAZONIAN (CA) AND MARONI-ITACAIUNAS (MI) PROVINCES, NORTHEASTERN AMAZONIAN CRATON

TABLE 2. MAIN GEOLOGIC AND GEOCHRONOLOGIC CHARACTERISTICS IN THE PROTEROZOIC TECTONIC PROVINCES, SOUTHWESTERN AMAZONIAN CRATON

The primitive nuclei, the Xingu-Iricoumé and Roraima blocks, make up the Central Amazonian province (CA in Fig. 1) of Cordani et al. (2000). Nevertheless, genuine Archean continental crust is restricted to the relatively large Carajás granite-greenstone terrain of the Xingu-Iricoumé block (Fig. 2), whose rocks yielded radiometric ages between 2600 and 3200 Ma (Table 1). The Roraima block occurs in the northern part of the craton. Its basement rocks are covered by extensive and practically undeformed volcano-sedimentary sequences (Surumu, Iricoumé, etc., Fig. 3), interpreted by Tassinari et al. (2000) as stable foreland deposits marginal to the Paleoproterozoic Maroni-Itacaiunas province. These sequences yield well-constrained U-Pb zircon ages, always younger than 2000 Ma.

Figure 2. Reconstruction of the relative positions of the ancient nuclei of West Africa and northern South America in Paleoproterozoic times. Adapted from Nomade et al., 2003

Figure 2. Reconstruction of the relative positions of the ancient nuclei of West Africa and northern South America in Paleoproterozoic times. Adapted from Nomade et al., 2003

Figure 3. Tentative boundaries of the Proterozoic accretionary belts, north of the Amazon sedimentary basin. The principal volcanic and sedimentary sequences overlying the tectonic provinces are indicated in the figure. Urban settlements: BV—Boa Vista; Ca—Caracaraí; Ja—Japurá; Mi—Mitu; PA—Puerto Ayacucho; SFe—San Fernando de Atabapo; SFi—San Filipe; SG—São Gabriel da Cachoeira. (A) and (B) are specific geographic domains with predominance of basement rocks, discussed in the text.

Figure 3. Tentative boundaries of the Proterozoic accretionary belts, north of the Amazon sedimentary basin. The principal volcanic and sedimentary sequences overlying the tectonic provinces are indicated in the figure. Urban settlements: BV—Boa Vista; Ca—Caracaraí; Ja—Japurá; Mi—Mitu; PA—Puerto Ayacucho; SFe—San Fernando de Atabapo; SFi—San Filipe; SG—São Gabriel da Cachoeira. (A) and (B) are specific geographic domains with predominance of basement rocks, discussed in the text.

The clastic sedimentary rocks of the Roraima Formation and the roughly contemporary Urupi Formation (locations in Fig. 3) overlie in places the Iricoumé volcanics, which are well constrained in age to ca. 1880 Ma, dated by U-Pb age determinations in interbedded tuff layers (Santos et al., 2003). Moreover, the Roraima Formation is intruded by mafic sills and dikes of the voluminous Avanavero Suite (Gibbs and Barron, 1983, 1993). A tuff interbedded with undeformed sediments correlated with the Avanavero Suite yields a Rb-Sr whole-rock isochron age of ca. 1650 Ma (Priem et al., 1973). If these ages are significant, they imply a fairly long succession of distinct continental sedimentation events under relatively stable tectonic conditions within the Roraima block of the Central Amazonian province.

The Maroni-Itacaiunas province (MI in Fig. 1) is made up of mobile belts that surround the northern and northeastern border zones of the Central Amazonian province (eastern Venezuela, Guyana, Suriname, French Guyana, and parts of Amapá, Roraima, Pará, and Amazonas States of Brazil). It consists of metavolcano-sedimentary sequences associated with juvenile calc-alkaline granitoid rocks (Table 1), with U-Pb zircon crystallization ages between 2050 and 2250 Ma, and positive ϵNd(t) values. It was affected by the widespread Transamazonian orogeny, initially defined by Hurley et al. (1967), which produced different generations of granitoids with ages down to ca. 1950 Ma. Large parts of this province comprise Paleoproterozoic greenstone belts consisting of mantle-derived material (Gibbs and Barron, 1993; Gruau et al., 1985; McReath and Faraco, 2006), while restricted parts are likely to be recycled older crust. However, a detailed overview of Maroni-Itacaiunas rocks in French Guyana, based on ∼100 new Pb-Pb and Sm-Nd isotopic data (see Delor et al., 2003; Avelar et al., 2003, and references therein) confirms a complex multistage crustal growth for this province, with both juvenile Paleoproterozoic accretion (some ϵNd(t) values up to +3.4) and some recycled Meso- to Neoarchean materials. Regional cooling after the Transamazonian orogeny took place between 1900 and 1800 Ma, as indicated by the widely distributed K-Ar apparent ages of micas from the country rocks (Teixeira et al., 1989; Tassinari et al., 2000, and references therein).

High-grade metamorphic rocks are described from the Maroni-Itacaiunas province, some having originated under ultrahigh-temperature conditions (e.g., Bakhuis felsic granulites), indicating the importance of Paleoproterozoic collisional episodes (Tassinari et al., 2004, and references therein). These granulites extend southwards to the state of Roraima, Brazil (see Fig. 2), and include the Falawatra and Kanuku Complexes as part of the WSW-ENE–trending Central Guyana granulite belt. Their radiometric ages are 2050–2020 Ma and 1940–1850 Ma (Priem et al., 1978), and their Sm-Nd TDM model ages are only generally older, closer to 2300 Ma (Ben Othman et al., 1984). These ages are clearly different from the typical ages of the Transamazonian orogeny, suggesting a slightly younger, Paleoproterozoic collisional event that may have taken place during a late stage of orogenic assembly. Moreover, the high-grade belt was intruded by anorogenic granitoids (Anorthosite-Mangerite-Charnokite-Granite [AMCG] Suite) at ca. 1560–1520 Ma (Gaudette et al., 1996; Reis et al., 2003) and was also affected by strong deformation associated with shearing and low-grade metamorphism at ca. 1100–1300 Ma (Gibbs and Barron, 1993). These geologic features are related to cataclastic belts and megashear zones that are widespread over large parts of the Guyana Shield and are designated as the Nickerie or K'Mudku event (Snelling and McConnell, 1969; Priem et al., 1971b).

Archean fragments are also present in this Paleoproterozoic province, such as the Imataca terrane at its western corner (Fig. 2), which may represent an allochthonous crustal fragment comprising Archean high-grade metamorphic rocks. Its rocks yielded SHRIMP U-Pb ages between 3200 and 2800 Ma. This terrane was probably juxtaposed against the province during the Transamazonian orogeny, along the Guri megafault zone, where 40Ar/39Ar ages of 1400–1200 Ma indicated a regional thermal episode of reactivation related to the K'Mudku event (Onstott et al., 1989). High-grade metamorphism (750–800 °C and 6–8 kbar) and crustal reworking at ca. 2000 Ma, in association with transpressive shearing and thrusting, was documented by Tassinari et al. (2004).

In addition, other inliers with Archean ages occur throughout the province, such as the Cupixi and Tartarugal Grande areas within Amapá State in Brazil (Tassinari and Macambira, 2004). More recent work by Rosa-Costa et al. (2006) greatly extended this Archean inlier to the entire southern portion of Amapá, an area more than 300 km long, and renamed it “Amapá block.” These authors indicated the original Meso- to Neoarchean ages of the rocks, obtained through U-Pb zircon evaporation ages, and the long-lived regional tectonic evolution, including reworking and crustal accretion in the Paleoproterozoic, as well as the intrusion by several Transamazonian granites.

An interesting synthesis of the polycyclic history of the northwestern region of the Amazonian Craton has been made on the basis of U-Pb SHRIMP dating of 49 detrital zircon crystals from a sand sample taken from the Orinoco River, where it flows over the eastern edge of the Imataca block (Goldstein et al., 1997). Without taking into account the young Phanerozoic crystals coming from the Andes, the U-Pb apparent ages reflect individual magmatic or metamorphic events, and the resulting histogram shows discrete age groupings at 2800, 2100, and 1100–1200 Ma. The two older peaks are representative of the most important periods of crust formation within the Guyana Shield. The relatively large Grenvillian-like population of zircons may indicate derivation from basement inliers within the Colombian and Venezuelan Andes (Cordani et al., 2005).

In a broader context, the Maroni-Itacaiunas province correlates well with the Birimian system in West Africa (Vidal et al., 1996), affected by the Paleoproterozoic Eburnean orogeny. Taking this into account, the different sectors of the province seem to be a result of amalgamation of at least four large continental masses: the already mentioned Xingu-Iricoumé and Roraima blocks, the Kenema-Man block of the West African Craton, and the Imataca terrane. Nomade et al. (2003) presented the results of a paleomagnetic study performed on granitic and metavolcanic rocks of Paleoproterozoic age from French Guyana and the Ivory Coast. These authors conclude that the northern part of the Amazonian Craton and the southern part of the West African Craton belonged to the same continental mass by ca. 2000 Ma, but were separated before that time. Figure 2, adapted and modified from Nomade et al. (2003), is a reconstruction of the proposed “proto-cratonic mass” in existence after the Transamazonian orogeny. The large oceanic domain that must have occurred to the southwest of such continental mass is also indicated in that figure.

THE PROTEROZOIC ACCRETIONARY BELTS OF THE AMAZONIAN CRATON

The position of the Proterozoic tectonic provinces (Ventuari-Tapajós, Rio Negro–Juruena, Rondonian–San Ignacio, and Sunsas-Aguapeí) that cover the entire southwestern half of the Amazonian Craton is indicated in Figure 1. Figures 3 and 4, located to the north and to the south of the Amazon sedimentary basin, respectively, include a more detailed view of the Proterozoic tectonic provinces. The proposed boundary of the accretionary systems is indicated, as well as the volcano-sedimentary covers established upon them after cratonization. However, their “basement” is not subdivided, because we recognize that geological mapping and geological research still has not progressed to an adequate stage for a better characterization. For instance, although it is known that the entire region is formed mainly by granitic rocks, sensu lato, it is still not possible, at present, to make a clear distinction between “basement granitoids” and syn-, late-, and posttectonic, or anorogenic granitoid intrusions of different type. More robust and precise geochronological work is needed.

Figure 4. Tentative boundary of the Proterozoic accretionary belts, south of the Amazon sedimentary basin. The principal volcanic and sedimentary sequences overlying the tectonic provinces are indicated in the figure. Urban settlements: AF—Alta Floresta; Arp—Aripuanã; Arq—Ariquemes; CO—Colorado do Oeste; GM—Guajará Mirim; JP—Ji-Paraná; Ju—Juina; PB—Pimenta Bueno; PV—Porto Velho; SI—San Ignacio; Vi—Vilhena. Location of Figure 5 is shown. (C) Specific geographic domain with predominance of basement rocks, discussed in the text.

Figure 4. Tentative boundary of the Proterozoic accretionary belts, south of the Amazon sedimentary basin. The principal volcanic and sedimentary sequences overlying the tectonic provinces are indicated in the figure. Urban settlements: AF—Alta Floresta; Arp—Aripuanã; Arq—Ariquemes; CO—Colorado do Oeste; GM—Guajará Mirim; JP—Ji-Paraná; Ju—Juina; PB—Pimenta Bueno; PV—Porto Velho; SI—San Ignacio; Vi—Vilhena. Location of Figure 5 is shown. (C) Specific geographic domain with predominance of basement rocks, discussed in the text.

Legends are the same for Figures 3 and 4. Regrettably, most of the sites mentioned in the text, as well as the locations of many geologic bodies considered in this work, cannot be shown in these figures, and the interested readers are kindly invited to consult the indicated references. The main purpose of both figures is to show the general areas in which some control for the existence of juvenile material is available, employing Sm-Nd analyses carried out on isolated samples or groups of samples. As previously mentioned, the positive ϵNd(t) values, mostly derived from granitoids and felsic volcanic rocks, are interpreted to indicate mantle-derived material accreted to the crust by B-subduction of oceanic lithosphere. In this work, for the Proterozoic accretionary belts of the Amazonian Craton, we envisage and will describe the establishment of specific tectonic settings typical of oceanic domains, including the formation and stacking of intra-oceanic magmatic arcs.

Beginning at ca. 2.0 Ga, a series of successive magmatic arcs began to be accreted along the southwestern margin of the Central Amazonian province, producing the juvenile material of the Ventuari-Tapajós and the Rio Negro–Juruena tectonic provinces (Fig. 1). It is noteworthy that the relatively large continental masses colliding on the northern side during the so-called Trans-amazonian orogeny (producing the Maroni-Itacaiunas belt) were matched by the contemporaneous subduction of oceanic lithosphere and juvenile magmatic arcs along the opposite side of the Central Amazonian province. The continued and mainly intra-oceanic soft-collision/accretion process produced a very large “basement” domain in which granites (sensu lato), gneisses, and migmatites predominate, at least 2700 km long and ∼1000 km wide, which started with the formation of the magmatic arcs of the Ventuari-Tapajós province.

The Ventuari-Tapajós Province

Felsic volcanics and granitic rocks (sensu lato), formed essentially between 2000 and 1800 Ma (Table 2), are the main constituents of the Ventuari-Tapajós province. Most of them exhibit juvenile isotopic signatures, with positive ϵNd(t) values indicating their formation as magmatic arcs within oceanic domains. Moreover, these rocks have Sm-Nd model ages only slightly older than their U-Pb or Rb-Sr radiometric ages. There is no evidence of Archean basement inliers within the Ventuari-Tapajós province, and regions with high-grade metamorphics are absent or restricted. In our view, geologic evidence, coupled with the isotopic constraints, are consistent with a scenario of successive amalgamation of intra-oceanic magmatic arcs, essentially mantle-derived, of which the roots are now exhumed.

The main isotopic features of the Ventuari-Tapajós province can be divided according to three geographic domains (A and B in Fig. 3; C in Fig. 4. In Venezuela and northern Brazil (Fig. 3, A domain), calc-alkaline granite-gneiss complexes dominate, with U-Pb and Rb-Sr ages between 1980 and 1830 Ma (Tassinari et al., 1996; Wynn et al., 1993). Volcanic associations are also present. These rocks exhibit NW-SE structural trends, truncating the older NE-SW structures of the Imataca block in the area to the northeast of Puerto Ayacucho (Fig. 3). Both volcanic and plutonic rock types are mostly juvenile, yielding positive ϵNd(t) values between +0.70 and +3.05 (Sato and Tassinari, 1997; Tassinari et al., 1996).

Within this domain, a very large area is dominated by ano-rogenic granites with rapakivi textures, such as the El Parguaza, Surucucus, and Auaris plutons. In the regional geologic maps, the El Parguaza granite is a huge batholith for which a U-Pb zircon age of ca. 1550 Ma is reported (Tassinari et al., 1996). Santos (2003) reported a precise U-Pb SHRIMP age of 1551 ± 5 Ma for the Surucucus pluton. Several areas are marked by undeformed sedimentary cover rocks previously correlated with the Roraima Group (Fig. 3). Reis et al. (2003) reported a maximum age of ca. 1550 Ma for one of these rocks (U-Pb SHRIMP age of detrital zircon crystals), and included all of such sedimentary units as belonging to the Quasi Roraima Group.

The best control for age and geochemical features of the Ventuari-Tapajós province is found in the B and C domains (Figs. 3 and 4). Distinct types of granitoids occur within domain B, located north of Manaus, yielding Pb-Pb and U-Pb SHRIMP zircon ages of ca. 1970 Ma, cut by late- to posttectonic plutons with U-Pb ages from 1890 to 1810 Ma (Santos, 2003). In the area between the Tapajós and Jamanxin Rivers, (domain C) occurs a succession of calc-alkaline to sub-alkaline rocks with ϵNd(t) values between +1.3 and +2.1 (Santos, 2003; Santos et al., 2004). These rocks (Table 2) were formed within juvenile magmatic arcs between 1970 and 1880 Ma, as indicated by Pb-Pb and U-Pb SHRIMP ages. Posttectonic granites in this area, typified by the Maloquinha granite and the felsic volcanics of the Uatumã Group, yield ages of 1870–1860 Ma (Santos et al., 2004).

Similar Paleoproterozoic granites were studied by Lamarão et al. (2005) in the region of Riozinho do Anfrísio, to the north of the Ventuari-Tapajós province, well within the area covered by the felsic volcanics of the Iriri Formation, in the Xingu-Iricoumé block, Central Amazonian province (Table 1). They exhibit negative ϵ Nd(t) values between −0.7 and −5.2, and were interpreted by the authors, among other alternatives, as related to subduction in a Andean-type tectonic setting, indicating source magmas bearing an important juvenile component but contaminated by an older continental substratum. Moreover, the nondeformed Maloquinha-type granites in the same area contain inherited Archeanage zircons (Lamarão et al., 2002), indicating their at least partial crustal derivation, and reflecting heritage from ancient basement rocks belonging to the cratonic area of the Central Amazonian province.

In the southern part of the Ventuari-Tapajós province, near the town of Alta Floresta (Fig. 4), domain C contrasts with the A and B domains mainly by the occurrence of the greenschist-type supracrustal sequences of the Jacareacanga Suite. Santos (2003) reported some U-Pb ages from the adjacent Parauari granitoids intrusive into the country rocks and from detrital zircons. The ages were quite similar, within the 1900–2100 Ma interval. In the easternmost portion of the area (just outside Fig. 4), detrital zircon grains from a very low-grade metasedimentary sequence were dated by the U-Pb SHRIMP method, indicating a maximum depositional age of ca. 2080 Ma, as well as inheritance of Archean zircon grains up to 3100 Main age. In the same area, the 1872 ± 12 Ma intrusive Matupá granitic massif (Moura and Botelho, 2002) yielded a ϵNd(t) signature of about −3.0, suggesting reworking of basement and therefore a possible proximity to an ancient continental nucleus such as the Xingu-Iricoumé block.

Several large rifts and similar structures (not shown in Figures 3 and 4), associated with development of cratonic volcano-sedimentary basins and aulacogens, are found over the entire Ventuari-Tapajós province. Mafic and alkaline intrusions may also occur, related to these tectonic features, which are most likely formed by extensional tectonism, generally following the stacking of individual accretionary arcs.

After a period of ∼200 m.y. during which active subduction produced the described succession of accretionary zones, we envisage temporary cessation of the process and the cratonization of the previously accreted material of the Ventuari-Tapajós province. This accretion at ca. 1800 Ma is delimited by the proposed boundary with the younger province, shown in Figures 3 and 4. In Figure 3, because of the general lack of structural and geo-chronological control, the boundary is located at the southernmost extension of the Quasi Roraima–type sedimentary cover.

The southern boundary is placed along what appears to be a major tectonic zone comprising a series of large overthrusts with WNW-ESE trends, located south of the town of Alta Floresta (Fig. 4). In that region, close to the Serra do Cachimbo, the Teles Pires Group, a practically unmetamorphosed volcano-sedimentary sequence bounded by extensional structures and located over the southernmost part of the Ventuari-Tapajós province, constrains the time of the cratonization stage. It comprises volcanic and plutonic rocks and underlies the very large Beneficente sedimentary basin, the age of which is bracketed between 1700 and 1400 Ma according to Tassinari et al. (2000).

Near Aripuanã, some shallow intrusives and extrusive felsic rocks of the Teles Pires Group exhibit gradational contacts. Zircon crystals from these rocks were dated by U-Pb SHRIMP (Neder et al., 2002), and yielded ages between 1762 and 1755 Ma. Their ϵNd(t) signatures showed a range between zero and −2. The Teles Pires–Beneficente volcano-sedimentary cover is located very close to the proposed boundary between the Ventuari-Tapajós and Rio Negro–Juruena provinces, and its geologic evolution is considered to reflect the main tectonic events of the latter province.

The Rio Negro–Juruena Province

A short time after the cratonization of the Ventuari-Tapajós province, subduction was resumed, initiating a second accretionary cycle, characterized by radiometric ages of granitoid rocks in the 1780–1550 Ma interval. The oldest ages are found in the region south of Alta Floresta, near the previously mentioned boundary with the province. In that region, some U-Pb SHRIMP zircon dates were obtained by Santos (2003) in granitic rocks of the São Romão and São Pedro Suites, which seem to characterize a magmatic arc of ca. 1780 Ma, formed adjacent to a previously cratonized region or cutting through the marginal zone within an Andean-type tectonic setting.

The Rio Negro–Juruena province is characterized by ages between 1780 and 1550 Ma, with a general younging of accretionary wedges from northeast to southwest (Tassinari et al., 1996). Like the Ventuari-Tapajós, the Rio Negro–Juruena province is composed predominantly of granite-gneiss and granitoid rocks, with positive to slightly negative ϵNd(t) signatures, roughly between +4.0 and −2.0, suggesting that juvenile accretionary events played a major role in their tectonic evolution. However, large parts of this province are poorly controlled by U-Pb geochronology and Nd isotope constraints.

Figure 5 shows the geologic setting of the Alto Jauru granite-greenstone terrain, at the southeastern end of the Rio Negro–Juruena province, where the geochronological pattern (see Table 2) is well constrained (Pinho et al., 1997; Geraldes et al., 2001, 2004b). The oldest rocks are the metavolcano-sedimentary sequences and the granite-gneissic rocks of the Alto Jauru Complex, yielding U-Pb zircon ages of 1750–1790 Ma and positive ϵNd(t) values between +2.2 and +2.6. In addition, toward the northwestern portion of Mato Grosso State, basement granitoid rocks yield whole-rock Rb-Sr and Pb-Pb ages of ca. 1700 Ma (Tassinari et al., 1996), with low initial Sr ratios and Pb µ1 values of 8.1. The granitic rocks of the Cachoeirinha magmatic arc are intrusive into the Alto Jauru granite-greenstone terrain, with U-Pb zircon ages between 1587 and 1522 Ma and ϵNd(t) values between −0.8 and +1.0. This geochronological framework can be roughly compared with that of the extreme northwest of the Rio Negro–Juruena province (border between Brazil and Venezuela, Fig. 3). There, conventional U-Pb and Rb-Sr ages between 1860 and 1810 Ma are available for calc-alkaline granitoids making up the basement complex (Tassinari et al., 1996; Gaudette and Olzewski, 1981), while U-Pb SHRIMP zircon ages around 1510–1540 Ma were obtained for younger granitic intrusions such as the Içana and Uaupés Suites (Santos et al., 2000).

Figure 5. Simplified geological map of the Jauru region, western part of Mato Grosso State of Brazil, near the Bolivian border. Adapted from Geraldes et al., 2001.

Figure 5. Simplified geological map of the Jauru region, western part of Mato Grosso State of Brazil, near the Bolivian border. Adapted from Geraldes et al., 2001.

In east-central Rondonia (Fig. 4), some U-Pb TIMS and SHRIMP zircon dates were obtained by Tassinari et al. (1996) and by Payolla et al. (2002) in the regional gneissic rocks. These results were grouped according to three distinct lithologic associations, with ages (1) ca. 1750 Ma, (2) between 1600 and 1530 Ma, and (3) ca. 1430 Ma. The oldest of these rocks, tonalitic gneisses and enderbitic granulites, are related to the development of the Rio Negro–Juruena province, exhibit calc-alkaline affinities, and their ϵNd(t) values range from −1.5 to +0.1, indicating a substantial contribution from juvenile magmatic source protoliths. The intermediate ones are more potassic and include the Serra da Providência Intrusive Suite, with its charnockitic and mangeritic granitoids, some of them with rapakivi texture. They occur in the southern part of the region near the border with the Rondonian–San Ignacio province, and exhibit ϵNd(t) values between −0.6 and +2.0, suggesting derivation from a mixture of predominantly juvenile sources and a slightly older crust. The youngest lithologies include fine-grained gneisses and granulites (Bettencourt et al., 1999a). Detrital zircons from a paragneiss within this region yielded U-Pb SHRIMP ages down to 1670 Ma, indicating that their provenance included sources from rocks of both the Ventuari-Tapajós and Rio Negro–Juruena provinces (Payolla et al., 2002).

As with the Ventuari-Tapajós province, volcano-sedimentary covers, practically undeformed and of a very low metamorphic grade, can also be found in the Rio Negro–Juruena province, filling rift-type or aulacogen-type basins (Figs. 3 and 4). Along the Juruena and Roosevelt Rivers, well into the province, felsic volcanics considered coeval with the Teles Pires Group were dated by Santos et al. (2004) to between 1773 and 1786 Ma (U-Pb SHRIMP zircon ages), and showed ϵNd(t) values between −1.4 and +0.6. In the southern part of the province, the Roosevelt-Aripuanã volcano-sedimentary sequence occurs, from where a dacite yielded a U-Pb zircon age of 1740 Ma (Santos et al., 2000). In central Rondônia, the Comemoração volcano-sedimentary sequence crops out along the border of the Serra da Providência Intrusive Suite. Some zircon crystals extracted from acid-ash tuffs at the base of the sequence yielded a conventional U-Pb zircon age of 1690 Ma (concordia plot, upper intercept), indicating a maximum age for the sedimentation. Moreover, the low-grade Mutum-Paraná volcano-sedimentary formation, located close to Porto Velho, was dated using the U-Pb SHRIMP method in zircon grains from interbedded tuff layers, with an age of 1746 ± 4 Ma (Santos, 2003). Other sequences of probably similar tectonic origin, such as the Caiabis and Dardanelos, crop out discontinuously along more than 900 km within the Rio Negro–Juruena province. Where isotopic analytical data are available, the volcanic rocks exhibit positive ϵNd(t) values, and Sm-Nd TDM model ages are quite close to their age of crystallization (Santos, 2003).

The southern boundary of the Rio Negro–Juruena with the Rondonian–San Ignacio province is very complex. It comprises several metamorphic belts, some of which reach granulite facies, several large shear zones, and recurrent plutonism (e.g., Bettencourt et al., 1999a; Payolla et al., 2002; Tohver et al., 2004, 2005b). Such features seem to be a good indication of the collisional nature of the boundary.

For the Ventuari-Tapajós and Rio Negro–Juruena provinces, the present authors envisage a practically continuous accretionary process involving oceanic domains, active during ∼500 m.y. In this phase, crustal evolution combined predominantly juvenile processes with some reworking of crustal material formed in previous accretionary phases. In addition, the geochronological pattern of the “basement” granitoids seems to indicate a younging from northeast to southwest, suggesting the development of successive accretionary units, with a great deal of juvenile material formed in subduction environments. In our view, this large, continental region was formed as the result of two possible, successive Proterozoic mega-accretionary cycles, followed by cratonization, the earliest at 1980–1830 Ma and the younger at 1780–1550 Ma.

Many volcano-sedimentary rift-type structures of Meso-proterozoic age are found over both cratonized provinces. These basins may accompany numerous, circular intrusions of granitic composition, formed by anorogenic magmatism, indicating major processes of disruption of the already cratonized continental crust. A complete understanding of the tectonic and magmatic evolution of this very large and complex region cannot be reached without a much larger set of geochronological and isotopic data. In spite of this handicap, we suggest that many of the extensional structures located over the Ventuari-Tapajós province represent the reflection of the tectonic processes produced by the younger subduction regime outboard, especially those near the marginal zone. An example of basin formation linked with extensional tectonics is the large Beneficente Group at the Serra do Cachimbo. A similar tectonic context is visualized for the Rio Negro–Juruena province, where several aulacogenic basins and anorogenic structures may be related to the tectonic processes that took place as a reflection of the collisional episodes that established the younger Rondonian–San Ignacio, or even the Sunsas-Aguapeí provinces formed toward the southwest.

The Rondonian–San Ignacio Province

The Rondonian–San Ignacio province, characterized by the collisional-type orogeny bearing the same name, occurs in the southwestern part of the Amazonian Craton, and its tectonomagmatic episodes are attributed to the early to middle interval of the Mesoproterozoic, roughly between 1500 and 1300 Ma (Tassinari et al., 2004). This province includes large parts of the Pre-cambrian shield of the Brazilian states of Rondônia and Mato Grosso, as well as a large area of the Santa Cruz province of Bolivia (Fig. 4). The total area exposed is at least 2000 km long and ∼800 km wide.

The tentative boundary with the Rio Negro–Juruena province is not easy to trace, taking into account all the presently available geological evidence, as already presented (Figs. 3 and 4). It seems that the accretionary regime with soft-collision stacking of oceanic features, which was the rule for the Ventuari-Tapajós and part of the Rio Negro–Juruena provinces, was interrupted by a continental collision with a relatively large microcontinent, made up by a relatively thick continental crust. For the purpose of this paper, we may call it “Parecis microcontinent,” speculating that it may occur below the sediments that fill up the very large Pimenta Bueno graben basin, which includes a thick Precambrian to Paleozoic sedimentary sequence that was at its turn covered by the Mesozoic Parecis Formation. Appropriate geophysical evidence, such as seismic data, necessary to confirm such possibility is scanty. However, the kimberlite provinces (Fig. 4) of Juina at the northern border of the Pimenta Bueno sedimentary basin, and of Colorado do Oeste at its southern border, may be an indirect indication of the existence of continental lithosphere with substantial thickness, below the Paleozoic and possibly Pre-cambrian sediments (Tohver et al., 2004). These authors propose that this region represents the westward continuation of the Nova Brasilândia belt on the basis of large-scale aeromagnetic anomalies. A recent work using continental-size seismic tomography by Heintz et al. (2005), through surface wave inversion, seems to confirm the existence of a fairly large cratonic area with a lithospheric keel at least down to 200–250 km, under the Pimenta Bueno–Parecis basin, but extending to the neighboring areas, well into the Rio Negro–Juruena province.

Along the border region between the Rio Negro–Juruena and the Rondonian–San Ignacio provinces, in western Rondônia, additional evidence of a major collisional tectonometamorphic episode is given by large shear zones, such as the Ji-Paraná shear zone as described by Tohver et al. (2002). A few metamorphic belts are also indicated (Fig. 4), with variable metamorphic grade, up to granulite facies in the Nova Brasilândia belt (Tohver et al., 2004, 2005b). Some U-Pb SHRIMP measurements on zircon overgrowths from rocks of the Rio Negro–Juruena basement in east-central Rondônia yielded metamorphic ages around 1300–1350 Ma (Tassinari et al., 2000; Payolla et al., 2003; Bettencourt et al., 2006). These ages agree well with Sm-Nd whole-rock garnet ages of gneissic rocks (Payolla et al., 2002), diagnostic for the timing of collision-related, high-grade regional metamorphism of the Rondonian–San Ignacio orogeny. Moreover, a series of 40Ar/39Ar ages on amphibole and biotite from different rocks, between 1330 and 1300 Ma (Rizzotto et al., 2002; Tohver et al., 2005b, 2006b; Teixeira et al., 2006), provides additional evidence for this main Mesoproterozoic metamorphic-thermal event of the Rondonian–San Ignacio province.

The Rio Crespo Intrusive Suite, which comprises fine-grained granitic gneisses and charnockitic granulites, and occurs just south of Ariquemes (Fig. 4), is one of the oldest units of the Rondonian–San Ignacio province, with an age of 1492 ± 12 Ma obtained by Bettencourt et al. (2006), using the U-Pb SHRIMP method applied to the cores of several zircon crystals. These rocks exhibit slightly positive ϵNd(t) values of +0.6 to +1.2, suggesting that they represent a mixture of predominantly juvenile material and some older crust, possibly within a Andean-type continental margin. Bettencourt et al. (2006) obtained younger ages of ca. 1350 Ma for the overgrowth rims of the same zircons, evidence of the medium- to high-grade metamorphic imprint that affected the entire region during the collisional processes of Rondonian–San Ignacio orogeny.

Existence of an older continental substratum in the northern part of the Rondonian–San Ignacio province is supported by the polymetamorphic nature of some basement rocks, and by the several negative ϵNd(t) values of granitoid rocks found there, indicating some degree of crustal reworking. Litherland et al. (1986) identified some pre–San Ignacio basement regions, including the high-grade metamorphic rocks of the Lomas Maneches Complex in Bolivia, based on geological evidence as well as on a few very imprecise Rb-Sr whole-rock ages. More recently, Boger et al. (2005) presented more precise U-Pb SHRIMP zircon ages from two high-grade rocks of the Lomas Maneches Complex, which yielded ca. 1660 and ca. 1690 Ma.

The Rondonian–San Ignacio province also contains numerous anorogenic rapakivi granites that were emplaced over a long period of time. The ages of the following intrusive suites have been constrained by U-Pb geochronology (Bettencourt et al., 1999a; Payolla et al., 2002): Santo Antônio (1410 Ma), Teotônio (1390 Ma), Alto Candeias (1340 Ma), and São Lourenço–Caripunas (1310 Ma). These granites exhibit slightly positive ϵNd(t) values (Bettencourt et al., 1999b), indicating a high proportion of juvenile material in their magmas. In our view they may well be late or posttectonic-type intrusions related to inboard magmatism of different pulses of the Rondonian–San Ignacio orogeny.

Litherland et al. (1986) identified voluminous syn- to post-tectonic magmatism in the bulk of the Rondonian–San Ignacio province along its western margin. This magmatism is related to their San Ignacio orogeny, with Rb-Sr ages in the 1280–1380 Ma interval (Darbyshire, 2000). This region was named “Paragua Craton” because it was considered to be tectonically stable during the Meso- to Neoproterozoic deformation of the Sunsas belt (see below). The ubiquitous granitic rocks were all included in the “Pensamiento Granites,” and their predominantly juvenile character was identified by Darbyshire (2000) on the basis of ϵNd(t) values between −0.9 and +3.9. Boger et al. (2005) shed some light on the evolution of the San Ignacio belt in Bolivia by means of some key U-Pb SHRIMP zircon ages. Firstly, U-Pb zircon ages of 1340–1320 Ma were obtained on the San Rafael granite, indicating emplacement synchronous with the San Ignacio orogeny, similar to those already mentioned, in Rondonia (Payolla et al., 2002; Rizzotto et al., 2002). Secondly, on detrital grains from paragneisses ages between 1690 and 1760 Ma were obtained. Moreover, R. Matos (2006, personal commun.) obtained U-Pb SHRIMP zircon ages from some rocks of the “Pensamiento Granites,” with ages within the 1320–1400 Ma interval, coupled to positive to slightly negative ϵNd(t) values. This isotopic pattern strongly suggests that a relevant part of the “Paragua Craton” is made up by oceanic-type material, formed in magmatic arcs within an accretionary belt.

The southeasternmost part of Figure 4 may provide some better hints for the tectonic development of the Rondonian–San Ignacio province. In the Alto Jauru region (Fig. 5), much better access to the area, and more detailed studies by Geraldes et al. (2001, 2004a, 2004b), provide better understanding of the tectonic relations between the Rio Negro–Juruena province and the evolution of the Rondonian belt. First, a cratonized portion of the province is identified in the northeastern corner of Figure 5, which is covered by the undeformed Rio Branco Igneous Suite. The volcanic rocks of this suite are bimodal. They yield U-Pb zircon ages between 1470 and 1420 Ma and ϵNd(t) values between −1.0 and +1.9, indicating some assimilation of basement material (Geraldes et al., 2001). Extension of the Mesoproterozoic basement along the boundary of the Parecis sedimentary basin seems to be confirmed by a whole-rock Pb-Pb age of 1780 Ma (Tassinari et al., 1996) for a gneissic sample.

Two magmatic arcs of juvenile and possibly intra-oceanic character, related to the Rondonian–San Ignacio province, have been characterized in the same region (Fig. 5). The Rio Alegre Complex contains mafic-ultramafic bodies, BIFs (banded iron formations), chert, and granitoid plutonic rocks with U-Pb ages between 1510 and 1480 Ma and ϵNd(t) values between +2.5 and +4.7. The plutonic rocks of the Santa Helena calc-alkaline batholith exhibits U-Pb ages between 1450 and 1420 Ma and ϵNd(t) values between +2.6 and +4.0 (Geraldes et al., 2001, 2004b; Ruiz, 2005).

It is our view that the Rio Alegre and Santa Helena are juvenile crustal domains progressively built or amalgamated to the evolving continental margin. The U-Pb zircon ages, as well as the Sm-Nd constraints, indicate the onset of intermittent convergent-margin magmatism and soft accretion by stacking of outboard juvenile magmatic arcs during a continued subduction regime. Ancient basement has not been found here, high-grade metamorphics are not present, and some mafic rocks, as well as deep-ocean sediments, are described (Geraldes et al., 2001), indicating the presence of oceanic-type material. The Rio Alegre Complex is coeval with the already mentioned Pensamiento granitoids, and the entire region may well be regarded as assembled by several accretionary intra-oceanic magmatic arcs, revealing a large oceanic domain occurring in Bolivia at the end of Mesoproterozoic time.

In the northwestern corner of the Amazonian Craton, outcrops of Rondonian–San Ignacio province rocks are limited, being concealed beneath the Phanerozoic cover (Fig. 3). For the low-grade Tunui Group metasediments, 40Ar/39Ar muscovite ages indicate that the metamorphic imprint of the Rondonian–San Ignacio orogeny occurred at ca. 1320 Ma (Santos, 2003). In addition, some detrital zircon grains from the same unit were dated by U-Pb SHRIMP and yielded ages between 1875 and 1720 Ma (Santos, 2003), indicating provenance from rocks of the nearby Rio Negro–Juruena province.

The precise time of cratonization of the Rondonian–San Ignacio province is difficult to establish, because of the multiple tectonic reactivations related to the younger Sunsas-Aguapeí collisional belt (see below). The best approximation of the time of tectonic stabilization seems to be provided by the youngest K-Ar mica dates obtained from the Pensamiento Complex in Bolivia (Litherland et al., 1986), and by the 40Ar/39Ar muscovite and hornblende ages of Rizzotto et al. (2002) for the Colorado schists of Rondônia, which are slightly older than 1300 Ma. These authors used this interpretation to characterize their “Paragua Craton” as a region that was not affected by their Sunsas orogeny. Tohver et al. (2006a) have reviewed the available 40Ar/39Ar hornblende ages for the entire basement of Rondônia. The ages encompass the 1350–1550 Ma interval and seem to mime the duration of the Rondonian–San Ignacio orogeny. After cratonization, several rift-type structural basins located within the Rondonian–San Ignacio province were formed over the stabilized region, possibly as a result of collisional processes that established the younger Sunsas province to the southwest. The more important basins are the Pacaás Novos, Nova Brasilândia, and Uopione, to be dealt with later.

The Sunsas-Aguapeí Province

The collisional-type Sunsas orogeny occurred roughly between 1250 and 1000 Ma at the southwestern end of the Amazonian Craton. It is made of low- to medium-grade metamorphics and associated granitoid plutons (Litherland et al., 1986, 1989; Tassinari et al., 2000). According to Litherland et al. (1986), and later to Sadowski and Bettencourt (1996), the Sunsas belt was formed in an extensional environment, consisting of a passive margin sedimentary sequence (the Vibosi and Sunsas belts) that was subsequently deformed during a collisional event. This deformed sequence was intruded by syn- to late-tectonic granitoids, followed by the emplacement of posttectonic and later ano-rogenic plutons.

The evolution of this orogen involved early deposition of the Sunsas and Vibosi Groups, and their subsequent deformation and metamorphism (Litherland et al., 1986). These authors placed the boundary of the Sunsas belt along the southern boundary of Paragua Craton at the Rio Negro Front and Santa Catalina shear zone, where several mylonitic belts are observed (Fig. 4). Throughout the Paragua Craton, the unmetamorphosed and practically flat-lying sedimentary sequences of the Huanchaca Group (Lither-land and Power, 1989), correlated with the Sunsas Group, overlie the Paragua basement rocks (Fig. 4). In Bolivia, the main phase of the Sunsas orogeny is constrained by the U-Pb SHRIMP zircon age of the undeformed posttectonic Taperas granite, dated by Boger et al. (2005) at 1076 ± 18 Ma.

Over the cratonic area, in Brazil, the Nova Brasilândia and Aguapeí belts were formed in rift-type structures that were affected by later transpression and crustal shortening. The history of deformation and metamorphism within these tectonic units seems to match in time the orogenic evolution of the Sunsas belt.

Within the Nova Brasilândia belt, U-Pb SHRIMP zircon ages of ca. 1120 Ma were produced by Santos et al. (2000). Moreover, Rizzotto et al. (2002) constrained the age of this unit more by U-Pb zircon geochronology, dating some detrital zircon crystals (1215 Ma), deformed granite intrusions (1113 Ma), basic intrusions (1110 Ma), and posttectonic granites (1005 Ma). Subsequent granulite-facies metamorphism took place at 1.09 Ga with cooling through 920 Ma, as recorded by the U-Pb dating of monazite and titanite, as well as 40Ar/39Ar ages from hornblende and biotite, respectively (Tohver et al., 2004). In addition, some mafic lava flows within the Pacaás Novos basin in Rondonia were dated by the K-Ar method between 1000 and 1200 Ma (Teixeira and Tassinari, 1984), and more recently a mafic sill yielded a 40Ar/39Ar age of 1198 ± 3 Ma (igneous biotite; Tohver et al., 2002). A maximum age for the sedimentary sequence making up the upper section in the basin was obtained by Santos (2003) at 1050 Ma, based on U-Pb determinations on detrital zircon grains.

The Aguapeí intracontinental rift, or aulacogen (Saes, 1999), is a structure more than 500 km long, with NW-SE trend, located in Mato Grosso State, Brazil, along the Bolivian border (Fig. 4). It is made up by clastic sediments, gently folded and affected by low-grade metamorphism. Santos (2003) and Santos et al. (2005) presented U-Pb SHRIMP ages from detrital zircon grains, the youngest of which was of 1161 ± 27 Ma, placing an upper limit on the age of deposition of the sediments.

The widespread anorogenic granitic magmatism of the Younger Granites of Rondônia, also known as the Rondônia Tin Province, which intruded large parts of the Rio Negro–Juruena province southeast of Porto Velho, can also be considered as a reflection of the Sunsas orogeny. This suite comprises alkali-granites, including rapakivi varieties, and associated mafic rocks. A few intrusive bodies were dated by several methods (Table 2), yielding ages between 970 and 1100 Ma (e.g., Priem et al., 1971a, 1989; Bettencourt et al., 1995, 1999a). Examples could be the ones dated by U-Pb SHRIMP method, such as the Santa Barbara Massif (1082–978 Ma; Leite et al., 2003), the Maçangana granite (990–980 Ma), and the Santa Clara Intrusive Suite (1082–1074 Ma). The Younger Granites of Rondônia usually exhibit negative ϵNd(t) values (Bettencourt et al., 1999b), such as the Santa Barbara pluton, whose values are from −2.9 to −4.6, indicating crustal reworking (Sparrenberger et al., 2002). To the southeast, in Mato Grosso State, additional anorogenic granitic bodies are synchronous with the Younger Granites of Rondônia, such as the São Domingos and Guapé plutons, with Rb-Sr and U-Pb ages of ca. 980–920 Ma (Geraldes et al., 2001; Ruiz, 2005). The Rincón del Tigre basic complex, in Bolivia, dated at 990 Ma by the Rb-Sr method (Litherland et al., 1986) may also have taken part in the same tectonic history of the Sunsas orogeny. According to Bettencourt et al. (1999a), these intrusions were formed during the collisional stage, as inboard manifestations over the older and already cratonized Rio Negro–Juruena and Rondonian–San Ignacio provinces.

From the comprehensive 1000–950 Ma Rb-Sr and K-Ar dates reported by Litherland et al. (1986, 1989), the timing for the final cratonization, cooling, and exhumation of the Sunsas orogenic belt seems well constrained. A few 40Ar/39Ar mica ages in the 900–1000 Ma range, obtained by Ruiz (2005) in gneissic rocks on the basement of the Aguapeí aulacogen, close to the boundary between the Rondonian–San Ignacio and the Rio Negro–Juruena border, can also be attributed to the thermal reactivation followed by cooling as a response to the Sunsas orogeny.

The history of deformation and magmatism of the Sunsas-Aguapeí province matches relatively closely the tectonic events of the Laurentian Grenville province, prototype of the Grenvillian orogenic system. The main features are the high-grade metamorphism and three successive tectonometamorphic events (Rivers et al., 1989): the older at ca. 1190–1140 Ma (Elzevirian pulse), the intermediate and more widespread at 1080–1020 Ma (Ottawan), and the younger at 1000–980 Ma (Rigolet). The Elzevirian episode has been related to backarc closure and arc accretion, and the younger episodes have been related to continental collision (Rivers, 1997; Wasteneys et al., 1995). The Sunsas collisional belt is considered by most to be the South American counterpart of the Grenville belt, with a role in the agglutination of the supercontinent Rodinia (e.g., Hoffman, 1991; Sadowski and Bettencourt, 1996; Kröner and Cordani, 2003). However, high-grade metamorphic rocks, typical of the two younger pulses within the Grenville belt in Canada, are not present within the Sunsas belt, and therefore the details of the probable Grenville-Sunsas link are far from resolved.

A history similar to the Grenville belt is encountered in a few basement inliers with high-grade metamorphic rocks in the northern Andes (Cordani et al., 2005). The common tectonic events found in all of these Proterozoic rocks of South America, as well as in their correlative terranes in Mexico and the central Andes, strongly suggest that they are fragments of the once coherent Grenvillian collisional belt that participated in the agglutination of Rodinia. When the tectonic inliers from Colombia are merged with the correlative Mexican and central Andean domains, as well as with Laurentia and Amazonia, an extensive and continuous belt is formed, in the central region of Rodinia, as envisaged firstly by Hoffman (1991), and later by many others involved with global paleogeographic reconstructions.

ACCRETIONARY BELTS AT THE EASTERN MARGIN OF THE AMAZONIAN CRATON

The Sunsas belt is the youngest orogen in the Amazonian Craton. If we consider the southwestern half of the craton, the described continuous migration of the tectonic and magmatic processes from northeast to southwest must be considered as one of the major features of Earth's geodynamics for Proterozoic times. It must be linked to successive subduction processes during ∼1.0 b.y., related to large-scale and systematic movement of the asthenosphere.

More or less synchronous with the termination of the tectonic events of the Sunsas belt is early Neoproterozoic continental breakup and fragmentation affecting the eastern margin of the craton, producing a passive margin and the observed truncation of the major structures. The intra-oceanic Goiás magmatic arc (Pimentel and Fuck, 1992; Pimentel et al., 2000) is evidence of the large oceanic basin that existed at 900–1000 Ma between the rifted margins of the Amazonian Craton and some other unidentified cratonic fragments. Candidates for a possible pre-Neoproterozoic correlation are the Baltica and São Francisco–Congo Cratons, the Trans-Saharan “metacraton,” and the other smaller cratonic fragments in South America, such as the Paranapanema cratonic fragment (Mantovani et al., 2005). The Goiás magmatic arc is part of the Tocantins province of Central Brazil (Figs. 1 and 6), which, together with the Borborema province and the Trans-Saharan belt of Africa, were affected by a series of orogenic events that produced West Gondwana at the end of the Proterozoic (Kröner and Cordani, 2003).

Figure 6. Tocantins province and its main tectonic domains. AR—Araguaia belt; PA—Paraguay belt; GO—Goiás magmatic arc; CX—Crixás granite-greenstone terrain; NC—Natividade-Cavalcante block; BR—Brasilia belt. Adapted from Cordani et al., 2000.

Figure 6. Tocantins province and its main tectonic domains. AR—Araguaia belt; PA—Paraguay belt; GO—Goiás magmatic arc; CX—Crixás granite-greenstone terrain; NC—Natividade-Cavalcante block; BR—Brasilia belt. Adapted from Cordani et al., 2000.

Pimentel et al. (2000, and references therein) detailed the lithology, geochemistry, and the tectonic evolution of the geologic units belonging to the Tocantins province (Fig. 6). The main tectonic element in the region is a very large fault zone, a megasuture that was active throughout the Neoproterozoic, the Transbrasiliano Lineament, which crosses the entire region from northeast to southwest. The Araguaia and Brasilia marginal belts are identified at the borders of the Amazonian and São Francisco Cratons, respectively, evolving from passive margins with thick sedimentary piles into orogenic belts. Moreover, the Tocantins province comprises a mosaic of tectonic blocks, a series of juxtaposed terranes with variable lithologies, bounded by shear zones of different sizes. They make up a unit named Central Goiás Massif, considered a single heterogeneous cratonic fragment by Brito Neves and Cordani (1991).

The tectonic evolution of the Central Goiás Massif is very complex. Several ancient rock units are recognized in it, including the Archean nucleus with the Crixás, Goiás Velho, and other greenstone belts, some Paleoproterozoic medium- to high-grade complexes, and the three large layered mafic-ultramafic complexes: Canabrava, Niquelandia, and Barro Alto. These mafic-ultramafic complexes are very similar, and may have originated in the same large continental rift. They comprise an older lower layered series of gabbronorites, pyroxenites, and dunites, and a younger upper layered series in which gabbro-anorthosites predominate. The complexes are in contact with supracrustal sequences in their western part and were affected in their eastern part by high-grade metamorphism. Although many attempts were made (see Pimentel et al., 2000), the age of igneous crystallization of the mafic-ultramafic rocks is far from resolved. Age of metamorphism seems to be better constrained, ca. 750 Ma (Ferreira-Filho et al., 1998; Moraes et al., 2006). Pimentel et al. (2000) considered this high-grade metamorphic event to be related to the collision between the Central Goiás Massif and the São Francisco Craton. In addition, a high-grade metamorphic complex occurs in the southern part of the Central Goiás Massif (Fischel et al., 1998), where Sm-Nd garnet/whole-rock metamorphic ages are 630–610 Ma. These ages probably date the collision between the São Francisco Craton and the Paranapanema cratonic fragment, as suggested by Campos-Neto (2000), preceding other collisional events leading to the final amalgamation of the Gondwana super-continent, accomplished some 100 m.y. later.

The Goiás magmatic arc is a major area of soft collision and accretion of the Amazonian Craton margin and contains Neoproterozoic granitoids, with the oldest of these clearly juvenile (Pimentel et al., 1999). It is a large and elongated region, closely associated with the Transbrasiliano megasuture, where calc-alkaline orthogneisses and associated rocks predominate. The principal rock types of this unit are plutonic, ranging in composition from tonalite to granodiorite. Several supracrustal belts occur in this region, with volcano-sedimentary sequences with mafic and ultramafic bodies, considered being ophiolite fragments (Pimentel et al., 1997). A deep seismic study carried out by Assumpção et al. (2004), using teleseismic P-wave tomography, showed that beneath the Goiás magmatic arc the crust is thinner and the lithospheric upper mantle has lower velocities compared to those of the adjacent cratons. This seems to characterize a differential uplift in the Neoproterozoic, accompanied by mafic intrusions in the lower crust, and eventually producing the observed gabbroic/granitic magmatism in the region of the Goiás magmatic arc. Many postorogenic granitic intrusions are also found in the region. The geochronological systematics of the Goiás magmatic arc indicates ages between 900 and 530 Ma. The juvenile signature of the granitoid rocks is demonstrated by their low initial Sr isotopic ratios, their positive ϵNd(t) values, and their Sm-Nd TDM and model ages between 0.9 1.2 Ga (Cordani and Sato, 1999; Pimentel et al., 2000). The interpretation of these authors is that the material represents the roots of a series of juvenile intra-oceanic island arcs produced by consumption of oceanic lithosphere. This implies the existence of a large oceanic domain, the Goiás Ocean, since at least 900 Ma, separating the Amazonian Craton from the São Francisco–Congo Craton and other smaller cratonic units. The disappearance of this ocean is one of the major processes bearing on the amalgamation of Gondwana.

THE POSITION OF THE AMAZONIAN CRATON WITHIN THE PROTEROZOIC SUPERCONTINENTS

Supercontinents are formed by the amalgamation of preexisting continental masses, with the concomitant disappearance of the intervening oceans. Reconstructions are based on the available paleomagnetic data that are normally affected by large uncertainties, especially regarding paleolongitudes. If it were possible to estimate the size of the oceans that separated the existing continental masses at a given time, the task of producing a reconstruction would be facilitated. Following the ideas put forward by Cordani et al. (2003), we maintain that intra-oceanic granitoid magmatic arcs, with juvenile isotopic signatures, indicate the existence of significantly large oceanic basins. In the case of the Amazonian Craton, this conclusion applies not only to the Ventuari-Tapajós and Rio Negro–Juruena provinces, but also to the Pensamiento–Rio Alegre territory in the southwestern corner and to the Goiás magmatic arc. In contrast, when granitoid material yields isotopic signatures indicating crustal reworking, consumption of small oceans is a more likely interpretation, indicating the relative proximity of the convergent continental masses. Such tectonic environments, which are common for the eastern part of the Brazilian Shield (see Cordani et al., 2000), are related to Wilson cycle processes of ocean opening and closing that usually lead to collisional belts.

Pre-Rodinia Times

It is not possible to indicate with confidence the possible position of the cratonic nuclei related to the Amazonian Craton before the onset of the Transamazonian-Eburnean orogeny, in the Paleoproterozoic. After this event, the northern part of the Guyana Shield was attached to West Africa. Between ca. 1900 and 1500 Ma, the series of intra-oceanic magmatic arcs found in the Ventuari-Tapajós and Rio Negro–Juruena tectonic provinces attests to the existence of a very large ocean. This means that in Paleo- and Mesoproterozoic times Amazonia was much smaller, and this must be taken into account in the reconstructions envisaged for those times.

The age and the character of the Paleo- to Mesoproterozoic belts of southwestern Amazonia permit correlation with the Sve-cofennian domain and the Transscandinavian igneous belt within Baltica, an idea first advanced by Almeida (1978), and also with the Midcontinent region of Laurentia. Pesonen et al. (2003) attempted to reconstruct the relative position of these three cratonic masses at ca. 1500 Ma. An additional fact indicating such a correlation is the global collisional event that produced Rodinia, described for the Grenville belt in Laurentia, the Sveconorvegian belt in Baltica, and the Sunsas and Nova Brasilândia belts in Amazonia.

The relation of Amazonia and Laurentia is a key issue for the reconstruction of the Mesoproterozoic. In our view, the juvenile rocks of Pensamiento–Rio Alegre, and related magmatic arcs, dated between 1400 and 1500 Ma, could well correspond to the accreted intra-oceanic material that attests to the existence of a large ocean that separated Amazonia from Laurentia.

From then on, different scenarios have been proposed by several authors, most of them envisaging a final Sunsas collision at 1000 Ma, as part of the global Grenvillian orogeny that formed Rodinia. In our view, the granitoid rocks forming the Paragua Craton of Litherland et al. (1986, 1989) and those of Mato Grosso and Rondonia States, south of the boundary with the Rio Negro–Juruena province (Fig. 4), were already united at the end of the Rondonian–San Ignacio orogeny, ca. 1320–1340 Ma. Therefore, we are at variance with the view of Tohver et al. (2005a, 2005b) and Boger et al. (2005), according to which the collision of the Paragua Craton with the Amazonian Craton took place at ca. 1.1 Ga, during the Sunsas orogeny.

We prefer one of the alternative models outlined by Boger et al. (2005) (Fig. 7), according to which the collision of the Paragua Block (our Pensamiento–Rio Alegre intra-oceanic accretionary terrane) with the southern part of the already cratonized Rio Negro–Juruena province originates the Rondonian–San Ignacio orogeny. In our interpretation, rifting of the Aguapeí and Nova Brasilândia Groups correspond to the final orogenic collapse within the Rondonian–San Ignacio province. The later activation of these could be a reflection of the successive collision that produced the Sunsas belt. Final suturing would be, more properly, between Amazonia and Laurentia, with the Sunsas as part of the Grenvillian belts.

Figure 7. Cartoon of a possible scenario for the accretion of the Paragua block and the Arequipa-Antofalla terrane to the southwestern margin of the Amazonian Craton, after Boger et al., 2005.

Figure 7. Cartoon of a possible scenario for the accretion of the Paragua block and the Arequipa-Antofalla terrane to the southwestern margin of the Amazonian Craton, after Boger et al., 2005.

As a somewhat different alternative, Sadowski and Bettencourt (1996) envisaged a large continent (a proto-Laurentia plus a proto-Amazonia?) that would be disrupted by the onset of two successive Wilson cycles. In this view, Laurentia and Amazonia were close together, as parts of the same continental mass, since at least 1.5 Ga. Their preferred evolutionary model includes formation of a reasonably large ocean floor during the first cycle (ca. 1.5 Ga) and only a small ocean during the second (ca. 1.25 Ga). Following this hypothesis, the onset of the second Wilson cycle may have produced the passive margin that later was inverted to produce the Sunsas belt, and also the initial rifting of the Aguapeí aulacogen, more or less along the weakness zone represented by the previous suture of the Rondonian–San Ignacio collision. The final closing of the second ocean would have produced a high-grade collisional belt on the Laurentian side and a series of transtensional features in Amazonia. Tohver et al. (2006a) produced recently a summary of the available robust paleomagnetic constraints for the Laurentia-Amazonia link, where a common Rodinian paleogeography for both cratons was determined at least since 1200 Ma.

Amazonia in Rodinia

Rodinia was initially defined as a long-lived supercontinent that assembled all the continental fragments around Laurentia and remained stable from 1000 to 750 Ma (McMenamin and McMenamin, 1990; Hoffman, 1991). In most reconstructions (e.g., Hoffman, 1991; Sadowski and Bettencourt, 1996; Weil et al., 1998), Amazonia is placed against the eastern side of Laurentia by matching the Grenville and Sunsas belts. Tohver et al. (2002, 2004, 2005) suggested that a better matching for Amazonia would be against the Llano segment of Laurentia. Finally, other reconstructions, although keeping the general correlation for the ca. 1000 Ma mobile belts, place Laurentia and Amazonia in different positions (Starmer, 1996; Keppie and Ortega-Gutierrez, 1999) in order to accommodate other smaller cratonic fragments such as Oaxaquia of southern Mexico, Arequipa-Antofalla of western South America, and the Garzón Massif of Colombia (not shown in the figures).

Many of the basement units of the northern Andes, such as the Santa Marta, Santander, and Garzón Massifs in Colombia, yielded Grenvillian radiometric ages of ca. 1000 Ma (Tschanz et al., 1974; Cordani et al., 2005), a fact that suggested their correlation with the Sunsas belt. Kroonenberg (1982) suggested that Grenvillian basement may occur below the entire eastern Cordillera of Colombia. Restrepo-Pace et al. (1997) also suggested possible links with the southern terranes of the Arequipa Massif in Peru (Wasteneys et al., 1995) and the Precordillera in Argentina (Ramos, 1988). A review of the available evidence on the Andean basement fragments can be found in Ramos and Aleman (2000). A correlation of all Grenvilleage terranes scattered in Mexico and their possible comparison with the Grenvillianage belts of Laurentia, Amazonia, and Baltica was attempted by Keppie and Ortega-Gutierrez (1999), concluding for a close similarity between the tectonic evolution of all them. Following the model of Starmer (1996), these terranes may initially have formed at 1200 ± 100 Ma within a large magmatic arc that was generated by consumption of a Mesoproterozoic ocean located between Laurentia on one side and Amazonia plus Baltica on the other. These terranes subsequently collided during the Grenvillian orogeny.

Amazonia in Gondwana

According to Cordani et al. (2003) and Kröner and Cordani (2003), the geological, geochronological, and paleomagnetic database accumulated for South America and Africa in the last decade seems to demonstrate that most of these continental fragments were not part of Rodinia. Tohver et al. (2006a) reached a similar conclusion on the basis of their review of the available paleomagnetic data from Africa and South America. In the transition from Rodinia to Gondwana, it is necessary to envisage the disappearance of at least two very large oceans: the Goiás (or Brasiliano) Ocean in the West, and the Mozambique Ocean in the East, this latter related to the collage of West and East Gondwana (Cordani et al., 2003). Figure 8 reconstructs the paleogeography for the time of amalgamation of West Gondwana at ca. 600 Ma, and shows that the Goiás Ocean extended into the Pharusian Ocean to the northeast (Brito-Neves et al., 1984; Trompette, 1994; Fetter et al., 2003; Caby, 1989, 2002). It may also have extended to the southwest, toward the Neoproterozoic to Cambrian Pampean belt, assuming that the granitoid rocks of that belt originated through subduction processes. Figure 8 also indicates that the megasuture of the Transbrasiliano Lineament plus the Hoggar 4°50′ Lineament, related to the closing of both ocean domains, crosses West Gondwana from northeast to southwest. Based on the close correlation between northeastern Brazil and west-central Africa, a large oceanic domain separating the West African Craton from the Borborema–Trans-Saharan cratonic fragment in the Neoproterozoic was already proposed by Castaing et al. (1994).

Figure 8. Relative position of cratons and mobile belts at ca. 600 Ma, during the final events of agglutination of West Gondwana. The Trans-brasiliano-Hoggar 4°50′ megasuture is emphasized. For geographic reference, the present outline of South America is indicated. SFC—São Francisco Craton; PH—Pharusian belt; GO—Goiás magmatic arc; Tb—Transbrasiliano Lineament; Ho—Hoggar 4°50′ Lineament. Cratonic fragments: LA—Luiz Alves; MA—Maranhão; PR—Pa-ranapanema; RA—Rio Apa.

Figure 8. Relative position of cratons and mobile belts at ca. 600 Ma, during the final events of agglutination of West Gondwana. The Trans-brasiliano-Hoggar 4°50′ megasuture is emphasized. For geographic reference, the present outline of South America is indicated. SFC—São Francisco Craton; PH—Pharusian belt; GO—Goiás magmatic arc; Tb—Transbrasiliano Lineament; Ho—Hoggar 4°50′ Lineament. Cratonic fragments: LA—Luiz Alves; MA—Maranhão; PR—Pa-ranapanema; RA—Rio Apa.

The oldest granitoids in the Goiás magmatic arc are 900–850 Ma (Pimentel et al., 1997), indicating that the oceanic lithosphere consumed in the production of the granitoid rocks must have been generated some 100 m.y. earlier. This seems to imply that, when the main nucleus of Rodinia formed around Laurentia, including Amazonia, a large ocean separated several continental masses and fragments from this nucleus, such as the São Francisco–Congo, Rio de La Plata, and Kalahari Cratons, plus the Borborema–Trans-Saharan provinces, the Central Goiás Massif, and the Paranapanema block (Cordani et al., 2003; Kröner and Cordani, 2003). This large oceanic domain would include the well-documented Goiás and Pharusian Oceans as well as the less well-defined Pampean Ocean.

The Pampean-Goiás-Pharusian oceanic lithosphere started its consumption in the earlier Neoproterozoic, by forming intra-oceanic magmatic arcs. Later these units were united in a series of soft collisions, in the process of amalgamation of Gondwana, along the Pampean, Paraguay, Araguaia, and Pharusian mobile belts. There is evidence for extensive granitoid magmatism in the period between 840 and 530 Ma, corresponding to the tectonomagmatic episodes of the Brasiliano/Pan-African orogenic cycle. The predominantly calc-alkaline chemistry of these magmatic rocks indicates subduction-related, active margin processes.

In conclusion, considering the possible relative positions of the ancient cratonic fragments now within South America, it is not difficult to envisage West Gondwana, still attached to Laurentia, at ca. 600 Ma (see for instance Cawood et al., 2001; Tohver et al., 2006a). When Laurentia rifted away, forming the Iapetus Ocean at ca. 580 Ma, many fragments of the Grenville belt were left behind, including the Blue Ridge province of Laurentia, which is derived from the Amazonian Craton (Tohver et al., 2006b). Along the western margin of Gondwana, these Gren-villian terranes, such as the Garzón Complex and the Dibulla, Bucaramanga, and Jojoncito gneisses of Colombia (see Cordani et al., 2005), as well as the Arequipa-Antofalla and Precordillera terranes (see Ramos, 2000), were later redistributed and accreted back to South America in Paleozoic times as part of the Hercynian-Alleghanian orogeny, in the process of amalgamation of Pangea.

The authors thank colleagues Jorge Bettencourt and Colombo Tassinari for their comments and helpful suggestions, which improved an earlier version of the manuscript. They are also grateful to Eric Tohver for valuable discussions and especially for his careful and thoughtful review of the final version, which included an important amelioration of the English. The constructive reviews made by Bob Hatcher and Randy Van Schmus are also greatly appreciated. The authors also acknowledge the National Council of Scientific and Technological Development of Brazil (CNPq), for its continued support through grants 302851/2004-6 and 304300/2003-9.

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Figures & Tables

Figure 1. Main geotectonic provinces of South America as of the late Neoproterozoic, with emphasis on the Amazonian Craton and its tectonic provinces. The São Francisco Craton, the small São Luís and Luiz Alves cratonic fragments, as well as the Neoproterozoic Brasiliano/Pan-African orogens are shown in the eastern part of the figure. These belts are the Borborema province (B) in northeastern Brazil, the Tocantins province (T) in central Brazil, and the Mantiqueira province (M) in eastern and southeastern Brazil. The Andean belt is indicated in the figure, with its main Precambrian inliers. Adapted from Cordani et al., 2000. Locations of Figures 3, 4, and 6 are shown.

Figure 1. Main geotectonic provinces of South America as of the late Neoproterozoic, with emphasis on the Amazonian Craton and its tectonic provinces. The São Francisco Craton, the small São Luís and Luiz Alves cratonic fragments, as well as the Neoproterozoic Brasiliano/Pan-African orogens are shown in the eastern part of the figure. These belts are the Borborema province (B) in northeastern Brazil, the Tocantins province (T) in central Brazil, and the Mantiqueira province (M) in eastern and southeastern Brazil. The Andean belt is indicated in the figure, with its main Precambrian inliers. Adapted from Cordani et al., 2000. Locations of Figures 3, 4, and 6 are shown.

Figure 2. Reconstruction of the relative positions of the ancient nuclei of West Africa and northern South America in Paleoproterozoic times. Adapted from Nomade et al., 2003

Figure 2. Reconstruction of the relative positions of the ancient nuclei of West Africa and northern South America in Paleoproterozoic times. Adapted from Nomade et al., 2003

Figure 3. Tentative boundaries of the Proterozoic accretionary belts, north of the Amazon sedimentary basin. The principal volcanic and sedimentary sequences overlying the tectonic provinces are indicated in the figure. Urban settlements: BV—Boa Vista; Ca—Caracaraí; Ja—Japurá; Mi—Mitu; PA—Puerto Ayacucho; SFe—San Fernando de Atabapo; SFi—San Filipe; SG—São Gabriel da Cachoeira. (A) and (B) are specific geographic domains with predominance of basement rocks, discussed in the text.

Figure 3. Tentative boundaries of the Proterozoic accretionary belts, north of the Amazon sedimentary basin. The principal volcanic and sedimentary sequences overlying the tectonic provinces are indicated in the figure. Urban settlements: BV—Boa Vista; Ca—Caracaraí; Ja—Japurá; Mi—Mitu; PA—Puerto Ayacucho; SFe—San Fernando de Atabapo; SFi—San Filipe; SG—São Gabriel da Cachoeira. (A) and (B) are specific geographic domains with predominance of basement rocks, discussed in the text.

Figure 4. Tentative boundary of the Proterozoic accretionary belts, south of the Amazon sedimentary basin. The principal volcanic and sedimentary sequences overlying the tectonic provinces are indicated in the figure. Urban settlements: AF—Alta Floresta; Arp—Aripuanã; Arq—Ariquemes; CO—Colorado do Oeste; GM—Guajará Mirim; JP—Ji-Paraná; Ju—Juina; PB—Pimenta Bueno; PV—Porto Velho; SI—San Ignacio; Vi—Vilhena. Location of Figure 5 is shown. (C) Specific geographic domain with predominance of basement rocks, discussed in the text.

Figure 4. Tentative boundary of the Proterozoic accretionary belts, south of the Amazon sedimentary basin. The principal volcanic and sedimentary sequences overlying the tectonic provinces are indicated in the figure. Urban settlements: AF—Alta Floresta; Arp—Aripuanã; Arq—Ariquemes; CO—Colorado do Oeste; GM—Guajará Mirim; JP—Ji-Paraná; Ju—Juina; PB—Pimenta Bueno; PV—Porto Velho; SI—San Ignacio; Vi—Vilhena. Location of Figure 5 is shown. (C) Specific geographic domain with predominance of basement rocks, discussed in the text.

Figure 5. Simplified geological map of the Jauru region, western part of Mato Grosso State of Brazil, near the Bolivian border. Adapted from Geraldes et al., 2001.

Figure 5. Simplified geological map of the Jauru region, western part of Mato Grosso State of Brazil, near the Bolivian border. Adapted from Geraldes et al., 2001.

Figure 6. Tocantins province and its main tectonic domains. AR—Araguaia belt; PA—Paraguay belt; GO—Goiás magmatic arc; CX—Crixás granite-greenstone terrain; NC—Natividade-Cavalcante block; BR—Brasilia belt. Adapted from Cordani et al., 2000.

Figure 6. Tocantins province and its main tectonic domains. AR—Araguaia belt; PA—Paraguay belt; GO—Goiás magmatic arc; CX—Crixás granite-greenstone terrain; NC—Natividade-Cavalcante block; BR—Brasilia belt. Adapted from Cordani et al., 2000.

Figure 7. Cartoon of a possible scenario for the accretion of the Paragua block and the Arequipa-Antofalla terrane to the southwestern margin of the Amazonian Craton, after Boger et al., 2005.

Figure 7. Cartoon of a possible scenario for the accretion of the Paragua block and the Arequipa-Antofalla terrane to the southwestern margin of the Amazonian Craton, after Boger et al., 2005.

Figure 8. Relative position of cratons and mobile belts at ca. 600 Ma, during the final events of agglutination of West Gondwana. The Trans-brasiliano-Hoggar 4°50′ megasuture is emphasized. For geographic reference, the present outline of South America is indicated. SFC—São Francisco Craton; PH—Pharusian belt; GO—Goiás magmatic arc; Tb—Transbrasiliano Lineament; Ho—Hoggar 4°50′ Lineament. Cratonic fragments: LA—Luiz Alves; MA—Maranhão; PR—Pa-ranapanema; RA—Rio Apa.

Figure 8. Relative position of cratons and mobile belts at ca. 600 Ma, during the final events of agglutination of West Gondwana. The Trans-brasiliano-Hoggar 4°50′ megasuture is emphasized. For geographic reference, the present outline of South America is indicated. SFC—São Francisco Craton; PH—Pharusian belt; GO—Goiás magmatic arc; Tb—Transbrasiliano Lineament; Ho—Hoggar 4°50′ Lineament. Cratonic fragments: LA—Luiz Alves; MA—Maranhão; PR—Pa-ranapanema; RA—Rio Apa.

TABLE 1. MAIN GEOLOGIC AND GEOCHRONOLOGIC CHARACTERISTICS IN THE CENTRAL AMAZONIAN (CA) AND MARONI-ITACAIUNAS (MI) PROVINCES, NORTHEASTERN AMAZONIAN CRATON

TABLE 2. MAIN GEOLOGIC AND GEOCHRONOLOGIC CHARACTERISTICS IN THE PROTEROZOIC TECTONIC PROVINCES, SOUTHWESTERN AMAZONIAN CRATON

Contents

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