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INTRODUCTION
In models of global supercontinent reconstruction, the São Luís Craton and the northeastern portion of the Amazonian Craton (to the east of the Guayana Shield) have been considered remnants of the West African Craton preserved in the northern South American Platform after the breakup of the Pangea Supercontinent (Hurley et al. 1967, Torquato & Cordani 1981, Lesquer et al. 1984, Brito Neves et al. 2000, Klein and Moura 2008).
On the African side, several studies based on structural, geochemical, geophysical and geochronological data demonstrate the existence of Archean and dominantly juvenile Paleoproterozoic crust (Abouchami et al. 1990, Boher et al. 1992, Gasquet et al. 2003). On the Brazilian side, the northern part of the Amazonian Craton and São Luís Craton have demonstrated geochronological and evolutionary similarities. In this part of the Amazonian Craton, the ancient continental crust stabilized in the Archean is bordered by meta-volcanosedimentary sequences (Rosa-Costa et al. 2006) and 2.02-2.25 Ga granitic terranes (Cordani et al. 1979, Cordani & Brito Neves 1982, Tassinari & Macambira 1999, Santos et al. 2000, Tassinari et al. 2000, Tassinari & Macambira 2004, Rosa-Costa et al. 2006).
The main outcrop area of the São Luís Craton crops out for some 100 km near the Atlantic coast. The rocks are discontinuously exposed in erosive or tectonic windows within the sedimentary cover (Gorayeb et al. 1999). The main lithological associations are meta-volcanosedimentary sequences and granitoids (Gorayeb et al. 1999, Klein et al. 2005b).
Despite recent advances in knowledge of the evolution of the São Luís Craton, systematic and more detailed studies are needed owing to the wide variety of the rocks of this tectonic unit, the difficulties of access, the restriction of outcrops and the extensive Phanerozoic cover.
The study area is located approximately 70 km south of São Luís, in Maranhão State, northeastern of Brazil, where the easternmost fragment of the craton is exposed as a set of granitic rocks named by Rodrigues et al. (1994) and Gorayeb et al. (1999) as the Rosário Suite (Fig. 1). The present work involves the study of a varied set of granitoid rocks included in the Rosario Suite, still little known from the cartographic, geochemical, geochronological and petrological point of view. In addition, the age range of magmatism is not well determined, the geochemical signature is not fully known, and it is unclear whether they represent juvenile crust or older reworked crust.
Figure 1: Simplified geological map of São Luis Craton and Gurupi Belt with the location of the study area in the northern state of Maranhão, northern Brazil. Adapted from Gorayeb et al. (1999), Vasquez and Rosa-Costa (2008), Klein et al. (2012) and Sousa et al. (2012). The cited geochronological ages are subject to variable analytical uncertainties.
This research includes new data on granitoids of the Rosário Suite, particularly petrographic, geochemical, geochronological (LA-ICP-MS) and isotopic (Sm-Nd) data. Considering along available data in the literature, it allows us to discuss the crustal evolution of the suite, to make correlations with granitoids of other portions of the São Luís Craton, and to contribute to the advancement of the evolutionary models.
REGIONAL GEOLOGY CONTEXT
The São Luís Craton consists generally of three main Paleoproterozoic rock associations: a meta-volcanosedimentary succession, volcanic sequences and granitoids (Fig. 1). Older rocks (2240 ± 5 Ma) belong to the Aurizona Group, which comprises a meta-volcanosedimentary succession of schists, felsic and mafic meta-volcanic rocks, quartzites and meta-cherts.
The Tromaí Suite (2168 to 2,148 Ma, single zircon Pb-evaporation) (Klein & Moura 2001) is the most extensive igneous unit. It is formed of tonalite, trondhjemite, granodiorite and granite that belong to a juvenile calc-alkaline series related to an intra-oceanic island-arc to transitional setting (Klein & Moura 2001, Klein et al. 2008). Volcanic rocks with similar ages were included into the Serra do Jacaré and Rio Diamante units, with the chemical characteristics of a transitional arc in an active continental margin (Klein et al. 2009). The Rosilha volcanic unit is younger (2069 Ma) than the other two volcanic units (~2160 Ma), and has a post-orogenic tectonic setting (Klein et al. 2009).
Gorayeb et al. (1999) characterized the Rosário Suite as a set of composite tonalitic, granodioritic and granitic plutons with Paleoproterozoic ages (2.08-2.13 Ga). The rocks exhibit partial textural, structural and mineralogical transformations along transcurrent shear zones.
Other granitoids that have biotite and muscovite, peraluminous and S-type characteristics are represented by the Ourém, Japiim, Jonasa, Tracuateua and Mirasselvas bodies, aged 2.14 to 2.06 Ga (Palheta et al. 2009, Klein et al. 2012). The Negra Velha Granite (2056-2,076 Ma) consists of small granitic bodies intruded into the Tromaí Granitic Suite and associated with felsic volcanic and pyroclastic rocks of the same age (Klein et al. 2008, 2009). The Caxias Microtonalite, with age of 2009 ± 11 Ma (Klein et al. 2014), represents the youngest magmatic plutonic activity of this cratonic area.
The Gurupi Belt is interpreted as a Neoproterozoic-early Cambrian orogen with NNW-SSE orientation, developed in the south-southwestern margin of the São Luís Craton (Almeida et al. 1976, Abreu et al. 1980 Costa 2000, Klein et al. 2005a, 2012). The belt and its reworked basement include rock units of varied nature and ages ranging from Archean to Eocambrian (Klein et al. 2005b, Palheta et al. 2009).
Several plutonic bodies are exposed as basement units of the Gurupi Belt and represent a variety of granitoid types emplaced at different times. They show zircon inheritance and chemical and isotopic features that imply participation in the magma genesis of reworked Archean to Paleoproterozoic crust, in clear contrast to the juvenile characteristics of the predominant magmatic unit of the neighboring Tromaí Suite (Klein et al. 2012).
GEOLOGY OF THE ROSÁRIO REGION
The study area is located in the northwestern Maranhão State around the towns of Rosário, Bacabeira, Perizes, Axixá, Morros and Presidente Juscelino, where the Rosário Suite granitoids crop out (Fig. 2). The granitoids are exposed only in erosive and tectonic windows and are largely covered by Paleozoic sedimentary rocks of the Parnaíba Basin, in the southern portion, and the Cenozoic Barreiras Formation, in the north. The main exposures are found in mines and river valleys. Figure 2 shows the distribution of the principal units and the sampling locations.
Figure 2: Geological map of the Rosário region with the localities of outcrops studied in this work. Modified from Gorayeb et al. (1999) and Sousa et al. (2012).
In this work, we identified five main lithological types: meta-melatonalite, meta-tonalite, meta-granodiorite, meta-monzogranite, and andesite dykes. They are generally exposed in hill tops, gravel-extraction quarries and outcrop slabs on the banks and beds of rivers, such as the Rio Munim, in the town of Presidente Juscelino (Fig. 2) (in Appendix A are the coordinates of the points in the map).
The contact relationships between the rocks are not registered directly, but temporal relationships are recognized by the presence of enclaves or by injecting veins. The meta-tonalites contain many leucotonalite veins, pegmatites and aplites, which are genetically related to granodiorite nearby and the youngest magmatic phases of the suite, probably the most evolved felsic phases of magmatic differentiation of the suite (Gorayeb et al. 1999).
ANALYTICAL PROCEDURES
Petrography
Petrographic analyses of 16 thin sections from granitoids of the Rosário Suite were performed by conventional optical microscopy, involving mineralogical characterization and quantification and textural/microstructural analysis. Modal mineralogical analyses were performed using a Swift automatic point counter, with 2,800 points for each thin section (Table 1). Petrographic classification was defined according to Streckeisen (1976), Le Maitre (2002), Fettes and Desmons (2008) and Paschier and Trouw (1996), and the modal results were plotted in Q-A-P and Q (A + P) -M’ diagrams.
Geochemistry
The geochemical analyses were performed on 27 samples at the ACME Analytical Laboratories Ltd. (Vancouver, Canada) and the analytical results are in Table 2. The analytical package included major and minor oxides and trace elements, including rare earth elements (REE). SiO2, TiO2, Al2O3, Fe2O3t, MgO, CaO, MnO, Na2O, K2O and P2O5 were analyzed by inductively-coupled plasma atomic emission spectrometry (ICP-AES), with detection limits of SiO2 = 0.02%; Al2O3 = 0.03%; Fe2O3 = 0.04%; and K2O, CaO, MgO, Na2O, MnO, TiO2, P2O5 = 0.01%. Trace elements were analyzed by inductively-coupled plasma atomic mass spectrometry (ICP-MS) with detection limits of: Ba, Ga, Hf, Nb, Rb, Sr, V, Zr, La, Ce, Eu, Gd, Dy, Ho, Er, Tm, Yb, Co and Zn = 0.5 ppm; Cs, Sn, Cu e Ni = 1 ppm; Hg, Ta, Th, Ti, U, W, Y, Sm, Lu = 0.1 ppm; Bi, Cd e Sb = 0.1 ppm; Pr and Pb = 0.02 ppm; Nd = 0.4 ppm.
Table 2: Chemical analyses of major, minor (in wt %) and trace elements (in ppm) for the Rosário Suite.
Analytical accuracy was monitored by the analysis of the standard STD SO-18, chemical blanks and one duplicate analysis (sample 2013/SR-03). The detailed analytical procedures performed by ACME labs are available on http://www.acmelab.com. The concentrations of major elements were recalculated using the conversion factor for volatile correction, following the procedures of Rollinson (1993), Wilson (1989) and Gill (2010).
U-Pb Geochronology
U-Pb zircon analyses were performed on five samples by laser inductively-coupled plasma mass spectrometry (LA-ICP-MS) at the Geochronology Laboratory of University of Brasília (UnB). The analytical procedures followed the recommendations of Bühn et al. (2009) and Chemale Jr. et al. (2012). The zircon crystals were concentrated using conventional techniques at the Pará-Iso Laboratory of the Federal University of Pará, in Belém, Brazil, including mineral sieving (250-180 µm and 180-125 µm), magnetic separation and gravimetric separation by heavy liquid. The least magnetic zircon fraction was concentrated using an isodynamic Franz magnetic separator, and the least altered crystals were picked under a stereo microscope. Selected zircon grains were mounted in circular epoxy mounts and polished to obtain a smooth surface. Cathodoluminescence images were obtained using a scanning electron microscope (SEM) at the Geochronology Laboratory of UnB. U-Pb analyses were performed on a New Wave UP213 Nd:YAG laser (λ = 213 nm) coupled to a Thermo Finnigan Neptune Multicollector ICP-MS at frequency rate of 10 Hz, energy of approximately 100 mJ/cm2, and spot size varying from 15 to 30 µm. The instrumental mass discriminations were corrected by the analyses of zircon standard GJ-1 (Jackson et al. 2004), and the instrumental mass discriminations were corrected by the standards GJ-1 zircon (Jackson et al. 2004) and 91500 zircon (Wiedenbeck et al. 1995).
Age calculations and U-Pb plots in the Concordia diagram were performed using Isoplot/Ex 3.0 software (Ludwig 2003). The estimate of common Pb was performed using the model of Stacey and Kramers (1975), taking as reference the age 206Pb/208Pb uncorrected for common Pb. The calculation and calibration procedures follow the routine of the Laboratory of Geochronology of the University of Brasilia and are presented in Bühn et al. (2009). Only the uncertainties of the GJ-1 were propagated to the sample values; 91500 was treated as a secondary standard and analyzed as an unknown.
Sm-Nd Isotopic Analyses
Sm-Nd isotopic analyses of four granitoids were performed at the Isotope Geology Laboratory (Pará-Iso Lab) of Geoscience Institute of Federal University of Pará following the analytical procedures of Gioia and Pimentel (2000) and Oliveira et al. (2008). Approximately 100 mg of whole-rock powders were mixed with 100 mg and 149Sm-150Nd spike solution and dissolved in Savillex capsules using the HNO3, HF and HCl acids. Two-step ion-exchange chromatography was performed in Teflon columns, using the Ln Eichrom resin for Sm and Nd separation.
The Sm and Nd isotopic analysis was performed in a Thermo Finnigan Neptune Multicollector ICP-MS. For the correction of mass discrimination, the 143Nd/144Nd ratio was normalized to 146Nd/144Nd = 0.7219, using the exponential law (Russell et al. 1978). The accuracy and reproducibility of results were controlled by standards using BCR-1 [(143Nd/144Nd ranged from 0.512573 ± 12 (2σ) to 0.512669 ± 10 (2σ)], with the average value of 0.512622 ± 28 (2σ)) and La Jolla (143Nd/144Nd isotopic ratios ranged from 0.511793 ± 9 to 0.511883 ± 5, with most values being above 0.5118) (Oliveira et al. 2008). The decay constant used was 6.54 × 10-12 a-1 (Lugmair & Marti 1978), and the Nd model ages (TDM) were calculated according to the model of depleted mantle evolution of DePaolo (1981). During the period of Sm and Nd procedures, total chemical blanks were lower than 0.1% of the elements concentration and were considered negligible.
PETROGRAPHY OF THE ROSÁRIO SUITE
The plutonic rocks studied were classified according to Streckeisen (1976) and Le Maitre (2002) as quartz diorite, melatonalite, tonalite, granodiorite and monzogranite (Table 1, Fig. 3). Leucotonalite, pegmatite and aplite occur as veins, preferentially intruded into tonalitic rocks, which are also cross-cut by dykes of porphyry andesite. In general, the granitoids show variable deformation and low-grade metamorphism; primary igneous features are largely preserved (Fig. 4).
Figure 3: QAP and Q(A+P)M diagrams (Streckeisen 1976, Le Maitre 2002) with the modal composition of Rosário Suite rocks and displaying the composition trends of granitoid series from Lameyre and Bowden (1982).
Figure 4: Textural and structural features of the Rosário Suite Granitoids: (A, B) relicts of igneous textures in granite and granodiorite; (C, D) metamelatonalites showing tectonic fabric; (E) preferred orientation of minerals in metagranodiorite; (F) metaleucotonalite with tectonic transposition banding and veins.
The granitoids are generally coarse-grained plutonic rocks with partially preserved hypidiomorphic granular texture. They are deformed by shearing in transcurrent zones, inducing partial mineralogical re-equilibration under greenschist metamorphic conditions that partially modified the plutonic igneous fabric, turning them into protomylonites. In this process, new mineral associations were generated (Chl, Ab, Act, Cc, Ep, Qtz), which changed the original grey and pink colors into green tones in these granitoids. Because of such characteristics, designation as metaplutonic rocks is more appropriate.
Hornblende metatonalite
The hornblende metatonalites are phaneritic coarse-grained, and leucocratic to mesocratic (M = 16-32), with greenish and whitish light grey colors. Medium- to fine-grained portions are related to comminution in shear zones and show discrete foliation defined by the preferred orientation of feldspars, quartz, biotite and amphibole. The preserved textural aspects in these rocks have two main characteristics: plutonic igneous textures (e.g., hypidiomorphic granular type) and the superposition of a tectono-metamorphic fabric recognized by overlapping mineral grains resulting in rock anisotropy, which becomes mylonitic foliation in shear zones or shear bands. This anisotropy is a common feature in the Rosário Suite rocks. The essential primary mineralogy comprises oligoclase (An24), quartz, hornblende and biotite. Accessory minerals are titanite, apatite, zircon and opaque minerals. Secondary phases, related to metamorphic transformations, are represented by tremolite-actinolite that partially replaces hornblende, and plagioclase transformed into epidote and sericite by saussuritization.
Hornblende-biotite metatonalite
Of restricted occurrence (2013/SR-05), it represents a variation of the hornblende metatonalite. This sample exhibits coarse grain-size and a melanocratic colour index (M = 60-70): dark grey, with greenish and whitish tones. Microscopically, it shows hypidiomorphic granular texture and mineralogy represented by oligoclase (An25), quartz, microcline, hornblende, biotite and titanite, with accessory apatite, zircon and opaque minerals. Tremolite-actinolite, epidote and sericite represent secondary phases related to metamorphic transformation.
Hornblende metagranodiorite and Hornblende metaquartz diorite
The hornblende metagranodiorite and hornblende metaquartz diorite are leuco- to mesocratic (M = 9-40), coarse-grained and pinkish grey rocks, showing hypidiomorphic granular texture. Plagioclase, quartz, microcline, hornblende and titanite are the main mineralogical phases, with biotite, apatite, zircon and opaque minerals as accessories. The alteration phases are tremolite-actinolite, sericite and epidote. The textural aspects are similar to those of metatonalite, e.g., preserved plutonic features in a mylonitic fabric. The feldspars, amphibole and quartz crystals are rotated and slightly stretched, forming an incipient foliation. In these rocks, centimetre-thick dioritic or amphibole-rich mafic enclaves are also found, representing fractions of partially digested tonalite (Fig. 5A).
Metamonzogranite
Monzogranitic rocks are coarse-grained, slightly richer in quartz and alkali-feldspar, plagioclase and biotite, and lacking hornblende. The textures are similar to the types described before with minerals slightly imbricated due to deformation and incipient recrystallization. The plagioclase crystals exhibit green tones due to saussuritization, and epidote, sericite and calcite transformation.
The field and petrographic data of granites series studied reveals an important plutonic event in the region in which all rocks show petrographic similarities with common textural and mineralogical features, reflecting slight variations in mineral quantities. As shown in Figure 4, the compositional trends suggest magmatic differentiation processes in the evolution of this suite, as pointed out by Gorayeb et al. (1999).
STRUCTURAL GEOLOGY AND THERMAL-TECTONIC PROCESSES
The granitoids of Rosário Suite exhibit textural/structural and mineralogical changes related to shear tectonics recorded in other parts of the São Luís Craton, as well as faults and joints. However, except along the shear zones, these transformations did not destroy the original igneous fabric, which preserves the history of plutonic origin. The main structural features superimposed on igneous textures are marked by imbrications and light stretching of the primary minerals (quartz, plagioclase, hornblende and titanite), creating anisotropy and developing a discrete foliation in the rocks, transforming them into protomylonites.
Shear bands and ductile shear zones with sinistral movement, centimetric to decimetric widths and lengths up to tens of meters, are common in the study area. Along these zones, there is a 50-70 Az mylonitic foliation that dips between 60 and 70º SE and stretching lineation dipping 30-35 to 200-220 Az. In some narrow zones, mylonite and ultramylonite developed, marked by darker colour and comminution of minerals, resulting in aphanitic rocks. The microstructural features are highlighted by almond-shaped hornblende and relict plagioclase, and quartz ribbons in a fine-grained matrix. The matrix is composed of sericite, epidote and carbonates replacing plagioclase, associated with quartz and plagioclase microgranular aggregates, chlorite, acicular tremolite-actinolite and microgranular titanite derived mainly from the substitution reaction of hornblende.
In general, the granitoids are slightly deformed with incipient metamorphic transformation that generated new mineral assemblages, imposing green tonalities on the original grey and pink colours and these granitoids. The metamorphic paragenesis Ab + Ser + Ep + Chl + Act + Cc + Qtz coexists with relict primary minerals (quartz, plagioclase, hornblende, alkali-feldspar), which allows us to estimate the metamorphic conditions in the low greenschist facies.
The transformations recorded in Rosário Suite may be related to the same context of thermo-tectonic processes that took placed on other areas, such as to the boundary between the craton and the Gurupi Belt (Klein & Lopes 2011).
GEOCHEMISTRY OF THE ROSÁRIO SUITE
In general, the studied granitoids demonstrated high contents of SiO2 (50-79%), Al2O3 (10-16%) and Na2O (2.3-6.7%) and low concentrations of TiO2 (0.03-0.77%), K2O (0.6-3.5%), MnO (0.02-0.22%) and P2O5 (0.02-0.3%). Other major elements showed low and moderate variations: MgO (0.12-8%); CaO (1.5-9%); Fe2O3 Total (0.8-12%); Na2O (1.8-7%); and low K2O/Na2O ratio (0.1-1.9). The trends of compositional types (diorite, tonalite, granodiorite and granite) showed continuous variation in the contents of main major and trace elements, with increasing SiO2, with positive covariance between Na2O and SiO2, and negative covariance between CaO, Fe2O3t, K2O, MgO, TiO2 and P2O5 (Fig. 6). These variations are probably related to magmatic differentiation.
Figure 6: Harker diagrams with major and trace elements vs. SiO2 for granitoids of the Rosário Suite.
In classificatory diagrams, as R1-R2 (La Roche 1980) and diagram Total-Alcali vs. Silica (TAS) (Cox et al. 1979), all granitoids plot in the fields of diorite, tonalite, granodiorite and granite (Fig. 7A, B), in accordance with the petrographic classification. The samples that fall in the gabbro field are melanocratic types (melatonalites and diorites).
Figure 7: Geochemical diagrams with plotted data of the Rosário Suite Granitoids: (A) R1-R2 classification diagram (La Roche 1980); (B) TAS classification diagram (Cox et al. 1979).
In the aluminum-saturation Shand diagram (Shand 1950) (Fig. 8), the granitoids plot within the metaluminous field, followed the petrographic data that show significant presence of hornblende and minor biotite. In the diagram Alkali oxides, Fe oxides e Magnesium (Mg), the rocks define a trend compatible with the calc-alkaline series (Fig. 9).
Figure 9: AFM diagram for magmatic series classification (Irvine & Baragar 1971) with the Rosário Suite trends.
In the multielement diagram, the compositional groups showed consistent signatures (Fig. 10), such as large-ion lithophile elements (LILE)-enrichment relative to light rare earth elements (LREE) and high field strength (HFS) elements. Furthermore, the geochemical pattern of quartz diorites and granodiorites are similar, showing positive Ba and negative Th anomalies. Metatonalites are similar to tonalites, exhibiting accentuated negative Th and Nb anomalies with fractionated patterns. Granites also demonstrate more accentuated sub-horizontal pattern with the most intense high field strength elements (HFSE) depletion.
Figure 10: Chondrite-normalized multielement diagram (Thompson 1982) for the granitoids of the Rosário Suite.
The rare earth elements (REE) patterns are very similar for all analyzed granitoids. However, three groups can be discriminated. The first one, composed of quartz diorite and tonalite, shows sub-horizontal heavy rare earth elements (HREE) pattern and steep LREE pattern, which is slightly fractionated, with La/Yb ratio between 2 and 13, and incipient negative Eu anomalies. The second group, consisting of granodiorites and granites, exhibits steeper REE patterns than the other two groups (La/Yb = 3-22), with slight heavy REE depression and incipient negative Eu anomalies [(Eu/Eu*)N = 0.9-2.0]. In general, the total REE content is lower (∑REE = 13 to 89 ppm) than in the quartz diorite and tonalite (∑ETR = 55 to 144 ppm) (Fig. 11).
Figure 11: Rare earth element (REE) diagrams normalized to chondrite (Boynton 1984) for the Rosário Suite rocks.
In the Y+Nb versus Rb (Pearce et al. 1984) and Zr versus (Nb N /Zr N) (Thiéblemont & Tégyey 1994) diagrams the rocks plot in the field of volcanic arc granites (VAG) related to subduction setting and calc-alkaline affinity (Fig. 12A, 12B). In the log [CaO/(Na2O+K2O)] versus SiO2 diagram (Brown et al. 1984), the studied rocks correspond to granites of normal continental arc, similar to the Sierra Nevada and Peru batholiths of North and South America, respectively (Winter 2001, McBirney & White 1982, Thorpe et al. 1982) (Fig. 12C).
Figure 12: Geochemical diagrams for tectonic environment classification: (A) Y+Nb versus Rb (Pearce et al. 1984); (B) log [(CaO/Na2O+K2O)] versus SiO2 (Brown et al. 1984); (C) Zr versus NbN/ZrN (Thiéblemont & Tégyey 1994).
U-PB GEOCHRONOLOGY
Petrographic analyses under optical microscope and stereomicroscope observations of zircon grains complemented by cathodoluminescence (CL) images identified mostly euhedral zircon crystals with well-defined faces, showing clear concentric magmatic zoning (Fig. 13). The least magnetic zircon grains were chosen for analysis, and the analytical points were chosen considering the more homogeneous portions of the crystal, without inclusions or fractures. Analyses of the nucleus and edge of the crystals presented similar results. The results of the geochronological analyses are in Tables 3, 4, 5, 6 and 7. Most of the analyzed zircon grains have Th/U ratios between 0.23 and 0.90, within the normal range for magmatic zircons.
Figure 13: Cathodoluminescence images of analyzed zircon grains from the Rosário Suite Granitoids. Open circles mark spots analyzed by LA-ICP mass spectrometer (15-30 µm-size).
Table 3: Summary of U-Pb zircon in situ data from sample obtained by LA-MC-ICP-MS from metatonalite (SR-04) of the Rosário Suite.
f 206: the percentage of the common Pb found in 206Pb; #: ratios corrected for common Pb; *zircons excluded from the calculation of age; **data used for concordant age calculation. Th/U ratios and amount of Pb, Th and U (in pmm) are calculated relative to GJ-1 reference zircon, Conc.: degree of concordance = (206Pb/238U age / 207Pb/235U age)*100. Rho is the error correlation defined as the quotient of the propagated errors of the 206Pb/238U and the 207Pb/235U ratio.
Table 4: Summary of U-Pb zircon in situ data from sample obtained by LA-MC-ICP-MS from metatonalite (SR-05) of the Rosário suite.
f 206: the percentage of the common Pb found in 206Pb, #: ratios corrected for common Pb; *data used for concordant age calculation. Th/U ratios and amount of Pb, Th and U (in pmm) are calculated relative to GJ-1 reference zircon, Conc.: degree of concordance = (206Pb/238U age/207Pb/235U age)*100 Rho is the error correlation defined as the quotient of the propagated errors of the 206Pb/238U and the 207Pb/235U ratio.
Table 5: Summary of U-Pb zircon in situ data from sample obtained by LA-MC-ICP-MS from metatonalite (SR-06) of the Rosário suite.
f 206: the percentage of the common Pb found in 206Pb; #: ratios corrected for common Pb; Th/U ratios and amount of Pb, Th and U (in pmm) are calculated relative to GJ-1 reference zircon. Rho is the error correlation defined as the quotient of the propagated errors of the 206Pb/238U and the 207Pb/235U ratio, Conc.: degree of concordance = (206Pb/238U age / 207Pb/235U age)*100.
Table 6: Summary of U-Pb zircon in situ data from sample obtained by LA-MC-ICP-MS from Metatonalite (SR-09) of the Rosário suite.
f 206: the percentage of the common Pb found in 206Pb, #: ratios corrected for common Pb, *zircons excluded from the calculation of age. Th/U ratios and amount of Pb, Th and U (in pmm) are calculated relative to GJ-1 reference zircon, Conc.: Degree of concordance = (206Pb/238U age / 207Pb/235U age)*100. Rho is the error correlation defined as the quotient of the propagated errors of the 206Pb/238U and the 207Pb/235U ratio.
Table 7: Summary of U-Pb zircon in situ data from sample obtained by LA-MC-ICP-MS from metagranodiorite (SR-08) of the Rosário suite.
f 206: the percentage of the common Pb found in 206Pb; #: ratios corrected for common Pb; *zircons excluded from the calculation of age. Th/U ratios and amount of Pb, Th and U (in pmm) are calculated relative to GJ-1 reference zircon, Conc.: degree of concordance = (206Pb/238U age / 207Pb/235U age)*100. Rho is the error correlation defined as the quotient of the propagated errors of the 206Pb/238U and the 207Pb/235U ratio.
The age uncertainties calculated are all 2-sigma, or 95% confidence, limit uncertainties based on internal reproducibility of the sample data, but they do not take into account the equivalent uncertainty in U/Pb calibration against the standards, normally no better than around 0.3%.
The total of 29 zircons were analyzed from sample 2013/SR-04 (metatonalite), which fall around a line with an upper intercept at 2170 ± 4 Ma (Mean Square of Weighted Deviated - MSWD = 1.0, Fig. 14A). A more robust age estimate for the original crystallization is a Concordia age (as evaluated by Ludwig 2003) of 2166 ± 7 Ma (MSWD = 0.02) for the seven most concordant data points.
Figure 14: 207Pb/235U versus 206Pb/238U Concordia diagrams and weighted mean 207Pb/206Pb age diagrams for the zircon grains analyzed by LA-ICP-MS.
Twenty-nine analyses from a second metatonalite sample (2013/SR-05) gave a slightly more concordant dataset (Fig. 14B) with an upper intercept age of 2170 ± 6 Ma (MSWD = 0.7) and a poorly constrained lower intercept of -158 ± 370 Ma. In this case, nine of the data points provided a Concordia age of 2170 ± 7 Ma (MSWD = 1.3).
Twenty-five zircons were analyzed from metatonalite sample 2013/SR-06; all are somewhat discordant (Fig. 14C), and the best age that can be obtained is the weighted mean 207Pb/206Pb age of 2170 ± 7 Ma (MSWD = 1.7) (Fig. 14D).
The single metagranodiorite sample analyzed (2013/SR-08) yielded 23 data points with a wide spread in the Wetherill diagram (Fig. 14E), but with variable non-linear discordance. The best estimate for the original crystallization age is taken as the weighted mean 207Pb/206Pb age of 2176 ± 8 Ma for the nine most concordant data (MSWD = 0,5) (Fig. 14F).
For the final metatonalite sample (2013/SR-09), a set of 29 zircons was analyzed (Table 6, Fig. 14G). The data are variably discordant and do not fit a straight line in the Wetherill diagram. Too few are sufficiently concordant to define a Concordia age, but 21 analyses that are less than 5% discordant, given a weighted mean 207Pb/206Pb age of 2,161 ± 4 Ma (MSWD = 0.7). 207Pb/206Pb ages are equivalent to forcing a Discordia though zero, which in this case gives a close minimum age for crystallization (Fig. 14H).
The graphical representation of T (Ga) versus εNd (Fig. 15) also shows that all the new results fall within the field corresponding to juvenile Paleoproterozoic crust of the São Luís Craton, compiled from the data of Klein et al. (2005a, 2012).
Figure 15: εNd versus time diagram, showing the isotopic composition of the Rosário Suite. The field of the Paleoproterozoic São Luís crust is from Klein et al. (2005a, 2012).
These results and the geochemical data reveal the juvenile nature of these rocks reinforcing the interpretation that this region may be part of the Rhyacian juvenile continental magmatic arc, that extends through other parts of São Luís Craton with correspondence in the northeast portion of the Amazonian Craton and in West Africa (Abouchami et al. 1990, Boher et al. 1992, Wright et al. 1995, Hirdes et al. 1996).
WHOLE-ROCK SM-ND RESULTS
The Sm and Nd isotopic analytical results of four samples of metatonalites from the Rosário Suite (Table 8) showed acceptable value ranges for both 147Sm/144Nd ratio (0.08 to 0.13) and fractionation degree (-0.54 to -0.39), according to Sato and Tassinari (1997).
The εNd values calculated according to the crystallization age obtained in this work (t = 2.2 Ga) are in the range +3.2 to +1.9 and the data yield similar T DM model ages for separation from depleted mantle of 2.21 to 2.31 Ga.
CONCLUSIONS
The geochemical characteristics combined with the field, petrographic, geochronological and isotopic data indicate that studied rocks are co-genetic and that compositional variations are associated with magmatic fractionation process. The different petrographic-compositional types possibly represent successions of magmatic pulses in an arc-related environment, but the samples dated here are essentially coeval. All five samples analyzed have yielded consistent U-Pb zircon ages with preferred results of 2165 ± 7 Ma, 2170 ± 7 Ma, 2170 ± 7 Ma, for metatonalite and 2161 ± 4 Ma for metagranodiorite, and 2175 ± 8 Ma for metagranodiorite. Allowing for the inherent calibration uncertainty, these data suggest that the Rosário Suite plutonic rocks were emplaced during a single magmatic episode between 2155 and 2175 Ma. These ages are slightly older than previous results of 2.08 to 2.13 Ga presented by Gorayeb et al. (1999) using the Pb zircon evaporation method. Younger ages are commonly expected by Pb evaporation method, providing minimum ages.
Our data show that emplacement of the Rosário Suite between about 2.15 and 2.18 Ma represents an important event of Paleoproterozoic crust formation during the Rhyacian period. Whole-rock Sm-Nd isotopic study provided TDM model ages between 2.21 and 2.37 Ga, with low positive εNd values, indicating that the Rosário Suite magmas had a short time of crustal residence, which implies an essentially juvenile nature.
The area where the Rosário Suite is located represents the most eastern exposures of the São Luís Craton, which are part of a large batholith of felsic to intermediate composition (diorites, tonalites, granodiorites, granites, leucogranites and andesites). Multiple plutons are probably involved, but it is not possible to delimit them on the scale of the mapping that has been carried out.
Geochemical data have demonstrated systematic variation in the major, minor and trace elements. In geochemical diagrams, all granitoids show trends of magmatic differentiation compatible to arc-related environment of the calc-alkaline series. They are metaluminous, calc-alkaline, I-type granitoids related to subduction environment of the continental magmatic arcs.
The structural data indicate the deformational effects of a regional transcurrent tectonic system, probably at more advanced stages of the Paleoproterozoic Transamazonian orogeny or subsequent Neoproterozoic tectonics of the Brasilian/Pan-African cycle that produced new structural features, such as mylonitic fabrics with comminution, rotation and overlapping processes of feldspars, biotite and hornblende. This tectonic condition also imposed different grades of stretching, recrystallization of quartz, saussuritization of plagioclase and neoformation of tremolite-actinolite and chlorite. The metamorphic conditions reached the greenschist facies. The deformation and metamorphic transformations are related to the collisional tectonic Transamazonian orogenesis in the Rhyacian period in other regions of the São Luís and Amazonian cratons.
The Rosário Suite is part of an extensive Rhyacian continental juvenile magmatic arc which is found in other parts of the São Luís Craton, which in the literature has been considered a fragment of the West African Craton. In Brazil, it is possible to correlate with the northwestern part of the Amazonian Craton, in which Rhyacian accretional magmatic arcs were amalgamated to form Archean terrains, more specifically in the northwest of the Pará state and Amapá. The Paleoproterozoic evolution of these cratons (2.24-2.1 Ga) is related to the Transamazonian orogenies, and the Rosário granitoids may represent the main accretion phase in the arc magmatic evolution.
