Geology, petrology and U-Pb geochronology of metavolcanic rocks in the Mundo Novo greenstone belt, eastern São Francisco Craton, NE Brazil: considerations about its tectonic setting

Field, petrological and LA-ICP-MS U-Pb geochronological data of metavolcanic rocks were used to interpret the petrological processes and to propose the tectonic setting for the Mundo Novo greenstone belt (MNGB) in the eastern São Francisco Craton. The metavolcanic rocks studied are metakomatiite, eastern and western metabasalts, and metadacite with subordinate metarhyolite, which host ocean floor hydrothermal alteration zones and are covered by ocean floor lithological associations composed of chemical metasedimentary rocks. Fractional crystallization and heterogeneous intraoceanic contaminations explain the mineralogical differences between the two metabasalts and the high (La/Yb)N ratio values of metakomatiite and metadacite. The metakomatiite and the eastern and western metabasalts feature a vector from the MORB-OIB array to the volcanic arc array in the Nb/Yb-Th/Yb diagram, similar to the Archean intraoceanic arc-basin systems. The geochemical pattern of the eastern and western metabasalts in the Zr-Zr/Y diagram suggests volcanism in nearby island arc and backarc basin settings, respectively. The 2595 ± 21 Ma U-Pb zircon crystallization age of the metadacite allowed the determination of the timing of volcanism in the MNGB. Therefore, an intraoceanic provenance in an arc-basin system is proposed for the MNGB in the Neoarchean, which was later compressed between cratonic blocks during the Rhyacian-Orosirian event.


INTRODUCTION
The Mundo Novo greenstone belt (MNGB), in the eastern boundary of the Gavião Block (Barbosa and Sabaté 2002, 2003, 2004 and eastern portion of the São Francisco Craton (Figs. 1A and 1B), lies within the Contendas-Jacobina Lineament, NE Brazil (Sabaté et al. 1990), which in the area under study is situated between the Gavião and Mairi blocks (Fig. 1C). That greenstone sequence has been a subject of study since the 1970s and hosts the Zn-Pb Fazenda Coqueiro deposit (Mascarenhas et al. 1975, Couto et al. 1978, Loureiro 1991, Mascarenhas and Silva 1994, Mascarenhas et al. 1998, Reis et al. 2017.
Greenstone belts are highly varied Archean geological entities that contain a vast diversity of rocks (Anhaeusser 2014). Generally, their rocks experienced multiple stages of deformation, metamorphism, and metasomatic alteration due to their great age and diversity of geotectonic settings, which were intruded by mafic, ultramafic, and granitoid rocks (Anhaeusser et al. 1969, Anhaeusser 2014. Moreover, the intraoceanic or intracontinental provenance of volcanic rocks in Archean greenstone belts has been a recurring discussion topic and whole-rock chemical tools have been constantly applied for such research purposes (Polat and Kerrich 2001, Polat et al. 2002, Pearce 2008, 2014.
The predominance of subaqueously deposited basalt and komatiite has been interpreted in a wide variety of geological settings proposed for greenstone belt terrains in intraoceanic crust, ranging from primitive island arcs to plume-related submarine plateaus, mid-ocean ridges (including ophiolites) and back-arc basins (De Wit et al. 1987, Storey et al. 1988, Parman et al. 2001, Chavagnac 2004, Furnes et al. 2013. Furthermore, the intraoceanic Archean greenstone belts present a set of evidence as follows: basalt and komatiite occurrence, absence of zircon xenocrysts, mid-ocean ridge basalt (MORB), and island arc tholeiitic (IAT) geochemical patterns, and Th enrichment due to intraoceanic crustal input processes (Pearce 2008). The intraoceanic crustal input, however, would have been produced by subduction components, high-grade metamorphism, intraoceanic contamination, crustal recycling, high Th-Nb proxy and delamination (Pearce 2008(Pearce , 2014.
Field, petrographic, whole-rock chemical, and mineral chemistry data were combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon age from the MNGB. Thus, this study focused on metavolcanic rocks of the lower sequence, which comprise the metakomatiite, and middle sequence of the MNGB, comprising the eastern metabasalt, the metadacite, and the western metabasalt that occur mainly at the Fazenda Coqueiro deposit. The objective was also to propose the age and the tectonic setting of the MNGB regarding the intraoceanic provenance of the volcanic rocks rather than the intracontinental provenance taking into account petrological processes. In addition, the comparison with other Archean greenstone belts and the insertion of the MNGB within the regional geologic context in the eastern portion of the São Francisco Craton, which comprises the Gavião, Mairi, Jequié, and Serrinha blocks and the subsequent tectonic events were also contemplated.

GEOLOGICAL SETTING
The eastern portion of the São Francisco Craton, where the MNGB is situated, was formed through the amalgamation of three Archean blocks during the Paleoproterozoic continent-continent collision (Barbosa and Sabaté 2002, 2003, 2004: the Gavião, Serrinha, and Jequié blocks (Fig. 1C). The Paleoproterozoic event compressed the MNGB and surrounding crust between the cratonic blocks, and the uplift caused by this event possibly resulted in the erosion and the formation of Paleoproterozoic sedimentary basins, such as the uppermost sequence of the MNGB and the Saúde Complex (Zincone et al. 2017).
MNGB is in contact to the west with 3.4 Ga (Mougeot 1996) tonalite-trondhjemite-granodiorite (TTG) basement rocks and subordinate metagranites in the Gavião Block and to the east and the south with paragneiss in the Saúde Complex with a maximum age between 2.20 and 2.06 Ga (Zincone et al. 2017;Fig. 2). To the north and northwest, the MNGB is in contact with quartzites in the Jacobina Group, which were deposited between 3.55 and 3.22 Ga (Teles et al. 2015), and with granitic intrusive aged 2080 ± 18 Ma (Leite 2002).
The eastern margin of the Gavião Block is in tectonic contact with the lithologies of the MNGB (Figs. 1C and 2) and is composed of TTG gneiss and migmatites that host mafic rock enclaves, metagranites, and metarhyolites . This block corresponds to the basement of the MNGB. Three groups of TTG gneiss are described in the Gavião Block: two groups are trondhjemitic with U-Pb zircon ages (SHRIMP) of 3403 ± 5 and 3158 ± 5 Ma (Barbosa 1997, Leal 1998, and the other group, with granodioritic compositions, includes the 3225 ± 10 Ma Aracatu granitoid (Barbosa et al. 2012a). The Gavião Block age, near the MNGB, is 3.4 Ga (Mougeot 1996), but metarhyolites aged 3303 ± 11 Ma (Peucat et al. 2002 and metagranites, such as Boa Sorte at 3291 ± 2.5 Ma, occur as well . The Mairi Block, composed of gneiss, migmatites, and granitic and tonalitic orthogneiss, with some occurrences of basic and ultrabasic bodies (Peucat et al. 2002) to the east and southeast of the MNGB, is in tectonic contact with thrust zones with a west vergence. The LA-ICP-MS (U-Pb, zircon) ages of 3.33 and 3.30 Ga (Sousa et al. 2018) for the orthogneiss in the Mairi Block indicate that this complex is coeval with the Gavião Block and, possibly, the two could have been joined together at the time of their formation.
The Jacobina Group is in tectonic contact with the MNGB along thrust zones, with all zones striking north-south and verging to the west (Figs. 1C and 2) with the Gavião Block in the footwall. The Jacobina Group comprises metaconglomerates that host an important gold deposit, and quartzites, metarenites, phyllites, chlorite schists and quartz-sericite schists  Loureiro (1991), Mascarenhas and Silva (1994), and Souza et al. (2002). (Mascarenhas et al. 1998) deposited in a passive margin setting . This group has a depositional age, based on detrital zircons, between 3500 and 3220 Ma (Teles 2013, Teles et al. 2015, Barbuena et al. 2016, with a large portion of the zircon populations situated between 3.3 and 3.4 Ga (Magee et al. 2001, Teles et al. 2015. Jacobina Ridge represents an Archean supracrustal sequence, with a maximum age of 3.28 Ga, whose sources are likely rocks from both the plutonic-volcanic system and the TTG suite in the Gavião Block . The MNGB, which Zincone et al. (2016) referred to as the Mundo Novo supracrustal belt, is inserted into the Contendas-Jacobina Lineament and is divided into three stratigraphic sequences -a lower sequence (ultramafic rocks), a middle sequence (mafic and felsic igneous rocks and clastic and chemical metasedimentary rocks), and an upper one (siliciclastic metasedimentary rocks; Spreafico et al. 2018). Carbonate and argilic-chloritic hydrothermal alteration zones in the ocean floor setting have been identified and described in the Fazenda Coqueiro deposit related to Zn-Pb mineralization hosted in the western metabasalt of the middle sequence (Spreafico 2017). Two ductile and compressional and progressive Paleoproterozoic deformational phases in the MNGB, D 1 and D 2 , are described in the area in a previous study (Spreafico 2017). The D 1 deformational phase is characterized by isoclinal and recumbent folds vergent to the west that generated greenschist metamorphic facies rocks. The D 2 deformational phase is characterized by a refolding that generated vertical and subvertical axial planes that eventually resulted in the formation of a coaxial interference pattern (Ramsay and Huber 1987) or compressive and transpressive shear zones, which bound the MNGB lithologies and generated greenschist rocks to amphibolite metamorphic facies. The most prominent brittle structures are east-trending faults and fractures. The age of the MNGB has been previously studied, and initial geochronological studies have defined the Neoarchean age of the volcanism (Spreafico et al. 2018), such as the sedimentation at the top of the sequence (Barbuena et al. 2016), which has a maximum age coeval with the Rhyacian-Orosirian tectonothermal event (Leite 2002, Spreafico 2017. These rocks lie upon the 3.4-3.2 Ga basement rocks of the northern part of the Gavião Block, composed of TTG gneiss, metagranites, and metarhyolites (Mougeot 1996, Barbosa 1997, Leal 1998, Peucat et al. 2002, Barbosa et al. 2012a along west-vergent thrust zones. This complex comprises aluminous paragneiss, biotite gneiss, and subordinate quartzites that are widely distributed in a north-south trend with significant occurrences in the Mundo Novo region and in the eastern portion of the Jacobina Ridge (Couto et al. 1978, Mascarenhas et al. 1998, Leite et al. 2007, Reis et al. 2017Fig. 1C). The maximal depositional age of 2.06 Ga (Zincone et al. 2017) for the Saúde Complex once more indicates the presence of a sedimentary basin near the MNGB; however, the rocks in the Saúde Complex were subjected to a higher metamorphic degree than the sedimentary rocks at the top of the MNGB along the tectonic contact (Spreafico 2017, Zincone et al. 2017). Finally, Rhyacian-Orosirian granites are present along the Contendas-Jacobina Lineament (Leite 2002, Spreafico 2017Fig. 1C). In general, these granites are undeformed leucogranites, comprising quartz, feldspar, biotite, and muscovite with some occurrences of garnet and sillimanite (Barbosa et al. 2012b). The Cachoeira Grande granite, for example, is a peraluminous leucogranite situated to the northeast of the MNGB, which has an average age of 2080 ± 18 Ma (Leite 2002), and is coeval with the Rhyacian-Orosirian granitic intrusions in the MNGB (Spreafico 2017, Spreafico et al. 2018).

ANALYTICAL METHODS
The study of the metavolcanic rocks of the MNGB and considerations regarding the tectonic setting involved petrographic, mineral, and whole-rock chemistry and geochronologic analyses.
For petrography, we analyzed 127 thin sections of metakomatiite, eastern and western metabasalts and metadacite to determine the mineralogical composition and microstructures of the rocks using the ZEISS Axio Scope.A1 microscope provided by Companhia Baiana de Pesquisa Mineral (CBPM). The mineral abbreviations used on photomicrographs mainly follow those of Kretz (1983) and Siivola and Schmid (2007), and are completed with abbreviations of Whitney and Evans (2010).
Six of the thin sections were used for mineral chemistry analysis to detail the mineral differences between the two metabasalt types complementing the petrographic studies. Thus, a CAMECA SX50 electron microprobe was used with four wavelength-dispersive spectroscopes (WDS) and one Kevex energy-dispersive spectroscope (EDS) of the University of Brasília. The standards used are natural and synthetic with defined compositions, such as albite (for the element Na 2 O), andradite (for the elements CaO and FeO), forsterite (for the element MgO), microcline (for the elements Al 2 O 3 , SiO 2 , and K 2 O), MnTiO 3 (for the element TiO 2 ), and MnTiO 3 (for the element MnO). The analyzed spots were selected in polished sections in plagioclase (6 spots in 5 samples), amphibole (6 spots in 5 samples), pyroxene (4 spots in 2 samples), ilmenite (2 spots in 2 samples), titanite (2 spots in 2 samples), and biotite (1 spot in 1 sample) grains. The chemical contents are expressed by SiO 2 , TiO 2 , Al 2 O 3 , FeO, MnO, MgO, CaO, Na 2 O, and K 2 O and the structural formula is provided based on cationic contents of Si, Ti, Al, Fe, Mn, Mg, Ca, Na, and K of each spot analyzed. The data were processed using the Gabbrosoft spreadsheets (http://www.gabbrosoft.org) and the plagioclase results were plotted in the ternary diagram of feldspar; the amphiboles were plotted in the calcic amphibole diagrams (Leake et al. 1997) and the pyroxene in the classification diagram of pyroxenes (Morimoto 1988) using the Minpet program.
The whole-rock chemical analysis of 49 samples was conducted in the SGS-Geosol laboratory. The samples were dried and crushed so 75% of the sample was smaller than 3 mm. A 300 g sample was quartered and pulverized (until 95% was smaller than 105 microns) to form a powder for subsequent processes. The powders were melted at a high temperature with lithium metaborate, and the major, minor, trace, and rare earth elements (REE) were determined using ICP-MS and inductively coupled plasma optical emission spectrometry (ICP-OES) analysis. The international standard samples used are TILL-3 (description and values are in Lynch 1996) and GRE-05 (reference material from Geostats PTY Ltd.). The error for all analyzed elements in each sample was calculated based on analytical accuracy according to the content of the analyte in the sample, the statistical detection (Supplementary Data) and repeatability limits, and represented in terms of standard deviation (1σ) (Thompson 1988). The coefficient of variation of the analytical results for each element by sample analyzed, calculated from the standard deviation (1σ), was predominantly lower than 15%, which corresponds to well-represented results around the arithmetic mean. Only the samples with a loss on ignition (LOI) values of ≤ 5% were considered. The geochemical data were plotted and interpreted using the GCDKit software ( Janousek et al. 2006). The data in the REE diagram and the (La/Yb) N , (Gd/Yb) N and Eu/Eu* ratios were normalized by chondrite values (Boynton 1984), and the data in the multielement diagram were normalized by N (normal)-MORB values (Hofmann 1988).
The U-Pb geochronologic analyses of one metadacite sample were conducted at the Center of Geochronology Research at the Institute of Geosciences, University of São Paulo. The zircon grains from the sample were separated using binocular microscopy and placed in a 2.5 cm epoxy support. Then, the zircons were polished with sandpaper, and photomicrographs were captured. The internal structures of the zircon grains were characterized using cathodoluminescence (CL) images obtained by a FEI Quanta 250 scanning electron microscope (SEM) and a XMAX CL detector (Oxford Instruments), and the analysis was acquired using in situ LA-ICP-MS. The analysis was performed with a Neptune (Thermo) multicollector instrument and an ArF-193 nm Photon laser system (frequency of 6 Hz) with a spot diameter of 32 µm. The final results match the average obtained after calculating two standard deviations. Isotopic ratios are reported at level 1σ. Finally, the discordant values for the zircon data greater than 10% were eliminated. Corrections for the laser-induced elemental fractionation of the 206 Pb/ 238 U ratio and instrumental discrimination were based on the GJ-1 zircon standard (U-Pb mean age of 601 ± 3.5 Ma; Elhlou et al., 2006), which yielded an age of 600.7 ± 0.69 Ma in the analyzed period. The raw data were processed online and reduced in an Excel spreadsheet adapted from SQUID 1.02 (Ludwig 2001). The data were plotted on a concordia diagram using ISOPLOT/Ex®3.00 (Ludwig 2003).

Lower sequence
The lower sequence is composed of metakomatiite located at the base of the MNGB and comprises the ultramafic volcanic component of the MNGB. The metakomatiite has field relationships with the mafic volcanic rock of the middle sequence; however, these rocks are separated into different sequences due to the mineral content and the microstructural particularities. There are four restricted occurrences of metakomatiite in the central portion of the MNGB, which are northeast of Mundo Novo city and northeast of Piritiba city (Fig. 2).
Generally, the metakomatiite of the MNGB has a relict spinifex microstructure composed of skeletal grains with planar growths that intersect each other (Figs. 3A and 4A) and do not intercept former structures for igneous relicts' microstructure in komatiites, as described by Arndt (1994). The fine-grained spinifex microstructure is identified only in hand samples or by using a hand lens or microscope.
The metakomatiite of the MNGB has a light green color, a silky aspect, and is not magnetic (Fig. 3A). The rock is isotropic and the olivine and pyroxene crystals are entirely replaced by acicular and prismatic pseudomorphic grains of anthophyllite and tremolite (75-80% of the rock) with a grain size of 0.5 mm (Tab. 2; Fig. 4A). The fine-grained groundmass is composed of talc and clinochlore (20-25% of the rock) without a preferred orientation. Traces of pyrite and pyrrhotite are dispersed in the samples.

Middle sequence
The middle sequence is composed of metabasalt and, subordinately, tremolitite, calc-silicate rock, aluminous schist, BIF, ferruginous metachert, metadacite and subordinate metarhyolite; metabasalt and metadacite are the main topics of this study.
Metabasalt and metadacite are the terms used in this paper to define the mafic and felsic volcanic components, respectively, of the middle sequence of the MNGB. These terminologies were adopted based on the respective protolith due to its usefulness in determining the original nature of the rock, even though in many cases, a protolith name does not reflect the principal minerals and structural features of the rocks under observation ). Moreover, some occurrences of metavolcanic mafic rocks in the MNGB preserve primary structures such as pillow lava. Therefore, the term metabasalt is mainly used in this manuscript rather than amphibolite, which is also correctly used if the microstructure and mineral content are considered as proposed by Fettes et al. (2007).
The metabasalts are distributed along a north-south trend ( on petrography, supported by mineral chemistry analysis, and whole-rock chemical data. The first group, defined as the eastern metabasalt, occurs along the eastern portion of the MNGB, has field relationships with the metakomatiite of the lower sequence and corresponds to the main outcrops of the sequence near Piritiba city and extending to Ruy Barbosa city (Fig. 2). The second group, defined as the western metabasalt, occurs along the western portion of the MNGB, mainly within the Fazenda Coqueiro deposit (Mundo Novo city), and in a restricted area with a north-south trend near Piritiba city (Fig. 2).

Western metabasalt
The western metabasalt has a green to grey color and hosts the Zn and Pb sulfides of the Fazenda Coqueiro deposit, particularly in carbonate hydrothermal alteration zones, and is easily observed from drill hole samples obtained by CBPM (Fig. 3C). Based on petrography and mineral chemistry results, this rock is very fine-grained and is mainly composed of oligoclase and andesine (50-55%), actinolite, magnesiohornblende, and ferrotschermakite (25-30%) with a low percentage of quartz, biotite and igneous relicts of hypersthene micrograins (10-15%), as well as ilmenite and titanite as accessory minerals (5-10%; Tab. 2 and 3; Figs. 5A, 5B, 5C and 5D). Traces of manganiferous ilmenite, pyrrhotite, pyrite, chalcopyrite, galena, sphalerite, and arsenopyrite are also observed. The rock is anisotropic, inequigranular and aphanitic, has a polygonal granonematoblastic microstructure and the grains of biotite and actinolite are oriented defining schistosity planes (Fig. 4C).

Metadacite and subordinate metarhyolite
These rocks have restricted occurrences and correspond to the top of the MNGB middle sequence based on field relationships and on a felsic mineral content typical of the later stages of volcanism. These units occur to the northeast of Piritiba city and were identified in drill hole samples and in outcrops (Fig. 2). These rocks are distributed along a northeast-southwest trend and are in contact with eastern metabasalt and ferruginous metachert (Fig. 2).
The metadacite is gray colored, it has a porphyroclastic microstructure, and is not magnetic. This rock contains submillimetric porphyroclasts of plagioclase with sericitized borders and quartz (35-40% of the rock), dispersed in a fine-grained groundmass composed of quartz, plagioclase, K-feldspar, biotite, muscovite, and sericite (60-65% of the rock; Tab. 2; Figs. 3D and 4D), and traces of zircon and other opaque minerals.

U-PB GEOCHRONOLOGY IN ZIRCON
For LA-ICP-MS U-Pb zircon age determination, the metadacite (sample FD-052) of the middle sequence was selected (see sample site in Figs. 1 and 2).
Zircons in the metadacite sample FD-052 are light to dark brown, translucent, subhedral, and prismatic with subrounded terminations. Grain lengths range from 77 to 550 μm, and the aspect ratios range from 2.0 to 3.0. Some crystals show concentric oscillatory zoning and some unzoned ones. Generally, the zircons present thin metamorphic rims and resorbed portions. Twenty-four analyses (Tab. 4) were performed in the cores,    Table 3. Electron microprobe data of the oxides of the elements analyzed reported in wt.% and the cationic structural formula of the minerals in the eastern and western metabasalts.
*For amphibole and biotite were added 2.0 atomic units of H 2 O. The structural formula for the feldspar was provided based on 8 oxygens, for the amphibole based on 23 oxygens, for the hypersthene based on 6 oxygens, for the biotite based on 11 oxygens, for the titanite based on 5 oxygens and for the ilmenite based on 6 oxygens. and the results define the crystallization age. The data in the concordia diagram yield an upper intercept age of 2595 ± 21 Ma, and the MSWD = 2.3 (Fig. 6); thus, this age is considered to represent crystallization age. These data present concordances of between 55 and 91%, and the Th/U values range from 0.31 to 1.03 (values of igneous zircons). The lower intercept age of 616 ± 25 Ma indicates a loss of Pb, possibly from low-intensity geological processes.

GEOCHEMICAL CHARACTERIZATION
The bivariate diagrams of TiO 2 and Al 2 O 3 (the least mobile major elements), FeO t and MgO (mobile major elements), Ni (conservative trace element), Y (immobile trace element), and La and Ce (light REE) against Zr were drawn for the metakomatiite from the lower sequence and the eastern and western metabasalts and metadacite from the middle sequence of the MNGB; the diagrams are shown in Figure 7, and show important correlations. Zr was used as a crystal fractionation index due to its immobility during alteration and metamorphism and its large range of concentration in igneous suites, resulting from varying degrees of partial melting and fractional crystallization (Furnes et al. 2013).
TiO 2 defines a positive pattern versus Zr, and the eastern and western metabasalts form separate groups with the same slope. The Al 2 O 3 diagram features a positive asymptotic pattern, in which the metakomatiite samples plot near the origin, the metabasalt samples form a trend where the western metabasalt exhibits relatively high values of Al 2 O 3 , and the metadacite is approximately aligned in the trend. Four distinct and dispersed groups are formed in the FeO t plot, possibly due to the mobility of Fe during the alteration. The negative asymptotic patterns in the MgO and Ni diagrams form highly defined trends that can be explained by the fractional crystallization of olivine and pyroxene in the metakomatiite and eastern and western metabasalts. Y, La, and Ce, which are considered immobile elements, show highly defined positive correlations versus Zr. Therefore, they were used to demonstrate that metakomatiite, eastern and western metabasalts, and metadacite from the MNGB can be related by fractional crystallization.

Lower sequence
The metakomatiite of the MNGB is ultramafic, with MgO concentrations of 17-25 wt.% (Tab. 5; Fig. 8A). The chondrite-normalized REE geochemical pattern (Boynton 1984) indicates enrichment in light REE in the metakomatiite in the MNGB, similar to the komatiite pattern from the Onverwacht Suite of the Barberton greenstone belt, South Africa (Jahn et al. 1982;Tab. 6 resulted from low values of Nb (≤ 2.45 ppm) and Ti (≤ 1500 ppm), in addition to the enrichment in U and Th (Tab. 7) can be observed in Fig. 9A. High CaO/Al 2 O 3 ratios (Herzberg 1995) in the metakomatiite of the MNGB, with three samples greater than 1.10 (Tab. 8), allow this rock to be classified as Al-depleted. The (La/Yb) N ratios in the MNGB's metakomatiite show high values, with an average of approximately 6.79, standard deviation (σ) of 4.21 and a minimum value of 3.00, and the (Gd/Yb) N ratios values show an average of 1.34 (σ = 0.21) and a minimum of 1.09 (Tab. 6 and 8). The Eu/Eu* ratios values for the metakomatiite have an average of 0.95 (σ = 0.06), suggesting a subtle negative europium anomaly (Tab. 8). The Ce/ Ce* ratios show values around 3.72 and the ΣREE indicates an average of 29.32 ppm (σ = 5.33, Tab. 6 and 8).  In the multielement diagram normalized to N-MORB (Hofmann 1988), the Cs, Ba, Th and U elements show high and anomalous concentrations in the metakomatiite as compared to the other elements in the diagram, a flat pattern from Lu to Nd and a negative anomaly of Nb as compared to the neighboring elements in the diagram of the Fig. 9A.
The tectonic setting discrimination diagram of immobile elements, the Nb/Yb-Th/Yb diagram, indicates that the metakomatiite plots form a trend with other mafic volcanic rocks of the MNGB which extends from the N-MORB point to the volcanic arc array (Fig. 9E).

Eastern and western metabasalts
The division of MNGB metabasalt into the two eastern and western groups, as previously discussed based on petrography, was further confirmed by the REE patterns. These patterns are reliable because of the immobility of such elements during low-grade metamorphism, weathering and hydrothermal alteration, and therefore, a degree of confidence can be placed in the obtained patterns (Rollinson 1993).
The two groups have subtle differences. The eastern metabasalt is high-iron tholeiitic, showing higher percentages of Fe and Ti than the western metabasalt (Fig. 8A), which suggests a small andesitic trend starting from the tholeiitic field in some western metabasalt samples. In the AFM diagram (Irvine and Baragar 1971), two distinct groups of samples are also present, both in the tholeiitic series field, with just three samples of the western metabasalt plotting in the calc-alkalic series field (Fig. 8B).
In the chondrite-normalized REE diagram (Boynton 1984), the western metabasalt is more fractionated and enriched in light rare earth elements (LREE) than the eastern metabasalt, which shows a flat REE pattern (Tab. 6; Figs. 8E and 8F). The fractionation difference is also observed in the average (La/Yb) N ratio: the western metabasalt has a value of 4.47 and the eastern one has a value of 1.91 (Tab. 8). The (Gd/Yb) N ratios show         Abitibi Xie 2002, Xie andKerrich 1994), Barberton ( Jochum et al. 1991, Parman et al. 1997, Chavagnac 2004 and Isua Archean greenstone belts Hoffmann 2003, Polat et al. 2002)   The geochemical data of the major and trace elements normalized to the fertile mantle MORB (FMM) values as a tectonic setting marker (Pearce and Parkinson 1993) are similar for both metabasalt types of the MNGB. The patterns in which normalized Nb (24.06) and Zr (11.48) > TiO 2 (6.47) and in which Y (6.98) and Yb (7.04) ≥ CaO (3.00), Al 2 O 3 (3.91) and V (4.20) into the two metabasalts are similar to the patterns in ocean floor basalts in a back-arc basin system, as demonstrated in the Barberton greenstone belt (Furnes et al. 2013).
In the multielement diagram of trace elements normalized to N-MORB (Hofmann 1988;Figs. 9B and 9C), both metabasalt groups show enrichments of Cs, Ba, Th, U and LREE relative to Ta, Nb, Zr, Hf, Ti, Y and heavy rare earth elements (HREE), a flat pattern from Lu to Zr, and a negative Ta anomaly. The difference between the two groups is subtle but consistent, as for example, the negative Ti anomaly is more accentuated in the western metabasalt than in the eastern one, the western metabasalt is more enriched in La, Ce, and Nd than the eastern, and the Th, U, and Cs values of the western metabasalt are greater than the values of the same elements in the eastern metabasalt.
The Nb/Yb-Th/Yb discrimination diagram of immobile elements in the eastern and western metabasalts of the MNGB indicates a trend that extends from N-MORB in the MORB-Ocean island basalts (OIB) array to the volcanic arc array, with a principal axis of dispersion of the plots oblique to the MORB-OIB array (Fig. 9E). This pattern is similar to oceanic subduction-related basalts of the Mariana Arc and Isua, Barberton and South Abitibi Archean greenstone belts (Fig. 9E). The eastern metabasalt has an IAT-type pattern, with some samples overlapping the MORB field, and the western metabasalt mainly features a MORB pattern with a few occurrences plotting in the within-plate basalt field (WPB; Fig. 9F). Thus, the Zr/Y ratio values for the eastern metabasalt with an average of 3.11 (σ = 0.46) are lower than the values of the western metabasalt, that show an average of 5.07 (σ = 0.89; Tab. 8).

Metadacite and subordinate metarhyolite
The felsic metavolcanic rocks of the middle sequence of the MNGB are classified as metadacite and subordinate metarhyolite in the SiO 2 vs. Na 2 O + K 2 O diagram (Middlemost 1994 ;  Fig. 8C). The chondrite-normalized REE diagram (Boynton 1984) for these felsic metavolcanic rocks subtly slopes from Lu to Gd, as shown by (Gd/Yb) N ratios average of 1.56 (σ = 0.09; Tab. 8), with a negative Eu anomaly (Fig. 8G), as indicated by the average Eu/Eu* ratio of 0.72 (σ =0.07; Tab. 8). There is enrichment in LREE with strong fractionation from Sm to La also shown by the average of the (La/Yb) N ratio of 9.66 (σ = 1.71; Tab. 8). The average of the Ce/Ce* ratio is 8.86 (σ = 2.31; Tab. 8) and the ΣREE value for the metadacite sample is 74.52 and for the metarhyolite sample the ΣREE value obtained was 149.47 (Tab. 6).
In the N-MORB-normalized multielement diagram (Hofmann 1988;Tab. 7 ; Fig. 9D), the metadacite is enriched in Nb and Ta; high Cs, Ba, Th and U values, a negative Ti anomaly and moderate values of Hf (2.72-4.88 ppm) and Zr (83-152 ppm) are shown. Moreover, the metadacite and subordinate metarhyolite plot in the volcanic arc field in the tectonic diagrams of Pearce et al. (1984;Figs. 9G and 9H).

DISCUSSION
The fine-grained microstructure of the metakomatiite in the MNGB is composed of skeletal grains with planar growths that intersect each other and do not intercept former structures, which indicates a relict spinifex microstructure preserved in chilled margins of the komatiite flows. This spinifex microstructure in the metakomatiite and the occurrence of pillow lava structure in the eastern metabasalt suggest rapid and subaquatic crystallization such as that widely observed and interpreted in other greenstone belt terrains (Anhaeusser 2014). The lithological association of the volcanic rocks of the MNGB with the metachert and BIF, the absence of zircon xenocrysts, the manganiferous ilmenite occurrence in the western metabasalt and the carbonate and argilic-chloritic hydrothermal alteration zones developed on the western metabasalt indicate ocean floor settings (Zucchetti et al. 2000a, 2000b, Grachev et al. 2011, Spreafico 2017.
The effect of the fractional crystallization and heterogeneous intraoceanic crustal contamination during the rise of the magma in the eastern and western metabasalts of the MNGB may explain the differences in the mineral paragenesis. The eastern metabasalt, for example, is composed mainly of anorthite, bytownite, magnesiohornblende, ferrohornblende, augite, edenite, and quartz; the western metabasalt comprises oligoclase, andesine, actinolite, ferrotschermakite, magnesiohornblende, biotite, and quartz.
Enrichments of Cs, Ba, Th, and LREE relative to Ta, Nb, Zr, Hf, Ti, Y and HREE and the flat pattern from Lu to Nd in the multielement diagram (Figs. 9A, 9B and 9C) show that the metakomatiite and the two metabasalts of the MNGB were generated from the metasomatized mantle above the subducting altered oceanic crust; Furnes et al. (2013) interpreted a similar enrichment in the Onverwacht Suite in the Barberton greenstone belt. However, the enrichment of LREE relative to HREE in the chondrite-normalized REE diagram (Boynton 1984;Figs. 8E and 8F) was more accentuated in the metakomatiite, which shows average values of the (La/ Yb) N and (Gd/Yb) N ratios of 6.79 and 1.34, respectively, and in the tholeiitic-calc-alkalic western metabasalt, which shows higher average values of the (La/Yb) N and (Gd/Yb) N ratios of 4.47 and 1.38, respectively, relative to the highiron tholeiitic eastern metabasalt, which shows average values of 1.91 and 1.21, respectively. In this sense, the ΣREE average value to the western metabasalt of 120.06 is also significantly greater than the ΣREE average value to the eastern metabasalt of 53.68 and the ΣREE average value to the metakomatiite of 29.32.
High Ce/Ce* ratio values to metadacite (6.63) and metarhyolite (9.90) may be explained by the trace concentration of zircon in these rocks. Thus, it opens the possibility of the occurrence of zircon in the western metabasalt, which also has high Ce/Ce* ratio average values of 9.15.
Eu/Eu* ratio values form a downward trend from the eastern metabasalt (average of 0.93) to metadacite (value of 0.77) with the western metabasalt (average of 0.83) showing intermediate values. The greater concentration of plagioclase, with emphasis on calcic plagioclase, in the eastern metabasalt than the western metabasalt and metadacite may explain the Eu/ Eu* ratio relation among them. In this case, we consider the fractional crystallization of plagioclase with the early crystallization of the calcic plagioclase in the eastern metabasalt, the main crystallization of sodic plagioclase in the western metabasalt and the low plagioclase content in the metadacite.
Both the eastern and western metabasalt samples of the MNGB plot in the IAT, MORB and WPB fields in the Zr-Zr/Y diagram (Fig. 9F); the eastern metabasalt plots in the IAT and MORB fields, due to low values of Zr and Zr/Y ratio, and the western metabasalt, which has higher values of the same element and ratio, plots in the MORB and WPB field. The duplicity of fields by each metabasalt type in the Zr-Zr/Y diagram appears initially uncertain; however, the proximity of settings during volcanism and the transition from one setting to the other due to tectonic events must be considered. Therefore, it is possible that the eastern and western metabasalts are cogenetic and consistent with nearby intraoceanic settings in the MNGB, such as an island arc, which is more consistent with the eastern metabasalt data, and a back-arc basin, which is more coherent with the western metabasalt data, although with different levels of intraoceanic crustal contamination during the formation, as for example, by subduction components.
The plots of the metakomatiite and eastern and western metabasalts of the MNGB in the Nb/Yb-Th/Yb diagram feature a steep vector oblique to the MORB-OIB array (Fig. 9E). Those MNGB plots extend from near the N-MORB point, in the MORB-OIB array, and enter the field of the volcanic arc array, avoiding the OIB point. This plot distribution is similar to the South Abitibi Xie 2002, Xie andKerrich 1994), Barberton ( Jochum et al. 1991, Parman et al. 1997, Chavagnac 2004, and Isua Hoffmann 2003, Polat et al. 2002) Archean greenstone belts (Fig. 9E), which are interpreted as an intraoceanic provenance as discussed in Pearce (2008), and similar to the modern Mariana intraoceanic arc-basin system . Moreover, the MNGB geochemical pattern observed in the Nb/Yb-Th/Yb diagram shows that a subduction-related setting, in this case an intraoceanic arc, contributes to the increase in Th content in the rocks displacing the samples from the MORB-OIB array to the volcanic arc array. Therefore, the eastern metabasalt samples remaining in the MORB-OIB array register the initial ocean crust in the MNGB and, with increasing island arc input, the plots displace from the MORB-OIB array. However, the western metabasalt, which could initially be formed in a back-arc basin, was affected by intraoceanic crustal contamination, probably because of the subduction components of the island arc during the rise of the magma, which totally displaced the samples from the MORB-OIB array.
The metadacite samples of the MNGB plot in the volcanic arc field in the diagrams of Pearce et al. (1984;Figs. 9G and 9H), thus excluding intracratonic possibilities. In addition, the (La/Yb) N ratio of 8.45 in the metadacite may be interpreted as an oceanic crust setting with a slight crustal input subducting the oceanic crust according to Condie and Kronër (2013). Rios (2017) interpreted an intraoceanic setting with a back-arc provenance for basalts in the Neoarchean Contendas-Mirante volcano-sedimentary sequence, which is also inserted in the Contendas-Jacobina Lineament (southern part); this volcano-sedimentary sequence is similar and coeval relative to the MNGB. In the Archean Rio das Velhas greenstone belt, southern São Francisco Craton, ocean floor metabasalts and the felsic volcanic rocks were interpreted as occurrences of island arc or back-arc basin settings (Zucchetti et al. 2000a(Zucchetti et al. , 2000b, which we also interpreted in the MNGB. Differently, an intracontinental provenance has been described for the Umburanas greenstone belt, in the southern part of the Gavião Block in the eastern São Francisco Craton (Leal et al. 2003), with deposition over a continental crust. This provenance suggests a diversity of settings for the greenstone belts in the São Francisco Craton, more specifically in the Gavião Block.
Therefore, the intraoceanic arc-basin system appears suitable for defining the tectonic setting of the MNGB, such as other greenstone and volcano-sedimentary sequences in the São Francisco Craton. In addition, the volcanic rocks would have formed in back-arc basin and island arc settings at 2595 ± 21 Ma, considering the hypothesis that these rocks of the lower and middle sequences sourced from the same magma. Thus, the oceanic crust in which the MNGB was formed was amalgamated between cratonic blocks (Gavião, Mairi, Jequié and Serrinha blocks and Itabuna-Salvador-Curaçá Belt) of the northern and eastern São Francisco Craton in the Orosirian period (Leite 2002;Figs. 10A and 10B), forming the northsouth trend Contendas-Jacobina Lineament within which the greenstone lies.

CONCLUSIONS
The mineralogical and geochemical particularities between the eastern and western metabasalts are products of the intraoceanic crustal contamination during the rise of the magma, fractional crystallization and the formation in nearby but different settings such as the back-arc basin and island arc tholeiitic. Thus, we strongly consider the possibility that the volcanic rocks of the MNGB have sourced from the same magma.
The volcanic rocks in the MNGB were formed in an intraoceanic arc-basin system at 2595 ± 21 Ma. The eastern and western metabasalts were formed in the near back-arc basin and island arc settings, with an IAT pattern for the eastern metabasalt and a MORB pattern for the western metabasalt, suggesting the back-arc basin setting to the latter. The intraoceanic crustal input in the oceanic arc system enriched the Cs, Ba, Th, and LREE content in the metakomatiite, eastern and western metabasalts, and the metadacite of the MNGB, with most accentuated values in the western metabasalt and metadacite suggesting that these rocks were the most affected by intraoceanic contamination during the volcanic processes.
The intraoceanic provenance of the MNGB is comparable to the Contendas-Mirante volcano-sedimentary sequence. Furthermore, both of which are placed in the Contendas-Jacobina Lineament and have geological and tectonic similarities relative to other greenstones in other parts of the São Francisco Craton, such as the Rio das Velhas greenstone belt.
The MNGB rocks were compressed between cratonic blocks of the eastern São Francisco Craton during the Rhyacian-Orosirian period, thus forming the Contendas-Jacobina Lineament within which the greenstone lies.
In order to improve the interpretations present in this manuscript, additional geochronology and Lu-Hf isotope studies could contribute to a more thorough understanding of the geology and tectonic history of the MNGB. Figure 10. Intraoceanic setting proposed for the MNGB formation based on new data presented in this study and compiled ages for the Gavião Block, MNGB and Rhyacian-Orosirian granites (Mougeot 1996, Leite 2002, Peucat et al. 2002, Spreafico 2017, Spreafico et al. 2018, Mairi Block (Sousa et al. 2018), Jacobina Group (Teles 2013, Teles et al. 2015, Barbuena et al. 2016, Serrinha Block (Oliveira et al. 2002a, 2002b, Rios et al. 2009), Itabuna-Salvador-Curaçá Belt (Silva et al. 1997, Oliveira et al. 2010, and Saúde Complex (Barbuena et al. 2016, Zincone et al. 2017. (A) Island arc tholeiitic and back-arc basin between the Gavião and Mairi blocks and formation of the MNGB at 2595 Ma. (B) Rhyacian-Orosirian tectonic event that compressed the MNGB between the cratonic blocks of the eastern São Francisco Craton. A B