Age , provenance and tectonic setting of the high-grade Jequitinhonha Complex , Araçuaí Orogen , eastern Brazil Idade

2Département des Sciences de la Terre et de l’Atmosphère, GEOTOP, Université du Québec à Montréal, Montréal, Québec, Canada. E-mail: stevenson.ross@uqam.ca 3Geological Survey of Brazil, CPRM-SUREG-BH, Belo Horizonte (MG), Brazil. E-mail: luiz.silva@cprm.gov.br 4Universidade Federal de Ouro Preto, Departamento de Geologia, Morro do Cruzeiro, Ouro Preto (MG), Brazil. E-mail: alkmim@degeo.ufop.br 5IG-Laboratório de Geocronologia, Universidade de Brasília, Asa Norte, Brasília (DF), Brazil. E-mail: marcio@unb.br, marcio.pimentel@ufrgs.br


INTRODUCTION
The process of West Gondwana assembly during the Neoproterozoic left significant sutural orogenic scars among the cratons of Brazil, resulting in the diachronous Brasiliano system of orogens (Trompette 1994, Brito Neves et al. 1999, Cordani et al. 2003).One example is the Araçuaí orogen (Pedrosa-Soares et al. 1992, 2008), bound by the eastern edge of the São Francisco craton (Almeida 1977) and the Atlantic continental margin (Fig. 1).
Prior to the opening of the South Atlantic Ocean, the Araçuaí orogen with its African counterpart, the West Congo belt (Fig. 1), constituted an important branch of the Brasiliano-Pan African orogenic system, surrounded on three sides by the São Francisco-Congo craton.This unusual configuration probably represented an inland-sea basin partially floored by oceanic crust, i.e. a gulf-like branch of the Adamastor Ocean (Pedrosa-Soares et al. 1998, 2001, Cordani et al. 2003, Alkmim et al. 2006).This configuration implies that the São Francisco-Congo palaeocontinent, assembled during the Rhyacian-Orosirian orogenies, was not completely disaggregated by Neoproterozoic rifting, but remained linked by a continental bridge (Fig. 1: the Bahia-Gabon bridge) at the northern end of the Araçuaí-West Congo orogenic system (e.g.Trompette 1994, Pedrosa-Soares et al. 2001, 2008, Alkmim et al. 2006, Noce et al. 2007).
This unique palaeogeographic interpretation has been checked by studies on the nature, age, and tectonic setting of stratigraphic units located in the northern portion of the Araçuaí orogen.In this context, the Jequitinhonha Complex is a key unit represented by an extensive clastic sedimentary succession metamorphosed to the amphibolite-granulite facies (Fig. 1 and 2).In this work, we present new geochronological, field, petrographic, lithochemical, and isotopic data of the Jequitinhonha Complex type-area in order to characterize the age, depositional environment, provenance, and tectonic setting of this unit.Based on this dataset, we suggest a correlation of the Jequitinhonha Complex with the upper Macaúbas Group, representing the precursor basin of the Araçuaí orogen, and discuss its role in the evolution of the São Francisco -Congo paleocontinent.

GEOLOGICAL SETTING
The precursor basin to the Araçuaí Orogen is represented by the Macaúbas Group (Pedrosa-Soares et al. 2011b and references therein), an up to 10 km thick sedimentary unit that can be subdivided into three major sucessions: a pre-glacial sucession, composed by the Matão, Duas Barras, and Rio Peixe Bravo formations (quartzite, metaconglomerates and metabreccias); a glacial-related succession, composed of the diamictite-bearing Serra do Catuni, Nova Aurora, and Chapada Acauã formations; and a post-glacial sucession, composed of the diamictite-free Upper Chapada Acauã and Ribeirão da Folha succession, containing fine-grained turbidites and exhalites interleaved with metamafic and ultramafic rocks interpreted as remnants of a Neoproterozoic oceanic crust (Pedrosa-Soares et al. 1998, Queiroga et al. 2007).
Detrital zircon U-Pb data and the ages of related anorogenic igneous provinces suggest that deposition of the Macaúbas Group is related to at least three extensional events that took place during the Neoproterozoic (Pedrosa-Soares & Alkmim 2011): (i) a ca.999 Ma event in the West Congo Belt; (ii) a ca.930 -870 Ma event which gave rise to continental rift basins (Zadinian and Mayumbian groups) and anorogenic magmatism of the Salto da Divisa granites and Pedro Lessa mafic dikes (Machado et al. 1989, Tack et al. 2001, Silva et al. 2008, Pedrosa-Soares et al. 2008, 2011a, Thiéblemont et al. 2011, Babinski et al. 2012); and (iii) a ca.735 -675 Ma continental rifting event (Rosa et al. 2007, Pedrosa-Soares et al. 2011a) that evolved to a passive margin setting with seafloor spreading, forming a confined oceanic basin, that is, a large gulf partially floored by oceanic crust.The post-glacial, diamictite-free units of the Macaúbas Group represent the infilling of this Cryogenian passive margin to ocean basin system (Pedrosa-Soares et al. 1998, 2001, 2008, 2011a, Queiroga et al. 2007).
Recently, Kuchenbecker et al. (2015) presented a comprehensive account of detrital zircon U-Pb data for various units of the Macaúbas Group.Overall, three main age peaks occur, at 900 -1000 Ma, 1900 -2200 Ma, and 2600 -2800 Ma.The lack of Archaean zircons in the pre-glacial units and the relative abundance of Tonian zircons in the upper sequences demonstrate an important change in the source areas, coherent with the climate change to glacial conditions and the change of tectonic setting from rift-related to a passive margin basin.
Subsequently closure of the ocean basin generated calc-alkaline magmatism and eventual collision resulted in the Araçuaí orogen.Orogenic calc-alkaline magmatism started around 630 Ma and lasted until ca.585 Ma in the core of the basin, building a pre-collisional magmatic arc represented by the G1 supersuite and volcano-sedimentary successions of the Rio Doce Group (Fig. 2) (Nalini et al. 2000, Pedrosa-Soares et al. 2001, 2011a;Martins et al. 2004, Vieira 2007, Paes et al. 2010, Silva et al. 2011).The Nova Venécia Complex, composed of peraluminous paragneiss with intercalations of calcsilicate rocks, represents pre-collisional deposition in the back-arc basin related to the G1 supersuite (Noce et al. 2004, Pedrosa-Soares et al. 2008, Gradim et al. 2014).
In addition to regional deformation and metamorphism, the syn-collisional stage generated a large volume of S-type granitic rocks, mostly represented by the biotite-garnet granite and two-mica granite of the G2 supersuite, dated at ca. 585 -560 Ma (Pedrosa Soares et al. 2011a) (Fig. 2 and 3).

THE JEQUITINHONHA COMPLEX
The Jequitinhonha Complex was formerly defined by Almeida and Litwinski (1984) in the surroundings of Jequitinhonha and Almenara towns, northeastern Minas Gerais (Fig. 2 and 3).The unit mostly consists of paragneiss with thin intercalations of calcsilicate rock, lenses and layers of quartzite and graphite gneiss (Fig. 3 and 4).Metamorphic T-P conditions of 791 ± 42 o C at 5 ± 0.5 kbar were determined by multi-equilibrium thermobarometry (AvPT module in Thermocalc software, Powell & Holland 1994) using mineral chemistry data from a sillimanite-garnet-cordierite-biotite gneiss sampled close to Almenara (Belém 2006).These data, with the metamorphic mineral assemblages, indicate regional metamorphism in the amphibolite-granulite facies transition, accompanied by abundant partial melting of the kinzigitic gneiss (see also Uhlein et al. 1998).New multi-equilibrium thermobarometry data presented by Moraes et al. (2015) on migmatites and granulites correlative to the Jequitinhonha Complex in southern Bahia indicate metamorphic peak conditions of 850ºC and 7 kbar.
The first partial melting of the paragneiss produced biotite-garnet granite, locally rich in cordierite.This S-type granite underwent the regional deformation and represents the G2 supersuite (Pedrosa-Soares et al. 2011a).A second melting episode led to the generation of veins and patches of garnet-cordierite leucogranite free of the regional foliation, representing the G3 supersuite (Pedrosa-Soares et al. 2011a). Siga Jr. et al. (1987) presented a U-Pb (TIMS) age of late-stage zircon extracted from a kinzigitic gneiss sample, constraining the age of the main melting and metamorphic event at around 590 ± 20 Ma.This age overlaps with modern metamorphic ages found throughout the Araçuaí orogen that cluster around 575 Ma (Pedrosa-Soares et al. 2011a, Silva et al. 2011).
In the uppermost portion of the kinzigitic gneiss pile, an intercalated quartzite-rich unit up to 100 m thick forms a series of high plateaus and hills, in contrast to the lower smooth relief associated with the paragneiss.Paes et al. (2010) designated this quartzite layer as the Mata Escura Formation.We here consider this as a quartzite layer interleaved in the topmost portion of the paragneiss package, related to the other quartzite layers that occur within this package (Fig. 3).

MATERIALS AND METHODS
For geochemistry and isotopic analysis, only samples that include both the neosome and the paleosome of the gneisses were analysed, in order to obtain a full characterization of samples.For this, ca. 2 kg of each sample were crushed for analysis, and the fine powders obtained were thoroughly mixed before separation of a small fraction for geochemical analysis (ca.300 g).Whole-rock geochemical analysis of nine paragneiss samples was conducted at the ACME Analytical Laboratories Ltd., Vancouver, Canada, via ICP-MS, with 5 % precision for oxides and 10 -15% for most of the trace and rare earth elements.The Sm-Nd isotopic analyses were conducted at the GEOTOP Research Center, Université du Québec à Montréal, Canada, on a ThermoScientific Triton Plus Mass Spectrometer operating in static mode, using both the JNdi standard and USGS standard BHVO-2 as control.The Sm and Nd concentrations and the 147 Sm / 144 Nd ratios have an reproducibility of 0.5% that corresponds to an average error on the initial εNd value of ± 0.5.For details on the methodology used in the geochemical and Sm-Nd analysis, see Caxito et al. (2015).
For the zircon U-Pb analysis, 10 kg of a quartzite sample from the Mata Escura Formation (JE03) was crushed in carefully cleaned equipment, and grains were separated through standard magnetic and hand-picking techniques.Morphological features and internal structures of zircon grains were revealed by electron backscattered electron (BSE) and cathodoluminescence (CL) images.Analyses were conducted at the Research School of Earth Sciences, The Australian National University, in a Neptune Multicollector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS).Results were corrected for common lead content using the 204 Pb / 206 Pb ratio.For the full U-Pb analytical procedures followed in this study and machine parameters, see Kuchenbecker et al. (2015).

The Jequitinhonha Complex in the studied area
Peraluminous paragneiss is the most common gneiss variety in the studied area, typically banded and showing distinct migmatite structures owing to different degrees of partial melting (Fig. 4A and 4B).In addition to quartz and feldspars (plagioclase > K-feldspar), the blueish grey paleosome is rich in biotite, cordierite, garnet and/or sillimanite (Fig. 4C and 4D), with traces of graphite, resembling the so-called kinzigite sensu strictu (Mehnert 1971).The neosome includes the granitic leucosome and a quartz-feldspar poor melanosome to mesosome variably rich in biotite, garnet and/or cordierite.In fact, the paragneiss is a rock assemblage, called by the general name "kinzigitic gneiss", derived from a sedimentary series with variable contributions of clay minerals and carbonaceous material, now composed of sillimanite-graphite gneiss (whose protolith was the richest in carbonaceous material and iron-free clay), graphite-sillimanite-garnet-cordierite-biotite gneiss (richest protolith in clay fraction and the most abundant variety), garnet-cordierite-biotite gneiss, garnet-biotite gneiss, and biotite gneiss (poorest protolith in clay minerals, but the richest in sand fraction).These gneisses show different degrees of partial melting and preserve migmatite features, such as ptigmatic, stromatic, augen, schollen, and schlieren structures (Fig. 4).Centimetric to metric lenses of calcsilicate rocks, intercalated within the paragneiss, consist of quartz, plagioclase, microcline, light pink (Ca-rich) garnet, clinopyroxene and orthopyroxene, and represent mud-carbonate (marl) sediment.
The kinzigitic gneiss package also includes thin lenses to thick layers of quartzite (quartz sandstone), feldspathic quartzite, and sillimanite-graphite-biotite quartzite that grades to the paragneiss.The quartzite typically shows a coarse-grained sacaroidal texture and massive to foliated structure (Fig. 4E and 4F).

Lithochemistry
New major and trace element analyses of 9 paragneiss samples (Tab. 1) are compared with data presented by Reis (1999), Teixeira (2002), Daconti (2004), and Paes et al. (2010), totalling 35 paragneiss samples that spatially represent the varieties of this rock in the Jequitinhonha Complex (Fig. 5  and 6).Samples of the Macaúbas Group schists that crop out nearby (Teixeira 2002) are also plotted for comparison.
The bivariant diagrams for major and trace elements (Fig. 5 and 6) show decreasing trends of Al 2 O 3 , TiO 2 and MgO relative to SiO 2 , and increasing MgO, TiO 2 , Cr, and V relative to Al 2 O 3 , reflecting variable mixtures of clay-size (phyllosilicate) and sand (quartz-feldspar) fractions in the protoliths.Al, Ti, Mg, Cr and V are preferentially concentrated in the clay minerals of mud deposits.Trends of increasing MgO and V in relation to Al 2 O 3 reflect the original sedimentary composition.The decrease of K 2 O and the corresponding increase of Al 2 O 3 relative to silica suggest the preferential absorption of potassium by clay minerals, in contrast to the contribution of clastic K-feldspar.This interpretation is also supported by the decrease of Na 2 O + CaO as K 2 O increases, i.e, most sodium and calcium was provided by the sand fraction (probably clastic plagioclase and carbonate), but most potassium was provided by the mud contribution.TiO 2 variation relative to silica suggests a similar interpretation, that is, most titanium is   the northern to eastern border of the complex, where silica contents are also high, reflecting the predominance of plagioclase over K-feldspar and even less abundant biotite.In general, relatively high SiO 2 / Al 2 O 3 and low K 2 O / Na 2 O ratios suggest sand-mud protoliths richer in plagioclase than in K-feldspar.McLennan et al. (1990) pointed out that the Th / Sc ratio is a sensitive indicator of sediment provenance, because Th is highly incompatible whereas Sc is relatively compatible, so that it can be used as an indicator of the predominance of continental versus juvenile sources.The Th / Sc ratio of the paragneiss samples varies from 0.6 to 1.1 (Tab.1), similar to trailing edge (i.e., passive plate margin) sediments (0.73 -1.4), but quite distinct from those of juvenile arc-related sediments (0.003 -0.7; Fig. 7) or the high values (up to 1.8) of continental arc basins (Taylor & McLennan 1985).
Chondrite-normalized rare earth elements (REE) patterns of the paragneiss are moderately enriched in light rare earth elements (LREE) (La N / Yb N = 6.7 -14.4) and show prominent negative Eu anomalies (Eu / Eu* = 0.5 -0.8; Fig. 8A), compatible with modern passive margin turbidite muds (La N / Yb N = 4.4 -13.6;Eu / Eu* = 0.58 -0.75; McLennan et al. 1990).In contrast, sediments from arc-related basins typically show lower enrichments of LREE and less prominent negative Eu anomalies (La N / Yb N = 2.09 -11.7;Eu / Eu* = 0.7 -0.96; McLennan et al. 1990), reflecting erosion of less fractionated sources.The rare earth element contents of the Jequitinhonha samples are very similar to NASC, showing a flat NASC-normalized pattern (Fig. 8B; Grommet et al. 1984).The only exceptions are sample RP64, which shows slight heavy rare earth element (HREE) depletion, probably owing to trapping of HREE in garnet; and sample AL16A, which is the richest in quartz so that the total amount of REE is lower than in the other samples.The REE patterns of other gneiss samples from the same region (Paes et al. 2010; La N / Yb N = 6.6 -11.4,Eu / Eu* = 0.4 -0.8) are very similar to the new data presented here.

U-Pb (LA-ICP-MS) data
U-Pb analysis of detrital zircon from one quartzite sample of the Jequitinhonha Complex (AT-128) was formerly presented by Gonçalves-Dias et al. (2011).Another sample (JE-03) from the uppermost thick quartzite (Mata Escura Formation) NNW of Jequitinhonha city was analysed (Fig. 3, Fig. 9, and Fig. 10).Results are displayed in a histogram and also in a probability density plot calculated using the Isoplot 3.6 software (Ludwig 2008).
A total of 52 zircon grains were recovered and analysed from sample JE03.Most of these grains are well-rounded to sub-rounded, but some of them show subhedral shapes, ranging in size from 100 to 450 µm.Oscillatory zoning is also a common feature (Fig. 9).From these 52 zircons, 49 analysed spots in the same namber of zircons yielded concordant data (< 10% discordance; Tab. 2).Most Th / U values range from 0.2 to 1.4, with some reaching up to 3.2, and are consistent with a magmatic origin for these zircon grains.The concordant analyses yielded a 207 Pb / 206 Pb age spectrum with five main peaks (Fig. 10) at ca. 1.0, 1.2, 1.5, 1.8, and 2.2 Ga.The age of the youngest concordant zircon grain is 916 ± 24 Ma (spot number 58.D), with 98% concordance (Table 2).These results are similar to those published for sample AT-128 (Gonçalves-Dias et al. 2011), which yielded a 207 Pb / 206 Pb age spectrum with six main peaks at 1.0, 1.2, 1.5, 1.8, 2.0, and 2.5 Ga (Fig. 11).The main difference between the two samples is the minor Archean peak, which is absent in sample JE-03.

Sm-Nd data
Nd isotopic data were obtained for nine paragneiss and one quartzite sample (Table 3).The initial isotope ratios were recalculated to 575 Ma, which is the main age of the metamorphic peak in the Araçuaí orogen (Pedrosa-Soares et al. 2011a).The paragneiss samples yield a very homogeneous Nd isotopic signature, with 143 Nd / 144 Nd ratios from 0.511919 to 0.511980, εNd (575 Ma) around -7.5 and T DM model ages (De Paolo 1981) from 1.6 Ga to 1.8 Ga.Sm / Nd ratios in the range of 0.18 -0.20 are typical of the upper continental crust (Faure 1986).
Sample AT-128, a quartzite layer interleaved within paragneiss, clearly shows an isotopic bias toward older sources,   with T DM = 2.4 Ga, and εNd (575 Ma) = -17.9.Fine-grained and/or clay-rich sediments are more likely to represent large and distant source regions, whereas coarse-grained rocks can be biased towards specific nearby source areas (Frost & Winston 1987, Evans et al. 1991).Celino (1999) also presented Sm-Nd data for the Jequitinhonha Complex.Four paragneiss samples (one of them is a xenolith within a syn-collisional granite) yielded similar T DM model ages (1.6 -1.7 Ga).Daconti (2004) also presented results from two paragneiss samples collected in the surroundings of Almenara region, which yielded quite similar T DM model ages to ours (1.76 Ga and 1.83 Ga).

Sedimentary Provenance
Excluding the few Archaean zircon grains of sample AT-128 (Fig. 11), the two quartzite samples of the Jequitinhonha Complex (JE-03 and AT128) show very similar age spectra, with main age peaks in the ca.1.0, 1.2, 1.5, 1.8, and 2.15 Ga, with a minor Archean peak.Potential sources for the Archaean and Palaeoproterozoic zircon grains are common in the basement of São Francisco-Congo craton and in the basement of the Araçuaí-West Congo orogen (e.g.Teixeira et al. 2000, Silva et al. 2002, Barbosa & Sabaté 2004, Noce et al. 2007).The Espinhaço-Chapada Diamantina basin system developed upon the São Francisco craton and associated magmatism are the most probable Statherian and Mesoproterozoic sources (e.g.Danderfer et al. 2009, Pedrosa-Soares & Alkmim 2011, Chemale et al. 2012).The youngest zircon population can be assigned to the A-type magmatism of the Tonian precursor basin of the orogen, representing erosion of rift shoulders and internal horsts in the cratonic margin bordering the northern and eastern Araçuaí orogen (e.g.Tack et al. 2001, Silva et al. 2008, Pedrosa-Soares & Alkmim 2011).
Figure 11 shows a comparison of the detrital zircon age spectra of the two analysed samples and samples from the upper Macaúbas Group (upper Chapada Acauã Formation and Ribeirão da Folha Formation), which are considered to represent the distal passive margin of the precursor basin to the Araçuaí Orogen, and also a comparison with samples from the Nova Venécia Complex paragnaisses and the Rio Doce Group, which are believed to represent syn-orogenic basins related to the erosion of the G1 magmatic arc (Noce et al. 2004, Pedrosa-Soares et al. 2000, 2011a, Gonçalves-Dias et al. 2011, Novo 2013, Gradim et al. 2014, Peixoto et al. 2015, Kuchenbecker et al. 2015).
Despite some differences, such as the relative abundance of Archean detrital zircons in the Upper Chapada Acauã Formation and the peak of 1.2 Ga zircons in the Jequitinhonha Complex, the range of detrital zircon ages of the Jequitinhonha Complex is more similar to those of the upper units of the Macaúbas Group.In particular, both the Jequitinhonha Complex and the upper Macaúbas Group samples lack the distinctive Cryogenian -Ediacaran peak found in samples of metasedimentary units related to the G1 magmatic arc (Rio Doce Group and Nova Venécia Complex).
The younger detrital zircon populations of the Jequitinhonha Complex samples constrain the maximum
depositional age at about 900 Ma, and the main epoch of the syn-collisional metamorphism and anatexis suggests a minimum depositional age around 575 Ma (Siga Jr. et al. 1987, Pedrosa-Soares et al. 2011a, Silva et al. 2011, Gradim et al. 2014).

Discussion of the Sm-Nd data
The Nd evolution diagram for the paragneiss samples (Fig. 12) shows a comparison with the main possible sources as suggested by the U-Pb data from detrital zircon grains, that is, the Archaean-Palaeoproterozoic basement of the São Francisco craton and the Tonian rift-related volcanic rocks (Fig. 13; data from Teixeira et al. 1996, Noce et al. 2000, Tack et al. 2001).The positioning of the Jequitinhonha Complex samples in between the São Francisco craton and West Congo rift volcanics fields (Fig. 12A) suggests variable mixing between these two broad source areas as the main sedimentary provenance for the Jequitinhonha Complex protholits.The εNd evolution diagram also suggests an important role for the Tonian rift-related magmatism in the isotopic inheritance of the clay-rich protoliths such as those represented by the peraluminous gneiss of the distal Jequitinhonha Complex.
In comparison to the Jequitinhonha Complex, Nd isotope data for the Macaúbas Group metasedimentary and metavolcanic rocks shows a broader variation, with εNd (575 Ma) from -2.0 (basic metavolcanics) to -18.0 (metasedimentary rocks) and T DM model ages from 1.5 to 2.5 Ga (Babinski et al. 2012).Nevertheless, the homogeneous results found for the Jequitinhonha Complex plot within the range of samples from the Macaúbas Group in the Nd isotope evolution diagram (Fig. 12B).

Tectonic setting of the Jequitinhonha Complex and stratigraphic correlations
In the eastern portion of the Araçuaí orogen, mediumto high-grade metamorphic rocks whose sedimentary protoliths were deposited in syn-orogenic basins related to the G1 supersuite magmatic arc are quite common (Pedrosa-Soares et al. 2011a).This is the case of the Rio Doce Group (Nalini et al. 2000, Pedrosa-Soares et al. 2001, 2011b, Martins et al. 2004, Vieira 2007, Paes et al. 2010, Silva et al. 2011, Novo 2013) and the Nova Venécia Complex, composed of peraluminous paragneiss with intercalations of calcsilicate rocks (Noce et al. 2004, Pedrosa-Soares et al. 2008, Gradim et al. 2014).Both of those units bear an important detrital zircon U-Pb age peak at ca. 630 Ma, indicating provenance from the Araçuaí orogen magmatic arc.The Jequitinhonha Complex, on the other hand, yielded no Ediacaran zircons, but, instead, shows patterns which are more similar to the precursor passive margin basin of the orogen (upper Macaúbas Group), with younger zircons at around 900 Ma.Thus, the above petrographic, litochemical, isotopic, and geochronologic data suggests that the Jequitinhonha Complex represents a sedimentary package deposited in a passive margin environment of the precursor basin of the Araçuaí orogen, between ca.914 Ma (youngest peak from detrital zircon ages) and 580-540 Ma (age of the   (Teixeira et al. 1996, Noce et al. 2000) and Tonian rift volcanics of the West Congo belt (Tack et al. 2001); and (b) Macaúbas Group samples (Babinski et al. 2012).
syn-collisional metamorphism; Pedrosa Soares et al. 2011a).The Jequitinhonha Complex is then not correlative with other paragneiss successions of the eastern Araçuaí orogen, such as the Nova Venécia Complex, which is related to the back-arc region of the G1 supersuite magmatic arc (Fig. 2) (Noce et al. 2004), but, instead, probably represents a higher metamorphic grade chronostratigraphic equivalent of the upper units of the Macaúbas Group (Ribeirão da Folha and upper Chapada Acauã formations; Pedrosa-Soares et al. 2011b, Babinski et al. 2012, Kuchenbecker et al. 2015) with both series representing sand-mud shelf deposits.This is supported by trace element data of paragneiss samples, which suggest a trailing-edge environment of deposition, and by the very similar U-Pb detrital zircon age spectra and Sm-Nd isotope data for the Jequitinhonha and Macaúbas units.
The exclusively sedimentary nature of the Jequitinhonha Complex and the absence of any ophiolite slivers in the region reinforce the interpretation that it represents the ensialic part of the precursor gulf-like basin of the Araçuaí-West Congo orogen (Fig. 13).This scenario also supports the suggestion that the São Francisco-Congo paleocontinent was not broken to the north of the studied region as a consequence of the opening of the Macaúbas-Jequitinhonha basin.This interpretation provides further evidence that the São Francisco-Congo paleocontinent acted as a single plate during West Gondwana amalgamation in Ediacaran time, joined by the Bahia -Gabon cratonic bridge, as shown by virtually all paleogeographic reconstructions (e.g.Cordani et al. 2003, D'Agrella et al. 2004, Li et al. 2008).

Trace and REE element patterns of the Jequitinhonha
Complex suggest erosion of an evolved continental crust, similar to present-day passive margin (trailing edge) turbidites, and are very different from present-day active margin (arc-related) basins.2. The detrital zircon U-Pb age spectra of two quartzite samples indicates main source areas of ca.1.0, 1.2, 1.5, and 2.2 Ga.The absence of Ediacaran zircon excludes, in principle, the Araçuaí orogen magmatic arc (G1 supersuite) as a probable source area.On the other hand, the detrital zircon age patterns of the Jequitinhonha Complex samples are very similar to those of the upper units of the Macaúbas Group (the upper Chapada Acauã and Ribeirão da Folha formations).The main suggested source areas are the São Francisco craton basement and the extensive Tonian bimodal volcanic units of the West Congo belt.3. Sm-Nd isotope data on samples from the Jequitinhonha Complex are homogeneous, and suggest a mixing of

Figure 1 .
Figure 1.Geotectonic setting of the Jequitinhonha Complex (cj), located in the northeastern part of the Araçuaí Orogen (modified from Pedrosa-Soares et al. 2008).
incorporated in the mud fraction, rather than in heavy minerals in the sand fraction.Higher Na 2 O + CaO and Sr + Ba in comparison to K 2 O and Rb contents characterize most paragneiss located along 205 Brazilian Journal of Geology, 46(2): 199-219, June 2016 Tatiana Gonçalves Dias et al.

Figure 5 .Figure 6 .
Figure 5. Major element bivariant diagrams for kinzigitic gneiss samples from different parts of the Jequitinhonha Complex, compared with schist samples of the Macaúbas Group.
Figure 12.Nd isotopic evolution diagram for paragneiss samples of the Jequitinhonha Complex, as compared with (a) the São Francisco Craton basement(Teixeira et al. 1996, Noce et al. 2000)  and Tonian rift volcanics of the West Congo belt(Tack et al. 2001); and (b) Macaúbas Group samples(Babinski et al. 2012).

Figure 13 .
Figure 13.Tectonic model for the precursor basin of the Jequitinhonha Complex and its relations to the Macaúbas Group.

Table 1 .
Lithochemistry data from paragneiss samples of the Jequitinhonha Complex.

Table 2 .
U-Pb (LA-ICP-MS) data for detrital zircon grains from quartzite sample (JE-03) from the Mata Escura Formation of the Jequitinhonha Complex.Shaded rows highlight more than 10% discordant data.

Table 3 .
Nd isotopic data for rocks from the Jequitinhonha/Almenara region.T DM is calculated after DePaolo's (1981) model.