ABSTRACT:

The Jequitinhonha Complex of the northeastern Araçuaí orogen is an extensive sedimentary unit metamorphosed in the amphibolite-granulite facies transition around 580-545 Ma. The unit consists of Al-rich (kinzigitic) paragneisses with decametric intercalations of graphite gneisses and quartzites, and centimetric to metric lenses of calcsilicate rocks. A new detrital zircon U-Pb age spectrum is reported for a sample of quartzite, and whole-rock geochemical (major and trace elements, 9 samples) and Sm-Nd isotope data (10 samples) for Jequitinhonha Complex paragneiss. Together with published data these show that: (1) the geochemistry of paragneiss samples of the Jequitinhonha Complex are similar to those of passive margin sedimentary protoliths; (2) detrital zircon data yield U-Pb age populations between ca. 0.9 and 2.5 Ga; and (3) Sm-Nd T_DM model ages range from 1.6 to 1.8 Ga and εNd_{(575 Ma)} around -7.5. The data reveal a mixture of Cryogenian to Mesoproterozoic rift-related igneous rocks with the Palaeoproterozoic-Archaean basement rocks of the São Francisco-Congo palaeocontinent as the main source areas, and also support the correlation between the Jequitinhonha Complex and the passive margin units of the upper Macaúbas Group, constituting the precursor basin of the orogen. Our results, with the absence of ophiolites in the Jequitinhonha Complex, reinforce the interpretation that the São Francisco-Congo palaeocontinent was not divided to the north of the focused region, suggesting an ensialic termination of a gulf during the Neoproterozoic.

KEYWORDS:
Kinzigite gneiss; Jequitinhonha Complex; Araçuaí Orogen; West Gondwana

RESUMO:

O Complexo Jequitinhonha é uma unidade sedimentar extensa da porção nordeste do orógeno Araçuaí, metamorfizada na transição entre as fácies anfibolito-granulito há cerca de 580-540 Ma. A unidade é composta por paragnaisses kinzigíticos com intercalações de grafita-gnaisse, quartzito e rochas calcissilicáticas. Dados U-Pb de zircão detrítico de uma amostra de quartzito e novos dados geoquímicos (nove amostras) e isotópicos (Sm-Nd) (dez amostras) são aqui apresentados. De maneira concomitante a dados previamente publicados, esses dados mostram que: (1) a geoquímica dos paragnaisses sugere uma afiliação do tipo margem passiva para as rochas metassedimentares; (2) zircões detríticos apresentam populações de idade U-Pb entre 0,9 e 2,5 Ga; e (3) dados isotópicos de Nd apresentam idades modelo T_DM entre 1,6 e 1,8 Ga e εNd _{(575 Ma)} ao redor de -7,5. Esses dados revelam uma mistura de fontes, envolvendo magmatismo de rift Criogeniano a Mesoproterozoico e o embasamento Paleoproterozoico-Arqueano do paleocontinente São Francisco-Congo, e sugerem uma forte correlação entre o Complexo Jequitinhonha e o Grupo Macaúbas, compondo a mais importante bacia precursora do orógeno Araçuaí. Além da natureza exclusivamente sedimentar do complexo, fatias ofiolíticas não foram encontradas na área, reforçando a interpretação da terminação ensiálica de um golfo e que o paleocontinente São Francisco-Congo não foi separado ao norte da região, agindo como uma única peça durante o Neoproterozoico.

PALAVRAS-CHAVE:
Gnaisse kinzigítico; Complexo Jequitinhonha; Orógeno Araçuaí; Gondwana Ocidental

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 1994Trompette R.R. 1994. Geology of Western Gondwana (2000-500 Ma) Pan-African-Brasiliano aggregation of South America and Africa. A.A. Rotterdam, Balkema, 350 pp., Brito Neves et al. 1999Brito Neves B.B., Campos Neto M.C., Fuck R.A. 1999. From Rodinia to Western Gondwana: An approach to the Brasiliano-Pan African Cycle and orogenic collage.Episodes, 22(3):155-166., Cordani et al. 2003)Cordani U.G., Brito Neves B.B., D'Agrella M.S., Trindade R.I.F. 2003. Tearing-up Rodinia: the Neoproterozoic paleogeography of South American cratonic fragments. Terra Nova , 15:343-349.. One example is the Araçuaí orogen (Pedrosa-Soares et al. 1992Pedrosa-Soares A.C., Noce C.M., Vidal P., Monteiro R., Leonardos O.H. 1992. Toward a new tectonic model for the Late Proterozoic Araçuaí (SE Brazil) - West Congolian (SW Africa) Belt. Journal of South American Earth Sciences , 6:33-47., 2008Pedrosa-Soares A.C., Alkmim F.F., Tack L., Noce C.M., Babinski M., Silva L.C., Martins-Neto M.A. 2008. Similarities and differences between the Brazilian and African counterparts of Neoproterozoic Araçuaí-West Congo orogen. : Pankhrust R., Trouw R., Brito-Neves B.B., Wit M. (eds). The Gondwana Peleocontinent in the South Atlantic Region Geological Society of London, Special Publications, 294:153-172.), bound by the eastern edge of the São Francisco craton (Almeida 1977Almeida F.F.M. 1977. O Cráton do São Francisco. Revista Brasileira de Geociências , 7: 349-364. ) 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. 1998Pedrosa-Soares A.C., Vidal P., Leonardos O.H., Brito-Neves B.B. 1998. Neoproterozoic oceanic remnants in eastern Brazil: Further evidence and refutation of an exclusively ensialic evolution for the Araçuaí- West Congo Orogen. Geology 26:519-522., 2001Pedrosa-Soares A.C., Noce C.M., Wiedemann C.M., Pinto C.P. 2001. The Araçuaí-West Congo orogen in Brazil: An overview of a confined orogen formed during Gondwanland assembly. Precambrian Research , 110:307-323., Cordani et al. 2003Cordani U.G., Brito Neves B.B., D'Agrella M.S., Trindade R.I.F. 2003. Tearing-up Rodinia: the Neoproterozoic paleogeography of South American cratonic fragments. Terra Nova , 15:343-349., Alkmim et al. 2006Alkmim F.F., Marshak S., Pedrosa-Soares A.C., Cruz S., Peres G.G., Cruz S.C.P., Whittington A. 2006. Kinematic evolution of the Araçuaí-West Congo orogen in Brazil and Africa: Nutcracker tectonics during the Neoproterozoic assembly of Gondwana. Precambrian Research , 149:43-64.). 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 1994Trompette R.R. 1994. Geology of Western Gondwana (2000-500 Ma) Pan-African-Brasiliano aggregation of South America and Africa. A.A. Rotterdam, Balkema, 350 pp., Pedrosa-Soares et al. 2001Pedrosa-Soares A.C., Noce C.M., Wiedemann C.M., Pinto C.P. 2001. The Araçuaí-West Congo orogen in Brazil: An overview of a confined orogen formed during Gondwanland assembly. Precambrian Research , 110:307-323., 2008Pedrosa-Soares A.C., Alkmim F.F., Tack L., Noce C.M., Babinski M., Silva L.C., Martins-Neto M.A. 2008. Similarities and differences between the Brazilian and African counterparts of Neoproterozoic Araçuaí-West Congo orogen. : Pankhrust R., Trouw R., Brito-Neves B.B., Wit M. (eds). The Gondwana Peleocontinent in the South Atlantic Region Geological Society of London, Special Publications, 294:153-172., Alkmim et al. 2006Alkmim F.F., Marshak S., Pedrosa-Soares A.C., Cruz S., Peres G.G., Cruz S.C.P., Whittington A. 2006. Kinematic evolution of the Araçuaí-West Congo orogen in Brazil and Africa: Nutcracker tectonics during the Neoproterozoic assembly of Gondwana. Precambrian Research , 149:43-64., Noce et al. 2007Noce C.M., Pedrosa-Soares A.C., Silva L.C., Alkmim F.F. 2007. O Embasamento Arqueano e Paleoproterozóico do Orógeno Araçuaí. Geonomos , 15(1):17-23.).

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. 2011bPedrosa-Soares A.C. , Alkmim F.F. 2011. How many rifting events preceded the development of the Araçuaí-West Congo orogen? Geonomos 19(2): 244-251. 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. 1998Pedrosa-Soares A.C., Vidal P., Leonardos O.H., Brito-Neves B.B. 1998. Neoproterozoic oceanic remnants in eastern Brazil: Further evidence and refutation of an exclusively ensialic evolution for the Araçuaí- West Congo Orogen. Geology 26:519-522., Queiroga et al. 2007Queiroga G.N., Pedrosa-Soares A.C., Noce C.M., Alkmim F.F., Pimentel M.M., Dantas E., Martins M., Castañeda C., Suita M.T.F., Prichard H. 2007. Age of the Ribeirão da Folha ophiolite, Araçuaí Orogen: The U-Pb zircon dating of a plagiogranite. Geonomos 15:61-65.).

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 2011Pedrosa-Soares A.C. , Alkmim F.F. 2011. How many rifting events preceded the development of the Araçuaí-West Congo orogen? Geonomos 19(2): 244-251.): (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. 1989Machado N., Schrank A., Abreu F.R., Knauer L.G., Almeida-Abreu P.A. 1989. Resultados preliminares da geocronologia U/Pb na Serra do Espinhaço Meridional. : SBG, Simpósio de Geologia de Minas Gerais, 5, Anais , p. 1-4., Tack et al. 2001Tack L., Wingate M.T.D., Liégeois J.P., Fernandez-Alonso M., Deblond A. 2001. Early Neoproterozoic magmatism (1000-910 Ma) of the Zadinian and Mayumbian groups (Bas-Congo): Onset of Rodinian rifting at the western edge of the Congo craton. Precambrian Research , 110:277-306., Silva et al. 2008Silva L.C, Pedrosa-Soares A.C., Teixeira L., Armstrong R. 2008. Tonian rift-related, A-type continental plutonism in the Araçuaí Orogen, eastern Brazil: New evidence for the breakup stage of the São Francisco Congo Paleocontinent. Gondwana Research , 13:527-537., Pedrosa-Soares et al. 2008Pedrosa-Soares A.C., Alkmim F.F., Tack L., Noce C.M., Babinski M., Silva L.C., Martins-Neto M.A. 2008. Similarities and differences between the Brazilian and African counterparts of Neoproterozoic Araçuaí-West Congo orogen. : Pankhrust R., Trouw R., Brito-Neves B.B., Wit M. (eds). The Gondwana Peleocontinent in the South Atlantic Region Geological Society of London, Special Publications, 294:153-172., 2011aPedrosa-Soares A.C., Campos C.P., Noce C., Silva L.C., Novo T., Roncato J., Medeiros S., Castañeda C., Queiroga G., Dantas E., Dussin I., Alkmim F.F. 2011a. Late Neoproterozoic-Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Sial A.N., Bettencourt J.S., De Campos C.P., Ferreira V.P. (eds). Granite-Related Ore Deposits . Geological Society, London, Special Publications, 350:25-51. , Thiéblemont et al. 2011Thiéblemont D., Prian J.P., Goujou J.C., Boulingui B., Ekogha H., Kassadou A.B., Simo-Ndounze S., Walemba A., Préat A., Theunissen K., Cocherie A., Guerrot C. 2011. Timing and characteristics of Neoproterozoic magmatism in SW-Gabon: First geochronogical and geochemical data on the West-Congolian orogen in Gabon (SYSMIN project, Gabon 2005-2009). 23 Colloquium of African Geology, Posters and Abstracts ., Babinski et al. 2012Babinski M., Pedrosa-Soares A.C., Trindade R.I.F. , Martins M., Noce C.M., Liu D. 2012. Neoproterozoic glacial deposits from the Araçuaí orogen, Brazil: Age, provenance and correlations with the São Francisco craton and West Congo belt. Gondwana Research 21(2-3):451-465. ); and (iii) a ca. 735 - 675 Ma continental rifting event (Rosa et al. 2007Rosa M.L.S., Conceição H., Macambira M., Galarza M.C., Cunha M., Menezes R., Marinho M.M., Cruz-Filho B.E., Rios D.C. 2007. Neoproterozoic anorogenic magmatism in the Southern Bahia Alkaline Province of NE Brazil: U-Pb and Pb-Pb ages of the blue sodalite syenites. Lithos , 97:88-97. , Pedrosa-Soares et al. 2011aPedrosa-Soares A.C., Campos C.P., Noce C., Silva L.C., Novo T., Roncato J., Medeiros S., Castañeda C., Queiroga G., Dantas E., Dussin I., Alkmim F.F. 2011a. Late Neoproterozoic-Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Sial A.N., Bettencourt J.S., De Campos C.P., Ferreira V.P. (eds). Granite-Related Ore Deposits . Geological Society, London, Special Publications, 350:25-51. ) 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. 1998Pedrosa-Soares A.C., Vidal P., Leonardos O.H., Brito-Neves B.B. 1998. Neoproterozoic oceanic remnants in eastern Brazil: Further evidence and refutation of an exclusively ensialic evolution for the Araçuaí- West Congo Orogen. Geology 26:519-522., 2001Pedrosa-Soares A.C., Noce C.M., Wiedemann C.M., Pinto C.P. 2001. The Araçuaí-West Congo orogen in Brazil: An overview of a confined orogen formed during Gondwanland assembly. Precambrian Research , 110:307-323., 2008Pedrosa-Soares A.C., Alkmim F.F., Tack L., Noce C.M., Babinski M., Silva L.C., Martins-Neto M.A. 2008. Similarities and differences between the Brazilian and African counterparts of Neoproterozoic Araçuaí-West Congo orogen. : Pankhrust R., Trouw R., Brito-Neves B.B., Wit M. (eds). The Gondwana Peleocontinent in the South Atlantic Region Geological Society of London, Special Publications, 294:153-172., 2011aPedrosa-Soares A.C., Campos C.P., Noce C., Silva L.C., Novo T., Roncato J., Medeiros S., Castañeda C., Queiroga G., Dantas E., Dussin I., Alkmim F.F. 2011a. Late Neoproterozoic-Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Sial A.N., Bettencourt J.S., De Campos C.P., Ferreira V.P. (eds). Granite-Related Ore Deposits . Geological Society, London, Special Publications, 350:25-51. , Queiroga et al. 2007Queiroga G.N., Pedrosa-Soares A.C., Noce C.M., Alkmim F.F., Pimentel M.M., Dantas E., Martins M., Castañeda C., Suita M.T.F., Prichard H. 2007. Age of the Ribeirão da Folha ophiolite, Araçuaí Orogen: The U-Pb zircon dating of a plagiogranite. Geonomos 15:61-65.).

Recently, Kuchenbecker et al. (2015Kuchenbecker M., Pedrosa-Soares A.C., Babinski M., Fanning M. 2015. Detrital zircon age patterns and provenance assessment for pre-glacial to post-glacial successions of the Neoproterozoic Macaúbas Group, Araçuaí orogen, Brazil. Precambrian Research , 266:12-26.) 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. 2000Nalini Jr. H.A., Bilal E., Neves J.M.C. 2000. Syn-collisional peraluminous magmatism in the Rio Doce region: mineralogy, geochemistry, and isotopic data of the Neoproterozoic Urucum Suite (eastern Minas Gerais State, Brazil). Revista Brasileira de Geociências , 30:120-125., Pedrosa-Soares et al. 2001Pedrosa-Soares A.C., Noce C.M., Wiedemann C.M., Pinto C.P. 2001. The Araçuaí-West Congo orogen in Brazil: An overview of a confined orogen formed during Gondwanland assembly. Precambrian Research , 110:307-323., 2011aPedrosa-Soares A.C., Campos C.P., Noce C., Silva L.C., Novo T., Roncato J., Medeiros S., Castañeda C., Queiroga G., Dantas E., Dussin I., Alkmim F.F. 2011a. Late Neoproterozoic-Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Sial A.N., Bettencourt J.S., De Campos C.P., Ferreira V.P. (eds). Granite-Related Ore Deposits . Geological Society, London, Special Publications, 350:25-51. ; Martins et al. 2004Martins V.T.S., Teixeira W., Noce C.M., Pedrosa-Soares A.C. 2004. Sr and Nd characteristics of Brasiliano-Pan African granitoid plutons of the Araçuaí orogen, southeastern Brazil: Tectonic implications. Gondwana Research , 7:75-89., Vieira 2007Vieira V.S. 2007. Significado do Grupo Rio Doce no Contexto do Orógeno Araçuaí Tese de Doutorado, Instituto de Geociências, Universidade Federal de Minas Gerais, Belo Horizonte., Paes et al. 2010Paes V.J.C., Raposo F.O., Pinto C.P., Oliveira F.A.R. 2010Projeto Jequitinhonha, Estados de Minas Gerais e Bahia: texto explicativo. Geologia e Recursos Minerais das Folhas Comercinho, Jequitinhonha, Almenara, Itaobim, Joaíma e Rio do Prado. Programa Geologia do Brasil. Belo Horizonte, CPRM, 376 p., Silva et al. 2011Silva L.C., Pedrosa-Soares A.C., Armstrong R., Noce C.M. 2011. Determinando a duração do período colisional do Orógeno Araçuaí com base em geocronologia U-Pb de alta resolução em zircão: uma contribuição para a história da amalgamação do Gondwana Ocidental. Geonomos 19(2):180-197.). 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. 2004Noce C.M, Pedrosa-Soares A.C., Piuzana D., Armstrong R., Laux J.H., Campos C.M., Medeiros S.R. 2004. Ages of sedimentation of the kinzigitic complex and of a late orogenic thermal episode in the Araçuaí orogen, northern Espírito Santo State, Brazil: Zircon and monazite U-Pb SHRIMP and ID-TIMS data. Revista Brasileira de Geociências , 349:587-592., Pedrosa-Soares et al. 2008Pedrosa-Soares A.C., Alkmim F.F., Tack L., Noce C.M., Babinski M., Silva L.C., Martins-Neto M.A. 2008. Similarities and differences between the Brazilian and African counterparts of Neoproterozoic Araçuaí-West Congo orogen. : Pankhrust R., Trouw R., Brito-Neves B.B., Wit M. (eds). The Gondwana Peleocontinent in the South Atlantic Region Geological Society of London, Special Publications, 294:153-172., Gradim et al. 2014Gradim C., Roncato J., Pedrosa-Soares A.C., Cordani U.G., Dussin I.A., Alkmim F.F., Queiroga G., Jacobsohn T., Silva L.C., Babinski M. 2014. The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation. Brazilian Journal of Geology , 44(1):155-180.).

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. 2011aPedrosa-Soares A.C. , Alkmim F.F. 2011. How many rifting events preceded the development of the Araçuaí-West Congo orogen? Geonomos 19(2): 244-251.) (Fig. 2 and 3).

THE JEQUITINHONHA COMPLEX

The Jequitinhonha Complex was formerly defined by Almeida and Litwinski (1984Almeida F.F.M. , Litwinski N. 1984. Província Mantiqueira: setor setentrional. : Almeida F.F.M. , Hasui Y. (eds.) O Pré-Cambriano do Brasil . São Paulo, Editora Edgar Blücher, p. 282-307.) 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^oC at 5 ± 0.5 kbar were determined by multi-equilibrium thermobarometry (AvPT module in Thermocalc software, Powell & Holland 1994Powell R. , Holland T. 1994. Optimal geothermometry and geobarometry. American Mineralogist 79:120-133.) using mineral chemistry data from a sillimanite-garnet-cordierite-biotite gneiss sampled close to Almenara (Belém 2006Belém J. 2006. Caracterização mineralógica, física e termobarométrica de minérios de grafita da Província Grafítica Bahia-Minas . Dissertação de Mestrado, Instituto de Geociências, Universidade Federal de Minas Gerais, 165 p.). 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. 1998Uhlein A., Egydio-Silva M., Bouchez J.L., Vauchez A. 1998. The Rubim pluton (Minas Gerais, Brazil): a petrostructural and magnetic fabric study. Journal of South American Earth Sciences , 11:179-189.). New multi-equilibrium thermobarometry data presented by Moraes et al. (2015Moraes R., Nicollet C., Barbosa J.H.F., Fuck R.A., Sampaio A.R. 2015. Applications and limitations of thermobarometry in migmatites and granulites using as an example rocks of the Araçuaí Orogen in southern Bahia, including a discussion on the tectonic meaning of the current results. Brazilian Journal of Geology , 45(4):517-539.) 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. 2011aPedrosa-Soares A.C., Campos C.P., Noce C., Silva L.C., Novo T., Roncato J., Medeiros S., Castañeda C., Queiroga G., Dantas E., Dussin I., Alkmim F.F. 2011a. Late Neoproterozoic-Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Sial A.N., Bettencourt J.S., De Campos C.P., Ferreira V.P. (eds). Granite-Related Ore Deposits . Geological Society, London, Special Publications, 350:25-51. ). 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. 2011aPedrosa-Soares A.C., Campos C.P., Noce C., Silva L.C., Novo T., Roncato J., Medeiros S., Castañeda C., Queiroga G., Dantas E., Dussin I., Alkmim F.F. 2011a. Late Neoproterozoic-Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Sial A.N., Bettencourt J.S., De Campos C.P., Ferreira V.P. (eds). Granite-Related Ore Deposits . Geological Society, London, Special Publications, 350:25-51. ). Siga Jr. et al. (1987Siga Jr. O., Cordani U.G., Basei M.A.S., Teixeira W., Kawashita K., Van Schmus W.R. 1987. Contribuição ao estudo geológico e geocronológico da porção nordeste de Minas Gerais. : 48º Simpósio de Geologia de Minas Gerais. SBG-MG, Boletim , 7:29-44.) 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. 2011aPedrosa-Soares A.C., Campos C.P., Noce C., Silva L.C., Novo T., Roncato J., Medeiros S., Castañeda C., Queiroga G., Dantas E., Dussin I., Alkmim F.F. 2011a. Late Neoproterozoic-Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Sial A.N., Bettencourt J.S., De Campos C.P., Ferreira V.P. (eds). Granite-Related Ore Deposits . Geological Society, London, Special Publications, 350:25-51. , Silva et al. 2011Silva L.C., Pedrosa-Soares A.C., Armstrong R., Noce C.M. 2011. Determinando a duração do período colisional do Orógeno Araçuaí com base em geocronologia U-Pb de alta resolução em zircão: uma contribuição para a história da amalgamação do Gondwana Ocidental. Geonomos 19(2):180-197.).

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. (2010Paes V.J.C., Raposo F.O., Pinto C.P., Oliveira F.A.R. 2010Projeto Jequitinhonha, Estados de Minas Gerais e Bahia: texto explicativo. Geologia e Recursos Minerais das Folhas Comercinho, Jequitinhonha, Almenara, Itaobim, Joaíma e Rio do Prado. Programa Geologia do Brasil. Belo Horizonte, CPRM, 376 p.) 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 ¹⁴⁷Sm / ¹⁴⁴Nd 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. (2015Caxito F.A., Uhlein A., Dantas E.L., Stevenson R., Pedrosa-Soares A.C. 2015. Orosirian (ca. 1.96 Ga) mafic crust of the northwestern São Francisco Craton margin: Petrography, geochemistry and geochronology of amphibolites from the Rio Preto fold belt basement, NE Brazil. Journal of South American Earth Sciences , 59:95-111. ).

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 ²⁰⁴Pb / ²⁰⁶Pb ratio. For the full U-Pb analytical procedures followed in this study and machine parameters, see Kuchenbecker et al. (2015Kuchenbecker M., Pedrosa-Soares A.C., Babinski M., Fanning M. 2015. Detrital zircon age patterns and provenance assessment for pre-glacial to post-glacial successions of the Neoproterozoic Macaúbas Group, Araçuaí orogen, Brazil. Precambrian Research , 266:12-26.).

RESULTS

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 1971Mehnert K.R. 1971. Migmatites and the Origin of Granitic Rocks . Amsterdan, Elsevier, 405 pp.). 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 (1999Reis L.B. 1999. Estudos de mineralizações de grafita no extremo nordeste de Minas Gerais . Dissertação de Mestrado, Instituto de Geociências, Universidade Federal de Minas Gerais, Minas Gerais.), Teixeira (2002Teixeira L.R. 2002. Relatório Temático de Litogeoquímica. Projeto Extremo Sul da Bahia . 2002. Programa de Levantamentos Geológicos Básicos do Brasil. CPRM/CBPM.), Daconti (2004Daconti B.C. 2004. Contexto geológico, controle e correlação regional das mineralizações de grafita da região de Almenara, Província Grafítica do Nordeste de Minas Gerais Dissertação de Mestrado, Instituto de Geociências, Universidade Federal de Minas Gerais.), and Paes et al. (2010Paes V.J.C., Raposo F.O., Pinto C.P., Oliveira F.A.R. 2010Projeto Jequitinhonha, Estados de Minas Gerais e Bahia: texto explicativo. Geologia e Recursos Minerais das Folhas Comercinho, Jequitinhonha, Almenara, Itaobim, Joaíma e Rio do Prado. Programa Geologia do Brasil. Belo Horizonte, CPRM, 376 p.), 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 2002Teixeira L.R. 2002. Relatório Temático de Litogeoquímica. Projeto Extremo Sul da Bahia . 2002. Programa de Levantamentos Geológicos Básicos do Brasil. CPRM/CBPM.) are also plotted for comparison.

Regionally, the paragneiss samples show a wide range of Al₂O₃ (11.49 - 19.80%) reflecting the relative abundance of peraluminous silicates (biotite, cordierite, garnet and/or sillimanite), that is, the clay fraction in the protolith, as so the silica contents (57.82 - 75.93%) in relation to the abudance of sand (quartz + feldspars). K₂O (0.24 - 4.93%) compared to Na₂O (0.87 - 3.24%) and CaO (0.33 - 6.85%) contents, together with modal contents from thin sections (biotite up to 35%, plagioclase up to 30%, K-feldspar up to 10%), point to the predominance of the phyllosilicates and plagioclase over K-feldspar.

The bivariant diagrams for major and trace elements (Fig. 5 and 6) show decreasing trends of Al₂O₃, TiO₂ and MgO relative to SiO₂, and increasing MgO, TiO₂, Cr, and V relative to Al₂O₃, 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 Al₂O₃ reflect the original sedimentary composition. The decrease of K₂O and the corresponding increase of Al₂O₃ 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 Na₂O + CaO as K₂O 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. TiO₂ variation relative to silica suggests a similar interpretation, that is, most titanium is incorporated in the mud fraction, rather than in heavy minerals in the sand fraction.

Higher Na₂O + CaO and Sr + Ba in comparison to K₂O and Rb contents characterize most paragneiss located along 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 SiO₂ / Al₂O₃ and low K₂O / Na₂O ratios suggest sand-mud protoliths richer in plagioclase than in K-feldspar.

McLennan et al. (1990McLennan S.M., Taylor S.R., McCulloch M.T., Maynard J.B. 1990. Geochemical and Nd-Sr isotopic composition of deep-sea turbidites: Crustal evolution and plate tectonic associations. Geochimica et Cosmochimica Acta 54:2015-2050.) 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 1985Taylor S.R. , McLennan S.M. 1985. The Continental Crust: Its Composition and Evolution . Blackwell, Oxford. ).

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. 1990McLennan S.M., Taylor S.R., McCulloch M.T., Maynard J.B. 1990. Geochemical and Nd-Sr isotopic composition of deep-sea turbidites: Crustal evolution and plate tectonic associations. Geochimica et Cosmochimica Acta 54:2015-2050.). 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. 1990McLennan S.M., Taylor S.R., McCulloch M.T., Maynard J.B. 1990. Geochemical and Nd-Sr isotopic composition of deep-sea turbidites: Crustal evolution and plate tectonic associations. Geochimica et Cosmochimica Acta 54:2015-2050.), 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. 1984Grommet L.P., Dymek R.F., Haskin L.A., Korotev R.L. 1984. The "North American shale composite": Its composition, major and trace element characteristics. Geochimica et Cosmochimica Acta , 48:2469-2482.). 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. 2010Paes V.J.C., Raposo F.O., Pinto C.P., Oliveira F.A.R. 2010Projeto Jequitinhonha, Estados de Minas Gerais e Bahia: texto explicativo. Geologia e Recursos Minerais das Folhas Comercinho, Jequitinhonha, Almenara, Itaobim, Joaíma e Rio do Prado. Programa Geologia do Brasil. Belo Horizonte, CPRM, 376 p.; 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. (2011Gonçalves-Dias T., Pedrosa-Soares A.C., Dussin I.A., Alkmim F.F., Caxito F.A., Silva L.C., Noce C.M. 2011. Idade máxima de sedimentação e proveniência do Complexo Jequitinhonha na área-tipo (Orógeno Araçuaí): primeiros dados U-Pb (LA-ICP-MS) de grãos detríticos de zircão. Geonomos 19(2):121-130.). 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 2008Ludwig K.R. 2008. User's manual for Isoplot 3.6. A geochronological toolkit for Microsoft Excel. Berkeley Geochronologic Center, Special Publication No. 4, Berkeley, USA.).

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 ²⁰⁷Pb / ²⁰⁶Pb 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. 2011Gonçalves-Dias T., Pedrosa-Soares A.C., Dussin I.A., Alkmim F.F., Caxito F.A., Silva L.C., Noce C.M. 2011. Idade máxima de sedimentação e proveniência do Complexo Jequitinhonha na área-tipo (Orógeno Araçuaí): primeiros dados U-Pb (LA-ICP-MS) de grãos detríticos de zircão. Geonomos 19(2):121-130.), which yielded a ²⁰⁷Pb / ²⁰⁶Pb 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. 2011aPedrosa-Soares A.C. , Alkmim F.F. 2011. How many rifting events preceded the development of the Araçuaí-West Congo orogen? Geonomos 19(2): 244-251.). The paragneiss samples yield a very homogeneous Nd isotopic signature, with ¹⁴³Nd / ¹⁴⁴Nd 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 1986Faure G. 1986. Principles of Isotope Geology . 2nd Edition. New York, John Wiley & Sons, 589 p.).

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 1987Frost C.D. , Winston D. 1987. Nd isotopic systematic of coarse- and fine-grained sediments: examples from the Middle Proterozoic Belt Purcell Super Group. Journal of Geology , 95:309-327., Evans et al. 1991Evans J.A., Stone P., Floyd J.D. 1991.Isotopic characteristics of Ordovician greywacke provenance in the Southern Uplands of Scotland. : Morton A.C., Todd S.P., Haughton P.D.W. (eds.). Developments in sedimentary provenance studies . Geological Society Special Publication 57:161-172.).

Celino (1999Celino J.J. 1999. Variação composicional em suítes de granitóides neoproterozóicos e sua implicação na evolução do Orógeno Araçuaí (Brasil) - Oeste Congolês (África) Tese de Doutorado, Instituto de Geociências, Universidade de Brasília.) 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 (2004Daconti B.C. 2004. Contexto geológico, controle e correlação regional das mineralizações de grafita da região de Almenara, Província Grafítica do Nordeste de Minas Gerais Dissertação de Mestrado, Instituto de Geociências, Universidade Federal de Minas Gerais.) 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).

DISCUSSION

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. 2000Teixeira W., Sabaté P., Barbosa J.S.F., Noce C.M., Carneiro M.A. 2000. Archean and Paleoproterozoic Tectonic evolution of the São Francisco Craton, Brazil. Cordani U.G., Milani E.J., Thomas Filho A., Campos D.A. (eds.). Tectonic Evolution of the South America International Geological Congress, 31, Rio de Janeiro, Brazil, p. 101-137., Silva et al. 2002Silva L.C., Armstrong R., Noce C.M., Carneiro M., Pimentel M.M., Pedrosa-Soares A.C., Leite C., Vieira V.S., Silva M., Paes V., Cardoso-Filho J. 2002. Reavaliação da evolução geológica em terrenos pré-cambrianos brasileiros com base em novos dados U-Pb SHRIMP, parte II: Orógeno Araçuaí, Cinturão Móvel Mineiro e Cráton São Francisco Meridional. Revista Brasileira de Geociências , 32:513-528., Barbosa & Sabaté 2004Barbosa J.S.F., Sabaté P. 2004. Archean and Paleoproterozoic crust of the São Francisco Craton, Bahia, Brazil: geodynamic features. Precambrian Research , 133:1-27., Noce et al. 2007Noce C.M., Pedrosa-Soares A.C., Silva L.C., Alkmim F.F. 2007. O Embasamento Arqueano e Paleoproterozóico do Orógeno Araçuaí. Geonomos , 15(1):17-23.). 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. 2009Danderfer A., de Waele B., Pedreira A. J., Nalini Jr. H.A. 2009. New geochronological constraints on the geological evolution of Espinhaço basin within the São Francisco craton - Brazil. Precambrian Research , 170:116-128., Pedrosa-Soares & Alkmim 2011Pedrosa-Soares A.C. , Alkmim F.F. 2011. How many rifting events preceded the development of the Araçuaí-West Congo orogen? Geonomos 19(2): 244-251., Chemale et al. 2012)Chemale Jr. F., Dussin I.A., Alkmim F.F., Martins M.S., Queiroga G., Armstrong R., Santos M.N. 2012. Unravelling a Proterozoic basin history through detrital zircon geochronology: The case of the Espinhaço Supergroup, Minas Gerais, BrazilGondwana Research22(1):200-206. . 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. 2001Tack L., Wingate M.T.D., Liégeois J.P., Fernandez-Alonso M., Deblond A. 2001. Early Neoproterozoic magmatism (1000-910 Ma) of the Zadinian and Mayumbian groups (Bas-Congo): Onset of Rodinian rifting at the western edge of the Congo craton. Precambrian Research , 110:277-306., Silva et al. 2008Pedrosa-Soares A.C., Alkmim F.F., Tack L., Noce C.M., Babinski M., Silva L.C., Martins-Neto M.A. 2008. Similarities and differences between the Brazilian and African counterparts of Neoproterozoic Araçuaí-West Congo orogen. : Pankhrust R., Trouw R., Brito-Neves B.B., Wit M. (eds). The Gondwana Peleocontinent in the South Atlantic Region Geological Society of London, Special Publications, 294:153-172., Pedrosa-Soares & Alkmim 2011)Pedrosa-Soares A.C. , Alkmim F.F. 2011. How many rifting events preceded the development of the Araçuaí-West Congo orogen? Geonomos 19(2): 244-251..

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. 2004Noce C.M, Pedrosa-Soares A.C., Piuzana D., Armstrong R., Laux J.H., Campos C.M., Medeiros S.R. 2004. Ages of sedimentation of the kinzigitic complex and of a late orogenic thermal episode in the Araçuaí orogen, northern Espírito Santo State, Brazil: Zircon and monazite U-Pb SHRIMP and ID-TIMS data. Revista Brasileira de Geociências , 349:587-592., Pedrosa-Soares et al. 2000Pedrosa-Soares A.C., Cordani U.G., Nutman A. 2000. Constraining the age of Neo-proterozoic glaciation in eastern Brazil: first U-Pb (SHRIMP) data of detrital zircons. Revista Brasileira de Geociências , 30:58-61., 2011aPedrosa-Soares A.C., Campos C.P., Noce C., Silva L.C., Novo T., Roncato J., Medeiros S., Castañeda C., Queiroga G., Dantas E., Dussin I., Alkmim F.F. 2011a. Late Neoproterozoic-Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Sial A.N., Bettencourt J.S., De Campos C.P., Ferreira V.P. (eds). Granite-Related Ore Deposits . Geological Society, London, Special Publications, 350:25-51. , Gonçalves-Dias et al. 2011Gonçalves-Dias T., Pedrosa-Soares A.C., Dussin I.A., Alkmim F.F., Caxito F.A., Silva L.C., Noce C.M. 2011. Idade máxima de sedimentação e proveniência do Complexo Jequitinhonha na área-tipo (Orógeno Araçuaí): primeiros dados U-Pb (LA-ICP-MS) de grãos detríticos de zircão. Geonomos 19(2):121-130., Novo 2013Novo T.A. 2013. Caracterização do Complexo Pocrane, magmatismo básico mesoproterozóico e unidades neoproterozóicas do sistema Araçuaí-Ribeira, com ênfase em geocronologia U-Pb (SHRIMP e LA-ICP-MS) . Phd Thesis, Universidade Federal de Minas Gerais, Belo Horizonte, 193 p., Gradim et al. 2014Gradim C., Roncato J., Pedrosa-Soares A.C., Cordani U.G., Dussin I.A., Alkmim F.F., Queiroga G., Jacobsohn T., Silva L.C., Babinski M. 2014. The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation. Brazilian Journal of Geology , 44(1):155-180., Peixoto et al. 2015Peixoto E.N., Pedrosa-Soares A.C., Alkmim F.F., Dussin I.A. 2015. A suture-related accretionary wedge formed in the Neoproterozoic Araçuaí orogen (SE Brazil) during Western Gondwanaland assembly. Gondwana Research 27:878-896., Kuchenbecker et al. 2015Kuchenbecker M., Pedrosa-Soares A.C., Babinski M., Fanning M. 2015. Detrital zircon age patterns and provenance assessment for pre-glacial to post-glacial successions of the Neoproterozoic Macaúbas Group, Araçuaí orogen, Brazil. Precambrian Research , 266:12-26.).

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. 1987Siga Jr. O., Cordani U.G., Basei M.A.S., Teixeira W., Kawashita K., Van Schmus W.R. 1987. Contribuição ao estudo geológico e geocronológico da porção nordeste de Minas Gerais. : 48º Simpósio de Geologia de Minas Gerais. SBG-MG, Boletim , 7:29-44., Pedrosa-Soares et al. 2011aPedrosa-Soares A.C. , Alkmim F.F. 2011. How many rifting events preceded the development of the Araçuaí-West Congo orogen? Geonomos 19(2): 244-251., Silva et al. 2011Silva L.C., Pedrosa-Soares A.C., Armstrong R., Noce C.M. 2011. Determinando a duração do período colisional do Orógeno Araçuaí com base em geocronologia U-Pb de alta resolução em zircão: uma contribuição para a história da amalgamação do Gondwana Ocidental. Geonomos 19(2):180-197., Gradim et al. 2014Gradim C., Roncato J., Pedrosa-Soares A.C., Cordani U.G., Dussin I.A., Alkmim F.F., Queiroga G., Jacobsohn T., Silva L.C., Babinski M. 2014. The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation. Brazilian Journal of Geology , 44(1):155-180.).

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. 1996Teixeira W., Carneiro M.A., Noce C.M., Machado N., Sato K., Taylor P.N. 1996. Pb, Sr and Nd isotope constraints on the Archaean evolution of gneissic-granitoid complexes in the southern São Francisco Craton, Brazil. Precambrian Research 78:151-164., Noce et al. 2000Noce C.M., Macambira M.B., Pedrosa-Soares A.C. 2000. Chronology of Neoproterozoic-Cambrian granitic magmatism in the Araçuaí Belt, Eastern Brazil, based on single zircon evaporation dating. Revista Brasileira de Geociências , 30:25-29., Tack et al. 2001Tack L., Wingate M.T.D., Liégeois J.P., Fernandez-Alonso M., Deblond A. 2001. Early Neoproterozoic magmatism (1000-910 Ma) of the Zadinian and Mayumbian groups (Bas-Congo): Onset of Rodinian rifting at the western edge of the Congo craton. Precambrian Research , 110:277-306.). 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. 2012Babinski M., Pedrosa-Soares A.C., Trindade R.I.F. , Martins M., Noce C.M., Liu D. 2012. Neoproterozoic glacial deposits from the Araçuaí orogen, Brazil: Age, provenance and correlations with the São Francisco craton and West Congo belt. Gondwana Research 21(2-3):451-465. ). 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, medium- to 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. 2011aPedrosa-Soares A.C. , Alkmim F.F. 2011. How many rifting events preceded the development of the Araçuaí-West Congo orogen? Geonomos 19(2): 244-251.). This is the case of the Rio Doce Group (Nalini et al. 2000Nalini Jr. H.A., Bilal E., Neves J.M.C. 2000. Syn-collisional peraluminous magmatism in the Rio Doce region: mineralogy, geochemistry, and isotopic data of the Neoproterozoic Urucum Suite (eastern Minas Gerais State, Brazil). Revista Brasileira de Geociências , 30:120-125., Pedrosa-Soares et al. 2001Pedrosa-Soares A.C., Noce C.M., Wiedemann C.M., Pinto C.P. 2001. The Araçuaí-West Congo orogen in Brazil: An overview of a confined orogen formed during Gondwanland assembly. Precambrian Research , 110:307-323., 2011bPedrosa-Soares A.C., Babinski M., Noce C., Martins M., Queiroga G., Vilela F. 2011b. The Neoproterozoic Macaúbas Group (Araçuaí orogen, SE Brazil) with emphasis on the diamictite formations. : Arnaud E., Halverson G., Shields G. (eds). The Geological Record of Neoproterozoic Glaciations . Geological Society of London, Memoir 36, chapter 49., Martins et al. 2004Martins V.T.S., Teixeira W., Noce C.M., Pedrosa-Soares A.C. 2004. Sr and Nd characteristics of Brasiliano-Pan African granitoid plutons of the Araçuaí orogen, southeastern Brazil: Tectonic implications. Gondwana Research , 7:75-89., Vieira 2007Vieira V.S. 2007. Significado do Grupo Rio Doce no Contexto do Orógeno Araçuaí Tese de Doutorado, Instituto de Geociências, Universidade Federal de Minas Gerais, Belo Horizonte., Paes et al. 2010Paes V.J.C., Raposo F.O., Pinto C.P., Oliveira F.A.R. 2010Projeto Jequitinhonha, Estados de Minas Gerais e Bahia: texto explicativo. Geologia e Recursos Minerais das Folhas Comercinho, Jequitinhonha, Almenara, Itaobim, Joaíma e Rio do Prado. Programa Geologia do Brasil. Belo Horizonte, CPRM, 376 p., Silva et al. 2011Silva L.C., Pedrosa-Soares A.C., Armstrong R., Noce C.M. 2011. Determinando a duração do período colisional do Orógeno Araçuaí com base em geocronologia U-Pb de alta resolução em zircão: uma contribuição para a história da amalgamação do Gondwana Ocidental. Geonomos 19(2):180-197., Novo 2013Novo T.A. 2013. Caracterização do Complexo Pocrane, magmatismo básico mesoproterozóico e unidades neoproterozóicas do sistema Araçuaí-Ribeira, com ênfase em geocronologia U-Pb (SHRIMP e LA-ICP-MS) . Phd Thesis, Universidade Federal de Minas Gerais, Belo Horizonte, 193 p.) and the Nova Venécia Complex, composed of peraluminous paragneiss with intercalations of calcsilicate rocks (Noce et al. 2004Noce C.M, Pedrosa-Soares A.C., Piuzana D., Armstrong R., Laux J.H., Campos C.M., Medeiros S.R. 2004. Ages of sedimentation of the kinzigitic complex and of a late orogenic thermal episode in the Araçuaí orogen, northern Espírito Santo State, Brazil: Zircon and monazite U-Pb SHRIMP and ID-TIMS data. Revista Brasileira de Geociências , 349:587-592., Pedrosa-Soares et al. 2008Pedrosa-Soares A.C., Alkmim F.F., Tack L., Noce C.M., Babinski M., Silva L.C., Martins-Neto M.A. 2008. Similarities and differences between the Brazilian and African counterparts of Neoproterozoic Araçuaí-West Congo orogen. : Pankhrust R., Trouw R., Brito-Neves B.B., Wit M. (eds). The Gondwana Peleocontinent in the South Atlantic Region Geological Society of London, Special Publications, 294:153-172., Gradim et al. 2014Gradim C., Roncato J., Pedrosa-Soares A.C., Cordani U.G., Dussin I.A., Alkmim F.F., Queiroga G., Jacobsohn T., Silva L.C., Babinski M. 2014. The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation. Brazilian Journal of Geology , 44(1):155-180.). 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 syn-collisional metamorphism; Pedrosa Soares et al. 2011aPedrosa-Soares A.C., Campos C.P., Noce C., Silva L.C., Novo T., Roncato J., Medeiros S., Castañeda C., Queiroga G., Dantas E., Dussin I., Alkmim F.F. 2011a. Late Neoproterozoic-Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Sial A.N., Bettencourt J.S., De Campos C.P., Ferreira V.P. (eds). Granite-Related Ore Deposits . Geological Society, London, Special Publications, 350:25-51. ). 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. 2004Noce C.M, Pedrosa-Soares A.C., Piuzana D., Armstrong R., Laux J.H., Campos C.M., Medeiros S.R. 2004. Ages of sedimentation of the kinzigitic complex and of a late orogenic thermal episode in the Araçuaí orogen, northern Espírito Santo State, Brazil: Zircon and monazite U-Pb SHRIMP and ID-TIMS data. Revista Brasileira de Geociências , 349:587-592.), 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. 2011bPedrosa-Soares A.C., Babinski M., Noce C., Martins M., Queiroga G., Vilela F. 2011b. The Neoproterozoic Macaúbas Group (Araçuaí orogen, SE Brazil) with emphasis on the diamictite formations. : Arnaud E., Halverson G., Shields G. (eds). The Geological Record of Neoproterozoic Glaciations . Geological Society of London, Memoir 36, chapter 49., Babinski et al. 2012Babinski M., Pedrosa-Soares A.C., Trindade R.I.F. , Martins M., Noce C.M., Liu D. 2012. Neoproterozoic glacial deposits from the Araçuaí orogen, Brazil: Age, provenance and correlations with the São Francisco craton and West Congo belt. Gondwana Research 21(2-3):451-465. , Kuchenbecker et al. 2015Kuchenbecker M., Pedrosa-Soares A.C., Babinski M., Fanning M. 2015. Detrital zircon age patterns and provenance assessment for pre-glacial to post-glacial successions of the Neoproterozoic Macaúbas Group, Araçuaí orogen, Brazil. Precambrian Research , 266:12-26.) 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. 2003Cordani U.G., Brito Neves B.B., D'Agrella M.S., Trindade R.I.F. 2003. Tearing-up Rodinia: the Neoproterozoic paleogeography of South American cratonic fragments. Terra Nova , 15:343-349., D'Agrella et al. 2004D'Agrella-Filho M.S., Pacca I.G., Trindade R.I.F., Teixeira W., Raposo M.I.B., Onstott T.C. 2004. Paleomagnetism and 40Ar/39Ar ages of mafic dykes from Salvador (Brazil): new constraints on the São Francisco craton APW path between 1080 and 1010 Ma. Precambrian Research , 132:55-77., Li et al. 2008Li Z.X., Bogdanova S.V., Collins A.S., Davidson A., de Waele B., Ernst R.E., Fitzsimons I.C.W., Fuck R.A., Gladkochub D.P., Jacobs J., Karlstrom K.E., Lu S., Natapov L.M., Pease V., Pisarevsky S.A., Thrane K., Vernikovsky V. 2008. Assembly, configuration, and break-up history of Rodinia: A synthesis. Precambrian Research 160:179-210.).

CONCLUSIONS

ACKNOWLEDGEMENTS

Our gratitude to the Brazilian scientific research agencies (CAPES, CNPq, FAPEMIG) and the Geological Survey of Brazil (CPRM) for the financial support. Tatiana Gonçalves Dias and Fabrício Caxito were supported by the ELAP (Emerging Leaders in the Americas Program) of the Canadian Bureau for International Education, and the Merit Scholarship Program of the Ministère de l'Education, du Loisir et du Sports du Québec (MELS), during their stay at the GEOTOP/UQAM, Montréal, Canada. The original manuscript was greatly improved after comments and suggestions by S.O. Verdecchia, R.J. Pankhurst and an anonymous reviewer.

REFERENCES

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