Records of Mesoproterozoic taphrogenic events in the eastern basement of the Araçuaí Orogen, southeast Brazil

Registros de eventos tafrogênicos mesoproterozoicos no embasamento do Orógeno Araçuaí, sudeste do Brasil

ABSTRACT:

The history of palaeocontinents alternates long fragmentation to drift periods with relatively short agglutination intervals. One of the products of a Rhyacian-Orosirian orogeny was a palaeocontinent that brought together the basement of the Araçuaí-West Congo orogen (AWCO) with regions now located in the São Francisco and Congo cratons. From ca. 2 Ga to ca. 0.7 Ga, this large region of the São Francisco-Congo palaeocontinent was spared of orogenic events, but underwent at least five taphrogenic events recorded by anorogenic magmatism and/or sedimentation. The taphrogenic events are well documented in the AWCO proximal portions and neighboring cratonic regions, but lack evidence in the AWCO high-grade core. Our studies on amphibolites intercalated in the Rhyacian Pocrane complex, basement of the Rio Doce magmatic arc, allowed to the recognition of two Mesoproterozoic taphrogenic episodes. The oldest one, a Calymmian episode, is recorded by amphibolites with a zircon magmatic crystallization age at 1529 ± 37 Ma (U-Pb SHRIMP), and lithochemical signature of basaltic magmatism related to continental intraplate settings. Another set of amphibolite bodies records the youngest taphrogenic episode, a Stenian event, with a zircon magmatic crystallization age at 1096 ± 20 Ma (U-Pb SHRIMP), and lithochemical signature similar to mature magmatism of continental rift setting. The Calymmian episode (ca. 1.5 Ga) correlates to the Espinhaço II basin stage and mafic dikes of the northern Espinhaço, Chapada Diamantina and Curaçá domains, while the Stenian episode (ca. 1.1 Ga) correlates to the Espinhaço III basin stage. We also present U-Pb data for 87 detrital zircon grains from a quartzite lens intercalated in the Pocrane complex, the Córrego Ubá quartzite. Its age spectrum shows main peaks at 1176 ± 21 Ma (35%), 1371 ± 30 Ma (18%), 1536 ± 22 Ma (19%), 1803 ± 36 Ma (17%) and 1977 ± 38 Ma (12%), suggesting a Stenian (ca. 1176 Ma) maximum depositional age (although only one zircon with low discordance shows an age of 955 ± 66 Ma). Comparing with data from the western sector of the Araçuaí orogen and São Francisco craton, it is noteworthy that no igneous zircon from the three samples yielded an age older than early Orosirian (~2.05 Ga), showing age spectra essentially limited in the range of ca. 1-2 Ga; i.e., younger than the Late Rhyacian orogeny that amalgamated the basement, and older than the main anorogenic event (930-870 Ma) associated with the Early Tonian precursor basin of AWCO. All together, these continental taphrogenic events testify the several unsuccessful fragmentation attempts that affected the long-lived São Francisco-Congo palaeocontinent, which remained preserved from a complete break-up associated with ocean spreading from the Early Orosirian to the Atlantic opening in Cretaceous time.

KEYWORDS:
Taphrogenic events; São Francisco-Congo palaeocontinent; Araçuaí-West Congo orogeny

RESUMO:

A história dos paleocontinentes resume-se em longos períodos de fragmentação e pequenos intervalos de aglutinação. Um dos produtos de uma orogenia riaciana-orosiriana foi o paleocontinente São Francisco-Congo, representado nos crátons homônimos e no embasamento do Orógeno Araçuaí-Congo Ocidental (AWCO). Durante o período de ca. 2 Ga até ca. 0,7 Ga, aquela extensa região paleocontinental foi poupada de eventos orogênicos, mas por outro lado foi submetida a pelo menos cinco eventos tafrogênicos registrados por magmatismo anorogênico e sedimentação. Tais eventos tafrogênicos estão bem registrados nas zonas proximais do AWCO e regiões cratônicas adjacentes, mas suas evidências no núcleo de alto grau do AWCO são muito mais raras. Nossos estudos sobre lentes anfibolíticas intercaladas em ortognaisses do Complexo Pocrane, embasamento do Arco Magmático Rio Doce, permitiram reconhecer dois eventos tafrogênicos mesoproterozoicos distintos. O mais antigo deles, do Caliminiano, é caracterizado por anfibolitos com zircões cristalizados em 1529 ± 37 Ma (U-Pb SHRIMP). Outro conjunto de anfibolitos registra episódio tafrogênico mais jovem, do Esteniano, com zircões de idade magmática em 1096 ± 20 Ma (U-Pb SHRIMP). O episódio calimiano (ca. 1,5 Ga) é correlato com a bacia Espinhaço II e diques máficos dos domínios Espinhaço, Chapada Diamantina e Curaçá, enquanto o episódio esteniano (ca. 1,1 Ga) se correlaciona com a bacia Espinhaço III. Dados U-Pb de 87 grãos detríticos de zircão coletados em lentes do Quartzito Córrego Ubá, intercaladas no Complexo Pocrane, mostram um espectro de idades com picos em 1176 ± 21 Ma (35%), 1371 ± 30 Ma (18%), 1536 ± 22 Ma (19%), 1803 ± 36 Ma (17%) e 1977 ± 38 Ma (12%). Apesar de se ter encontrado um zircão com idade de 955 ± 66 Ma, o tratamento estatístico dos dados sugere a idade máxima de deposição em ca. 1176 Ma. O espectro de idades mostra-se essencialmente limitado no intervalo 1-2 Ga, contrastando com os dados do setor oeste do AWCO e do Cráton São Francisco, onde são comuns zircões do Toniano ou mais velhos que 2 Ga. Isso sugere uma bacia alimentada por fontes relativamente restritas. Os dois eventos tafrogênicos mesoproterozoicos aqui reportados evidenciam duas das tentativas de fragmentação mal sucedidas que incidiram sob o Paleocontinente São Francisco-Congo, o qual resistiu à completa fragmentação desde ca. 2 Ga até a abertura do Oceano Atlântico no Cretáceo.

PALAVRAS-CHAVE:
Eventos tafrogênicos; Paleocontinente São Francisco-Congo; Orógeno Araçuaí-Congo Ocidental

INTRODUCTION

The history of palaeocontinents includes short compressive orogenic processes, intercalated with large taphrogenic phases marked by fragmentation and drift (Nance et al. 1988, Murphy & Nance 1992, Rogers & Santosh 2004). In this context, a new-formed continent becomes a refractory cap that hinders heat release from the Earth’s mantle. The heat transmission is associated to magmas, that tend to settle beneath rift systems. The reconstruction of the rifting events is easily detected and characterized when it comes from undeformed domains. However, reconstruction becomes much more complex in orogenetic reworked cores, in which rocks associated with taphrogenic events are intensely deformed. A fundamental tool for the analysis of tectonized magmatism and/or sedimentation is U-Pb dating on zircon grains. Besides the magmatic and metamorphic age, the analyses of the zircon age spectra of sedimentary deposits may reflect the tectonic setting of the basin, providing important hints for tectonic and palaeogeographic interpretations (Cawood et al. 2012).

The study area, located in the Araçuaí Orogen, southeastern Brazil (Fig. 1), records a series of events related to the amalgamation of the São Francisco-Congo palaeocontinent in the Rhyacian-Orosirian boundary (Noce et al. 2007), as well as the evolution of the Araçuaí Orogen in Neoproterozoic time (Pedrosa-Soares et al. 2001, 2008; Alkmim et al. 2006). It lacks evidence that the region had experienced orogenic events from the Orosirian to the Ediacaran. Conversely, during that long period, the region between the São Francisco and Congo cratons records at least five major taphrogenic events which ultimately led to the development of the precursor basin system of the Araçuaí Orogen (Pedrosa-Soares & Alkmim 2011, Chemale Jr. et al. 2012, Guadagnin et al. 2015, Tupinambá et al. 2007). Although these anorogenic events are well documented in the proximal zone of the Araçuaí Orogen and in the São Francisco Craton, no solid evidence from them were previously found in the high-grade core of the Araçuaí Orogen.

Figure 1:
Tectonic situation of the São Francisco Craton and the neighbor orogens. Ages shown in the map are from: 1) Silveira et al. 2013; 2) Danderfer et al. 2009; 3) Evans et al. 2015; 4) Lobato et al. 2015; 5) Evans et al. 2015; 6) Menezes et al. 2012; 7) Silva et al. 2002; 8) Gonçalves-Dias et al. 2016; 9) Kuchenbecker et al. 2015b; 10) Souza 2016; 11) Castro et al. 2015; 12) Dussin 2016; 13) Kuchenbecker et al. 2015a; 14) Chemale Jr. et al. 2012; 15) Peixoto et al. 2015; 16) Dussin & Chemale Jr. 2012; 17) Silveira 2016; 18) Rolim et al. 2016; 19) Barrote 2016; 20) This Work; 21) This Work; 22) This Work; 23) Angeli et al. 2004; 24) Chaves 2001.

This paper focuses on metamafic rocks (amphibolites) and the Córrego Ubá quartzite, spatially associated with the Rhyacian Pocrane complex, located in the high-grade core of the Araçuaí Orogen (Fig. 1). Our field, petrographic, lithochemical and geochronological (zircon U-Pb SHRIMP and LA-ICP-MS) studies provide evidence of two distinct Mesoproterozoic taphrogenic events on the Rhyacian basement of the Araçuaí Orogen high-grade core. Based on a thorough compilation of the literature, we correlate these data with the main unsuccessful taphrogenic events which affected the São Francisco-Congo palaeocontinent before the evolution of the Araçuaí Orogen.

GEOLOGICAL SETTING

The Neoproterozoic Araçuaí Orogen together with its counterpart located in Africa, the West Congo Belt, evolved during the Brasiliano cycle, one of the tectonic cycles that led to West Gondwana amalgamation.

The focused area covers part of the basement of the high-grade core of the Araçuaí Orogen (Fig. 1), where the Ediacaran Rio Doce magmatic arc is an outstanding feature and metamorphic peaks ranges from the amphibolite to granulite facies (Novo et al. 2010; Tedeschi et al. 2015; Gonçalves et al. 2015).

The basement includes gneissic-migmatitic complexes in the amphibolite (e.g., the Pocrane Complex; Novo 2013) to granulite facies (e.g., the Juiz de Fora Complex; Noce et al. 2007). Those complexes formed in response to convergence related to an orogenic Rhyacian-Orosirian event that amalgamated the São Francisco-Congo palaeocontinent (Noce et al. 2007; Heilbron et al. 2010; Silva et al. 2016). After the basement consolidation, the region seems to have experienced a long period without any event of compressive nature (Pedrosa-Soares et al. 2001, 2008, 2011; Pedrosa-Soares & Alkmim 2011), until the onset of the Brasiliano orogeny in Early Ediacaran (Novo et al. 2010; Pedrosa-Soares et al. 2011; Tedeschi et al. 2015).

Pocrane Complex

The Pocrane Complex is mainly composed of tonalite-trondhjemite-granodiorite (TTG) orthogneiss with migmatitic portions. Stricto sensu amphibolite lenses (Fig. 2) usually occur interlayered in this gneiss (Silva et al. 1987; Novo 2013), as sills or tectonic slices. Previous reports on these metamafic rocks (Paes 1999, Silva et al. 2002) point out the variability of their genesis. In the available geological maps (Tuller 2000; Féboli & Paes 2000; Oliveira 2000), lenses and layers of ultramafic and metasedimentary rocks (e.g., calcissilicatic rock, mica schist, paragneiss, quartzites, marble and paraconglomerate) were described as intercalations in the Pocrane Complex orthogneiss. The structural pattern of the metasedimentary layers delineates a large fold with a plunging axis in the NNE-SSW direction and hinge concavity facing SSW. At first, the available geological data suggests that the lithological set (orthogneiss, metamafic and metasedimentary layers) should be a bundle of interpolations of igneous and sedimentary origins, simultaneously folded as a deformed and metamorphosed volcano-sedimentary sequence. However, the new data presented in this paper refuses this theory (of one single volcano-sedimentary association), showing that at least part of the metamafic rocks and the sedimentary strata is younger than the orthogneiss.

Figure 2:
Geological map of the Pocrane Complex and the Córrego Ubá Cross Section showing the field aspects of the Córrego Ubá quartzite and amphibolite lenses. Yellow star marks the sample site location.

Córrego Ubá quartzite

The Córrego Ubá quartzite occurs as a tectonic slice interlayered in the Pocrane Complex gneiss (Ubá Creek Cross Section in Fig. 2). It is a fine-grained and recrystallized quartzite that shows three foliation sets, whose main one (S_n) is marked by biotite and muscovite, north-south trend and moderate deep to east. Near tectonic contacts, the foliation develops a mylonitic texture. The biotite enrichment confers a strongly folded and crenulated gneiss banding marking S_n+1. A brittle deformation crosscuts all the previous structures materializing S_n+2. The quartzite is mainly composed by quartz with granoblastic polygonal texture and accessory biotite, muscovite, plagioclase, titanite and zircon.

Amphibolite lenses

The amphibolites we focus on outcrop as thick lenses (Fig. 2) with low lateral continuity (not more than six meters) interlayered with the Pocrane Complex gneiss. They show fine-grained and compact aspect, though foliation is visible with a magnifying glass and in thin section, or in partially weathered outcrops. The amphibolite consists of hornblende, plagioclase, biotite, quartz, chlorite, apatite, titanite, zircon and opaque minerals (Fig. 3). Biotite and chlorite are formed by hornblende alterations; biotite may be more abundant than hornblende. The foliation (Sn) of amphibolites is anastomosed, developed from a progressive deformational phase. Sometimes late ductile shear zones obliterate the foliation. Amphibole crystals and biotite palettes materialize the foliation, and the stretched amphibole crystals commonly form ocellar porphyroclasts, showing pressure shadows and recrystallization tails. Quartz and plagioclase occur as stretched crystals, parallel to the foliation, and may display dynamic recrystallization with subgrains individualization.

Figure 3:
Amphibolite microscopic aspects: Sn and Sn+1 foliation outlined by hornblende and biotite, which envelops hornblende porphyroclast.

AMPHIBOLITE LITHOCHEMISTRY

Analytical procedures

The whole-rock lithochemical analyses were conducted on three samples from distinct amphibolite lenses: TN-8A, TN-8B and P2A (Fig. 2). Sample powders were analyzed by Geosol Laboratories, in Belo Horizonte, Brazil. Major and trace element contents were determined using inductively coupled plasma optical emission spectroscopy (ICP-OS/MS) and inductively coupled plasma mass spectrometry (ICP-MS), respectively. Detection limits are 0.01% for oxides and 0.1 ppm for most trace elements, reaching values up to 0.01 ppm for Heavy Rare Earth Elements (HREE), such as Tb, Tm and Lu. The lithochemical analyses are available in the Supplementary data file labeled as “Lithochemical_Data”.

Results

Whole-rock chemical analyses were performed for major and trace elements of three samples of amphibolite lenses. Although the data are not sufficient for a statistical analysis, the analytical results provide a preliminary approach.

Classification diagrams for the magmatic protoliths of amphibolites indicate tholeiitic gabbro and sub-alkaline compositions (Figs. 4A and 4B). The Rare Earth Elements (REE) patterns distribution shows that samples TN-8A and TN-8B are markedly different from sample P2A (Fig. 4C). Sample P2A is low fractionated and very little depleted in Light Rare Earth Elements (LREE) [(La/Yb)n = 0.63], meanwhile Samples TN-8A and TN-8B are, on the whole, enriched in REE [(La/Yb)n = 2.91 - 7.39]; they show remarkable fractionation, with enrichment of LREE in relation to HREE.

Figure 4:
Classification of amphibolite samples in the diagrams: (A) TAS (Wilson 1989 and Xinhua et al. 2000) and alkaline and sub-alkaline diagram (Irvine & Baragar 1971 apud Rollinson 1993; (B) AFM diagram (Jensen, 1976); (C) Chondrite-normalized REE patterns (Taylor & McLennan 1985) in comparison to some general patterns - Normalized to Morb-N (Hofmann 1988), Continental Crust and Superior Crust (Weaver & Tarney 1984) and Ocean Island Basalt (OIB; Sun, 1982);

The lithochemistry presented here reflects the different amphibolites under study. Despite their field and petrographic similarities, their ages are different, as shown ahead.

GEOCHRONOLOGY

Two samples of amphibolites lenses were prepared and analyzed for U-Pb isotopes in the Centro de Pesquisas Geocronológicas (CPGeo), São Paulo University, Brazil, by using the sensitive high-resolution ion microprobe (SHRIMP IIe) equipment. One sample from Córrego Ubá quartzite was prepared and analyzed for U-Pb isotopes by using the laser ablasion multicollector inductively coupled plasma mass spectrometry (LAMC-ICP-MS) equipment in the CPGeo, São Paulo University, Brazil.

Material and Methods

About 20 to 40 kg of each rock sample, as for quartzite and amphibolite, were prepared for analyses in laboratories of São Paulo University, Brazil. Zircon grains were separated using conventional methods (crushing, grinding, gravimetric and magnetic-Frantz isodynamic separator) and handpicked under binocular microscope. For geochronological analysis of the magmatic rocks and their metamorphic equivalents, zircon crystals from the least magnetic fractions were selected. After mounted in epoxy disks and polished to expose grain centers, backscattered electron (BSE) and cathodoluminescence (CL) images revealed morphological features and internal structures of zircon grains. No analytical spot was performed on grain areas with inclusions, fractures and/or metamict features. Temora (417 Ma; Black et al. 2003) standard zircon was used in SHRIMP and M127 (Klötzli et al. 2009) and Plešovice (Sláma et al. 2008) standard zircons were used in LAMC-ICP-MS analytical routines. In this study, the spot size in SHRIMP analyses had 30 and 25 µm - LAMC-ICP-MS. Data reduction used the SQUID software (Ludwig 2003) for the SHRIMP data, and Glitter software (Van Achterbergh et al. 2001) and the Excel sheet developed by Ludwig (2003) for the LAMC-ICPMS data. Data evaluation for each spot took into account the common Pb contents, errors of isotopic ratios, percentages of discordance and Th/U ratios. From the selected spots, only those with discordance of lesser than 10% were used to age calculations and plotted in histograms and Concordia diagrams. The Concordia diagram and histograms were obtained with the Isoplot/Ex software (Ludwig 2003).

Only zircon crystals from the least magnetic fractions were selected from samples P2A and TN-8A (amphibolite lenses that occur interlayered in the Pocrane Complex orthogneiss). For the Córrego Ubá quartzite, a number up to 120 detrital grains were randomly picked. Grains were mounted in epoxy disks and polished to expose their centers. Morphological features and internal structures of zircon grains were revealed by BSE and CL images. U-Pb (SHRIMP and LAMC-ICP-MS) analysis was performed on zircon crystals recovered from the amphibolite lenses (samples P2A and TN-8A) in order to obtain igneous crystallization and metamorphic ages. Analysis on detrital zircon grains from sample P2D (Córrego Ubá quartzite) was performed by using U-Pb (LAMC-ICP-MS) in order to determine maximum depositional ages and sedimentary provenance. The new U-Pb analyses are available in the Supplementary data file labeled as U_Pb_data.

U-Pb SHRIMP results for the amphibolite lenses (samples TN-8A and P2A)

Two amphibolite lenses were collected for geochronological U-Pb analyses: TN-8A (UTM24S: 206564E/7856556S WGS84) and P2A (UTM24S: 219363E/7841825S WGS84) (Fig. 2).

• Sample TN-8A: amphibolite occurring as a decimeter-thick lens, interposed in the Pocrane Complex orthogneiss. It features as a fine-grained and massive rock, composed of hornblende and plagioclase, quartz, biotite, apatite, titanite and zircon as accessory minerals. Amphibole and biotite mark the regional foliation (Sn). Twenty-four zircon grains from sample TN-8A were analyzed by U-Pb SHRIMP. Cathodoluminescence images show two different families of zircon grains (Fig. 5A, 5B and 5C). The first one was formed by 2:1 and 3:1 ratio crystals, with preserved igneous features (e.g., 6.1 and 3.1 grains). The second family shows a prismatic proportion of 2:1 grains, with incipient to advanced metamict processes (e.g., 4.1 and 11.1 grains). Grains of the two families present a thin metamorphic overgrowth with high luminescence, which could not be dated due to the small length of its metamorphic edge. Fifteen spots with better analytical consistency result in an upper intercept age of 1,529 ± 37 Ma (MSWD = 0.25), interpreted as igneous crystallization age of TN-8A amphibolite igneous protolith (Fig. 5C). The amphibolite metamorphic age was calculated using the data from five spots with good analytical consistency, which resulted in a concordant age of 627 ± 20 Ma (MSWD = 1.6; Fig. 5A). The magmatic crystallization age and metamorphic age presented here for the TN-8A amphibolite are equal (considering the analytical errors) to the values obtained in another amphibolite lens in the nearby area (Bananal site) by Silva et al. (2002);

• Sample P2A: meter-thick boudin of amphibolite interleaved in the Pocrane Complex orthogneiss. It features a fine-grained and massive rock, but with quite a distinct foliation. It consists of amphibole, plagioclase and quartz; apatite, titanite and zircon are accessory minerals. For the P2A sample, eight zircon grains were analyzed with U-Pb SHRIMP. CL images show zircon grains composed of equidimensional prisms, with a high luminescence metamorphic overgrowth, which cannot be analyzed due to their small size (Fig. 5D). The U and Th contents of the analyzed spots are compatible with magmatic rocks. Four measures with good analytical consistency were selected for the construction of the Wetherill diagram, which provides the concordant age of 1,096 ± 20 Ma (MSWD = 3), considering the time of igneous crystallization of the amphibolite protolith (Fig. 5D). The sample also features four inherited zircon grains, which indicate Rhyacian, Statherian and Calymmian inheritance. The Rhyacian heritage relates to the Pocrane Complex, host rock of the P2A amphibolite. The Statherian heritage relates to magmatism associated with the Juiz de Fora Complex (Duarte et al. 2003; Heilbron et al. 2010). The Calymmian heritage relates to the mafic magmatism represented by amphibolites co-genetic to sample TN-8A (this paper) and Bananal (Silva et al. 2002).

Figure 5:
Ages of sample TN-8A and P2A: (A) Crystallization age of amphibolite magmatic protolith (TN-8A), given by the upper intercept Wetherill diagram, and cathodoluminescence images of analyzed grains; (B) All zircons concordia diagram of sample TN-8A; (C) Metamorphic concordant age and cathodoluminescence images of analyzed grains (spot size = 30 µm) from sample TN-8A; (D) Concordia age for igneous crystallization of P2A amphibolite protolith, and cathodoluminescence images of analyzed grain (spot size = 30 µm).

Detrital zircon U-Pb data

The 90 grains analyzed from sample P2D (219.592E/7.841.790N, UTM24S WGS84) - Córrego Ubá quartzite - are prismatic, short to elongated, generally showing rounded terminations and the maximum length of 300 µm. Some grains are pyramidal with a rounded core (Fig. 6). After data reduction, 87 spots could be used for age calculations. Representing 35% of analyzed zircon grains, the statistic peak around 1,176 ± 21 Ma encompasses the youngest grains of the sample (Fig. 7), which represents a younger component of the source, thus constraining the maximum depositional age of the Córrego Ubá quatzite. Furthermore, the youngest concordant grain dated shows the age of 995 ± 66 Ma. This youngest and most important population includes rounded grains and some euhedral to subhedral crystals with well-rounded shapes (Fig. 6). Other significant sources of P2A quartzite are indicated by age peaks at 1,371 ± 30 Ma (18%) and 1,536 ± 22 Ma (19%). Minor Paleoproterozoic ages point to sources around 1,803 and 1,977 Ma (Fig. 8). It is worth to mention the complete absence of Ediacaran to Ordovician sources and the lack of ages older than the Orosirian period.

Figure 6:
Cathodoluminescence (CL) images of representative zircon grains from the Córrego Ubá quartzite (sample P2D).

Figure 7:
Ages of main extensional events in the São Francisco-Congo Craton and Araçuaí and West Congo Orogen. References: 1) Kuchenbecker et al. 2015a; 2) Gonçalves-Dias et al. 2011; 3) Gonçalves-Dias et al. 2016; 4) This work; 5) Chemale Jr. et al. 2012; Mantiqueira, Juiz de Fora and Pocrane complexes data from Silva et al. 2016.

Figure 8:
Tectonic model for the Mesoproterozoic-Neoproterozoic taphrogenic events and Ediacaran orogenic event in the Rio Doce Arc region, Araçuaí Orogen. 1) João Pinto Formation (Rio Doce Group), Novo 2013; 2) Palmital do Sul Formation (Rio Doce Group), Novo 2013; 3) Macaúbas Group, Kuchenbecker et al. 2015b; 4) Jequitinhonha Complex, Gonçalves-Dias et al. 2011; 5) Jequitinhonha Complex, Gonçalves-Dias et al. 2016; 6) This Work; 7) São João da Chapada Formation, Chemale Jr. et al. 2012; 8) Bandeirinhas Formation, Chemale Jr. et al. 2012.

DISCUSSIONS

The history of the continental drift alternates long fragmentation periods with short agglutination intervals (Nance et al. 1988, Murphy and Nance 1992). Hence, one of the products of Rhyacian-Orosirian Orogeny would have been a paleocontinent that brought together inherited parts of the São Francisco and Congo cratons and the Araçuaí-West Congo Orogen basement (Atlantica paleocontinent; Rogers 1996, Rogers and Santosh 2004). Afterward, over a period of about 1.5 Ga (interval between the Rhyacian and Ediacaran orogenies), this paleocontinent region was spared of orogenic events with compressive nature. However, on the other hand, it experienced several tafrogenic episodes, revealed by magmatism and sedimentation periods (Pedrosa-Soares & Alkmim 2011). The new data presented in this study characterize the Córrego Ubá Quartzite and amphibolite lenses as evidence of taphrogenic events tectonically interlayered in the Pocrane Complex. These signs improve the knowledge about the Calymmian and Stenian episodes as unsuccessful fragmentation attempts of São Francisco-Congo paleocontinent now described in the metamorphic, highly deformed orogenic core.

The Calymmian distensive event marks one of the oldest taphrogenic episodes ever recorded in the region of Araçuaí-West Congo Orogen (the oldest one is the Statherian event). Despite being regarded as part of the Espinhaço-Chapada Diamantina System (Danderfer et al. 2009; Babinski et al. 1994, 2012; Chemale Jr. et al. 2012; Silveira et al. 2013; Guadagnin et al. 2015), there is only one reference to this event in the Araçuaí Orogen metamorphic core (U-Pb age by Silva et al. 2002). The new data from this paper displays a tholeiitic composition amphibolite with crystallization age at 1,529 ± 37 Ma. Its REE pattern is similar to the basic magmatism related to continental rifting, or continental plateau basalt. These features suggest this amphibolite sample represents basic magmatism related to continental rift during Calymmian times. Reports of this approximately 1.5 Ga distensive event are common in the São Francisco-Congo Craton region (Medium Espinhaço from Chemale Jr. et al. 2012 and Event 2 from Pedrosa-Soares & Alkmim 2011), as well as around the globe (Bingen et al. 2005,2008; Zhao et al. 2004; Ernst et al. 2000; Ernst et al. 2008; Wingate et al. 2009). The discovery of ca. 1.5 Ga intrusion in the reworked Araçuaí Orogen terrain is a major step to a more complete barcode record in São Francisco-Congo paleocontinent, which can be used in future paleo-continental reconstructions.

The Stenian episode is well portrayed in several studies, and thereby records a wide extension event (Tack et al. 2001; Tupinambá et al. 2007; Vicat and Pouclet 1995; Silveira et al. 2013). The sample P2A amphibolite from this work presents crystallization age of 1,096 ± 20 Ma and a tholeiitic gabbro composition. REE pattern correlated with advanced stage of magmatism relating to a continental rift, which confirms the tectonic environment discriminant graphics, with plots in the continental intraplate basalt fields. These features suggest the P2A amphibolite represents basic magmatism related to a Stenian continental rift. Castro et al. (2015) portrait similar U-Pb ages for amphibolites lenses occurred in the neighboring region. Angeli et al. (2004) described ultramafic rocks associated with this rift system, supporting the idea of continuous crustal thinning in the region during Stenian times. Therefore, considering this mafic-ultramafic sequence, intrusive into the continental crust, the geodynamic setting should be related to specific stages in the evolution of the São Francisco-Congo Craton.

The Córrego Ubá quartzite represents the basin formed during the abovementioned Stenian event, partially preserved in the Araçuaí-West Congo orogen metamorphic core. The U-Pb geochronology study of the Córrego Ubá quartzite provides important hints for their geotectonic and palaeogeographic interpretations. The results indicate that great part of the sedimentary supply should have come from secondary sources, located both in the São Francisco and Congo cratonic counterparts. The oldest intervals reveal contributions from Rhyacian to Stenian sources, and the absence of pre 2.05 Ga zircon grains discards São Francisco and Congo cratons basement complexes. They also require a younger source with no inheritance, which may correspond mainly to Juiz de Fora and Pocrane basement complexes from Araçuaí-West Congo Orogen (Novo, 2013; Teixeira et al. 2000, Silva et al. 2002, Barbosa & Sabaté 2004, Noce et al. 2007). Correspondingly, the source of this interval could also be associated to rift related granites (e.g., Borrachudos Suite). The Statherian to Cambrian peaks indicate reworked sources from Bomba Formation and Middle Espinhaço Supergroup (Danderfer et al. 2009, Pedrosa-Soares & Alkmim 2011, Chemale Jr. et al. 2012, Guadagnin et al. 2015). Statistical analyses point out a maximum age of deposition for the psammitic sediment, limited by 1,176 ± 21 Ma. However, the youngest concordant grain dated shows the age of 995 ± 66 Ma, suggesting the Tonian-Stenian limit.

Figure 8 shows a comparison of the detrital zircon age spectra of the Córrego Ubá quartzite, with plausible primary and secondary sources from the São Francisco-Congo Craton neighboring areas. Regardless of the lack of data from Upper Espinhaço (0.9 - 1.2 Ga; Chemale Jr. et al. 2012), those units show significant similitude with the Córrego Ubá quartzite spectra. Despite some differences, like the relative abundance of Archean zircon grains in the continental rift stages, units of the Upper Espinhaço, the Córrego Ubá quartzite age spectra feature great similarity to the age spectra of those units (Figs. 7 and 8A). The Córrego Ubá quartzite has no Cryogenian or Ediacaran zircons, but, instead, shows patterns that are more similar to a rift arrangement. The Córrego Ubá quartzite is then not correlative with other sedimentary successions of the Araçuaí Orogen metamorphic core (e.g., Rio Doce Group and Nova Venécia Complex - Vieira 2007, Gradim et al. 2014). However, instead, they probably represent a chronostratigraphic equivalent to Upper Espinhaço Basin (Chemale Jr. et al. 2012), an extension of the passive margin sediments formed in an intraplate setting.

Surprisingly, comparing with data from the western sector of the Araçuaí Orogen and São Francisco Craton, it is noteworthy that no igneous zircon from the three samples yielded an age older than early Orosirian (~2.05 Ga), showing age spectra essentially limited in the range of ca. 1 - 2 Ga; i.e., younger than the Late Rhyacian orogeny that amalgamated the basement, and older than the main anorogenic event (930 - 870 Ma) associated with the Early Tonian precursor basin of Araçuaí-West Congo orogen (AWCO). Therefore, it is possible to suggest that the Córrego Ubá quartzite could represent a restrict basin, confined in a juvenile basement, like the Pocrane and Juiz de Fora complexes.

During early Mesoproterozoic to late Neoproterozoic, the development of a continental rift sequence and the continued thinning of the lithosphere eventually led to plate separation and creation of a true oceanic crust (Santo Antônio do Grama Suit; Queiroga et al. 2006, 2007, Fig. 8B), extending to ca. 660 Ma. This basin evolved from a continental rift system surrounded by the São Francisco-Congo Paleocontinent, to an inland-sea basin (a gulf), partially floored by ocean crust (Pedrosa-Soares et al. 2011; Alkmim et al. 2006; Queiroga et al. 2007; Peixoto et al. 2015). In the Ediacaran Period, the pre-collisional Rio Doce magmatic arc marked the onset of a subduction-related convergent stage, followed by regional deformation and metamorphism in the collisional stage, that provided intense tectonism, basin inversion and obduction of oceanic crust slices (Queiroga et al. 2006, 2007; Tupinamba et al. 2007, Figs. 8C and 8D).

From a global tectonic standpoint, the Córrego Ubá quartzite and the mafic intrusives of the Pocrane Complex might be correlated to the extensional processes that led to the fragmentation of a supercontinent (Atlantica). It could also mark the dismembering of the São Francisco-Congo Paleocontinent, which was later amalgamated with other blocks in the Gondwana Supercontinent during the Brasiliano/Pan-African Orogeny (Tedeschi et al. 2015; Gonçalves et al. 2015; Pedrosa-Soares et al. 1998, 2007; Alkmim et al. 2006, 2007; Brito Neves & Sato 2001; Tack et al. 2001; Vicat & Pouclet 1995).

CONCLUSION

The mafic intrusions from southeastern Brazil characterize important regional geological events in the São Francisco-Congo Craton and its surrounding Neoproterozoic mobile belts. The amphibolite lenses are interpreted as mafic intrusions, related to the Calymmian and Stenian rifting stages. They might also be correlated to the extensional events that led to the break-up and dispersion of a Paleoproterozoic supercontinent, and thus relate to the mafic dike swarms and other intrusions throughout the São Francisco-Congo paleocontinent and surrounding areas (Figs. 1, 7 and 8A). The intrusions infer continued lithospheric thinning with associated volcanism. The amphibolite lenses, thus, provide a benchmark for the onset of the Calymmian and Stenian continental rifting in this area, and set an important marker for the registration of the Atlantica breakup and dispersion processes in South America.

The Córrego Ubá quartzite shows detrital zircon U-Pb age spectra, indicating sources limited in the range of ca. 1 - 2 Ga, with complete absence of Cryogenian-Ediacaran peak, excluding sources related to the Brasiliano cycle. Therefore, it cannot be associated to other metasediment units of the Araçuaí Orogen metamorphic core, like Rio Doce Group. Additionally, the detrital zircon age spectra of the Córrego Ubá quartzite displays great similarity to continental rift related units. Thus, it represents one of the several unsuccessful fragmentation attempts of São Francisco-Congo paleocontinent, deposited in a restrict basin, probably during Stenian times. It can be associated with the Tonian extensional tectonism at the São Francisco-Congo Craton area. This is the first time that such kind of basin is described in Araçuaí-West Congo Orogen reworked metamorphic core.

All together, these continental taphrogenic events testify the several unsuccessful fragmentation attempts that affected the long-lived São Francisco-Congo Palaeocontinent, which remained preserved from a complete break-up associated with ocean spreading from the Early Orosirian to the Atlantic opening in Cretaceous time.

ACKNOWLEDGEMENTS

The authors are grateful to the Brazilian research and development agencies (CNPq, CAPES, CODEMIG, CPRM, Petrobras) for financial support, and to technical staffs of the geochronological laboratories of São Paulo University, for isotopic analyses.

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Data available in the Supplementary data file labeled as Lithochemical_Data Lythochemical analysis of amphibolite samples (major elements in % weight; trace in ppm).

The U-Pb analyses are available in the Supplementary data file labeled as U_Pb_data Analytical data of zircons of TN-8 sample. In bold unused data to obtain igneous crystallization age. In italics data used for obtaining metamorphic age.

Analytical data of zircons of P2A sample. In bold unused data to obtain age. In italics inherited zircon grains not used to obtain age.

Analytical data of zircons of P2D sample. In bold unused data to obtain the age spectra.

Publication Dates

• Publication in this collection
  Jul-Sep 2017

History

• Received
  29 Mar 2017
• Accepted
  27 June 2017