U–Pb ages of detrital zircon grains for the Canastra Group and Passos Nappe units and U–Pb and Lu–Hf isotope analyses from orthogneisses: Provenance and tectonic implications, southern Brasília Belt, Brazil

doi:10.1016/j.precamres.2020.105771

Precambrian Research

Volume 346, 15 August 2020, 105771

https://doi.org/10.1016/j.precamres.2020.105771 Get rights and content

Highlights

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The Canastra Group records possible evidence of a rifting event at ca. 1300 Ma.
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Rhyacian substratum is covered by Meso- to Neoproterozoic metasedimentary rocks.
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Passos Nappe units show distinct source and tectonic setting of the Araxá Group.
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Rhyacian orthogneisses emplaced in the southern Brasília Belt.

Abstract

The age and tectonic setting of Mesoproterozoic to Neoproterozoic covers that occur along the southwestern margins of the São Francisco Craton (SFC) are still uncertain. This study reports new $^{207}$ Pb/ $^{206}$ Pb LA-ICPMS ages of detrital zircon grains from metasedimentary rocks of the Passos Nappe units and Canastra Group, and combined U–Pb and Lu–Hf analyses on igneous zircon from orthogneisses within the metasedimentary units. The U–Pb age spectra of detrital zircon grains of the Canastra Group comprises a wide range from $\sim$ 1.30–2.71 Ga with a significant Rhyacian component source ( $\sim$ 2.1 Ga). The second major source is represented by Statherian grains ( $\sim$ 1.6–1.7 Ga), and are similar to the Statherian interval at Espinhaço Basin. The youngest ages fall around $\sim$ 1.30 Ga. This peak age is compatible with the initial extension and sedimentation of the Carandaí and Middle Espinhaço basins, which mark the second intracratonic rifting of the São Francisco Craton. Zircon grains from the metasedimentary rocks the Passos Nappe yielded a very prominent Rhyacian peak at ca. 2.10 Ga with some Statherian and Neoarchean contributions. All samples from the metasedimentary units show a prominent peaks at ca. 2.10 Ga, suggesting that most sedimentary sources derived from a Rhyacian substratum similar to the Mineiro belt, Natividade-Cavalcanti block, Campinorte Complex and Juiz de Fora Complex. Rhyacian age is compatible with U-Pb crystallization ages of felsic orthogneisses (2122 ± 67 and 2117 ± 23), enclosed as tectonic lenses in the Passos Nappe and the Canastra Group. These rocks show initial $ε$ Hf(t) values between +2.1 to −13.0 and +10.0 and −1.8, indicating that these orthogneisses are derived from Rhyacian granitic rocks, which were formed either by fractionation or re-melting of juvenile magmas, and by the reworking of more ancient, mainly Archean crust. The new data and regional geologic correlations suggest evidence of Paleoproterozoic terrain in the southwestern margin of the SFC.

Introduction

In recent years, the number of isotopic data regarding absolute ages of the Meso- to Neoproterozoic metasedimentary rocks of the Southern Brasília belt (SBB) (Fig. 1), has exponentially increased (Pimentel et al., 2001, Piuzana et al., 2003a, Piuzana et al., 2003b, Valeriano et al., 2004, Rodrigues, 2008, Rodrigues et al., 2010, Rodrigues et al., 2012, Dias et al., 2011, Falci et al., 2018). These works provide the provenance patterns of the sedimentary units and the reconstruction of the paleogeography, mainly the external zone (Canastra, Ibiá, Paranoá groups) and Araxá Group (internal zone) in the north portion of the SBB. However, a correlation between passive margin sequence of the north portion with sedimentary rocks of south portion, represented by the Canastra Group and upper thrust sheets of the Passos Nappe units is still not clear. That is due to polyphase deformation and tectonic imbrication of SBB, in addition to the lack of geological maps at an appropriate scale.

In this paper, we investigate the provenance of metasedimentary rocks of Passos Nappe units and Canastra Group of northwest Passos nappe system in order to improve our understanding of source regions for the detrital zircon grains, emphasizing the reconstruction of the paleogeography and depositional systems. We also present the combination of U–Pb and Lu–Hf isotope data obtained from orthogneisses lenses within the metasedimentary rocks of the Passos Nappe units and Canastra Group. The new data enable us to provide evidence for the age of crust formation and crustal evolution of the southern portion of the São Francisco Craton.

Section snippets

Geological setting

The Brasília Orogenic Belt is a sector, which together with the Araguaia and Paraguay belts, constitutes the Tocantins Province (Almeida et al., 1981). It resulted from the convergence and collision of three major continental blocks: Amazonian, São Francisco and Paranapanema cratons (Fig. 1) (Strieder and Suita, 1999, Dardenne, 2000). The province developed during the amalgamation of West Gondwana through a long succession of Neoproterozoic collisional events during the Brasiliano orogeny.

The

Lithostratigraphy

The study area is located in the northwest sector of Passos Nappe (Fig. 3A), near the Peixoto Dam. The Passos Nappe units were thrusted over the metasedimentary rocks of the Canastra Group. This unit in the study area do not allow a stratigraphic correlations for formal units of the Canastra Group in the Paracatu-Coromandel region (see Freitas-Silva and Dardenne, 1994 for a review), due to intense deformation on transversal structures represented by lateral ramps and strike slip shear zones.

Samples

Seven samples (DCS-11, DCS-117, DCS-118, DCS-120, DCS-130, DCS-132 and DES-356) of the Canastra Group were collected for this study. These samples comprise quartzite, micaceous quartzites and metagraywacke. Two samples (DCS-02 and DCS-114) of the Passos Nappe were collected from B and D units, and comprises quartzite and paragneiss. Samples DCS-115, DCS-122 and NP-01 (not shown in Fig. 3) are representative of the felsic orthogneisses that occur in the form of tectonic lenses within the Passos

Analytical procedures

For in situ U–Pb analyses, about 10 kg of each sample was crushed at the Laboratório de Preparação de Amostras (LPA) of the Universidade Estadual Paulista (UNESP) and separated using bromoform and subsequent magnetic separation. Zircon grains were hand-picked ( $\sim$ 150 grains for sample) and mounted on adhesive tape, enclosed in epoxy resin and then polished to expose grain centers. Zircon grains were imaged by SEM cathodoluminescence (CL) and backscattered electron (BSE) to identify morphological

Samples DES-356 (S20° 3′ 57.3″; W47° 1′ 37.1″) and DCS-120 (S20° 4′ 39.2″; W46° 59′ 33.1″

These samples comprise micaceous quartzite from the Ciq1 Unit. The zircon grains are small ( $<$ 200  $μ$ m), rounded to sub-rounded, (Figs. 4A and B). The probability density plot (Fig. 5B) of sample DES-356 has ages in the range 2262–1932 Ma (n = 74; see Supplementary Table), with peaks at 2129 (n = 32, MSWD = 0.87) Ma and 2059 Ma (n = 37, MSWD = 0.63), indicating early Orosirian-Rhyacian sources.

Sample DCS-120 shows ages ranging between 3316 and 1522 Ma. Two main age are distinguished at ca. 1661

Sample DCS-122 (S20° 22′ 14.2″; W47° 1′ 38.3″)

Sample DCS-122 is inequigranular, medium- to coarse-grained felsic orthogneiss composed of plagioclase, microcline, quartz, biotite, muscovite and epidote, with zircon and opaque minerals as the accessory phases. The orthogneiss occurs as ca. 12 km-long and 1.5 km-wide within the Passos Nappe. The zircon crystals are dark brown with lengths of $\sim$ 80–230  $μ$ m and with length/width ratios of 3:1–2:1. CL images show that zircon grains are homogeneously dark (Fig. 8C) and strongly fractured. Fifty U–Pb

U–Pb geochronology of the Canastra Group

The U–Pb in situ ages of detrital zircon grains from samples of the Canastra Group show dominant Rhyacian source and minor contribution of Statherian, Calymmian, Orosirian, Neoarchean and Mesoarchean sources.

The U–Pb age modes of detrital zircon grains extracted from the Ciq1 and Ciq3 units (samples DES-356, DCS-120, DCS-118 and DCS-132) are characterized by major peak at ca. 2.1 Ga and a minor peak at 1.6 Ga for sample DCS-120 (Fig. 5D). For this sample, the two youngest grains yielded ages of

Conclusion

Detrital zircon grains of metasedimentary rocks from Passos Nappe units and Canastra Group, and combined U–Pb and Lu–Hf isotope analyses on zircon from orthogneisses, allow some conclusions and interpretations concerning its age and tectonic significance. They are summarized as follows:

(i) Samples from Canastra Group show Archean and Mesoproterozoic populations, with a strong contribution from Orosirian-Rhyacian sources (1.87–2.30 Ga), to a lesser extent, Statherian (1.62–1.79 Ga),

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge the Brazilian agencies for the financial support for field work and geochronology studies, such as São Paulo State Research Foundation (FAPESP; Grant No. 2017/03663-1), Coordination for the Improvement of Higher Education Personnel (CAPES) and CODEMIG-UFMG. The comments and reviews provided by anonymous reviewers are much appreciated and have greatly improved the quality of the manuscript.

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Archean–Ediacaran evolution of the Campos Gerais Domain — A reworked margin of the São Francisco paleocontinent (SE Brazil): Constraints from metamafic–ultramafic rocks
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Determining the “best age” of individual grains from UPb detrital zircon analyses remains problematic, with the current system generally relying on an arbitrary cut-off age for determining when to switch from ²⁰⁶Pb/²³⁸U ages to ²⁰⁷Pb/²⁰⁶Pb ages. Recently, some found that iteratively calculated IsoplotR single grain concordia ages provide estimates with increased precision and have turned to using these estimates as the preferred age. Here, we compile a new 604,553 record global detrital zircon database containing UPb ratios, ages, uncertainties, laboratory settings, among other variables. The new database provides a means for investigating four best age models from the standpoint of accuracy. The models are (1) 1700 Ma cut-off for best age, (2) 1000 Ma cut-off for best age, (3) iteratively calculated IsoplotR single grain concordia ages, and (4) a new non-iterative model developed by comparing UPb ages of zircon standards dated by both ID-TIMS and LA-ICP-MS spectrometry. We then classify all analyses by their degree of concordance and calculate unweighted age distributions. The age distributions are nearly identical for all models when highly concordant analyses are used. However, as discordance increases, the models with arbitrary cut-offs produce age distributions with artificial depressions at the cut-off age. After segregating database records into five concordance classes, cross-correlation analyses show the five non-iterative, class-based, age distributions are aligned and correlated better than those from the other age models. Rather than abruptly transitioning from ²⁰⁷Pb/²⁰⁶Pb ages to ²⁰⁶Pb/²³⁸U ages at a specific cut-off age, the results support a cumulative distribution model that defines a gradual transition from the opposing geochronometers over an interval of ~1200-myr, primarily encompassing the span from 1600 Ma to 400 Ma. Additionally, a supplementary template is developed to provide geochronologists with a means to validate UPb related data, calculate the new concordance classifications, and calculate non-iterative ages.
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Provenance of passive-margin and syn-collisional units: Implications for the geodynamic evolution of the Southern Brasília Orogen, West Gondwana
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Citation Excerpt :
Kuster et al. (2020) contested this interpretation and ascribed the ~0.76 Ga peak to sources in the passive margin domain (i.e. São Francisco Craton), such as rift-related A-type granites (e.g., Pedrosa-Soares and Alkmim, 2011) or zircon-bearing amphibolites (Heilbron et al., 2019). Although this remains a possibility, most of the rocks related to the western Neoproterozoic passive margin of the São Francisco paleocontinent, that have been dated so far, do not show peaks at ~0.76 Ga (e.g. Matteini et al., 2010; Rodrigues et al., 2010, 2012; Silva et al., 2019, 2020; Westin et al., 2019; Marimon et al., 2020a). Upon careful examination of the isotopic data obtained by Frugis et al. (2018), it is possible that the ~0.76 Ga peak (Arantina Unit) could represent concealed Pb-loss (see Section 3.5).
Collisional orogens are one of the most conspicuous features of plate tectonics on Earth. They record the dynamics of continent-continent amalgamation, often confined to linear deformational belts. The Southern Brasília Orogen is one of the most well-studied and complete orogens of West Gondwana. Newly obtained isotopic data, coupled with previously published analyses, expand the occurrence of metamorphosed passive margin sequences (maximum depositional age of ~900 Ma), with a main source in the São Francisco paleocontinent, to large sections of the orogen previously interpreted, by some investigators, as related to younger active margin deposits. This has implications for the location of the suture zone: as most of the upper and lower nappes are dominated by metamorphosed passive margin deposits, the suture is probably located at the base of the uppermost nappe, related to the active margin (Paranapanema paleocontinent). Zircon (U-Pb and Lu-Hf) and whole-rock (Sm-Nd and geochemistry) data obtained for syn-collisional meta-graywackes (~650–630 Ma), that uncomformably overlie passive-margin-related successions, point to a bimodal provenance from both the passive (São Francisco paleocontinent) and active (Paranapanema paleocontinent) margins, depending on the structural level from which the samples were collected. Despite the conspicuous zircon age differences, the maximum depositional age is the same for samples from the upper and the lower nappes. Furthermore, the meta-graywackes from the lower and upper nappes are petrographically and geochemically similar. This suggests concomitant deposition in a common collisional setting. Two metamorphic ages were obtained by U-Pb monazite dating at ~633 and 606 Ma, indicating that protracted medium-grade metamorphic conditions affected the Southern Brasília Orogen.

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U–Pb ages of detrital zircon grains for the Canastra Group and Passos Nappe units and U–Pb and Lu–Hf isotope analyses from orthogneisses: Provenance and tectonic implications, southern Brasília Belt, Brazil

Highlights

Abstract

Introduction

Section snippets

Geological setting

Lithostratigraphy

Samples

Analytical procedures

Samples DES-356 (S20° 3′ 57.3″; W47° 1′ 37.1″) and DCS-120 (S20° 4′ 39.2″; W46° 59′ 33.1″

Sample DCS-122 (S20° 22′ 14.2″; W47° 1′ 38.3″)

U–Pb geochronology of the Canastra Group

Conclusion

Declaration of Competing Interest

Acknowledgements

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