The Neoproterozoic Andrelândia group: Evolution from an intraplate continental margin to an early collisional basin south of the São francisco craton, Brazil
Introduction
Detailed geological mapping, structural geology, microtectonics and metamorphic analysis are essential to unravel the history of metamorphic Precambrian successions. These tools allow the recognition of isograds and the correlation of the internal stratigraphy of tectonic units. Likewise, it is often possible, in spite of deformation and metamorphism, to identify facies associations and even regional sedimentary features such as unconformities, offlap patterns and marine flood surfaces. Isotopic data, including detrital zircon geochronology with Lu–Hf signatures, improve geological interpretations, especially maximum depositional ages and provenance of the sediments. Zircon isotopic data also help to interpret basin tectonic setting (e.g. Cawood et al., 2012; Gehrels, 2012, 2014 and references therein). Similar approaches can be used to interpret the protoliths, maximum depositional age and provenance of the metasedimentary rocks of the Neoproterozoic Andrelândia Group. The dataset also allows interpretation of the tectonic setting of the Neoproterozoic basins related to the Andrelândia Group, south of the São Francisco Craton in southeast Brazil (Fig. 1).
The studied Neoproterozoic succession comprises amphibolite facies metasedimentary and metaigneous rocks of the Andrelândia Group, cropping out in close proximity to the Andrelândia town, Minas Gerais State. Since the pioneering studies of Ebert in the 1950s and 1960s (summarized in Ebert, 1984) distinct interpretations of stratigraphy, paleoenvironment and tectonics concerning the Andrelândia Group have been published. According to Ribeiro et al. (1995), the Andrelândia Group succession records a Neoproterozoic depositional cycle in the southern passive margin of the São Francisco paleocontinent. Paciullo et al. (2000, 2003a) recognized two depositional sequences (Fig. 2): the lower one consists of paragneiss, metapelite, micaceous quartzite, metawacke, metachert (Fe- and Mn-quartzite), calc-silicate, metamafic and metaultramafic rocks interpreted as constituting shelf and deep water facies associations (Fig. 2, Fig. 3). Maximum Tonian depositional ages were published by Valeriano et al. (2004), Valladares et al. (2004) and Belém et al. (2011). The upper sequence is a monotonous Cryogenian/Ediacaran biotite schist/metawacke succession (Fig. 2, Fig. 3). The rocks of these two sequences are alternatively interpreted by Campos Neto et al. (2004, 2007), Westin et al. (2016, 2019) and Frugis et al. (2018) as constituting different tectonic and stratigraphic units: these are the a) São Vicente Complex gneisses, considered as a Paleoproterozoic intra-arc unit; b) Carrancas Group quartzites and metapelites related to the São Francisco paleocontinent passive margin and; c) schists, quartzites, metawackes, amphibolites and metavolcano-sedimentary rocks of the Rio Capivari, Santo Antonio and Serra da Boa Vista units, interpreted as Cryogenian/Ediacaran subduction and collisional (Serra da Boa Vista) related deposits. The approximate correlations of these units with those of this paper are presented in Table 1.
The Andrelândia Group succession is distributed in a series of nappes, known as the Andrelândia Nappe System (Fig. 2; Campos Neto et al., 2004; 2007; Trouw et al., 2013). Trouw et al. (2011) consider the lower nappes to consist of Andrelândia Group shelf and deep water facies and the upper nappes of deep water facies association (Fig. 2, Fig. 3). The Cryogenian/Ediacaran biotite schist occurs in both lower and upper nappes. In contrast, Campos Neto et al. (2004, 2007) relate the lower nappes to the São Francisco paleocontinent passive margin and consider that each upper nappe contains a distinct stratigraphy related to erosion of Brasília orogen magmatic arcs situated to the west of the São Francisco paleocontinent and in the active margin of the Paranapanema paleocontinent (cf. Frugis et al., 2018). Therefore, the suture between the Paranapanema (upper plate) and São Francisco paleocontinents is either situated at the base of the upper nappes of the Andrelândia Nappe System (e.g. Campos Neto et al., 2004; Frugis et al., 2018), or at the base of the uppermost Socorro-Guaxupé nappe (Trouw et al., 2013; Coelho et al., 2017, Fig. 2).
In this paper, we focus on the geology, stratigraphy, depositional ages and provenance of the Andrelândia Group in its type area in order to discuss regional stratigraphy, tectonics and basin evolution. The work is mainly based on detailed geological mapping and U–Pb and Lu–Hf detrital zircon analysis; the results are discussed and interpreted together with a thorough compilation from the literature.
Section snippets
Geological setting
The Neoproterozoic southern Brasília Orogen extends through ca. 800 km oriented N–S along the western margin of the São Francisco Craton, in central and southeast Brazil (Fig. 1). The orogen resulted from the convergence of the São Francisco and Paranapanema paleocontinents (the latter now mostly covered by the Phanerozoic Paraná Basin; Fig. 1b; Valeriano et al., 2004, 2008; Valeriano, 2017). In its southern zone an Ediacaran (625-600 Ma; Campos Neto et al., 2010; 2011; Reno et al., 2009, 2011;
Geology of the Andrelândia Group in the studied section
The investigated area comprises an NW-SE section crosscutting the Lima Duarte, Andrelândia and Liberdade nappes, and the Serra da Natureza Klippe in the Andrelândia region (Fig. 2). According to Paciullo et al. (2003a, 2003b), these allochthons are structured in up to kilometric northwest and southeast verging open to tight folds with variable plunging axes (NW, NE, SW) attributed to a second deformation phase (D2). The older D1 deformation phase structures consist of thrust surfaces and
Analytical methods
Four quartzite samples were collected for zircon analysis, being three samples from the, considered here, deep water succession of the Andrelândia nappe (KA-53 and MAR-367) and Liberdade nappe (KA-109) and one sample from the platformal succession of the Lima Duarte nappe (BJM-08). Zircon grain separation was performed at Sample Preparation Laboratory, Federal University of Rio de Janeiro, Brazil. A total of 10 kg of each rock sample was crushed and powdered to the size of approximately 500 μm.
U–Pb detrital zircon ages
A total of 386 LA-ICP-MS analyses were performed on the detrital zircon grains extracted from four rock samples, resulting in 331 concordant ages. The CL images (Fig. 7) show the spot locations in representative zircon grains from each sample. The results are shown in calculated concordant age histograms and probability density plots created with the Isoplot 4.15 software (Ludwig, 2008) and are presented in Supplementary Table 1.
Provenance and possible source areas
The U–Pb data of detrital zircon grains of the Andrelândia Group show wide age spectra. The probability density plots (Fig. 8) show a variety of age peaks which could indicate several possible sources. The zircon grains are mainly derived from Paleoproterozoic and Mesoproterozoic source areas, while Archean and Tonian grains constitute, each, less than 5% of the grain population. The exception is sample BJM-8, which shows a major Paleoproterozoic contribution without Mesoproterozoic and
Andrelândia Group evolution
Field geology and isotopic studies helped to determine that the Andrelândia Group includes two major sequences (Fig. 3). The lower sequence records deposition in a passive margin basin at the southern border of the São Francisco paleocontinent. The upper sequence was deposited in a basin developed during the early collisional stage of the southern Brasília Orogen (Fig. 13). The isotopic data from this work and a thorough compilation from the literature help to confirm that the pronounced
CRediT authorship contribution statement
Klaus Kuster: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Visualization. André Ribeiro: Supervision, Conceptualization, Validation, Investigation, Resources, Project administration, Funding acquisition, Writing - review & editing. Rudolph A.J. Trouw: Supervision, Conceptualization, Validation, Investigation, Resources, Project administration, Funding acquisition, Writing - review & editing. Ivo Dussin: Supervision, Methodology, Resources, Formal
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.
Acknowledgments
K. Kuster acknowledges CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) for MSc. Grant. R.A.J. Trouw thanks the financial support received from the Brazilian Research Council (CNPq). The authors would like to thank the technical staff at the Geochronology Laboratory of the Universidade de Brasília (UnB) and at the CPGeo-USP. Careful editorial handling by Dr. M. Martini and suggestions frow two anonymous reviewers are greatly acknowledged.
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