Midcrust deformation regime variations across the Neoproterozoic Araçuaí hot orogen (SE Brazil): Insights from structural and magnetic fabric analyses
Introduction
Collisional orogenic belts characterized by high temperatures, slow cooling rates, and excessive amount of melt in the hinterland (“hot orogens”) behave with decreased rock strength due to the effects of large volume of magma in the crust (Fossen et al., 2017; Jamieson et al., 2011). These aspects have important consequences on the forces that control the dynamic of these hot orogens. Previous studies have demonstrated that crustal thickening induced by compression is accommodated by a transpressional regime in combination with orogen-parallel escape flow along sub-horizontal structures. This accommodation reflects the weakness of the hot lithosphere which would not be able to sustain the load of high topographies of the orogen (e.g., Beaumont et al., 2001; Rey et al., 2001; Chardon et al., 2009). In addition, under HT-LP conditions, partial melting is pervasive in the middle crust and strain repartition is less efficient, leading to homogeneous deformation of a large volume of rocks (Vauchez and Tommasi, 2003; Vauchez et al., 2007).
Anisotropy of magnetic susceptibility (AMS) has been used for retrieving the mineral preferred orientation fabrics of plutons and metasedimentary rocks (e.g., Bouchez, 2000; Park et al., 2005; Egydio-Silva et al., 2005; Archanjo et al., 2008; Mondou et al., 2012; Maffione et al., 2015; Parsons et al., 2016) to access information related to the flow of rocks in orogenic areas. The kinematic and geometric pattern of the fabric is required to understand how complex deformation regimes are accommodated as a result of the imposed deformation and consequently its distribution and tectonic processes active during collision. To better constrain the imposed deformation and consequently the tectonic processes active during collision, we studied the strain distribution through AMS in the hinterland portion and adjacent areas of a segment of the Araçuaí belt, which experienced slow (<5 °C/My) regional cooling from synkinematic high temperature (~750 °C) and low pressure (600 MPa) conditions (Petitgirard et al., 2009). In addition, we performed rock magnetism studies to reveal the carriers of the magnetic anisotropies and acquired zircon geochronological data from a syn-kinematic tonalitic body emplaced in arc-related metasedimentary rocks. Our review of the literature regarding structural and geochronological data combined with AMS measurements performed to obtain reliable structural maps, allow us to discuss the deformation history and structural patterns associated with the behavior of this orogenic setting.
Section snippets
Geological setting
The Neoproterozoic Araçuaí belt is the northern segment of the Ribeira-Araçuaí orogenic system, formed during the Brasiliano/Pan-African Orogeny during the amalgamation of West Gondwana (Fig. 1A) (Almeida, 1977; Bento dos Santos et al., 2015). The Araçuaí belt displays a N–S structural trend and shows a progressive increase in metamorphic conditions from the low-grade foreland to a high-temperature - low-pressure hinterland. Oliveira et al. (2000) and Vauchez et al. (2007) prioritized
The imbricated synkinematic magmatic São Vitor Tonalite and metasedimentary host rock
The CPU involves huge volumes of magmatic rocks emplaced within metasediments. The main magmatic bodies of this domain in the studied area are represented by the São Vitor and Galiléia tonalites (Fig. 1B). The former is gray in color, medium to coarse-grained, exhibiting inequigranular phaneritic texture and magmatic foliation marked by the preferential alignment of biotite and feldspar. Elongate mafic enclaves that are parallel to the magmatic foliation are also observable (Fig. 2A). The
Anisotropy of magnetic susceptibility (AMS)
AMS can be used to measure the petrofabric of rocks to determine the preferred orientation of magnetically-dominant minerals, and is commonly used as an effective approach for studying plutonic and metasedimentary rocks. AMS provides magmatic strain patterns in rocks where visible planar and/or linear structures are difficult to characterize (Tarling and Hrouda, 1993; Martín-Hernández et al., 2004). The AMS can be described by a symmetrical second rank tensor (Kij) that relates the intensity of
U–Pb zircon (LA-ICP-MS) geochronology
Integration of U–Pb age data with structural and isotopic data provides insights into a number of fundamental issues concerning batholith primary structure, emplacement mechanisms, chronology, and kinematics of regional deformation. The determination of whether a pluton was emplaced during a single magmatic event or resulted from multiple magmatic stages during batholith construction is important when dealing with kinematic interpretations and timing of the deformations characterized in the
Significance of magnetic fabrics
The integrated magnetic susceptibility measurements and magnetic mineralogy investigations revealed that the magnetic susceptibility of the Tumiritinga Formation, São Vitor Tonalite, and Mantiqueira Complex, with Km values of ~5 × 10−4 SI, is dominantly paramagnetic, further confirmed by the constant drop of susceptibility with increasing temperature (K-T curves). Small amounts of magnetite were detected in the hysteresis and IRM curves, although this mineral does not exert the dominant control
Conclusions
Detailed structural mapping using field and AMS measurements and the study of microstructures in the central portion of the Araçuaí belt revealed a complex strain distribution formed before solidification of the syn-to late plutonic bodies intruded in metasediments, with kinematics consistent with examples described in ‘hot orogens’. This study revealed that AMS results from the magnetocrystalline anisotropy of biotite and/or amphibole and the shape anisotropy of MD or PSD magnetite grains, and
CRediT authorship contribution statement
Tiago Valim Angelo: Conceptualization, Funding acquisition, Formal analysis, Writing - original draft, Writing - review & editing. Marcos Egydio-Silva: Conceptualization, Funding acquisition, Formal analysis, Writing - original draft, Writing - review & editing. Filipe Altoé Temporim: Conceptualization, Formal analysis, Writing - review & editing. Marina Seraine: Conceptualization, Formal analysis, Writing - review & editing.
Acknowledgments
This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (Capes) – Finance code CAPES/SIU 0013. This work has been partially performed at USPMag lab at Instituto de Astronomia, Geofísica e Ciências Atmosféricas (IAG), Oceanographic Institute of the University of São Paulo (LabGeo), and Magnetic Anisotropies and Rock Magnetism Laboratory of the Instituto de Geociências at the University of São Paulo (IGc/USP). The authors are grateful to the
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2021, TectonophysicsCitation Excerpt :So, the geometry expected in this case would be a flat-floored pluton. The northern post-collisional plutons in the AO might have experienced this mechanism of emplacement given their large horizontal dimensions, geothermobarometry data compatible with the crust brittle-ductile transition (2.4–3.5 Kbar, Serrano et al., 2018) and the gently dipping foliation and lineation (Angelo et al., 2020; Mondou et al., 2012). Considering the buoyancy of the magma and the ductile conditions of the crust as important parameters of diapirism mechanism, the continental crust might act as a natural density filter for such intrusions.
The role of Ediacaran synkinematic anatectic rocks and the late-orogenic charnockitic rocks in the development of the hot Araçuaí belt
2021, Precambrian ResearchCitation Excerpt :From microscopic observations we can identify textures related to melt crystallization and melt-solid reactions, mineral assemblages from which melt-forming reactions can be inferred and investigate whether the deformation occurred at magmatic or solid state (e.g., Sawyer, 2008; Cavalcante et al., 2013) and finally, utilizing thermobarometry we can constrain the P-T conditions during partial melting (e.g., Brown, 2002; Harris et al., 2004; Cavalcante et al., 2014; Clark et al., 2015). Several studies conducted over the last three decades have produced a large body of structural, geochronological, geochemical and petrological data that provides important constraints on the tectonic setting and orogenic processes of the hot Araçuaí belt (e.g. Trompette et al., 1993; Vauchez et al., 1994; Vauchez et al., 2007; Vauchez et al., 2019; Trompette, 1997; Pedrosa-Soares et al., 2001; Martins et al., 2004; Alkmim et al., 2006; Petitgirard et al., 2009; Mondou et al., 2012; Cavalcante et al., 2013; Cavalcante et al., 2014; Cavalcante et al., 2018; Richter et al., 2016; Melo et al., 2017a; Melo et al., 2017b; Angelo et al., 2020). However, key questions, especially those related to the geodynamic implications of the formation of a large migmatitic-anatectic area with evidence of being partially molten for a long time period (e.g., Cavalcante et al., 2018; Vauchez et al., 2019), have not received due attention.
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