Abstract
The Neoproterozoic Araçuaí-West Congo orogen in Brazil and Congo represents a branch of the Brasiliano/Pan-African orogenic system that is considered to terminate northward into a confined cratonic environment defined by the horseshoe-shaped pre-Atlantic São Francisco-Congo craton. The prevailing interpretation is that this orogen formed as a result of 50 m.y. of subduction leading to a classical continent–continent collision. This subduction-collision model hinges on interpretation of 630–580 Ma granitoids in the core of the orogen as being arc-related, and of locally exposed mafic and ultramafic metamorphic rocks as being ophiolitic. We show that when tested beyond geochemical signature, the model has fundamental problems that cannot be accounted for. In particular, there is an insurmountable oceanic space problem in this confined setting that is overlooked in most of the current literature. There are also problems with subduction initiation, lack of unambiguous evidence for oceanic crust, no trace of any high-P metamorphism and the abrupt termination of an ocean with no realistic way to transfer the large amount of oceanic opening displacement and subsequent convergence required by the model. We conclude that the prevailing subduction-collision model cannot possibly work and argue that the existing data are more consistent with hot intracontinental orogeny. We stress the importance of building tectonic models on more than one type of data, making realistic restorations and palinspastic reconstructions, and taking into account modern geotectonic knowledge.
Highlights
It is generally accepted that the Neoproterozoic Araçuaí orogen in Brazil and its African counterpart, the West Congo orogen (Fig. 1), are an unusual example of termination of an orogenic belt into a cratonic environment (e.g., Porada, 1989; Brito Neves and Cordani, 1991; Pedrosa-Soares et al, 1998; Alkmim et al, 2006; De Wit et al, 2008; Gray et al, 2008; Stampfli et al, 2013; Barbosa and Barbosa, 2017; Degler et al, 2018)
A major Tonian-Cryogenian rift system was shortened by convergent movements between the São Francisco and Congo cratons during the Brasiliano-Pan African orogeny (Porada, 1979, 1989), expressed by westward thrusting onto the São Francisco craton and eastward thrusting onto the Congo craton (Fig. 2)
The intracontinental model that these authors call for is that of extensive rift-related crustal thinning and basin formation, followed by convergent movements between the African (Congo craton) and Brazilian (São Francisco craton) sides
Summary
It is generally accepted that the Neoproterozoic Araçuaí orogen in Brazil and its African counterpart, the West Congo orogen (Fig. 1), are an unusual example of termination of an orogenic belt into a cratonic environment (e.g., Porada, 1989; Brito Neves and Cordani, 1991; Pedrosa-Soares et al, 1998; Alkmim et al, 2006; De Wit et al, 2008; Gray et al, 2008; Stampfli et al, 2013; Barbosa and Barbosa, 2017; Degler et al, 2018). The intracontinental (ensialic) model that these authors call for is that of extensive rift-related crustal thinning and basin formation, followed by convergent movements between the African (Congo craton) and Brazilian (São Francisco craton) sides. This model requires the formation of a wide Neoproterozoic rift system, possibly hyperextended, that was shortened to form the Araçuaí-West Congo orogen in the Ediacaran (Cavalcante et al, 2019). Others favor a subduction-collision model involving extensive subduction of oceanic crust and related arc activity in the Araçuaí-West Congo orogen that ended with wholesale collisional orogenesis (e.g., Pedrosa-Soares et al, 1992, 1998; Amaral et al, 2020). We suggest a hot intracontinental orogenic model that better explains the total body of data currently available from this interesting branch of the Brasiliano–Pan-African orogenic system
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