Discussion: Implications for the tectonic evolution of Western Gondwana
8.1. Pre‐Gondwana configuration
‐CHAPTER 8‐
Discussion: Implications for the tectonic evolution of Western Gondwana
8.1. Pre‐Gondwana configuration
Several contributions attempted to elucidate the Mesoproterozoic paleogeography (e.g., Powell et al., 1993; Dalziel et al., 2000; Kröner and Cordani, 2003; Pisarevsky et al., 2003; Tohver et al., 2006; Li et al., 2008; Evans, 2009), which represents a key point to understand the history of Gondwana amalgamation. Despite most authors agree on the fact that the Amazonas and West African cratons were part of Rodinia (Dalziel et al., 2000; Tohver et al., 2006; Li et al., 2008; Evans, 2009), the participation of southwestern Gondwanic blocks is still under discussion. Kröner and Cordani (2003) indicated that the Río de la Plata and Congo‐São Francisco cratons were not part of Rodinia, which was further supported by Tohver et al. (2006) and Rapalini et al. (2013). In contrast, Evans (2009) included both blocks within Rodinia. In the case of the Río de la Plata Craton, the pre‐
Brasiliano geological record is restricted to the Paleoproterozoic (Cingolani, 2011; Oyhantçabal et al., 2011a), pointing to lack of Mesoproterozoic events and isolation of the Río de la Plata Craton during Rodinia evolution. The Congo‐São Francisco Craton, in turn, exhibits several distinct Mesoproterozoic magmatic events at ca. 1.50, 1.38 and 1.10 Ga (Ernst et al., 2013). Although the youngest event is almost coeval with the timing of Rodinia assembly, it is unclear if it is related to a convergent or an intraplate setting (Ernst et al., 2013; Debruyne et al., 2015). A different situation can be observed in the Kalahari Craton, which was clearly part of Rodinia based on the existence of the Namaqua‐Natal Belt (e.g., Thomas et al., 1994; Dalziel et al., 2000; Cornell et al., 2006; Eglington, 2006; Spencer et al., 2015). In any case, most reconstructions do not consider Mesoproterozoic connections of the Congo‐
São Francisco and Kalahari cratons, even if the former was part of Rodinia (Evans, 2009), which is further supported by detrital zircon data from the Damara Belt (Foster et al., 2015) and paleomagnetic data (Bartholomew, 2008). Hence, the Río de la Plata, Congo‐São Francisco and Kalahari cratons did not interact with each other prior to their incorporation into Gondwana, being only the latter part of Rodinia.
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On the other hand, new data presented herein further supports the allochtony of the Nico Pérez Terrane regarding the Río de la Plata Craton and its derivation from the Congo Craton, previously suggested by Oyhantçabal et al. (2011a) and Rapela et al. (2011). The oldest Neoproterozoic record in the southern Dom Feliciano Belt, in turn, is constituted by the high‐grade rocks of the Cerro Olivo Complex, which contains zircons yielding U‐Pb concordant ages of ca. 800‐
750 Ma (Hartmann et al., 2002; Oyhantçabal et al., 2009a; Basei et al., 2011a; Lenz et al., 2011). Due to similar ages reported in the Coastal Terrane of the Kaoko Belt (Kröner et al., 2004; Konopásek et al., 2008), both regions were correlated and these ages were interpreted as the timing of rifting‐
related magmatism during the Rodinia break‐up (Kröner et al., 2004; Oyhantçabal et al., 2009a; Basei et al., 2011a; Frimmel et al., 2011; Rapela et al., 2011; Konopásek et al., 2014), although some contributions indicated a convergent setting for this event (Lenz et al., 2013; Koester et al., 2016).
Hence, the African crustal affinity of the Nico Pérez Terrane together with correlations between the Cerro Olivo Complex and the Coastal Terrane point to Cryogenian rifting of the Nico Pérez Terrane from the Congo Craton margin as indicated by Rapela et al. (2011), in contrast to contributions interpreting the Cerro Olivo Complex as the basement of a distinct terrane (Bossi and Gaucher, 2004;
Gaucher et al., 2010; Basei et al., 2011a). Similarities in the detrital zircon pattern of the southwestern Dom Feliciano Belt metasediments (Chapter 4) and a quartzite of the southeastern Dom Feliciano Belt basement (Nedrebø, 2014) would further support that the underlying Nico Pérez Terrane basement and the Cerro Olivo Complex, respectively, comprised a single African block.
Further to the north, the Nico Perez Terrane extends up to the Taquarembó block in Río Grande do Sul, southeastern Brazil (Fig. 1.4; Oyhantçabal et al., 2011a). To the east of this block, isolated outcrops of gneisses of the Encantadas Complex within the Pelotas Batholith record Paleoproterozoic magmatism at ca. 2.3‐2.0 Ga (Hartmann et al., 2000b, 2003a; Leite et al., 2000;
Saalmann et al., 2011), which is comparable to magmatism recorded in the Nico Pérez Terrane in Uruguay (Chapter 3) and the Taquarembó Block. Sm‐Nd model ages in the area are mostly Archean and Paleoproterozoic (Gastal et al., 2005), being also similar to those recorded in the Nico Pérez
Chapter ). Subduction location of a m e.
al et al., 201 00a; Koester ages is also 00c, 2003b;
ependently o rised a single ks were rifte . If so, the fi during Cryog ocean separat recorded in Basei et al., of whether A
e block or no ed during R irst rifting ph al extension a e la Plata Crat ges are indica zone at ca. 60 after Rapela e on and the Co ated in blue, 00 Ma betwee t inliers of gmatism may red area ind and Amazonas
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Rapela et al. (2011) considered that the rifting gave rise to the opening of the Adamastor Ocean. Nevertheless, the Adamastor Ocean was originally defined by Hartnady et al. (1985) for the ocean closed during the evolution of the Gariep and Damara belts, and was afterwards extended to the Dom Feliciano Belt (Fragoso Cesar, 1991). Fragoso Cesar (1991) considered that the Adamastor Ocean separated the Río de la Plata and Kalahari cratons prior to the collisional phase of the Dom Feliciano Belt, which gave rise to the closure of the oceanic basin. Considering new and available evidences summarized in this work (e.g., Chapter 3, 6, 7; Section 8.2), this phase was related to the Río de la Plata and Congo collision, which also included the amalgamation of minor crustal blocks such as the Nico Pérez Terrane. Likewise, the definition of the Río de la Plata Craton after Oyhantçabal et al. (2011a) also differs from the definition considered by Fragoso Cesar (1991), as the Sarandí del Yí Shear Zone comprises its eastern boundary. Hence, the existence of the Adamastor Ocean can be questioned, or it should be at least redefined. If the Adamastor is regarded as the ocean located to the east of the Río de la Plata Craton as indicated by Fragoso Cesar (1991), it may thus represent a probable pre‐Cryogenian remnant ocean (Fig. 8.1; Cordani et al., 2003), rulling out that its opening was related to the beginning of a “Brasiliano Wilson Cycle” (Fragoso Cesar, 1991) or to Cryogenian rifting (Rapela et al., 2011). On the other hand, if Cryogenian magmatism records crustal extension that triggered the ocean development, the Adamastor would have separated the Congo Craton from the Congo‐rifted blocks (Fig. 8.1), and not from the Río de la Plata Craton as originally defined by Fragoso Cesar (1991). However, no evidences of Cryogenian oceanic crust generation were founded so far as Cryogenian rocks show Sm‐Nd Paleo‐ to Mesoproterozoic TDM ages, and some contributions even indicated that this magmatism could be associated with a convergent setting (Lenz et al., 2013; Koester et al., 2016). In order to test the validity of these hypotheses, more data are nevertheless required.
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