Precise ID-TIMS U–Pb baddeleyite ages (1110–1112 Ma) for the Rincón del Tigre–Huanchaca large igneous province (LIP) of the Amazonian Craton: Implications for the Rodinia supercontinent
Graphical abstract
Introduction
Current knowledge of the Precambrian geology of Eastern Bolivia, SW portion of the Amazonian Craton, is largely a consequence of systematic mapping of the so-called Proyecto Precambrico – a British-Bolivian cooperative program of the 1980s. This program established the general geologic relations, structural framework and tectonic significance of the major lithostratigraphic units with the support of reconnaissance geochronological and chemical data (Litherland and Bloomfield, 1981, Berrangé, 1982, Litherland et al., 1986, Litherland et al., 1989). Subsequent efforts have led to a better understanding of the timing and nature of the particular magmatic–tectonic events (e.g., Boger et al., 2005, Vargas-Mattos, 2006, Cordani and Teixeira, 2007, Santos et al., 2002, Santos et al., 2008, Cordani et al., 2009, Matos et al., 2009, Teixeira et al., 2010).
The geologic and tectonic history of Bolivia correlates well with that of the Brazilian portion of the SW Amazonian Craton where the magmatic, sedimentary, and metamorphic histories have revealed a polycyclic evolution of the continental crust, highlighted within the Mesoproterozoic Rondonian-San Ignacio and Sunsas-Aguapeí provinces (e.g., Sadowski and Bettencourt, 1996, Geraldes et al., 2001, Ruiz, 2005, Teixeira et al., 2010, Bettencourt et al., 2010, Rizzotto et al., 2014) – Fig. 1.
From a paleogeographic perspective, Amazonia is a key landmass within supercontinent cycles and for large igneous provinces – LIPs (e.g., Ernst et al., 2013a, 2014; Reis et al., 2013, Nance et al., 2014). Nevertheless, the position of the proto-Amazonian Craton in plate tectonic reconstructions is a matter of debate, such as within Nuna (also known as Columbia) and Rodinia supercontinents (e.g., Sadowski, 2002, Tohver et al., 2002, Tohver et al., 2006, Li et al., 2008, Fuck et al., 2008, Bispo-Santos et al., 2008, Cordani et al., 2009, Johansson, 2009, Casquet et al., 2010, D’Agrella-Filho et al., 2012, Reis et al., 2013).
The Proterozoic growth of Amazonia occurred through a long-lived soft accretion/collision regime from ca. 2.25 to 1.00 Ga, developed outboard from one Archean nucleus (>2.6 Ga; Central Amazonian province) – see Cordani and Teixeira (2007) for a review. As a consequence of this accretionary history, lithostratigraphic units and metamorphic episodes are progressively younger in time and space toward the SW portion of Amazonia. Hence, the Proterozoic crustal architecture encompasses five sub-parallel, NW-trending tectonic provinces, namely: Maroni-Itacaiúnas (2.25–2.00 Ga), Ventuari-Tapajós (1.98–1.81 Ga), Rio Negro-Juruena (1.78–1.60 Ga), Rondonian-San Ignacio (1.56–1.30 Ga), and Sunsas-Aguapeí (1.28–0.97 Ga) – see inset; Fig. 1. The youngest province – where the Rincón del Tigre–Huanchaca magmatism (the focus of this study) occurs – results from the 1.11–1.00 Ga Sunsas collisional orogeny (see Teixeira et al., 2010 for review). This orogeny marks the final amalgamation of the Amazonian Craton during the assembly of Rodinia, and overlaps in time with the Grenville Orogen of Laurentia (e.g., Tohver et al., 2004, Tohver et al., 2006, Cordani et al., 2009, Chew et al., 2011).
Alternative scenarios have been put forward for the Precambrian architecture of Amazonia on the basis of geologic correlations of country rocks, U–Pb data, and regional structures and metamorphic patterns. For instance, a few models argue the Sunsas belt is autochthonous, and evolved from 1.45 to 1.10 Ga (e.g., Santos, 2003, Santos et al., 2000, Santos et al., 2008). Nevertheless, this idea contrasts with the observed allochtonous features of the Sunsás orogen (Litherland et al., 1986, Litherland et al., 1989), such as the coherent transport of the folded strata of the main belt against the structurally defined Paraguá Craton or terrane as described below (e.g., Litherland and Bloomfield, 1981). In our view, the Paraguá terrane played an important role in the consolidation of the Rondonian-San Ignacio province according to geologic and tectonic evidence (Bettencourt et al., 2010; see Fig. 1). The Santos model is also inconsistent with the observed decrease in U–Pb ages of granitoid rocks in time and spatially, in coherence with coeval geologic units that are ascribed to the evolution of Paleo- to Mesoproterozoic tectonic provinces (see above). These facts are consistent with a stepwise accretionary outgrowth of the SW Amazonia, as considered in most models (e.g., Sadowski and Bettencourt, 1996, Geraldes et al., 2001, Boger et al., 2005, Ruiz, 2005, Tohver et al., 2006, Cordani and Teixeira, 2007). Consequently, we follow the tectonic model of Cordani and Teixeira (2007) with refinements by Teixeira et al. (2010) and Bettencourt et al. (2010) that consider an evolving accretionary scenario akin to modern arc settings worldwide (e.g., Condie, 2007, Cawood et al., 2009). In addition, an increasing number of paleomagnetic poles obtained from mafic rocks emplaced into the Proterozoic tectonic provinces support a plausible correlation in time and space with other crustal blocks in the world, such as Baltica and Laurentia (e.g., Tohver et al., 2006, D’Agrella-Filho et al., 2008, D’Agrella-Filho et al., 2012, Bispo-Santos et al., 2012, Reis et al., 2013). However, systematic deep seismic data are needed to characterize better the internal crustal discontinuities of Amazonia and their potential relationship with Proterozoic global paleogeography.
This paper documents new, high quality U–Pb (ID-TIMS) baddeleyite ages from two widely separated late Proterozoic igneous complexes in the Precambrian shield of Bolivia, namely the Rincón del Tigre mafic–ultramafic layered intrusion and the Huanchaca mafic intrusive suite (e.g., Berrangé, 1982, Litherland et al., 1986, Litherland and Power, 1989, Ruiz, 2005, Lima et al., 2012). The tectonic significance of the ages is addressed in the context of the evolution of the Sunsas orogeny, and as a potential link of the mafic–ultramafic magmatism with a mantle plume event.
Section snippets
Geologic–tectonic framework of Eastern Bolivia and its Brazilian counterpart
For the purpose of this work, we summarize first the Precambrian framework of the Eastern Bolivian shield with special emphasis on the Late Mesoproterozoic Sunsas orogeny (Sunsas-Aguapeí province; after Teixeira et al., 2010) to which the Huanchaca and Rincón del Tigre igneous complexes are spatially associated.
The Huanchaca intrusive suite (see Fig. 1) crops out in the Paraguá terrane (or Paraguá Craton; after Litherland and Bloomfield, 1981). Much of the Paraguá terrane was formed during the
Geology and age background of the Rincón del Tigre igneous complex
The Rincón del Tigre complex is a mafic–ultramafic layered sill (4.5 km thick; e.g., Litherland et al., 1986, Annells et al., 1986a, Annells et al., 1986b, Prendergast et al., 1998, Prendergast, 2000). This complex crops out in the southeast tip of the Sunsas belt (see Fig. 1, Fig. 2), and shows local intrusive relationships with both the Sunsas (below) and Vibosi (above) groups. Both the Rincón del Tigre complex and the Sunsas/Vibosi metasedimentary envelope were subjected to a similar low
Geology and age background on the Huanchaca intrusive suite
Huanchaca mafic magmatism, here termed the Huanchaca intrusive suite (Ruiz et al., 2010, Lima et al., 2011, Lima et al., 2012, Sécolo et al., 2011), was previously termed the Huanchaca Dolerite Suite (Litherland et al., 1986). It is characterized by prominent mafic sills and related dykes that crosscut the Huanchaca platform cover (“Serrania Huanchaca-Ricardo Franco”) and/or the Pensamiento Granitoid Complex and older rocks of the Paraguá terrane at the Bolivia-Brazil border, nearby the town of
ID-TIMS U–Pb methodology and results
Grains of baddeleyite (ZrO2) were successfully recovered from both the Rincón del Tigre and Huanchaca intrusions, allowing, for the first time, precise ID-TIMS U–Pb isotopic measurements to constrain magmatic crystallization ages for these bodies. Despite its generally fine grain size (20–150 μm) and thin, delicate, blade-like forms, baddeleyite is widely recognized as the principal mineral suitable for accurate and precise dating of the emplacement and crystallization of gabbro and dolerite
Regional distribution of the Rincón del Tigre–Huanchaca LIP in Amazonia
The age for the Rincón del Tigre granophyre is identical to the age of the Huanchaca mafic sill, which is located some 500 km away (see Fig. 1), and both units have similar intraplate chemistry. Such a precise age match suggests that these two units belong to the same intraplate magmatic event. If so, then this anorogenic (intraplate) event would be regionally significant in the SW portion of the Amazonian Craton. Intraplate magmatism of large scale and potential short duration should be termed
Conclusions
High quality U–Pb (ID-TIMS) geochronology has provided the first precise ages for the Rincón del Tigre layered complex (granophyre; 1110 ± 2 Ma) and a Huanchaca mafic sill (1112 ± 2 Ma). The identical ca. 1110–1112 Ma crystallization ages obtained for each intrusion and the large distance between them (about 500 km), suggests that these belong to a previously unrecognized LIP in Amazonia. Additional dolerite sills and dykes are postulated to belong to this LIP, as well as related silicic magmatism with
Acknowledgements
The first author thanks the Brazilian National Research Council (CNPq) for its continuous support for the research projects. This is publication no. 36 of “The Large Igneous Provinces – Continental Reconstruction – Metallogeny, Industry Consortium Project (www.supercontinent.org)”. Finally the authors are grateful to the reviewers and the Editor-in-Chief for offering constructive comments and improvements in the early version of the manuscript.
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