The 2.26 to 2.18 Ga Arc-Related Magmatism of the Almas-Conceição do Tocantins Domain: An Early Stage of the São Francisco Paleocontinent Assembly in Central Brazil
Introduction
Continental crust is generated by magmatism dominantly within arc-related settings formed along subduction zones whereas the contribution of accretionary and collisional orogens to the formation of continental crust remain contentious (Cawood et al., 2009, 2013). Through partial melting and mixing of continental crust and mantle sources, such settings are important sources of felsic magmatism and the consequent diversity of granites (Frost et al., 2001).
Mixing of igneous and sedimentary sources originates peraluminous to strongly peraluminous granites (Barbarin, 1999; Collins and Richards, 2008). Peraluminous granites can be either generated by partial melting of metasedimentary rocks (S-type) or igneous rocks (I-type), a well-documented process in experimental petrology (Patiño Douce, 1999; Clemens and Stevens, 2012). Thus, the recycling of older igneous/sedimentary sources to generate new continental crust is a complex process that offers an opportunity to understand how the cycles of continental amalgamation and break-up evolved throughout Earth's history.
Widespread Paleoproterozoic felsic magmatic activity, particularly within 2.18 and 2.03 Ga, is linked to the amalgamation of the São Francisco (-Congo) Paleocontinent, one of the building blocks of the Columbia Supercontinent (Zhao et al., 2004; Cordeiro and Oliveira, 2017). Within such time span, older crustal blocks were welded together with multiple contemporaneous intra-oceanic arcs. However, contemporaneous orogenic systems are widely restricted to South American and African terranes, which still have obscure crustal recycling mechanisms.
Here we present geologic, geochronological (U–Pb in zircon) and whole-rock geochemical data for the Rhyacian magmatism related to the Almas-Conceição do Tocantins Domain. This area corresponds to the western part of the São Francisco palecontinent which, unlike the cratonic core, was affected by later deformation and is henceforth referred to as the São Francisco pericraton. We also discuss the role of magmatism associated with crustal recycling for generating multiple types of granitoids and the implications of this magmatism for the assembly of the São Francisco paleocontinent.
Section snippets
Geological setting
The Almas-Conceição do Tocantins Domain (Fig. 1B) is the northeastern most portion of the Goiás Massif, a ~65.000 km2 collage of dominantly Paleoproterozoic terranes cropping out as the basement of the Brasília Belt (Fuck et al., 2014; Cordeiro and Oliveira, 2017; and references therein). In the Neoproterozoic, The Goiás Massif represented the western edge of the São Francisco paleocontinent (Fig. 1A) during its collision against the Amazonian paleocontinent (Cordeiro and Oliveira, 2017; Hasui
Local geology
Several Rhyacian units occur within the Almas-Conceição do Tocantins Domain with ages spanning from 2.29 to 2.18 Ga. The description of these units will follow a geochronological order from the oldest to the youngest according to ages from this work and the literature. Geological units mapped in this work and their general macroscopic and petrographic features are summarized in Table 1 and Fig. 3, Fig. 4.
U–Pb LA-MC-ICPMS and SIMS (SHRIMP) geochronology
Four zircon U–Pb ages by Laser Ablation Microprobe Multi-Collector Inductively Coupled Plasma Mass Spectrometer (LAM-MC- ICPMS) and one by Secondary Ion Mass Spectrometer (SIMS) techniques were performed in the attempt to date the major magmatic events of the studied area. The new ages obtained in this work and the available geochronological data in the literature are reported in Table 1. Tables with analytical data are in the supplementary materials section (Tables A-E). The localization of
Monzogranite Unit: sample AS-378
Sample AS-378 is a pink, porphyritic, medium-to coarse-grained biotite meta-monzogranite from the Pau Ramalhudo Body (Fig. 4A) showing foliation along a N45°E trend. Quartz, K-feldspar, plagioclase and biotite are essential minerals whereas zircon is accessory and sericite with chlorite are subordinate. Zircon is represented by one family of medium bi-pyramidal crystals with some inclusions and abundant fractures and discrete oscillatory zonation under BSE images.
The isotopic composition of 33
Source and tectonic setting
Regarding granite classification, it is extremely important to identify the nature of the magma source (Chappell et al., 2012). The GTS and PS samples define different trends in Fig. 9a where the GTS compositions are similar to those of experimental melting of amphibolites and quartz-feldspathic (metagreywackes or orthogneisses) sources. The PS compositional trend is mainly related to experimental melts derived from quartz-feldspathic rocks, suggesting derivation from a more felsic crustal
Conclusions
The U–Pb ages indicate that the Pau Ramalhudo Body of the Monzogranitic Unit (2.29 Ga) and the Quartz-dioritic Suite (2.28 Ga) correspond to the earliest Rhyacian magmatic stages within the Almas-Conceição do Tocantins Domain. A later magmatic event around 2.26–2.2 Ga represents the development of a continental magmatic arc and the emplacement of the Granodioritic to Tonalic Suite-GTS and the Serra do Boqueirão Suite. The GTS is composed of metaluminous to peraluminous I-type granites likely
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.
Acknowledgements
We thank the Institute of Geosciences of the University of Brasilia (IG/UnB) and the Geological Survey of Brazil (CPRM) for the field work support and the laboratory analysis. The CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) is acknowledged for grants provided to CGO (process number 305769/2019–7). We are grateful to Dr. Natalia Hauser, Dr. Andres Folguera and Dr. Reinhardt Fuck for handling this paper and to the three anonymous reviewers for the valuable suggestions and
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