Crustal reworking and Archean TTG generation in the south Gavião Block, São Francisco Craton, Brazil
Introduction
Great part of the Earth’s Archean crust was formed by the addition of mantle-derived melts as pointed by recent models of crustal growth based on Hf isotopes (Belousova et al., 2010, Dhuime et al., 2012), with crust reworking processes being frequently overlooked. A large part of these juvenile melts evolved to form plutonic rocks, collectively known as tonalite-trondhjemite-granodiorites (TTGs) (Defant and Drummond, 1990, Martin, 1994). In general, TTGs are silica-rich (SiO2 ≈ 64 wt% average) and sodic (3.0 wt% < Na2O < 7.0 wt%; with low K2O/Na2O) rocks, with trace element patterns of typical modern “arc” rock types, including negative Nb-Ta and Ti anomalies, elevated Sr and Eu, but low Yb and Y contents (Moyen, 2011).
Partial melting of hydrated meta-mafic rocks leaving a garnet-rich residue or deep differentiation products of mafic melts at high pressures are commonly accepted as the source of the TTGs (Foley et al., 2002, Moyen and Martin, 2012, Rapp et al., 1991). The depletion in heavy rare earth elements (HREE) of these rocks are thought to reflect the garnet stability in the residual portion of the molten mafic rock providing qualitative-quantitative conditions of pressure and hence the depth of the TTG melt formation (Moyen and Martin, 2012 and references therein). Although there is a certain agreement regarding their petrogenesis, the geodynamic setting of TTGs formation is still a matter of debate. Some authors argue for subduction models, mainly driven by the partial melting of the subducting mafic oceanic plate, in a similar process to modern adakite formation (Defant and Drummond, 1990, Foley et al., 2002, Martin and Moyen, 2002, Rapp et al., 2003). However, partial melting experimental investigation within P-T window formation of TTGs demonstrates that melting of mafic lower crust can equally generate the adakitic signature, although with higher K2O/Na2O and enriched Sr-Nd-Hf isotopes (Qian and Hermann, 2013). On the other hand, some refer to intra-plate models for TTG generation, owing to the different behavior of the continental crust during the early Archean, at which TTGs are formed at the bottom of thick (>40 km) plateau-like mafic crust (Bédard, 2006, Johnson et al., 2017, Van Kranendonk et al., 2015). Additionally, models with variations on these two end-members, and consequently simultaneous operation of both processes have been also proposed (Laurent et al., 2019, Moyen and Laurent, 2018, Van Kranendonk, 2010), as well as stagnant lid models with combined downward dripping of mafic crust and upward diapirs of felsic rocks (Capitanio et al., 2019, Nebel et al., 2018).
Another ubiquitous feature of Archean continental crust is the transition from older, juvenile sodic TTG to the late-Archean high-K granitic magmatism (Laurent et al., 2014). This period differs in each craton, but, in general, it follows a two-stage chronologic sequence of: (1) long period of TTG emplacement (0.2–0.5 Gyr); and (2) a shorter period when high-K and hybrid granitoids are emplaced coeval or not with TTGs (Laurent et al., 2014). Several authors address this geochemical change to the beginning of collision tectonics, and thus crustal reworking of previous juvenile crust (Johnson et al., 2019, Laurent et al., 2014). Moreover, this transition has been documented mostly from ca. 2.95 to 2.50 Ga at several cratons (Laurent et al., 2014), coinciding with major geochemical changes in the Archean plutonic record, mainly regarding to variations in the K2O/Na2O, Sr/Y, and (La/Yb)N ratios (Johnson et al., 2019).
The south Gavião Block (GB) of the São Francisco Craton (SFC), located in Bahia state of Brazil, is one of the best exposures of Eo- to Paleoarchean continental crust, including one of the oldest rocks of South America (ca. 3.6 Ga-old Mairi Complex; Oliveira et al., 2020). Also, the ca. 3.4–3.1 Ga-old Sete Voltas massif (Martin et al., 1997, Nutman and Cordani, 1993, Zincone, 2016) is a well-documented unit of the GB that includes TTGs derived from melting of low-K mafic crust, and granitoids resulted from the reworking of the former TTGs (Martin et al., 1997). The most extensive unit of this block is the homonymous ca. 3.3–3.2 Ga-old Gavião Complex (Barbosa and Sabaté, 2004, Bastos Leal, 1998, Santos-Pinto et al., 2012), consisting of TTGs, high-K granites, and diorites, whose precise timing of magmatism, their sources, and petrogenetic evolution are still open to investigation.
We report new SHRIMP U-Pb zircon ages and whole-rock geochemical data from rocks of the Gavião Complex and Caraguataí Suite, within the Gavião Block, augmented by an extensive compilation of published and unpublished data of the Gavião Block (zircon and monazite Pb-Pb and U-Pb dating, Hf-Nd isotopes and whole-rock geochemistry) and detrital zircon from São Francisco and Congo cratons. We demonstrate that the Gavião Complex is constituted by TTG pulses formed at different depths, as well as reworking of previous TTGs with high-K granite intrusions. The data altogether indicate that the main mechanism of crustal growth and TTG formation from 3.6 to 3.2 Ga is dominated by crustal reworking instead of juvenile accretion, as proposed to other cratons (e.g., Bolhar et al., 2017, Zeh et al., 2013). Additionally, we discuss on the secular evolution of sources, metamorphism and tectonic processes, comparing it worldwide.
Section snippets
Geological outline of the Gavião Block
The São Francisco Craton (SFC) is one of the five Brazilian cratons that formed the Gondwana Supercontinent and it is the South American counterpart of the Congo Craton (Africa), connected prior to the continental breakup and the formation of the South Atlantic Ocean, during Cretaceous (Cordani et al., 2003, Heilbron et al., 2017) (Fig. 1A). Situated in the central-northeastern part of Brazil, the craton’s basement is exposed in two segments, to the north and south, with most of the craton
Zircon U-Pb SHIRMP
Eight samples from three outcrops were collected near Brumado town and Ubiraçaba village. The descriptions and details about location can be found in Table 1. We focused on outcrops with complex intrusion and melting relations to constrain the timing of distinct thermal events within the Gavião Complex. The samples were crushed and powdered. Heavy minerals were concentrated on a shaking table and classified using a Frantz isodynamic separator. The zircon concentrates from the least magnetic
Zircon U-Pb dating
Three outcrops were selected and eight samples in total were dated by U-Pb SHRIMP (Fig. 3). Two of them contain rocks interpreted as the Gavião Complex (LL-341 and LL-324), the other one (LL-344) contains rocks of the Caraguataí Suite and one xenolith interpreted as from the Gavião Complex. The details about location, units, lithologies, and results are presented in Table 1. Lower intercept ages present elevated uncertainties, and they are not linked to any known Pb loss event, thus they should
Metamorphism and the effects for element mobility of Gavião Complex rocks
It has been shown that the Gavião Block was deformed by regional-scale metamorphism and therefore the mobility of elements in the TTGs and other granitoids must be addressed. Some studies have shown that post-magmatic alteration and metamorphism of Archean volcanic rocks and gneisses are minor on REE (except Eu), HFSE, and some transition elements (Cr, Ni, Co, V, and Sc), whereas LILE (K, Na, Cs, Rb, and Ba) can be easier mobilized (e.g., Polat and Hofmann, 2003). The samples of Gavião Complex
Conclusions
Based on field relationships, newly obtained and compiled data reported here, we conclude:
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The Paleo- to Mesoarchean basement of the south Gavião Block is principally composed of TTGs and high-K granites of the Gavião Complex, with magmatic activity at ca. 3.36–3.18 Ga, with reworking and anatexis between 3.27 and 3.22 Ga.
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The TTG rocks of Gavião Complex can be divided into high-Sr (high pressure) and low-Sr (low- to medium pressure), evidencing that the sodic magmatism likely occurred at
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
The authors wish to thank Ms. Wang Chen (Deputy Director of Beijing Shrimp Center) for her support in the SHRIMP zircon dating. This research was carried out by a cooperation agreement between the Geological Survey of Brazil and the China Geological Survey. The authors would like to thank the direction and the supporting staff of both institutions, whose efforts made this research possible.
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2022, Journal of Asian Earth Sciences: XCitation Excerpt :The worldwide Precambrian cratons such as Wyoming Craton, Superior Craton, Baltic Craton, Slave Craton, North China Craton, and Indian Cratons are the prime reservoirs for the potassic granitoids (high-K granitoids). These granitoids are mainly derived from the melting of pre-existing crustal materials, including TTGs and supracrustal rocks (Almeida et al., 2013; Champion and Smithies, 2007; Feng and Kerrich, 1992; Laurent et al., 2014; Lopes et al., 2021; Mikkola et al., 2011 and references therein). The BGC of the Aravalli craton is made up of largely granitoids that are calc-alkaline nature and have high-K contents (Ahmad et al., 2016, 2018, 2019, 2020; Mondal et al., 2020; Rahaman et al., 2019; Rahaman and Mondal, 2013, 2015; Kaur et at., 2019, 2021).