Crustal reworking and Archean TTG generation in the south Gavião Block, São Francisco Craton, Brazil

Highlights

• The Gavião Complex has TTGs, high-K granites, and minor diorites.

• Melting of low-K mafic crust at the base of an oceanic plateau produced the TTGs.

• One of the first registers of potassic magmatism at 3.35 Ga in the Gavião Complex.

• The oldest metamorphic age for the São Francisco Craton was dated at c. 3.27 Ga.

• The Archean felsic crust of Gavião Block was formed by crustal reworking.

Abstract

Origin of early Earth’s felsic crust is a matter of growing debate in which the rock record from old cratons plays a key role. In northeast Brazil, the São Francisco Craton preserves the oldest felsic rocks of South America and understanding its Archean evolution is an important piece of this debate. Here we present new SHRIMP U-Pb zircon ages and whole-rock geochemistry, as well as a large, compiled dataset to discuss the evolution of the south Gavião Block (GB), a major component of this craton. The GB contains Paleo- to Mesoarchean (3.36–3.18 Ga) TTGs, high-K granites, and minor diorites of the Gavião Complex followed by renewed magmatism, reworking and anatexis between 3.27 and 3.22 Ga. 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 different depths diachronically between 3.36 and 3.20 Ga. We report a crystallization age of a ca. 3.35 Ga for an Archean unusual high-K calc-alkaline meta-granite which marks the first potassic magmatism of the Gavião Complex, suggesting reworking of preexisting TTGs. Zircon U-Pb dates from a TTG migmatite constrained an age of ca. 3.27 Ga, interpreted as a metamorphic event within the basement of the GB, stretching back in ca. 120 My the oldest metamorphic age known so far for the whole São Francisco Craton. Additionally, at ca. 2.7 Ga, the GB was intruded by the intraplate, alkaline magmatism of the Caraguataí Suite, marking thus the stability of this Block during Neoarchean. We argue that crustal reworking was the main mechanism responsible for the formation of the GB during the Paleo- to Mesoarchean. The data suggest the GB rocks share an old source possibly including Hadean components. We propose that the formation of the TTGs of GB is a result of the melting of low-K mafic crust at the base of a progressively thicker oceanic plateau.

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.