January 2017 LIP of the Month

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3.30 Ga High-Silica Intraplate Volcanic-Plutonic System of the Gavião Block, São Francisco Craton, Brazil: possible Silicic Large Igneous Province

Stefano A. Zincone a,b, Elson P. Oliveira a, Oscar Laurent c, Hong Zhang d, Mingguo Zhai e

a Department of Geology and Natural Resources, Institute of Geosciences, University of Campinas, P.O. Box 6152, 13083-970 Campinas, Brazil

b Department of Geology, School of Mine, Federal University of Ouro Preto, Brazil

c Université de Liège, Département de Géologie B20, Quartier Agora, Allée du six Août 12, B-4000 Liège, Belgium

d State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, 229 Taibai road, Beilin District, Xi’an, 710069 China

e Institute of Geology and Geophysics, Chinese Academy of Sciences, 19 Beitucheng Xilu Street, Beijing, 100049 China

 

E-mail address: teozincone@gmail.com (S. Zincone)

 

Extracted and modified after: Zincone, S. A., Oliveira, E. P., Laurent, O., Zhang, H., Zhai, M. 2016. 3.30 Ga high-silica intraplate volcanic–plutonic system of the Gavião Block, São Francisco Craton, Brazil: Evidence of an intracontinental rift following the creation of insulating continental crust. Lithos, 266, 414-434.

For full details see that paper.

1. Introduction

Most intraplate volcanic systems, such as silicic large igneous provinces (SLIPs), bimodal volcanic systems, oceanic islands and oceanic plateaux, compositionally differ from magmas generated at plate boundaries, with the former being hotter, drier and induced by anhydrous decompression melting, and the latter are typically colder, H2O-rich and formed by fluid-driven flux melting (Bailey, 1983; Grove et al., 2012; Lee and Bachmann, 2014). Intraplate magmatism generally affects continental or oceanic interiors undergoing extension and is actively driven by narrow mantle plumes originating from relatively shallow asthenosphere upwelling (Anderson and King, 2014), or passively driven by plate motion (Ziegler and Cloetingh, 2004). The addition of underplating mafic magma in intraplate environment provides a “non-plate tectonics” way for crustal growth and thickening (Thybo and Artemieva, 2013), and may contribute to remelting of parts of the original crust. The continuous underplating of mafic magmas causes the progressive differentiation of the continental lithosphere and stabilization by lower crustal delamination events (e.g., Bédard, 2006; Johnson et al., 2013).

Tonalite-trondhjemite-granodiorite suites (TTGs) represent the dominant lithology of Archean crust (Arndt, 2013; Moyen, 2011), especially for the Eo- and Paleoarchean periods (4.0-3.2 Ga). However, several recent studies have emphasized that Eo- to Paleoarchean crustal segments often contain a diversity of granitoids that are not all akin to TTGs. Some examples are the ca. 4.02 Ga Idiwhaa tonalites in the Slave craton, NW Canada (Reimink et al., 2014); the ca. 3.60 Ga ferroan augen gneiss suite in the North Atlantic Craton of SW Greenland (Friend and Nutman, 2005; Nutman et al., 1996); as well as some components of the 3.66-3.45 Ga Ancient Gneiss Complex (Hoffmann et al., 2016) of the eastern Kaapvaal Craton in southern Africa. Although subordinate in volume, these felsic rocks may represent important geodynamic events in the process of craton building and stabilization and must be considered along with TTG to depict the processes of Archean crust evolution. Specifically, petrogenetic studies of these non-TTG components showed that they likely formed in intraplate settings in which the crust is heated up by mantle upwelling and mafic magma underplating (Nutman et al., 1984; Hoffmann et al., 2014; Reimink et al., 2014), which could be interpreted as the result of a mantle plume impingement at the base of a preexisting continental crust.

Therefore, in order to better constrain the role of these processes in the formation and evolution of continental lithosphere in the early Earth, it is necessary to better characterize and study the origin of non-TTG components in Eo- to Paleoarchean terranes. Felsic 3.30 Ga-old volcanic rocks and associated granitoids occur at the eastern margin of Gavião Block, São Francisco craton, in association with the Contendas-Mirante and Mundo Novo supracrustal belts (Marinho et al., 1994; Peucat et al., 2002). They are geologically and petrologically unrelated to classical TTGs. Zincone et al (2016) described the petrology, whole-rock geochemistry, zircon U-Pb ages and Lu-Hf isotopic signatures of these felsic and plutonic rocks. Based on those data and coupled with geochemical modeling those authors investigate the nature of the source and interpreted the magmatic evolution as an inter-related high-silica plutonic-volcanic system formed in an intracontinental tectonic setting following the creation of insulating continental crust. The combined results were used to discuss the tectonic environment of the volcanic-plutonic system related to the evolution of the Gavião Block and to explore the general role that intraplate magmatism might play in craton formation and evolution. Here, we used the combined results to speculate about the possibility of the 3.30 Ga plutonic-volcanic system be a SLIP.

2. Geological setting

The São Francisco Craton (Fig. 1) comprises Archean and Paleoproterozoic terranes of low-grade to granulitic metamorphic facies. The Gavião Block forms the western segment of the São Francisco basement. It mainly consists of 3.40 - 3.33 Ga granitic gneiss complexes with TTGs and medium- to high-K calc-alkaline composition (Martin et al., 1997; Nutman and Cordani, 1993; Santos-Pinto et al., 1998, 2012; Marchesin, 2015; Zincone, 2016) and the 3.30 Ga plutonic-volcanic system (Zincone et al., 2016).


Figure 1: Simplified geological map of the São Francisco Craton (modified after Silva et al., 2015). The letters indicate the four main complexes within the Gavião Block; P: Porteirinha Complex; G: Gavião Complex; M: Mairi Complex; S: Sobradinho Complex. The red rectangle represents the area covered by Fig. 2.

The rhyolites occur associated with two distinct supracrustal sequences that occur 200 km apart from each other (Fig. 2A). The north Mundo Novo supracrustal belt (Fig, 2 B; Mascarenhas and Silva, 1994; Peucat et al., 2002) represents an intracratonic basin, with felsic volcanism and a continental to marine sedimentary sequence (Loureiro and Santos, 1991). The 3.30 Ga Contendas Rhyolite occurs in the southern sector, partly overlain by the Paleoproterozoic Contendas-Mirante basin (Fig. 2 C; Nutman et al., 1994; Zincone and Oliveira, in review). The Jacobina Basin, occurring west of the Mundo Novo belt, comprises siliciclastic units containing only 3580 to 3280 Ma detrital zircon grains, with the most abundant population at 3300 Ma (Teles et al., 2015; Barbuena et al., 2016), and was developed on the stable platform represented by >3.4 Ga gray gneiss of the Gavião Block (see Mascarenhas et al., 1998 and Pearson et al., 2005 for a review). Similar 3350 to 3280 Ma zircon-bearing quartzite has been recently described in spatial association with the investigated rhyolites (Zincone, 2016; Zincone and Oliveira, in review; Barbuena et al., 2016).


Figure 2: Simplified geological map of Contendas-Jacobina lineament at the eastern border of Gavião Block (modified from; Mascarenhas et al., 1998). In the south the low metamorphic grade Paleoproterozoic Contendas-Mirante Basin is sandwiched between the Gavião and Jequié blocks, and it is correlated with the northern Saúde basin. The grey rectangle represents the area covered by Fig. B and C, showing the geological map for the Mundo Novo Rhyolite and the Contendas Rhyolite, respectively.

3. Geological setting and petrography of the rhyolites and coeval plutonic rocks

The contact between rhyolite and basement gneisses or metasedimentary rocks is not observed; the volcanic rocks occur as inliers within Paleoproterozoic cover. The rhyolites are not interlayered with any other meta-volcanic unit and the maximum metamorphic grade is lower greenschist facies. For details on the petrography of the units see Zincone et al. (2016).

Plutonic rocks occur in the vicinity of both investigated rhyolites. The Boa Sorte meta-syenogranite is a pluton emplaced within the grey gneisses complex. The 3.35 Ga Boa Vista dome (Nutman and Cordani, 1993) occurs in the southern part of the Contendas-Mirante basin. The northeastern portion is largely composed of biotite meta-granite (sample TZD-199A) cut by albite granite dykes. The Calderão diorite gneiss is located between the Sete Voltas and Boa Vista domes.

4. Zircon U-Pb-Hf studies and whole-rock geochemistry

4.1. U-Pb geochronology

Zincone et al (2016) investigated four samples for geochronology: two rhyolite and two granite. Zircon grains in all four samples show similar shapes (perfectly euhedral, translucent, colorless to light pink stubby prisms ranging from 150 to 400 μm and aspect ratio of 3:1), concentric oscillatory zoning with no overgrowths and concordant U-Pb ages (Fig. 3). The rhyolites lack zircon xenocrysts and core-rim relationships. The igneous zircon crystallization occurred in the late stages of magma cooling and holds high zircon saturation temperature (ranging from 833 to 915 oC). Zircon Th/U ratios are ~0.5 in the rhyolites and slightly higher (0.7-0.8) in the granites. The two rhyolites present the same Concordia age of 3304.3 ± 8.2 Ma and 3303.7 ± 11 Ma. The granites present distinct crystallization age of 3292 ± 3 Ma for Boa Sorte biotite meta-granite and of 3327.7 ± 3 Ma for the Boa Vista biotite meta-granite.


Figure 3: Concordia diagrams (206Pb/238U vs. 207Pb/235U) for LA-ICP-MS zircon analyses on samples from the Contendas (13°40'10.40"S; 40°54'22.46"W) and Mundo Novo (11°53'36.79"S; 40°29'25.08"W) rhyolites and from the Boa Sorte (12°14'1.82"S; 40°34'27.43"W) and Boa Vista (14°22'38.59"S; 40°44'47.77"W) granites, the plutonic equivalent of the rhyolites. Error ellipses are 2σ.

4.2. Zircon Hf isotopes

Hf-in-zircon analyses were carried on the rhyolites. The 176Hf/177Hf(3.3 Ga) ratios overlap for both rhyolites, but are very heterogeneous (ranging from 0.280474 to 0.280659). Most of the corresponding εHf(3.3 Ga) values are negative ranging between 0.35 and -6.26 (TDM from 3.54 and 3.88 Ga; Fig. 4).


Figure 4: Composite Hf isotope evolution diagram. (a) 40 analyses from Contendas Rhyolite; (b) 30 analyses from Mundo Novo Rhyolite. 176Lu/177Hf evolution lines are based on assumed 176Hf/177Hf crustal ratio of 0.0113 (Bouvier et al. 2008). The 176Lu decay constant of Söderlund et al. (2004) and Scherer et al. (2001) were used in the calculation of εHf(3.3 Ga) values.

4.3. Whole-rock geochemistry

The two spatially distinct rhyolites show strikingly similar major and trace element signatures. All samples are silica oversaturated with more than 43% normative quartz and SiO2 ranging from 74 to 79 wt.%. They have low Al2O3 (<11 wt.%) contents and K2O/Na2O ratios between 0.5 and 1. The MNR samples display higher MgO (average 1.07 wt.%) and lower CaO (average 0.18wt.%) than the CR (average MgO 0.48 wt.%; CaO = 0.73%wt). Intriguingly, the most silica-rich samples are also the richer in MgO. The two rhyolite units are also depleted in ferromagnesian elements (Fe2O3(T)+MgO+MnO+TiO2 < 4.4 wt.%), with relatively high Cr (5-24 ppm) and low Ni (<1 ppm) contents.

On classification diagrams the samples fall in the sub-alkaline rhyolite field and show a narrow compositional range from rhyolite to quartz keratophyre (Fig. 5 A-C). The CR samples classify as metaluminous to slightly peraluminous, ferroan calcic rocks whereas the MNR samples are slightly to strongly peraluminous, ferroan to magnesian calcic rocks (Fig. 5D). All the samples match the medium-K calc-alkaline fields on the AFM diagram and SiO2 vs K2O plot.


Figure 5: Classification diagrams. A) Total alkalis vs. silica diagram (Middlemost, 1994); B) Nb/Y vs. Zr/TiO2 (Winchester and Floyd, 1977). C) Ternary diagram of feldspar: Ab - albite; An - anortite; Or – orthoclase (O'Connor, 1965); black field represents the TTG series and gray field represents the calc-alkaline series commonly related to modern continental arcs (BADR” suites). D) Classification diagrams for granitic rocks. Alumina saturation index (molecular [Al2O3/(CaO+Na2O+ K2O)]) against partial saturation index (molecular [Al2O3/(Na2O+ K2O)]) (Shand, 1943). Fe# [(Fe2O3(T)/(Fe2O3(T)+MgO)] vs SiO2. The ferroan signature is given when Fe# > 0.486 + 0.0046 x SiO2 wt.% (Frost et al., 2001). MALI index (Na2O+ K2O-CaO) vs. SiO2 plot showing the calcic affinity from the rhyolites.

The rhyolites display strong depletions in Sr and Ti, and moderate Nb-Ta negative anomalies. In addition, they are depleted in Rb relative to Th and enriched in Ba relative to Sr (Fig. 6 A). The REE patterns (Fig. 6 B) of both rhyolite units show no difference and present slight enrichment of the light REE relative to heavy REE (average La/Yb(N) ratio = 4.75) and marked negative Eu anomalies (Eu/Eu* = 0.55).


Figure 6: Spider diagrams. A) Mantle-normalized multi-element diagram. Normalizing values after McDonough and Sun (1995). B) Chondrite-normalized REE diagram. Normalizing values from McDonough and Sun (1995).

The Calderão diorite is an intermediate (58 wt.% SiO2) metaluminous and alkali-calcic rock of ferroan affinity (Fig. 5 D). It is poorer in K2O, SiO2 and richer in Fe2O3(T), MgO, CaO and Al2O3 than the granite and rhyolites. Although it has comparable profiles in multi-element diagrams and REE patterns, the Calderão diorite is also distinguished from the more felsic rocks by its lower contents in highly incompatible elements, less pronounced Nb, Ta, Sr and Ti troughs in primitive mantle normalized trace element diagrams, the absence of a negative Eu anomaly (EuN/Eu* = 0.92) and lower HREE contents (La/YbN = 18.8).

5. Discussion

5.1. Element mobility

Some MNR samples are characterized by shear planes filled with recrystallized muscovite and present green biotite and chlorite, resulting in strongly peraluminous character (Fig. 5 D). Their peculiar composition might thus reflect superimposed metasomatism due to focused fluid-flow regimes during later tectonothermal events. The fluid-dominated alteration modified some of their mineralogy and major element chemistry, but did not affect the trace element patterns (Fig. 6).

Asides from the most deformed samples, element mobility due to post-crystallization events seems to be minor or even absent, which is in line with preserved igneous textures and mineralogy. Specifically, the lack of any clear negative correlation between silica, other major element, and trace element contents argues against any role played by silicification in the generating the high-SiO2 content of these rocks.

5.2. Petrogenetic modeling

In order to constrain the differentiation processes and the source, we compared the major element composition of the rhyolites with experimental melts relevant for crustal melts (i.e. P <1.5 GPa; T <1200°C; SiO2 >70 wt.%.). The tight compositional range of the rhyolites indicates that they represent near-liquid compositions. In contrast, the Boa Sorte and Boa Vista granites show compositions that are closer to the field of experimental leucogranites. These granites and the rhyolites share compositional affinities and define rough linear trends in Harker diagrams. We therefore propose that they are cogenetic and specifically that the rhyolites represent differentiated liquids from a parental composition close to that of the granites. This suggests two-step mechanism with (1) derivation of the granites by partial melting of crustal rocks or crystallization of a mafic magma and (2) further differentiation of the granitic bulk composition to form the rhyolites. This two-step model is consistent with petrogenetic models of recent high-SiO2 rhyolites, indicating that these represent extracted liquids from crystal mushes formed in magma chambers of intermediate-felsic composition (Bégué et al., 2014; Dufek and Bachmann, 2010; Graeter et al., 2015).

We used least-squares mass balance calculations on major elements to check if the inferred two-step petrogenetic model (Calderão diorite → Boa Sorte granite → MNR and CR rhyolites) was plausible. See details of modelling in Zincone et al. (2016).

It must be noted that the entire process modelled is consistent with the formation of large volumes of rhyolites by segregation of residual melt within long-lived, crystallizing mush zones, as described in modern settings in which granitoids represent the leftovers of large rhyolitic eruptions (Bachmann and Bergantz, 2004, 2008; Gelman et al., 2014; Lipman and Bachmann, 2015). Moreover, the model predicts that the bulk proportion of fractionated crystals relative to the initial amount of melt is c. 50%. This is consistent with the results of numerical models showing that the probability of residual melt extraction in crystal mushes (and thus ascent and eruption of high-SiO2 rhyolites) dramatically increases as soon as the crystal fraction reaches c. 50%, irrespective of the initial melt composition and size of the magma chamber (Dufek and Bachmann, 2010).

The two-step model was tested and supported using trace element modelling as detailed in Zincone et al. (2016).

5.3. Interpretation of Hf isotope data

The large variations observed in zircon εHf(3.3 Ga) most likely reflects heterogeneous Hf isotopic composition of the magma from which zircon crystallized. The results of geochemical modelling suggest that differentiation of a parent dioritic liquid formed a granitic magma, crystallized at relatively shallow (<10 km) reservoirs and from which highly silicic residual liquid was extracted to erupt as rhyolites. In the scope of this model, the heterogeneous Hf isotopic composition of the rhyolites could result from two (non-exclusive) scenarios: (i) the source (i.e. diorite parent) itself had a heterogeneous Hf isotopic composition (e.g., Belousova et al., 2006; Kemp et al., 2008; Appleby et al., 2010; Kröner et al., 2013); or (ii) the granites and rhyolites were formed by disequilibrium partial melting of the zircon-bearing diorite source, with zircon specifically remaining in the residual phase, which is able to produce large variations in Hf isotopic compositions of the resulting melts (Tang et al., 2014; Gerdes and Zeh, 2009; Laurent and Zeh, 2015).

Despite the variation, almost all zircon εHf(3.3 Ga) values in the rhyolites are negative. In this respect, the two possible differentiation mechanisms of dioritic material for the origin of granites and rhyolites have different implications. The crystal fractionation model implies that the latter had itself a negative and heterogeneous εHf(3.3 Ga), most likely implying crustal contamination of the parent, mantle-derived diorite magma. If partial melting is considered, then it means that the diorite may have had a juvenile isotope composition at the time of emplacement and, in that case, be much older (>3.5 Ga). In either case, the negative εHf(3.3 Ga) of zircons in the studied rhyolites indicate that the rhyolite magmatism contributed to the reworking of older crustal components, potentially as old as 3.5 - 4.0 Ga according to the Hf model ages.

5.4. Comparison with other Archean granitoids and tectonic settings

The studied granites and rhyolites are completely different compared with major and trace element features of the most common Archean granitoid types, such as TTGs, sanukitoid and crust-derived granites (e.g. Laurent et al., 2014a; Moyen, 2011; Martin et al., 2014; Condie, 2014), precluding any affinity with these rocks (Fig. 7 A-B). This observation, and the fact that the rhyolites are younger than the 3.40-3.33 Ga TTG and medium- to high-K calk-alkaline magmatism of the Gavião Block, contrasts strikingly with the relationship observed in most Paleoarchean terranes, where the felsic volcanics erupted in the greenstone belts are similar in composition to, and coeval with the associated TTG gneisses (Agangi et al., 2015; Smithies et al., 2007; Kohler and Anhaeusser, 2002).


Figure 7: Binary diagrams comparing the rocks of the volcanic-plutonic system from the Gavião Block with those from TTG suites and potassic granites of the calc-alkaline suites (fields from Moyen, 2011), Idiwhaa Tonalitic Gneiss (data from Reimink et al., 2014), and Itsaq Augen Gnaisse (data from Nutman et al., 1984). A and B) exploring the role of La, Yb, Y and Sr; TTG dataset from Martin et al. (1997) and normalizing values after McDonough and Sun (1995). A) La/Yb(N) vs. Yb(N). B) La/Yb(N) vs. Sr/Y (Condie and Kröner, 2013). C, D and E) Tectonic setting discrimination diagrams. C) Y+Nb vs Rb (Pearce et al. 1984). The tectonic fields were represented by syn-collision (syn-COL), volcanic arc (VA), within-plate (WP), oceanic ridge granite (ORG). TTG (dashed line) and potassic granites (grey) fields are from Moyen (2011). D) Nb vs Ga/Al (Whalen et al. 1987). E) Nb-Y-Ce (Eby, 1992).

The composition of coeval plutonic rocks regionally associated with the rhyolites and the Calderão diorite gneiss clearly plot along the same chemical trends as the rhyolites (Fig. 6). They are calc-alkaline, intermediate to felsic rocks ranging from 65 to 72 wt.% SiO2 and have high Fe2O3(T), CaO and Na2O (CaO/Na2O ratio of 0.5-0.6) contents. Like the rhyolites, they have slightly peraluminous and ferroan calcic compositions (Fig. 5 D) and similar trace element and REE patterns (Fig. 6 B), with slightly higher Rb, Th, Nb-Ta, Sr, La and Ti contents and lower Ba and HREE contents.

The granite and rhyolites have a distinctive trace element signature that makes the assessment of their tectonic setting even more ambiguous than it normally is for the Archean period (e.g., Bédard, 2006; Moyen and Martin, 2012; Fig. 7 A-B). Overall, the trace element signature of the rhyolites only indicates extensive plagioclase fractionation, thus depicting shallow melt segregation (<10 kbar; Moyen, 2011) and/or low H2O activity in the system (Beard and Lofgren, 1991; Bogaerts et al., 2006; Tatsumi and Suzuki, 2009).

On the other hand, the rhyolites and granites present a calcic ferroan affinity similar to some rare “A-type” magmas and, just like them, elevated HFSE and HREE contents together with low Al2O3 and CaO (Frost et al. 2001, 2011; Whalen et al., 1987). Although care must be taken when applying classical tectonic discrimination diagrams to Archean rocks (Condie, 2015; Moyen and Martin, 2012; Bédard, 2006), our samples consistently correspond to magmas formed away from convergent plate margins in all discrimination diagrams (Fig. 7 C-E). Altogether, these consistent observations suggest that the granites and rhyolites were formed in an intraplate environment.

5.5. Implications for the evolution of the Gavião Block

The plutonic-volcanic system studied here was emplaced forthwith a period of important continental lithosphere formation, represented by scarce 4.1-3.5 Ga remnants and major 3.42-3.33 Ga magmatism. The widespread magmatic activity is mainly constrained to the time span from 3.40 to 3.33 Ga and is represented by different 3.40-3.36 Ga TTG massifs (Martin et al., 1997; Guitreau et al., 2012; Nutman and Cordani, 1993; Santos-Pinto et al., 1998, 2012; Zincone, 2016) and 3.35-3.33 Ga medium- to high-K calk-alkaline magmatism (Marchesin, 2015; Zincone, 2016). The oldest Sete Voltas TTGs were interpreted as partial melts of Archean tholeiites leaving a garnet amphibolite residue (Martin et al., 1997). The partial melting of the older TTGs to produce younger porphyritic granodiorites was interpreted as reflecting crustal thickening to between 30 and 45 km by mechanisms similar to modern continental collision (Martin et al., 1997). In terms of Hf isotopic evolution, only zircon grains older than 3.42 Ga have positive εHf(t) values, while 3.43-3.37 Ga grains yield εHf(t) ranging from +2 to -5.4 (Guitreau et al., 2012).

The Jacobina basin and the related quartzites surrounding the investigated rhyolites contain abundant 3.55-3.28 Ga-old detrital zircons (Teles et al. 2015; Barbuena et al., 2016; Magee et al., 2001) (Fig. 8). Among those, many grains crystallized in the 3.43-3.30 Ga time span and show negative εHf(t) values between -0.1 and -7.4 (Teles et al., 2015) that overlap with both rhyolites, gbranites and TTGs. All lines of evidence suggest that the Jacobina basin and related quartzites represent an Archean supracrustal sequence deposited at a maximum age of ca. 3.28 Ga, and that the plutonic-volcanic system and TTGs of the Gavião Block were the main sources of detrital zircons and sediments. The abundance of ca. 3.30 Ga-old zircons in those siliciclastic units suggest that the volcanic-plutonic association described here represents the remnants of an eroded system that could have originally been largely.


Figure 8: U-Pb LA-ICP-MS age distribution frequency of less than 5% discordant detrital zircon grains from metasedimentary rocks associated with the Gavião Block using Kernel density estimation plot and data from Teles et al. (2015), Zincone and Oliveira (in review) and Zincone (2016).

The scenario that best reconciles all observations is that the ca. 3.30 Ga granite-rhyolite system represents the initial stage of an intra-continental rifting following a major 3.43-3.33 Ga period of continental lithosphere formation and stabilization; with continental break-up and deposition of siliciclastic sequences with maximum deposition age of ca. 3.28-3.26 Ga. Remarkably, the plutonic-volcanic system and the siliciclastic metasediments both occur along the paleo-margin of the Gavião Block, which supports that their emplacement was followed by continental break-up.

Moreover, close relationships between continental rift settings and silicic volcanism have frequently been observed in early Paleozoic intraplate magmatism (e.g. Carlton Rhyolite volcanic field; Hanson et al., 2013), modern extensional environments (e.g. Continental Rift: Yellowstone, EUA; Nash et al., 2006; Simakin and Bindeman, 2012; Drew et al., 2013, Huang et al. 2015, Rift Arc: Taupo Volcanic Zone, New Zealand; Allan et al., 2012; Bégué et al., 2014, Oceanic Rift: Iceland; Martin and Sigmarsson, 2007; Willbold et al., 2009), and specifically, represent the precursor stage for rift segment propagation events (e.g., Afar triple point; Lahitte et al., 2003).

5.6. Global perspective

To summary the interpretations above, the 3.30 Ga-old volcanic-plutonic system of the Gavião Block represents within-plate intracrustal magmatism following a burst of felsic crust magmatism during a period of transition between crustal stabilization and an early stage of continental break-up. Clearly, this model, together with the “crustal” zircon Hf isotopic signatures, suggest that reworking of a felsic protocrust and intracrustal differentiation processes did take place as early as 3.30 Ga in Earth's history, earlier than the 3.2 - 3.0 Ga or later age range proposed previously by other works (e.g., Laurent et al., 2014a and references therein).

Remarkably, the plutonic-volcanic system described here is similar in composition to some older high-SiO2 ferroan felsic rocks showing high HREE contents and negative Eu anomalies, such as the 4.02 Ga Idiwhaa tonalitic gneiss in Canada (Reimink et al., 2014) and the ca. 3.62 Ga iron-enriched augen gneisses and diorite intrusions at the Itsaq Gneiss Complex, Greenland (Nutman et al., 1984; Nutman et al., 1996) (Fig. 7). These rocks were respectively interpreted as reflecting shallow-level magmatic processes coupled with crustal assimilation of rocks previously altered by surface waters (Reimink et al., 2014); and differentiates from crustally-contaminated ferrodiorites and ferrogabbros ponded at the base of the crust (Nutman et al., 1984; 1996). In both cases, those rocks would have been formed in magmatically and/or tectonically thickened crust that underwent internal differentiation associated with intrusion of mafic mantle-derived magma in an intraplate extensional tectonic environment, similar to that inferred for the Gavião volcanic-plutonic system.

Such “intraplate” crustal differentiation processes appear to have been relatively common in the early Earth, even in the Eo- and Paleoarchean. It must be noted, however, that such intraplate magmatism did not produce TTGs but rather magmas of exotic, calcic ferroan compositions like the granites and rhyolites from the Gavião Block. This questions to some extent the validity (or at least the uniqueness) of “intraplate” models proposed to explain the genesis of TTGs.

5.7. Paleoarchean Silicic Large igneous Provinces?

Large Igneous Provinces (LIPs) comprise large (>100,000 km3) volumes of volcanic or near-surface intrusions often generated within 1-5 Ma in an intraplate setting or exhibiting intraplate characteristics (Bryan and Ernst, 2008; Bryan and Ferrari, 2013; Ernst, 2014), and reach life spans of 50 Ma when linked with volcanic rifted continental margins (Umhoefer, 2011). Those episodic intraplate provinces commonly are related to mantle plumes, either directly linked to plume-derived melts or indirectly associated to lower crustal-derived melts in which the heat source is probably a mantle plume itself (e.g., Campbell, 2005; Condie, 2016). Bryan and Ferrari (2013) (see also Bryan 2007) summarized that Silicic LIPs (SLIPs) “reflect their crustal setting along young, fertile continental margins built up by paleo subduction processes, and where crustal partial melting overwhelmed the igneous system”. However, for the Archean period (4.0-2.5 Ga) identifying SLIPs related to rifted continental margins is complicated by a more fragmentary, limited and ambiguous geological record. Furthermore, assuming a paleo subduction process in the building of Paleoarchean (>3 Ga) continental crust is also a matter of contention (e.g., Zegers and van Keken, 2001; Bédard, 2006; Smithies et al., 2003, 2009; Van Kranendonk, 2010, 2014; Martin et al., 2005, 2014; Dhuime et al., 2015). The effects of erosion, burial and tectonic fragmentation in the Paleoarchean are also matters of considered debate (e.g. Lowe and Tice, 2004; Hessler and Lowe, 2006) with the the plumbing system more likely preserved (Bryan and Ernst, 2008; Bryan and Ferrari, 2013). For the investigated 3.30 Ga rhyolites the interpretation about which part of the volcanic apparatus is still preserved and exposed is an open issue, mainly because they represent basement inliers.

One way to reveal SLIPs in the Archean can involve the integration of igneous and sedimentary records derived from erosion of the SLIP (e.g., Whitsunday; Bryan et al., 1997, 2012). In this case, the possibility that the evolution of the pre-rift 3.30 Ga plutonic-volcanic system, coupled with the development of the Jacobina rift basin, shows a reasonable hypothesis to integrate both as part of a Paleoarchean SLIP. Overall, the area extends for 600 km long, whereas the measured thickness of the Jacobina basin is 10 km thick and 20 km wide (e.g. see Pearson et al., 2005 for review). Moreover, if we consider that intracontinental rift ended up being tectonically fragmented, the true size and immensity of SLIP magmatism and basin infill could have originally been greater. The large volume of SLIP magmatism may have measurable effect on the detrital zircon age record within metasediments of the Jacobina basin (Fig. 8). Finally, it is important to call attention about the important links and feedbacks among the volcanism, lithospheric extension, crustal partial melting and continental rupture to generate SLIP magmatism (Ewart et al., 1992; Pankhurst and Rapela, 1995; Riley et al., 2001; Bryan et al., 2002, 2008), which is quite similar to the present model here proposed. The basaltic magma that may associate with the silicic magma can be thought as “hidden” mafic large igneous provinces (Ernst, 2014) or, as discussed on the geochemical modelling, be in part related to the Calderão diorites. Thus, we expect that this contribution will lead to new investigations aimed at overcoming possible contradictions and disagreements and point the way forward for understanding the evolution of the 3.30 Ga plutonic-volcanic system of the Gavião Block, and allow assessment of it as perhaps the world’s oldest SLIP.

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