Introduction

The 3.30 Ga high-silica rhyolites of the Gavião Block are unusual metaluminous to peraluminous ferroan calcic rocks formed in an intracontinental tectonic setting after the formation and stabilization of new continental crust (Zincone et al. 2016). These rhyolites, according to Zincone et al. (2016), are the products during the first stage of rifting and continental break-up. The Paleoarchean age of the rhyolites, combined with their well-preserved primary petrographic features, make them an excellent natural case of study for investigating tectono-magmatic processes in the early Earth. These rocks are not associated with mafic to intermediate units as other Paleoarchean felsic rocks interlayered with basaltic to komatiitic flows in eastern Pilbara and Barberton, South Africa (e.g. Champion and Smithies 2007; Kröner et al. 2013; Smithies et al. 2019). Instead, they are associated with granitic intrusions like Boa Sorte and Meiras granite of similar trace element compositions and ages (Zincone et al. 2016), providing an inter-related plutonic-volcanic system at a key time in the Earth’s history.

Throughout the Archean, felsic rocks acquired increasingly complex geochemical signature during petrogenetic processes, including partial melting of pre-existing continental crust, interactions between mantle peridotite and incompatible element-rich components and melting of meta-igneous older lithologies via anatexis (Laurent et al. 2014). Although Paleoarchean felsic volcanism is not significantly investigated worldwide, a few studies have demonstrated its important role in primitive crustal thickening associated with internal reworking (Agangi et al. 2018). For example, the felsic volcanism in the Pilbara Craton, located at Western Australia, reveals the interaction of magma differentiation and crustal contamination in the formation of protocontinent (Smithies et al. 2019).

In Archean terrains, few Paleoarchean outcrops of felsic magmatism have preserved primary minerals assemblages (e.g. Champion and Smithies 2007; Huston et al. 2007; Agangi et al. 2018; Condie 2019). The São Francisco Craton is an outstanding example with a well-preserved Archean crustal segment which can help to constrain igneous processes and provide clues to the magmatic system evolution (Martin et al. 2005; Zincone et al. 2016; Zincone and Oliveira 2017; dos Santos et al. 2022; Gordilho Barbosa et al. 2022). In-situ analysis of key minerals, such as plagioclase and biotite, which are known to preserve robust isotope, major, and trace element signatures, ensures confidence in what is being analyzed. Isotope studies provide detailed information of magmatic evolution that is challenging to reconstruct using bulk rock compositions (Kemp et al. 2007).

Plagioclase mineral chemistry and Sr isotope analyses may elucidate the processes that occurred during magma cooling (e.g. Tepley and Davidson 2003; Wolff et al. 2011; Chen et al. 2015; Gao et al. 2015), while the use of biotite can trace changes in physicochemical conditions and the nature of the parental magma (e.g. Abdel-Rahman and Abdel Fattah 1994), making these minerals useful tools for investigating igneous systems. The combination of mineral scale examination with petrography may be outstanding instruments for investigating crystallization processes such as assimilation, contamination, magma mixing, and mingling with magma sources. Plagioclase and biotite are two important minerals for investigating igneous systems. For these reasons, they were chosen to constrain the physicochemical composition of Contendas rhyolite during its formation. In this article, we evaluated petrographic textures, mineral chemistry, and plagioclase Sr isotopic signatures of the rhyolite to better understand the magma composition, its physicochemical conditions, and oxygen fugacity (fO2) during crystallization.

Geological context

The São Francisco Craton is one of the largest in South America, containing the ancient gneiss complexes dated at 3.60–3.64 Ga (Oliveira et al. 2020). The Archean rocks crop out in the northern (Bahia State) and southern (Minas Gerais State) portions of the craton (Teixeira et al. 2017). In the northern portion of the São Francisco Craton, the basement is represented by the Archean terranes of Gavião, Serrinha, Jequié, and Itabuna-Salvador-Curaçá blocks (Barbosa et al. 2003; Barbosa and Sabaté 2004). The Gavião Block is the oldest Archean nuclei in the São Francisco Craton comprising tonalite-trondhjemite-granodiorite (TTG) gneisses, greenstone belts, metavolcano-sedimentary sequences, and several granitic plutons (Nutman and Cordani 1993; Martin et al. 1997; Santos-Pinto et al. 2012; Zincone et al. 2016; dos Santos et al. 2022; Gordilho Barbosa et al. 2022; Fig. 1).

Fig. 1
figure 1

Geologic map showing the location of Contendas rhyolite samples, represented by a star in the northern of São Francisco Craton (modified after Marinho et al. 1993a, b). The abbreviations are: GB - Gavião Block; JB - Jequié Block; SV - Sete Voltas dome; SM - Serra dos Meiras dome; BV - Boa Vista/Mata Verde dome; CC - Caldeirão Complex

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The southern Gavião Block is limited to the east by the Paleoproterozoic Contendas-Jacobina Lineament (Sabaté et al. 1990). Geochronologic constraints protract the São Francisco Craton from Paleoarchean and Neoarchean (Teixeira et al. 2017), demonstrating a crustal evolution akin to the Kaapvaal-Zimbabwe and Pilbara cratons, particularly before 3.0 Ga (Gordilho Barbosa et al. 2022). The geologic evolution of the northern and southern portions of the Gavião Block indicates Eo-to-Paleoarchean crustal growth, as evidenced by TTG magmatism and partial melting of Eoarchean crust, with a minor contribution from a mantle source (Martin et al. 1997; Gordilho Barbosa et al. 2022). The 3.30 Ga high-silica volcanism is found in the northern and southern parts of the block and is associated with intraplate magmatism and intracrustal differentiation processes (Zincone et al. 2016).

The 3.30 Ga high-silica rhyolites of Contendas and Mundo Novo are found 200 km apart from each other (Zincone et al. 2016). They occur inliers within the Mundo Novo and Contendas-Mirante greenstone belts, but contacts between the rhyolites and gneisses or metasedimentary rocks are not observed in the field (Marinho et al. 1993a; Zincone et al. 2016). These volcanic rocks are also unrelated to any other meta-volcanic unit, indicating lower greenschist facies metamorphism (Zincone et al. 2016). The Contendas rhyolite, according to Zincone et al. (2016), represents the high-silica magmatism of the southern of the block formed in the early stages of rifting and continental break-up. Intracontinetal rifting may have occurred coeval with or following the high-silica intraplate magmatism and the Au-U-bearing Jacobina Basin siliciclastic sequence, in the northern of the block (Mascarenhas et al. 1998; Teles et al. 2015; Zincone and Oliveira 2017).

The 3.30 Ga high-silica plutonic-volcanic system were formed after TTG magmatism and stabilization of the Gavião lithosphere indicated by high-K magmatism (Zincone et al. 2016). Paleomagnetic research findings on the Contendas rhyolite revealed an eastward sub-vertical coherent lava flow direction preserved during its formation (Amaral 2021).

The Gavião Block rhyolites are unusual for a number of reasons (Zincone et al. 2016): (1) they are unusually high in silica (74–79 wt%); (2) they formed at very high temperature (915 − 820 °C); (3) the chemistry ranges from ferroan calcic metaluminous to peraluminous; (4) primary features are common, including flow textures, and euhedral phenocrysts of plagioclase (Zincone et al. 2016). These rocks also have a homogeneous trace element pattern and a heterogeneous zircon Hf isotope. The whole-rock geochemistry shows negative anomalies in Nb-Ta, Sr, and Ti, followed by relatively fractionated LREE, resembling the geochemical signature of continental arc magmas (Zincone et al. 2016). The Hf isotopes show large range of ƐHf (t) (0 to -7) and older model ages (TDM, where DM means depleted mantle) of 3.54–3.88 Ga and up to 4.02 Ga (Zincone et al. 2016). Such sub-volcanic rocks would have been interpreted to be generated from extraction and emplacing of highly silicic residual liquid formed by crystallization of granitic magma in a relatively shallow (< 10 km) reservoir, now represented by the granite massifs (Zincone et al. 2016).

Materials and methods

Samples

Four samples of Contendas rhyolite were used in this study for petrographic analysis, biotite and plagioclase mineral chemistry, and in-situ Sr isotope analysis on plagioclase. These samples are from the southern part of the Gavião Block. They were collected from distinct incidences within the same outcrop, making it difficult to differentiate contact relationships with other lithologies. They were collected at the location with coordinates 13°40’11"S and 40°54’22"W, and were designated as TZD-31 A, TZD-31 C, TZD-31D, and TZD-31E.

The 3.30 Ga high-silica rhyolite (Zincone et al. 2016) has a porphyritic texture with quartz and feldspar crystals larger than 1 mm size. Phenocrystals are embedded in fine-grained matrix that mainly consists of quartz, feldspars, and biotite minerals, with traces of chlorite and magnetite.

Major elements

Feldspars were investigated using the system electron probe micro-analyser (EPMA). A total of 156 individual measurements were taken from 34 crystals. EPMA chemical analysis was also used to analyse biotite composition, with 49 individual measurements from 21 crystals. Major element analyses of plagioclase and biotite were performed at Microscopy and Microanalysis Laboratory, using a JEOL JXA-8230 EPMA system with five wavelength dispersive spectrometers (WDS) coupled with one Oxford EDS detector. The analytical conditions were 15 kV accelerating voltage, 10nA beam current and spot size of 5 μm in plagioclase and biotite. The reference materials used were anorthoclase for Na, fluorite for F, quartz for Si, corundum for Al, olivine for Mg, barite for Ba, magnetite for Fe, scapolite for Cl, chromite for Cr, strontianite for Sr, fluorapatite for P, ilmenite for Ti, fluorapatite for Ca, microcline for K and MnO2 for Mn. ZAF matrix corrections were applied and total iron content was taken as FeO. Counting times on peak and background were 10/5 s for all elements (Na, F, Si, Al, Mg, Fe, Cl, Cr, P, Ti, Ca, K, Mn) except for Ba and Sr (30/15 s).

Trace elements

The trace elements content of feldspars was obtained from 31 spots on 14 selected feldspar crystals. The trace element analyses were carried out at Isotopic Geochemistry Laboratory, by laser ablation-quadrupole-inductively coupled plasma-mass spectrometry (LA-Q-ICP-MS) Agilent 7700x coupled with New Wave Research UP-213 (NdYAG 213). The working conditions were 10 Hz of frequency and 50 μm of size spot. Helium gas was used as conductor and then mixed with argon in the ablation cells. Si was used as the internal standard for correction of inter-element fractionation. External standards such as NIST-610, BCR-2 and BHVO were used for signal correction. GLITTER software (Griffin et al. 2008) was used to refine the final data.

In-situ Sr isotopes

In-situ Sr isotope analyses were carried out on 11 feldspar crystals in a total of 25 measurements. Plagioclase crystals were chosen based on the major populations observed in the petrography described in the following sections. The point analyses aim for core-rim of plagioclase phenocrysts and rims of rapakivi crystals for in-situ Sr isotope determination. The alternative analyses on solely rapakivi rims were due to the core’s K-feldspar composition. Sr isotope composition was performed by Thermo Scientific Neptune Plus Multicollector MC-ICP-MS coupled with laser ablation device 193 nm HelEX Photon Machine in the Isotopic Geochemistry Laboratory. Plagioclase crystals from the Contendas rhyolite were measured in spots of 50 μm (samples TZD-31 C; -31D; 31-E). Working conditions were 10 Hz for ablation frequency and 6 J/cm2 for energy density. The ablated material was carried into the ICP using helium gas. The laser ablation measurement was 60s long and the blank gas of Kr took was 30s for measuring. The correction for 84Kr to 84Sr and 86Sr interference was performed by subtracting the background from the signal intensity. The interference of 87Rb on 87Sr was corrected by the 85Rb/87Rb ratio exponential law. Repeated measurements of international and internal standards such as calcite standard MIR (reference material), coral, Madagascar apatite and BHVO-2 confirmed the accuracy of the 87Sr/86Sr ratio. Further detail on the methods of ICP-MS can be found in Wilson (1997) and Yang et al. (2011).

Thermobarometer for volcanic systems

Plagioclase- and alkali feldspar-liquid thermobarometers of Putirka (2008) were applied to plagioclase crystals to estimate temperature and pressure during crystallization. Equation 24a and 25a of Putirka (2008) were used. The whole rock composition of Zincone et al. (2016) was used as the ‘liquid’ composition, and the microanalysis data of this research were used for feldspars compositions. The KD(Ab-An) equilibrium test was used to validate the results by determining the equilibrium conditions between crystal and melt.

Results

Petrography and mineral chemistry

The Contendas rhyolite has a porphyritic texture characterized by coarse-grained plagioclase and K-feldspar phenocrysts with tabular shapes and coarse rounded quartz grains presenting undulose extinction embedded in a fine-grained groundmass of quartz, orthoclase, albite, biotite, and chlorite. Some quartz grains are fine-grained, being part of the matrix. Zircon and magnetite are important accessories, whereas chlorite, carbonate, sericite, and epidote are present as secondary minerals.

Feldspar phenocrysts can be found in the microcrystalline matrix as tabular individual crystals and glomerocrysts with planar and irregular contacts (Fig. 2a). Plagioclase grains are characterized by the presence of albite and polysynthetic twining in the groundmass. Rapakivi microstructure is another igneous texture found in the rhyolite and occurs tabular and subhedral feldspars, characterized mainly by a K-feldspar core and a plagioclase rim (Fig. 2b). Small feldspars crystals are also surrounded by mica (Fig. 2c). This microstructure is found both in individual feldspars and also in aggregates (Fig. 2d). All crystals that characterize the studied samples are clearly oriented in the felsic groundmass (Fig. 2e).

Fig. 2
figure 2

Cross-polarised transmitted-light photomicrographs. a Plagioclase phenocrysts embedded in a fine-grained matrix. b Plagioclase phenocryst with rapakivi-like microstructure showing K-feldspar core and plagioclase rim. c Microphenocrysts enveloped in biotite. d Aggregate of subhedral and anhedral feldspars and rapakivi-like microstructure. e Microscope view of TZD-31E thin section showing phenocryst orientation. Minerals abbreviations are: Pl - plagioclase; Qz - quartz; Kfs - K-feldspar; Bt - biotite, according to Whitney and Evans (2010)

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Plagioclase

Plagioclase crystals in the Contendas rhyolite are divided into three populations: phenocrysts (500–2300 μm), rapakivi (1400–2300 μm), and microphenocrysts enveloped in biotite (100–150 μm) (Fig. 2a–f). Meanwhile, the rhyolitic groundmass of quartz, plagioclase, K-feldspar, biotite, magnetite, and tiny flakes of chlorite, are fine-grained (< 50 μm). Plagioclase phenocrysts are euhedral to subhedral with tabular shapes. Rapakivi crystals are phenocrysts, euhedral to subhedral, characterized by K-feldspar cores and plagioclase rims. Both plagioclase crystals are commonly found as glomeroporphyritic aggregates in the felsic groundmass. Plagioclase and K-feldspar microphenocrysts are enveloped in biotite showing zircon inclusions and opaque minerals such as magnetite. Plagioclase populations have been moderately altered to white mica and locally calcite.

The major compositional variation in the three types is small, with albite composition (Fig. 3a, b). Results are available in Tables S1 and S3 in the electronic supplementary material (ESM). The phenocrysts have Ab96 − 98, total iron (FeOT) ≤ 0.13 wt% and MgO ≤ 0.02 wt%. Sr and Ba contents vary between 117 and 310 and 3-662 ppm, respectively. Rapakivi crystals have rims of Ab93 − 98, FeOT ≤ 0.26 wt% and MgO ≤ 0.08 wt%, while rapakivi core are Or95 − 98, FeOT ≤ 0,32 wt% and MgO ≤ 0.02 wt%. At the rapakivi rim, Ba and Sr concentrations frequently range ​between 15 and 495 and 79–286 ppm, respectively. Plagioclase microphenocrysts enveloped by biotite have Ab96 and FeOT contents ≤ 0.10 wt% and MgO below the detection limit and Sr and Ba contents of 249 − 134 ppm and 16–162 ppm, respectively.

Fig. 3
figure 3

Composition variation of plagioclase crystals from Contendas rhyolite. a Ternary diagram of plagioclase populations. b An versus Ab plot, showing the variations of Na and Ca in plagioclase. Mineral abbreviations are: An - anorthite; Or – orthoclase; Ab - albite

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In general, trace elements of plagioclase crystals define two groups (Fig. 4a–c). Both consist of phenocrysts and rapakivi crystals. Group 1 comprise plagioclases with Sr concentration ranging from 117 to 310 ppm, and Sr/Ba ratios ranging from 4 to 17. Group 2 has Sr/Ba ranging from 0 to 6 and Sr content from 53 to 148 ppm. Few data of microphenocrysts do not allow a clear determination of the trace elements groups. Results can be found in Table S2 in the ESM.

Fig. 4
figure 4

Plots visualizing the chemical composition of feldspars from Contendas rhyolite. a Distribution of Sr versus CeN/YbN. b Sr/Ba versus Sr/Y. c Sr versus U/Yh in plagioclase crystals

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The REE content of the rapakivi plagioclase shows LREE normalized enrichment (LaN/YbN = 2.02-61.00) and Eu anomaly portraying very negative to high positive anomalies [Eu/Eu* = 0.35-5.00; with Eu*=(Sm×Gd)0.5; Fig. 5]. The Th/U ratio is mostly between 1.35 and 5.78. The phenocrysts have more homogenous REE content than the rapakivi plagioclase, as well as less evident LREE enrichment (LaN/YbN = 0.18–25.24), negative to positive Eu anomalies (Eu/Eu* = 0.41–5.21), and Th/U ratio from 0.91 to 3.52.

Fig. 5
figure 5

Primitive mantle-normalized REE diagrams of plagioclases. Normalization values from McDonough and Sun (1995)

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Thermobarometry

Using Putirka (2008) approach temperature and pressure estimate, the Ab-An exchange coefficient represented by KD(An-Ab)pla−liq was calculated using T < 1050 ºC, 0.1 ± 0.05 and for T > 1050 ºC, 0.28 ± 0.11. The acquired results do not correspond to the values specified by Putirka (2008), making the estimate of plagioclase temperature problematic.

In situ Sr isotopes

Sr isotopes in plagioclase from Contendas rhyolite highlight the presence of two populations; however, only one population accounts for almost all the crystals analyzed Fig. 6. Results can be found in Table S4 in the ESM. The 87Sr/86Sr ratios of all crystals have a wide range from 0.70700 to 0.70994 (Table 1). The phenocrysts exhibit moderate to high isotopic variations with 87Sr/86Srcore (0.70762 and 0.70815) more radiogenic than the 87Sr/86Srrim (0.70704 and 0.70780). Meanwhile, the rims of rapakivi crystals show a bimodal distribution, 0.70700 and 0.70793. Some crystals show variations of 87Sr/86Srcore−rim from high to moderate ratios (Δ87Sr/86Sr = 69.10− 5, 53.10− 5 and 35.10− 5), while other crystals show higher 87Sr/86Srrim values with moderate variation (0.70994 and 0.70946), Fig. 7.

Fig. 6
figure 6

a Histogram of Sr isotope composition of Contendas rhyolite plagioclase. b Plot of Sr concentrations versus 87Sr/86Sr ratios

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Table 1 In-situ Sr isotope for plagioclase of Contendas rhyolite
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Fig. 7
figure 7

Cross-polarised transmitted light images showing a tabular crystal of plagioclase clustered to a rapakivi and b plagioclase with irregular contact to a rapakivi. c, d Sr isotope composition according to the distance to rims of crystals. e, f, g, and h Distribution of Ce, Eu, Sr, and Ba according to the distance from rim

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Biotite

Biotite is found enveloping feldspars and quartz with sizes of 100–200 μm (Bt-1), and small skinny flakes with sizes < 50 μm (Bt-2) with some of them partly chloritized in the groundmass (Figs. 8 and 9). The structural formula of biotite was calculated following Li et al. (2020). Both biotite types indicate a Fe-rich nature in the Al(vi) + Fe3+ + Ti - Mg - Fe2+ + Mn diagram (Fig. 10a). In comparison to Bt-2, Bt-1 has higher FeOT, TiO2, and lower MgO. Bt-1 biotite has a Fe/Fe + Mg ratio ranging from 0.75 to 0.79, whereas Bt-2 grains vary between 0.64 and 0.74.

Fig. 8
figure 8

Biotite composition of Contendas rhyolite in Mg/(Mg + Fe) versus Al(iv)/(Al(iv) + Si(iv)) plot (after Deer et al. 1962)

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Fig. 9
figure 9

Back-scattered electron images of a Bt-1 of sample TZD-31E, b Bt-2a of sample TZD-31 A and c Bt-2b of sample TZD-31Ce. Mineral abbreviations are: Bt – biotite; Or – orthoclase; Pl – plagioclase; Qz - quartz

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Fig. 10
figure 10

Chemical compositions of biotite Bt-1, Bt-2a, and Bt-2b. a Ternary diagram Al(iv) + Fe3++Ti-Mg-Fe2+-Mn, showing the Fe-rich nature of samples (after Foster 1960). b Composition variation of MgO versus FeO and c Al2O3 versus MgO, depicting a magmatic source classification (after Abdel-Rahman and Abdel Fattah 1994)

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Biotite Bt-2 can be distinguished into two sub-groups: Bt-2a, indicating MgO and TiO2 contents from 4.59 to 5.31 wt% and 1.77–2.59 wt%; and Bt-2b, with MgO and TiO2 ranging from 6.90 to 8.04 wt% and 0.99–1.56 wt%, respectively. Despite this, the Mg/(Mg + Fe) ratios in Bt-2 are higher (0.28–0.44) than in Bt-1 (0.25–0.28). The AlT (1.35–1.55) of both biotites varies slightly, straddling the alkaline and peralkaline domains (Fig. 10b, c).

The chemistry of these different types of biotite has also revealed that the Bt-1 biotite and Bt-2a grains retain their primary composition while the Bt-2b grains are re-equilibrated (Table S3; Fig. 11a). The relationship among MgO-FeO-Al2O3 indicate an influence of anorogenic alkaline suites in a crustal nature (Fig. 11b, c). Another significant finding is the fO2 of crystallization. The fO2 estimates of Bt-1 and Bt-2 biotite using Fe2+, Fe3+, and Mg diagram of Wones and Eguster (1965) plot between the Ni-NiO and Fe3O4-Fe2O3 buffers (Fig. 11d).

Fig. 11
figure 11

a Ternary diagram FeO-10TiO2-MgO indicating primary biotite for Bt-1 (after Nachit et al. 2005). b Ternary diagram FeO-MgO-Al2O3 depicting the magmatic nature of Bt-1, Bt-2a, and Bt-2b (after Abdel-Rahman and Abdel Fattah 1994). c FeO/(FeO + MgO) versus Mgo plot, showing the crustal nature of biotite (after Zhou 1986). d Fe2+-Fe3+-Mg ternary diagram of Bt-1, Bt-2a, and Bt-2b from Contendas rhyolite (after Wones and Eguster 1965). Fe3O4-Fe2O3 - hematite-magnetite; Ni-NiO - nickel-nickel oxide; Fe2SiO4-SiO2-Fe3O4 - quartz-fayalite-magnetite

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Discussion

Contendas rhyolite formed in a relatively shallow reservoir (< 10 km) as a result of the extraction of a felsic residual liquid (Zincone et al. 2016). According to the same authors, the high-silica rhyolite of ferroan affinity with signature of “A-type-like” composition generated from the cooling of granitic magmas, currently representing the remains of an eroded Paleoarchean volcanic-plutonic system in the São Francisco Craton.

Based on mineral scale chemistry fingerprints and petrographic features in this study, there is no evidence of distinct magmatic interactions of feldspars or biotite from the Contendas rhyolite, which could have influenced the composition and texture of minerals crystallized in the high-silica melt. The unusual geochemical features of this metasubvolcanic rock displayed composition similarities with Archean igneous rock from the Acasta Gneiss Complex (Zincone et al. 2016), whose genesis is linked to shallow-level magmatic processes and assimilation of rocks previously altered by surface waters (Reimink et al. 2014).

The Contendas rhyolite plagioclases are characterized by two important groups of crystals identified in the petrography: phenocrysts and rapakivi. Mineral scale investigation based mainly on the compositional variation of elements such as Sr, Ba, and LREE, record two groups of feldspars crystallization. Even though plagioclase has a negligible variation in the anorthite content, the variation in trace elements and Sr/Ba ratio may indicate the influence of distinct mechanisms or different stages of crystallization during plagioclase formation. Phenocrysts and rapakivi crystals are found in both groups, however phenocrysts predominate in group one, whereas rapakivi crystals, in the second. Given the inaccuracy of temperature-pressure of plagioclase estimation, the crystallization history is currently unknown, particularly for an order of crystallization inferences. Phenocrysts and rapakivi from the first group contain more Sr and LREE than the second one, suggesting that the melt was enriched in LREE at the time of phenocrysts crystallization.

The bulk of phenocryst Sr isotopes show a slight Sr isotopic variation, with a low Sr isotope ratio near the rims compared to the cores, reflecting the influence of less radiogenic continental crust. The assimilation of less radiogenic country rock, rather than mixing interaction, was the most likely process driving differentiation. Moreover, the observed plagioclase demonstrated no significant variation in anorthite, despite the trace element heterogeneity. Another distinguishing chemical feature is the common negative Eu anomaly, whose partitioning is totally dependent on temperature and fO2 (Weill 1973; Drake 1975). In the Contendas rhyolite, Eu and Sr were most likely removed from the melt through fractional crystallization of early plagioclase generation during intra-crustal differentiation.

Previous geochemical studies of the Contendas rhyolite recognized feldspars encased by biotite (e.g. Zincone et al. 2016), but due to the lack of sufficient data on this microstructure, it is too early to provide crystallization inferences. Despite this, petrographic evidences given by the size of the fine-grained feldspars encapsulated by biotite suggests a nucleation delay in the groundmass (e.g. Rusiecka and Martel 2022). Biotite crystals crystallized under iron-rich environment and oxidized conditions (fO2). The chemical signature of biotite crystals indicate an anarogenic source with a magmatic system generated from a continental crust, confirming earlier assumptions of Zincone et al. (2016) (Fig. 9c).

The negligible variation of the Sr isotope and the low Sr and LREE content in the rapakivi rims, indicate a magmatic scenario with low concentrations of these elements as plagioclase crystallization occurred in the magma. Furthermore, the irregular contact observed in rapakivi crystals between the K-feldspar core and the plagioclase rim suggests magmatic disequilibrium, most likely caused by changes in pressure and temperature. The shallow-level magmatism and the rising residual liquid could have reduced pressure and temperature conditions in the high-silica system, resulting in K-feldspar corrosion and the formation of plagioclase with patchy zoning (e.g. Vance 1965).

Glomerocrysts are common in the rhyolitic groundmass. They are composed of plagioclase, K-feldspar, and rapakivi crystals with planar and irregular internal contacts. The orientation of the aggregates and individual grains also suggests a magma flow. Previous paleomagnetism studies on the Contendas rhyolite revealed a coherent eastward sub-vertical lava flow direction during its formation (Amaral 2021). Such evidences indicate that aggregates formed during the turbulence of melt injection and ascent to the shallow reservoir (e.g. Turner and Campbell 1986). Cluster features, on the other hand, can be related to hydrodynamic forces in a magmatic flow through a process known as synneusis (e.g. Vance 1969). Vogt (1921) introduced this process and applied it to crystal segregation from magma at an early stage. This structure is commonly identified by the parallel relationship of the crystalline faces and the orientation of the twinning (Vance 1969). Despite this, the presence of rapakivi crystals in the clusters, reinforces the premise that individual feldspar crystals joined during the emplacement of the lava flow.

In this study, a detailed investigation from the mineral scale elucidates how a few magmatic mechanisms interacted and performed for the generation of distinctive plagioclase populations and primary biotite crystallization in high-silica magmatism. Although the thermobarometer investigation was restricted by mineral assemblage, the petrographic textures and chemical analyses highlight the primary aspects preserved in the mineral compositions of this unusual sub-volcanic rock from the Contendas-Mirante region.

Conclusions

The Contendas subvolcanic rock is a meta-rhyolite with well-preserved petrographic and chemical igneous characteristics. The following are the main findings of this study: (i) The microstructures and chemical signatures of plagioclase populations reveal two main stages of generation. One stage indicates the crystallization of plagioclase phenocrysts and rapakivi with a high Sr/Ba content and slight LREE, while another stage records the crystallization of feldspars with low Sr/Ba. (ii) The small variations of plagioclase in-situ Sr isotope most likely point to subtle assimilation of country rock and highlight crystal evolution with no interactions from other processes such as magma mixing or mingling, constraining plagioclase populations and biotite formation to a continental crust composition. (iii) The chemical characteristics of primary biotite crystals indicate an anorogenic setting sourced by continental crust under oxidized conditions.