Subvolcanic petrology of the Pico dos Três Estados region (SE Brazil): Implications for Alkaline Magmatic Evolution

Abstract

This paper presents a geological study of the subvolcanic bodies present in the Pico dos Três Estados (São Paulo (SP), Rio de Janeiro (RJ), and Minas Gerais (MG)) and its surroundings at the Passa Quatro Alkaline Complex. In this study, we used field-collected data, petrography, and geochemical analysis data. From a more detailed fieldwork, we verified that the region that appeared to present only two lithologies in previous works presented a faciological complex with at least six distinct facies, with emphasis on the phonolitic body that forms the Pico dos Três Estados (2,665 m high). Still regarding fieldwork, we found that there are signs of magma mingling processes in the area. Modeling indicates that 10% of fractional crystallization was able to derive the phonolitic rocks from a trachytic liquid based on the following elements: Y, Nb, Yb, Ce, Sm, Eu, La, Nd, Hf, and Pr. The modal batch partial melting shows the possible mantle sources for the magmatism in this area. This data pointed out the involvement of an enriched mantle source in the La/Yb ratio by at least 16 times more than an Oceanic Island Basalt (OIB)-type mantle.

KEYWORDS:
fractional crystallization; partial melting; magma mingling; subvolcanic rocks

1 INTRODUCTION

The studied area comprises the region that joins the States of São Paulo, Minas Gerais, and Rio de Janeiro, more precisely known as Pico dos Três Estados (Fig. 1). This peak is 2,665 m high and is located between the approximate latitudes of 22°20’ and 22°30’ (south) and the approximate longitudes of 44°45’ and 45°00’ (west) in the Passa Quatro Alkaline Complex (PQAC), which is an elliptical-shaped batholith with an approximate area of 165 km² (Chiessi, 2004). It is 70 million years old (Brotzu et al., 1992; Ulbrich & Gomes, 1981), and is located in the Serra do Mar Alkaline Province (Morbidelli et al., 1995; Thompson et al., 1998).

Figure 1.
Location of the studied region within the red rectangle. The figure shows the area corresponding to the Passa Quatro Alkaline Complex, highlighting the specific study area.

This paper will contribute to increasing the collection of systematic geological studies in the region (which has been scarce until now). It will help us to bring answers to petrogenetic questions related to the evolutionary processes and source of the felsic magmatism in this region.

The main objective of this study was to understand the petrogenesis associated with the subvolcanic bodies present in Pico dos Três Estados in the Passa Quatro Alkaline Complex and in the region around the peak. Furthermore, this work proposed to delimit the occurrence areas of these subvolcanic bodies, which allowed the creation of a geological map on a scale of 1:1,500 of the entire area involved in the study. The results achieved in this article were based on previous studies published in scientific literature, fieldwork, macroscopic and microscopic petrographic data, and lithogeochemical analyses (main oxides, traces, and rare earth elements).

We applied all these data to the construction of geochemical models to define the evolutionary processes involved in the petrogenesis of magmatism in the studied region.

2 GEOLOGY OF THE PASSA QUATRO ALKALINE COMPLEX

The PQAC constitutes one of the 26 alkaline intrusions that comprise the Cabo Frio magmatic alignment proposed by Almeida (1991) as an alignment of syenitic complexes WNW-ESE trending along Precambrian structures with approximately 60 km of wide and 1,150 km of extension in the southeast of Brazil. This aligned magmatic structure together with other alkaline intrusions (São Paulo coastal line and occidental sector of Mantiqueira Mountain Range) denotes, so previously called, Serra do Mar Alkaline Province (Morbidelli et al., 1995; Thompson et al., 1998; Ulbrich & Gomes, 1981).

The PQAC is a ring intrusion inserted in the Cenozoic continental rift system of southeast Brazil (Chiessi, 2004; Riccomini et al., 2005; Zalán & Oliveira, 2005) and crosscut the Precambrian Andrelândia domain (gneiss, schist, and quartzite) and collisional granitoids from the Ribeira orogen in the central sector of the Mantiqueira Province (Heilbron et al., 2020). The contacts between the basement and the PQAC are difficult to delimit due to the large amount of talus deposits and the extensive forest cover present on the complex edges (Chiessi, 2004; Sigolo, 1988).

Previous work (Brotzu et al., 1992; Chiessi, 2004; Guarino et al., 2019; Sigolo et al., 1992) has suggested that the PQAC is predominantly formed by moderately and strongly silica-undersaturated syenites (Fig. 2A).

Figure 2.
(A) Simplified geological map of the Passa Quatro Alkaline Complex. Modified from Brotzu et al. (1992) and Chiessi (2004). (B) Simplified geological map of the studied area showing only the outcropping lithologies represented on the map. UTM coordinate system. Datum: SIRGAS 2000/UTM zone 23S.

Centimeter-to-metric thickness dikes represent the mafic magmatism of the PQAC, which cut the basement rocks close to the contact with the complex and are possibly representatives of the felsic subvolcanic rocks’ parental magma characterized in the complex (Marins, 2012; Silva et al., 2024; Thompson et al., 1998). Additionally, Chiessi (2004) has described the occurrence of lamprophyre’s dikes with thicknesses ranging from 2 to 4 cm, which cut plutonic rocks located in the innermost portions of the PQCA. The K-Ar age for amphibole isolated in nepheline syenite is 66.7 ± 3.3 Ma (Sonoki & Garda, 1988). The Rb-Sr ages for whole-rock nepheline syenites are 77 ± 3 (Brotzu et al., 1992) and 70.4 ± 0.5 Ma (Montes-Lauar et al., 1995).

2.1 Geology of the study area

There are few geological studies published in the literature dedicated to the Pico dos Três Estados subvolcanic plug. Chiessi’s work (2004) refers to the plug as a trachytic body with an area of approximately 8.25 km², which corresponds to 5% of the total area of the PQAC. For the same author, the plug is characterized as a sigmoidal geometry with a main orientation in WNW-ESE, very fine grain, and aphyric to porphyritic textures.

The study region’s predominant plutonic rocks are classified as alkali syenite with nepheline and alkali syenites, cut by phonolite, lamprophyre, and trachyte dikes. In the central and highest region of the area, known as Pico dos Três Estados, we identified a phonolite (Fig. 2B). The area where this plug is located was classified in previous work (Chiessi, 2004) as trachyte (Fig. 2A); however, we propose a reclassification of this subvolcanic rock based on more detailed studies.

The different types of intrusions (ring dike, tabular dike, and plug), in addition to subvolcanic breccias, show a shallow crustal stage of installation of phonolites and trachytes in the PQAC with evidence of volcanism (Silva et al., 2022). However, there is no information on the spatial (e.g., possible juxtaposition) and temporal (e.g., coexistence) relationship of these distinct subvolcanic lithologies (phonolites and trachytes) in the study region. Furthermore, there are no phonolite and trachyte subtypes with detailed petrographic descriptions in previous works for this region of the PQAC. Adding to this, the lack of lithogeochemical data in the region configures the absence of the petrological hypothesis magmatic differentiation (cogeneticity) and the characteristics of the source generating the phonolite plug that makes up Pico dos Três Estados.

To solve these petrological problems, we integrated data from field geology, petrography, and lithogeochemistry (main oxides and trace elements) with modeling of magmatic processes (by trace elements) to evaluate the supposed trachyte-phonolite cogeneticity and the generating source of the alkaline magmatism present in the focus area of this study.

3 MATERIALS AND METHODS

The materials for this work are rocks collected from outcrops in Pico dos Três Estados and its surroundings during the field geology stages in the eastern region of the PQAC. Posteriorly, aliquots of these materials were disponible to polished section confection (for microscopic petrography) and whole-rock powder (for lithogeochemistry).

3.1 Fieldwork

The study region comprises one of the highest elevations in the Brazilian Southeast, and access is exclusively by trails, making it difficult to access. We need to walk approximately 15 h to reach the study area, which is approximately 25 km². For the geological mapping, we used the topographic sheet of Passa Quatro as a cartographic basis at a scale of 1:50,000, according to IBGE (1974).

Seventy description stations were systematically georeferenced and described. The occurrence of outcrops and pebbles was noted and photographed in the field, together with changes in the mineralogy, mode, texture, and structure (magmatic and non-magmatic) of these rocks.

3.2 Petrography

We have made 28 polished sections from 18 samples collected. SOLINTEC (Integrated Geology Services) was the company responsible for making the polished profiles. Transmitted light microscopic analyses were carried out using a ZEISS AXIO instrument, connected to a computer to acquire photomicrographs, at the Geological Modeling and Evolution Laboratory (LabMEG) of the Universidade Federal Rural do Rio de Janeiro (UFRRJ).

We based the granulometry (fine < 1 mm, medium 1–3 mm, and coarse > 3 mm), textural classification, and mineral grain habits, among other mineralogical characteristics, on Gill (2014), Hibbard (1995), and Mackenzie et al. (1982). The lithological classifications were according to Le Maitre et al. (2002).

3.3 Lithogeochemistry

Part of the samples previously described under the microscope were initially prepared for lithogeochemical analysis at the facilities of LabMEG and the Department of Petrology and Geotectonics at UFRRJ. This initial preparation consists of manually fragmenting rock samples to a size of 10 mm², followed by discarding non-representative fragments, washing the fragments with distilled water, drying, and storing.

Subsequently, the fragments underwent a grinding process in a Siebtechnik tungsten pan mill. For this stage, we used the Sample Preparation Laboratory at the Universidade Federal do Rio de Janeiro (UFRJ) and followed the criteria established by Best (2002). Approximately 10 g of powder was placed in appropriate vials and sent to Activation Laboratories Ltd. (ACTLABS) in Canada, where the main oxides and trace elements, including rare earth elements (REE), were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively conductively plasma mass spectrometry (ICP-MS).

3.3.1 Lithogeochemistry analytical procedures

The ACTLABS laboratory in Canada analyzed the samples using the 4litho package. The 4litho package involves the analysis of larger oxides by ICP-OES after fusing the sample with metaborate or lithium tetraborate. ICP-MS analyzes trace elements, and loss of ignition (LOI) was quantified by gravimetry. The main oxides are represented in percentage (% by weight). In contrast, trace elements are defined in parts per million (ppm), and all iron present in the samples was analyzed as total iron in the form of ferric iron (Fe₂O₃^t).

The detection limit for major oxides is less than 0.01%, while trace elements range from 20 to 0.002 ppm, and for REE it goes up to 0.05 ppm (Suppl. Table 1). Any analytical method involves precision and accuracy. Precision is the ability to repeat results, while accuracy is the closest you can get to the correct value. The 4litho package uses data from twelve international rock standards to evaluate the accuracy of the data obtained. We have adopted SY-4 syenite as the standard due to its similarity with the study area rocks.

Precision for major elements ranges from 0 to 1.9%, for minor elements it ranges from 0 to 13.3%, and for trace elements from 0 to 15.9%. Only Cr did not present detectable values. The accuracy for major elements ranges from 0.1 to 7.4%; for minor elements, it ranges between 7.1 and 12.5%; and for trace elements, the range is 0 to 6.2%.

The geochemical data obtained include main oxides (SiO₂, TiO₂, Al₂O₃, Fe₂O₃^t, MnO, MgO, CaO, Na₂O, K₂O, P₂O₅, and LOI), mobile incompatible trace elements (Ba, Rb, and Sr), immobile incompatibles (Zr, Y, and Nb), compatible (Ni, Cr, V, and Co), rare earth elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), and U, Th, Hf, and Pb (Suppl. Table 2).

The geochemical interpretation was based, fundamentally, on the software GeoChemical Data ToolKIT (GCDkit; Janoušek et al., 2006), suitable for series discrimination, rock classification, CIPW (Suppl. Table 3) calculation, and petrogenetic interpretation.

The LOI values of the samples selected for this study are, in general, below 2% (mean 1.73%; Suppl. Table 1). High LOI values in rocks are generally more altered than those with lower values (Valente et al., 2002). Some samples of this work had LOI values above 2%. However, it should be noted that in addition to the rock alteration effects, LOI determinations are susceptible to significant measurement errors, for example, weight gain due to ferrous iron oxidation (Lechler & Desilets, 1987).

A quality estimate of the analyses used here was the calculation of coefficients of variation (coefficient of variation = standard deviation/mean; Suppl. Table 1). According to Cox et al. (1979), Rollinson (1993), and Rollinson and Pease (2021), high values of coefficients of variation are indicative of:

• 1. Analytical error that can be tested with precision and accuracy data;

• 2. Weathering that this can be tested by checking the LOI values and variations of immobile incompatible trace elements, e.g., Y, Zr, and Nb.

Only P₂O₅, Ba, and Sr elements analyzed in the studied samples had a variation coefficient above 1. Possibly, this was due to the fact of some weathering according to item 2 above, since the precision and accuracy values were considered satisfactory. Analytical results of lithogeochemical data from elements of 18 samples are shown in Suppl. Table 1.

3.4 Geochemical modeling

It was intended that the petrogenesis of the magmatism associated with the study area would be based on regression analysis by the least squares method. Thus, variation diagrams would be elaborated for major elements, mobile incompatible, and immobile incompatible trace elements. In this way, it would be possible to discuss the evolutionary processes that would have occurred in the magmatic chambers during the generation of the trachytic and phonolitic bodies of the PQAC.

We have chosen to work with the modal batch partial melting model with trace elements to characterize the generating source of magmas from Três Estados plug in the PQAC. We used the Fractional Crystallization Model with trace elements to test the hypothesis of cogeneticity between the trachytes and phonolites of the Três Estados plug, for which we used the Rayleigh equation.

4 RESULTS

4.1 Fieldwork

We organized the field activities in two stages: firstly, we prioritized a geological profile on the north side of the study area, including the highest region where the intersection boundaries of the Three States are located. Second, we prioritized the south face in a profile that connects the top of Pico dos Três Estados to the base of the neighboring peak known as Cupim do Boi.

We collected 29 samples, divided into 16 subvolcanic and 13 plutonic rocks. In the subvolcanic rocks, the predominant texture is porphyritic, and the predominant color is dark to medium gray (Figs. 3A and 3B). The plutonic rocks are inequigranular to equigranular, with a light gray color due to the low volume of mafic minerals, not exceeding 10% of the rock’s volume.

Figure 3.
The image displays all the lithologies observed in the field: (A) porphyritic phonolite, (B) trachyte, (C) alkali syenite with inclusions of mesocratic monzosyenite (marked by yellow circles), (D) alkali syenite with nepheline, (E) nepheline syenite, and (F) alkali syenite with magnetite.

In some points of the alkali syenite, one of the plutonic rocks characterized in the field, we identified semicircular portions (Fig. 3C), with a major axis varying between 1 and 6 cm. This material is rich in mafic minerals in a proportion that varies from 35 to 45% of the modal composition, different from the host rock, which had a maximum of 10% of mafic minerals in its composition. According to Furman and Spera (1985) and Lai et al. (2008), characteristics such as the facoidal or rounded shape of enclaves and the mineralogical difference between the host rock and the enclaves suggest the occurrence of magma mingling.

The prominent feature in the area is the porphyritic phonolite plug, which represents a rock formation measuring approximately 1.2 km², including the highest peak in the study area. The predominant lithology in contact with the phonolite plug is alkali syenite with nepheline (Fig. 3D); however, in the northeast portion of the plug, we identified contacts with two other lithologies: alkali syenite and nepheline syenite (Figs. 2B and 3E, and 3F).

Figure 3 shows macroscopic images of the lithologies characterized in the field and confirmed with photomicrographs and geochemical analysis.

4.2 Petrography

The microscopic description of the 28 thin sections from samples collected revealed the existence of six distinct lithologies.

Four of these rocks (alkali syenite, alkali syenite with nepheline, trachyte, and porphyritic phonolite) were mapped as units, and the other two (nepheline syenite with sodalite and mesocratic monzosyenite) occur only as inclusions present in the mapped units. These lithologies will be described in the following section, and the photomicrographs are shown in Fig. 4.

Figure 4.
Photomicrograph showing the lithologies characterized in this work. (A) Alkali syenite. (B) Nepheline syenite with sodalite, where most of the dark-colored grains are interstitial sodalite, as indicated by the arrows. (C) Porphyritic phonolite. (D) Trachyte. (E) Alkali syenite with nepheline. (F) Mesocratic monzosyenite, comprising the entire area shown to the right of the yellow dotted line.

4.2.1 Lithology 1: Alkali syenite

This lithology (Fig. 4A) is holocrystalline isotropic with coarse to fine granulometry (5 to > 0.5 mm), composed essentially of alkali feldspar crystals. These alkali feldspar crystals have imbrication texture and represent 91% of the volume sample, and interstitial of them occur nepheline (4% vol.), brown hornblende (3% vol.), Fe-Ti oxide (1% vol.), sphene (1% vol.), and minor biotite, aegirine-augite, apatite, and zircon crystals.

4.2.2 Lithology 2: Nepheline syenite with sodalite

The nepheline syenite (Fig. 4B) with sodalite is holocrystalline, inequigranular, composed of approximately 76% alkali feldspar, 10% nepheline, 4% sodalite, 1% biotite, and 1% opaque minerals. The alkali feldspar and nepheline crystals are subhedral, medium to fine with habits columnar to granular, while sodalite is interstitial fine-grained. It also presents euhedral titanite grains up to 0.5 mm and enclaves with the same felsic minerals and with approximately 8% mafic minerals, including amphiboles, pyroxenes, and biotite.

4.2.3 Lithology 3: Porphyritic phonolite

The porphyritic phonolite (Fig. 4C) is holocrystalline, porphyritic with a fine to very fine-grained matrix and medium- to fine-grained macrocrystals of sanidine, nepheline, brown hornblende, aegirine-augite, and biotite. The mineralogy is composed of approximately 78% alkali feldspars, 7% amphibole, 5% aegirine-augite, 4% nepheline, and 3% biotite. The rock also presents titanite crystals (2%) and opaque minerals. The alkali feldspar macrocrystals are columnar, subhedral, and, in some cases, skeletal. Opaque minerals occur as dispersed crystals in the matrix and are associated with the alteration process of some grains of pyroxene, amphibole, and biotite.

4.2.4 Lithology 4: Trachyte

Trachyte (Fig. 4D) is holocrystalline and has 78% of alkali feldspar, with grain size ranging from fine to very fine, amphibole (6%), clinopyroxene (5%), and biotite (4%). Crystals of titanite (2%), apatite (1%), nepheline, and opaque minerals (3%) are also present. Cancrinite, clay minerals, and carbonates are characterized as secondary minerals.

Biotite occurs as microcrystal clusters, while opaque minerals have an anhedral habit and form dispersed clusters in the matrix, assuming the habit of some alkali feldspar phenocrysts.

4.2.5 Lithology 5: Alkali syenite with nepheline.

This lithology (Fig. 4E) is mainly composed of 80% alkali feldspar, 10% mafic minerals (biotite, aegirina augite, and arfvedsonite), and subordinately by opaque minerals (3%), nepheline (4%), nosean, sodalite, apatite (1%), and titanite (1%), while clay minerals are secondary minerals.

Opaque minerals occur in the form of microcrystals associated with mafic minerals. The nepheline grains occur spread in the matrix. Some noseana grains measure up to 1 mm. The grains of titanite are euhedral and do not exceed 0.1 mm. There are alkali feldspar porphyries up to 5 mm. Most of the corroded edges present change to clay minerals, some with poiquilitic texture and others with micrographic texture. Most of these porphyries have Carlsbad twinning, showing a tabular habit.

4.2.6 Lithology 6: Mesocratic monzosyenite

This lithology (Fig. 4F) occurs as an enclave in the alkali syenite. It is composed of 48% alkaline feldspar, 30% biotite, 8% opaque minerals, 5% pyroxene (augite), and 5% amphiboles, nepheline (2%), apatite (2%), and titanite. Opaque grains partially replaced most of the biotite grains. It features pyroxene grains (augite) with inclusions of nepheline and opaque minerals and acicular apatite of up to 2 mm. Nepheline and titanite are euhedral, while alkali feldspars are anhedral with reactions in contact with mafic minerals.

In addition to these six lithotypes, it was also possible to characterize two more widely occurring lithologies near the target area. However, the recognition of this lithology was restricted to interpretations and evidence observed in fieldwork. One of these lithologies was characterized as nepheline syenite that outcrops in the northeast portion of the studied area. The rock color is light gray, presenting at least 10% of euhedral grains of nepheline with intergranular features. It also presents euhedral titanite grains up to 0.5 mm and approximately 10% mafic minerals, including amphiboles, pyroxenes, and biotite (the latter occurs occasionally).

The other lithotype identified is an alkali syenite with magnetite, located in the south and southwest regions of the study area. The rock is light gray in color, with medium to coarse grain size (0.3–0.7 mm) and inequigranular porphyritic texture. Mafic minerals are mainly amphiboles. Alkaline feldspar porphyries measuring up to 13 mm in their longest axis have also been identified in this rock.

The presence of magnetite was verified using traditional methods applied in fieldwork. One such method involved adhering magnets to hand samples and parts of the outcrop. Two other methods used were observing the compass needle anomalous oscillation and attracting and dragging the dust removed from the samples with a magnet. Grains of titanite and biotite are also present in this rock, although they occur punctually.

From the interpretations obtained in the fieldwork and in the analysis of photomicrographs, we prepared a simplified geological map for the research region and its surroundings (Fig. 2B).

4.3 Lithogeochemistry

Lithogeochemical analyses processed at ACTLABS indicated a SiO₂ variable mass from 54.58 to 61.39%. This interval falls within the spectrum of intermediate composition igneous rocks (Le Maitre et al., 2002; Wilson, 1989). The MgO content varied between 0.05 and 1.58%, and according to Cox et al. (1979) and Wilson (1989), this is an indication of the rock’s evolved character.

Sample 141D did not present detectable P₂O₅ values. Sc and Ni elements were significant only in some samples, and Cr was below the detection limit for all samples (Suppl. Table 2).

4.3.1 Lithogeochemistry classification and series discrimination

All plutonic rock samples are located in the alkaline rock field and classified as nepheline syenite (Fig. 5A) in the total alkali vs. silica (TAS) diagram of Cox et al. (1979). Samples of subvolcanic rocks also constitute alkaline rocks and are classified as trachytes and phonolites (Fig. 5B) according to the TAS diagram (Cox et al., 1979).

Figure 5.
(A) SiO2 × (Na2O + K2O) (TAS) diagram from Cox et al. (1979) for series discrimination. Plutonic samples are blue circles. (B) SiO2 × (Na2O + K2O) (TAS) diagram from Cox et al. (1979) for series discrimination. Subvolcanic samples are red circles. Diagrams with trace elements of samples of (C) plutonic rocks and (D) subvolcanic rocks, normalized by the primitive mantle of Sun and McDonough (1989).

We used the rare earth elements normalized for the composition of the primitive mantle according to Sun and McDonough (1989) to construct the spider diagram in Figs. 5C, and 5D. In general, the behavior of light, medium, and heavy rare earth elements (LREE, MREE, and HREE, respectively) of plutonic and subvolcanic rocks is very similar. Both show negative P and Ti anomalies, with emphasis on sample 143B of plutonic rocks and sample 141D of subvolcanic rocks whose phosphorus did not reach the minimum limit for detection. This negative anomaly suggests a fractionation of apatite and titanite, both very well characterized in microscopic petrography and even macroscopically in the case of titanite.

The CIPW norm (Suppl. Table 3) calculation indicates the presence of quartz and normative hyperstene, which characterizes some rock samples as saturated to supersaturate in silica, despite the fact that most of the samples are silica-undersaturated, as indicated by the presence of nepheline normative. The calculation of the CIPW norm was done for both samples together and with the lithogeochemical data of the oxides recalculated for the anhydrous basis using a ratio FeO/Fe₂O₃ = 0.50 (Middlemost, 1989).

The presence of acmite, as indicated by the CIPW normative calculation (Suppl. Table 3), highlights the sodium nature of specific samples (138B, 141C, 141D, and 144A). This observation becomes evident by the distribution of samples in the graph of Fig. 6, according to the ratio Al/NK (Al₂O₃/ Na₂O+K₂O) by Al/CNK (Al₂O₃/ CaO+Na₂O+K₂O) according to Shand (1943), which shows a higher concentration of these samples in the field of peralkaline rocks and with few samples in the field of peraluminous rocks.

Figure 6.
The diagram A/CNK × A/NK (Al₂O₃/CaO + Na₂O + K₂O × Al₂O₃/ Na₂O + K₂O (mol.%)) adapted from Shand (1943), shows the distribution of samples in the plutonic rocks represented by blue circles and subvolcanic rocks by red circles. A greater concentration of samples in the field of peralkaline rocks is evident.

Most of the samples analyzed in this article have potassium affinity (Fig. 7). This characteristic was observed in other studies carried out close to the investigated area and in other alkaline intrusions present in Brazilian territory (Middlemost, 1975; Morbidelli et al., 1995; Silva et al., 2024).

Figure 7.
Discriminating diagrams of ultrapotassic, potassic, and sodic suites of alkaline series, with most samples from the study area plotting in the potassic suites field. Diagram adapted from Middlemost (1975). The blue circles represent agpaitic samples, and the red circles represent miaskitic samples.

Marins (2012) reports that all alkaline complexes analyzed in his scientific research presented potassic and sodic rocks, and, in contrast, ultrapotassic rocks are characterized only in the Itatiaia’s alkaline complexes, Tanguá, Passa Quatro, and Gericinó-Mendanha. Marins (2012), in the Passa Quatro Alkaline Complex, also observed the predominance of miaskitic rocks in the study area.

4.3.2 Geochemical modeling

The study of the petrogenesis of magmatism associated with the study area would initially occur with regression analysis using the least squares method. Thus, we could construct variation diagrams, both for studies of larger elements and for incompatible mobile and immobile trace elements. In this way, it would be possible to discuss the evolutionary processes that would have occurred in the magmatic chambers during the generation of the trachytic and phonolitic bodies of the Passa Quatro Alkaline Complex.

However, the study area presented a great lithological variety of plutonic and subvolcanic rocks, and the collected samples in the fieldwork were not enough to represent all this variety in sufficient numbers for petrogenetic analyses through variation diagrams. We chose to work with partial batch melt modal modeling in order to characterize the generating source of the magmatism studied in this work. Additionally, we tested the Fractional Crystallization Model in order to assess the possibility of this evolutionary process being associated with felsic subvolcanic magmatism (from trachyte to phonolite) since there is greater representativeness in terms of the sample.

4.3.3 Modal batch partial melting modeling

The modal batch partial melting modeling described by the equation C_L/C₀ = 1/ F+D−FD, according to Wood and Fraser (1976), was used to derive the composition of the generating source of the less evolved rocks from the studied region in the present article. In this equation, C_L is the concentration of a trace element in the liquid, C₀ is the concentration of a trace element in the original solid, F is the residual melt fraction, and D is the total crystal-liquid partition coefficient.

Wilson (1989) warns about the use of this and other forms of modeling (modal fractional, non-modal fractional, and non-modal batch) pointing out that they are based on idealized assumptions and, therefore, are not much more than an attempt to approximate reality, but they are still important tools that demonstrate the application of basic principles and the limiting effects of igneous processes.

In this modeling, we consider the composition and mode of a spinel lherzolite by Maaloe and Aoki (1977), with 50% olivine, 18% orthopyroxene, 28% clinopyroxene, and 4% spinel (Table 1). The partition coefficients used are those of basalt, calculated by Mckenzie and O’Nions (1991).

Table 1.
Adjusted mode of a spinel lherzolite according to Maaloe and Aoki (1977).

Mineral	Mode (%)	Kd-La	Kd-Yb
Olivine	0.50	0.0004	0.0015
Orthopyroxene	0.18	0.002	0.049
Clinopyroxene	0.28	0.054	0.28
Spinel	0.04	0.01	0.01
Total	1.00

We used the concentration of La (2.06) and Yb (0.08) of the xenolith KLxm1 from the Limeira 1 Kimberlite, located 26 km north of Monte Carmelo (MG), data obtained from the work of Almeida (2009). According to the author, this xenolith is classified as a spinel lherzolite and presents high large ion lithophile element/high field strength elements (LILE/HFSE) ratios, characteristic of metasomatism provided by subduction zone fluids. The ratio of the cited La and Yb concentrations, normalized by the Thompson chondrite (1982), reached 25.75 (Table 2).

Table 2.
Element concentrations (ppm) and ratios of samples derived from metasomatized mantle^*.

Elements	La	Yb	La/Yb**
Concentration	2,060	0.080	25,750
Normalization factor	0.329	0.22

• *According to Almeida (2009);
• **La/Yb ratio normalized to chondrite of Thompson (1982).

We also used the La and Yb concentrations and the ratio between these elements (La/Yb) normalized according to Thompson’s chondrites (1982) composition from the less evolved samples (trachytes, phonolites, and basanite) of this study to calculate the actual La/Yb ratios for each of these samples (Table 3). The other data used and the results obtained are presented in Suppl. Table 4.

Table 3.
Concentrations (ppm) and element ratios of less evolved samples.

Less evolved samples	Elements		Ratio between elements
	La	Yb	La/Yb*
Saturated trachytic liquid	227	3.44	44.13
Undersaturated phonolytic liquid	211	3.83	36.84
Undersaturated tephritic liquid (PQ-GM-06D)**	87.5	2	29.26

• *La/Yb ratio normalized to chondrite of Thompson (1982);
• **Data from Marins (2012).

Figure 8 presents the partial melting curve of a metasomatized mantle in a melting range that varies from 0 to 10%. According to the modeling presented in this graph, with approximately 3% partial melting, it is possible to produce trachytic liquid with a La/Yb ratio equal to 44.06. The phonolytic liquid is generated with approximately 5% partial melting, presenting a La/Yb ratio of 36.81, and with approximately 8% partial melting the liquid with a tephritic composition is generated with a La/Yb ratio of 29.30 (Table 3 and Suppl. Table 4).

Figure 8.
Graph illustrating the variation in La/Yb ratio as a function of partial melting of a metasomatized mantle within the melting range of 0 to 10%. In this simulation, the trachytic melt (red box) is generated at approximately 3% partial melting, the phonolitic melt (green box) at approximately 5% partial melting, and the tephritic melt (yellow box) at approximately 8% partial melting.

4.3.4 Fractional crystallization model

For subvolcanic rocks, the model involving fractional crystallization was the one that presented the most plausible results for the magmatic differentiation process. For this modeling, we use the Rayleigh fractionation equation: CL/C0=F^(D−1) according to Wood and Fraser (1976).

We used partition coefficients (Kd) resulting from phenocryst-matrix analysis of evolved rocks such as panterllerite-trachyte (Mahood & Stimac, 1990), per-alkaline trachyte (Larsen, 1979), phonolite (Schnetzler & Philpotts, 1970), trachyte-basalt (Villemant, 1988), trachyte (Mahood & Stimac, 1990), syenite (Marks et al., 2004), and phonolite (Olin & Wolf, 2012). There were no records of the partition coefficient of Nb for apatite, Eu for titanite, or Pr for biotite and apatite.

In general, the subvolcanic rock samples presented the following modal composition: 76% K-feldspar, 9% amphibole, 5% clinopyroxene, 4% nepheline, 3% titanite, 2% biotite, and 1% apatite (Table 4).

Table 4.
Modal composition and Kd^* of elements used in Fractional Crystallization modeling.

Mineral	Mode (%)	Kd Y	Kd Nb	Kd Yb	Kd Sm	Kd Ce	Kd Eu	Kd La	Kd Nd	Kd Hf	Kd Pr
K Feldspar	0.76	0.017	0.004	0.002	0.0004	0.0005	0.024	0.15	0.026	0.009	0.27
Nepheline	0.04	0.01	0.011	0.016	0.012	0.0011	0.043	0.01	0.013	0.008	0.013
Amphibole	0.09	0.017	1.25	0.33	0.89	2.06	1.02	2.06	0.37	7.29	0.45
Aeg-augite	0.05	1.12	0.0001	0.227	0.33	3.9	0.6	2.3	0.49	1.54	2.12
Titanite	0.03	25	5.4	13.5	0.0001	34		29	52	10	47
Biotite	0.02	0.007	0.085	0.042	0.031	0.034	0.03	0.57	0.032	0.06
Apatite	0.01	20		10	38	30.3	30.2	39	23	0.07
Total	1.00
D**		1,021	0.280	0.549	0.478	1,705	0.444	1,686	1,869	1,042	1,762

• *Partition coefficient;
• **total partition coefficient.

For modeling, we used trace element concentrations from the most evolved sample (trachyte 139B) and the least evolved sample (phonolite 138B) among the subvolcanic rock samples (Table 5).

Table 5.
Concentration of trace elements in the samples used in the Fractional Crystallization model.

Elements	Y	Nb	Yb	Ce	Sm	Eu	La	Nd	Hf	Pr
139Ba(trachyte)	37.10	238.00	4.76	386.00	8.27	0.956	273.00	80.80	22.00	34.90
138Bb(phonolite)	39.10	262.00	5.27	363.00	8.62	1,030	240.00	77.50	21.90	31.80
Enrichment rate	1.05	1.10	1.11	0.94	1.04	1.08	0.88	0.96	1.00	0.91

• ^aLess evolved samples;
• ^bMore evolved samples.

In Table 6, we have presented the modeled data, representing the trace element concentrations obtained by applying the data from Tables 4 and 5 to the model, fixed at a fractional crystallization rate of 10%. Subsequently, we compared these modeled data with the actual trace element concentrations (real data) of the most differentiated sample used in the modeling process.

Table 6.
Results obtained by applying 10% Fractional Crystallization to modeling.

Elements	Y	Nb	Yb	Ce	Sm	Eu	La	Nd	Hf	Pr
Modeled CL	37.02	256.77	4.99	358.39	8.74	1.01	253.96	81.58	21.90	32.21
Real CL	39.10	262.00	5.27	363.00	8.62	1.03	240.00	77.50	21.90	31.80
% Difference	5.32	2.00	5.28	1.27	1.36	1.59	5.82	5.27	0.02	1.28
Analytical precision	7.06	11.79	5.84	5.70	6.30	3.40	6.00	6.50	0.00	4.90
Average % difference	2.90
Average analytical precision	5.70

5 DISCUSSION

5.1 Petrographic aspects

In a comprehensive study of petrography and discrimination of magmatic series of PQAC rocks, Paula et al. (2019) identified the presence of two alkaline magmatic series. The first series is strongly unsaturated in silica, comprising miaskitic and agpaitic rocks. The other series is moderately alkaline, comprising unsaturated, saturated, and supersaturated rocks in silica. Also, according to Paula et al. (2019), the predominant lithotypes are nepheline syenite, alkali syenites, and phonolites.

In our study, the alkaline magmatic series moderately unsaturated in silica predominated. Our field studies, including investigations into the textural aspects of the rocks, degree of weathering, color, optical microscopy, and geochemical analyses, led us to propose the existence of at least six distinct lithotypes.

Among these six lithotypes, four demonstrate considerable extension and are capable of mapping: trachyte, alkali syenite, alkali syenite with nepheline, and porphyritic phonolite. The remaining two lithotypes are localized occurrences found as inclusions within the mappable lithotypes: the mesocratic monzosyenite included in the alkali syenite and the nepheline syenite with sodalite occurring as inclusions in the porphyritic phonolite. Adding the lithotypes described only through fieldwork and petrography (nepheline syenite and alkali syenite with magnetite), in total there were eight lithotypes characterized in the region covered by Pico dos Três Estados and its adjacent areas.

Most lithotypes observed in this study align with the lithotypes proposed by Almeida (1983) in the Serra do Mar Alkaline Province. In general, the rock’s mineralogical composition in this study is very similar to the mineralogical composition of the entire PQAC proposed by Brotzu et al. (1992). This includes a percentage of mafic minerals less than 10% by volume and a notable scarcity of plagioclase grains.

5.1.2 Possible evidence of a mingling process

The study of mixing and mingling mechanisms is crucial not only for understanding the diversity of volcanic rocks but also for understanding the entire geological scenario within the system. Both phenomena are recognized in several geological processes, each possessing distinct characteristics (Lai et al., 2008).

The occurrence or absence of these processes ultimately depends on the energy dynamics inherent to the system, specifically on the ratio between heat gain and loss. This ratio, as highlighted by Furman and Spera (1985), plays a fundamental role in determining the time required for magma mixing. Furthermore, the extent of this process and the effectiveness of mixing depend on both the intensity of convection and the viscosity ratio of the magmas involved in the process (Furman & Spera, 1985).

In addition to the globular features observed macroscopically (Fig. 3C), slide petrographic analysis reveals a substantial difference in crystal granulometry between mafic and felsic materials. The granulometry of mafic material is significantly smaller than that of crystals in felsic material. This contrast arises from the temperature disparity between the materials, causing the higher-temperature mafic magma to solidify more rapidly (Sklyarov & Fedorovskii, 2006). The materials’ distinct mineralogical compositions involved are another factor that suggests the mingling occurrence between these two components (Lai et al., 2008).

These globular structures (Figs. 9A and 9B) and this difference in granulometry and mineralogy (Figs. 9C and 9D) observed in some samples collected in our study suggest a coexistence between two magmas with different characteristics at a given time and restricted in the magmatic chamber that gave rise to the PQAC. The volume of the mafic material is restricted to felsic and suggests that the mafic magma is subordinate to felsic magmas, which in turn dominate the magmatic chamber.

Figure 9.
Spheroidal structures indicative of magma mingling processes. In panel A, we present an image of a sample collected from the study area, while panel B features an image from the work of Lai et al. (2008). Panels C and D show photomicrographs under parallel and crossed nicols, respectively, highlighting the mineralogical and granulometric differences between these two liquids.

5.2 Modal batch partial melting modeling

The least evolved samples in this paper presented a La/Yb ratio of 38.38 for miaskitic rocks (trachytes) and 30.51 for agpaitic rocks (phonolite). We used data from a sample of tephritic composition analyzed in previous work at PQAC (Marins, 2012), which generated a La/Yb ratio of 29.26 (Table 3). These are very high proportions compared to the La/Yb ratio of an Oceanic Island Basalt (OIB)-type mantle, which is equal to 1.54 (Komiya et al., 2004).

When applying the La and Yb concentrations of an OIB-type mantle to partial melting modeling in modal equilibrium, we verified that it is not possible to simulate the generation of an alkaline magmatic composition with the high La/Yb ratios observed in the samples. Clague and Frey (1982) and Thompson et al. (1998) propose that an ideal range for alkaline magma generation would normally be between 0.1 and 11% of partial melting.

Thompson et al. (1998), using REE ratios in partial melt modeling, suggest the possibility that the majority of Serra do Mar mafic magmas were generated in the range between approximately 0.1 and 1.0% partial melt from an OIB-type lherzolitic mantle containing garnet-spinel in the proportion of 70-30, respectively, at a depth of approximately 70 km from the lithosphere. In part, this explains the plagioclase absence in the samples evaluated in our study. According to Clague and Frey (1982), Honolulu volcanoes’ alkaline olivine basalt and basanites were obtained in a range ranging from 5% to approximately 11% partial melting of a lherzolitic garnet.

According to Wyllie (1987) in Comin-Chiaramonti et al. (1997), potassium magmatism is characterized by a high concentration of incompatible elements in addition to high K, which makes its genesis improbable by partial melting of conventional peridotites.

Given the aforementioned scenario, the involvement of a mantle subjected to an enrichment process becomes necessary. This could elucidate the high La/Yb ratio linked to the petrogenesis of the trachytic and phonolytic bodies within the Passa Quatro Alkaline Complex.

The enrichment of the mantle may take place in subduction zones through metasomatic processes. Comin-Chiaramonti et al. (1997), based on model ages, proposed Proterozoic metasomatic events as the catalysts for this enrichment. They argue that the gain in incompatible elements results from geodynamic processes, serving as precursor agents for the genesis of tholeiitic and alkaline magmatism in the Paraná Basin.

In this article, considering the data presented and the discussion mentioned above, we applied data from an enriched source in the La/Yb ratio to the model (Table 2). We work with the hypothesis that the generator mantle of alkaline magmatism analyzed in this study underwent metasomatic processes.

Most xenoliths collected in the Limeira 1 Kimberlite showed some degree of metasomatism, according to Almeida (2009). However, the KLxm1 xenolith, classified according to Streckeisen (1976) as Spinel Lherzolite, presented characteristics that fit the partial melting modeling in modal equilibrium, according to Wood and Fraser (1976).

Still, according to Almeida (2009), an important characteristic regarding the KLxm1 xenolith is the fact that it was subjected to refertilization, especially in relation to LILE, which guaranteed a high LILE/HFSE ratio, a characteristic resulting from metasomatism related to subduction zone fluids.

The range of alkaline magma generation predicted by our modeling (0–10%) is consistent with values reported in the literature. This indicates that the source of the alkaline magmatism has undergone some degree of metasomatism associated with subduction events. Such a finding supports the notion that subduction processes significantly influence the composition and characteristics of the magma source. The agreement with existing data enhances our understanding of the geochemical mechanisms driving alkaline magmatism and the implications for the geological processes at play.

5.3 Fractional crystallization model

Regarding the Fractional Crystallization Model, we operate under the hypothesis that phonolites originated from a trachytic liquid composition. This proposition is aligned with Marks and Markl (2017), who suggest that agpaitic rocks result from the differentiation processes of miaskitic magmas. For Wilson (1989), phonolites may be residual liquids of undersaturated silica and intermediate alkaline differentiation series.

Applying this model, we identified a correlation between the elements Y, Nb, Yb, Ce, Sm, Eu, La, Nd, Hf, and Pr with the mineral assembly presented in Table 4, applying 10% fractional crystallization. This correlation becomes evident when we compare the modeled liquid data with the real sample data (Table 6).

Among the analyzed elements, only Yb, La, and Nd deviate slightly in analytical precision from the average percentage difference between the modeled liquid and PQAC phonolite. The other elements have the average percentage difference below the average analytical precision (Table 6). The Kd absence for certain elements in the available literature made it unfeasible to apply the model to these elements.

6 CONCLUSIONS

The investigation into the alkaline rock suites presented in this study has revealed a complex interplay of geological processes, particularly in relation to magma mingling and petrogenesis. Consistent with findings from similar studies, our research underscores the intricacies involved in fieldwork and the interpretation of geochemical data, which are crucial for constructing robust geological models that advance our understanding of magmatic evolution.

The integration of comprehensive field investigations with petrographic and geochemical analyses has allowed for a more nuanced characterization of subvolcanic bodies in the study area. Notably, the majority of these bodies exhibit phonolitic characteristics, with the Pico dos Três Estados region predominantly composed of porphyritic phonolitic rocks, while only a minor fraction is classified as trachyte. This detailed examination has facilitated the development of a geological map that delineates the primary facies within the target area.

Field observations of structures indicative of restricted magma mingling processes suggest the presence of a relatively stable magma chamber characterized by low convective intensity. This stability appears to have preserved the distinct compositional characteristics of the coexisting magmas within the chamber. However, further investigations are warranted to deepen our understanding of these processes and to validate their occurrence in the PQAC study area.

Modeling results indicate that the less evolved compositions observed may be associated with varying degrees of partial melting of a common mantle source. This source is distinguished by a significant enrichment in the La/Yb ratio, which exceeds that of an OIB-type source by at least 16 times. Such enrichment is likely the result of metasomatic processes, predominantly occurring during Neoproterozoic subduction events. The modeling successfully simulates the generation of alkaline magma within the range of 3 to 8% partial melting, providing insights into the magmatic processes at play.

Moreover, our application of the fractional crystallization model to the differentiation of subvolcanic bodies yielded compelling results. We propose that phonolitic bodies may originate from the differentiation of trachytic magma, beginning at approximately 10% fractional crystallization. Given the recognized lithological complexity and heterogeneity of the region, we advocate for further research to elucidate the local geological framework and to explore the implications of these findings on broader magmatic processes.

In conclusion, this study contributes significantly to the understanding of alkaline rock petrogenesis and highlights the necessity for continued exploration in this geologically rich area. The insights gained herein not only enhance our comprehension of the specific geological phenomena observed but also pave the way for future investigations that could reveal further complexities in the magmatic systems of the region.

Supplementary material

https://doi.org/10.5281/zenodo.13993836

ACKNOWLEDGMENTS

We thank Cristiane Mara Silva da Costa, João Pedro Lopes, and Mayara Reis who gave support in the field trip and for their help in revising the English text. We also thank Actalabs for lithogeochemical analyses. We, in particular, are extremely grateful to the PETROMAGMATISM project (FAPUR 19/03 and SAP 4600581919—authorized by the National Agency of Petroleum, Natural Gas, and Biofuels of Brazil) that supported this research.

ARTICLE INFORMATION

• Manuscript ID: e2024009. Received on: 5 FEB 2024. Approved on: 13 SEP 2024.
  How to cite: Freitas, G. C., Corval, A., Silva, J. C. L., Pinheiro, H. S. K., & Valente, S. C. (2024). Subvolcanic petrology of the Pico dos Três Estados region (SE Brazil): Implications for Alkaline Magmatic Evolution. Brazilian Journal of Geology, 54(2), e20240009. https://doi.org/10.1590/2317-4889202420240009

• Funding:
  We, in particular, are extremely grateful to the PETROMAGMATISM project (FAPUR 19/03 and SAP 4600581919—authorized by the National Agency of Petroleum, Natural Gas, and Biofuels of Brazil) that supported this research.

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Publication Dates

• Publication in this collection
  03 Feb 2025
• Date of issue
  2025

History

• Received
  05 Feb 2024
• Accepted
  13 Sept 2024