Amphibole crystallization conditions as record of interaction between ultrapotassic enclaves and monzonitic magmas in the Glória Norte Stock, South of Borborema Province

The Glória Norte Stock (GNS) is made up of predominantly porphyritic biotite- amphibole-bearing quartz monzonite, enclosing a large number of microgranular mafic enclaves (MME). The GNS MME are fine-grained rocks with rounded and ellipsoid shapes that show sharp and gradational contacts with the host rocks, suggesting they coexisted with the host monzonite as magmas. The studied amphibole crystals of the two rock types are calcic and correspond to pargasite, edenite, and magnesium-hornblende. Their compositions are influenced by substitutions involving Al 3+ , Na + , Fe 2+ , Si 4+ and Mg 2+ ions, reflecting the decrease in temperature and the increase in oxygen fugacity. They present low Ti and Al contents, and varied Si content (6.2–7.7 apuf). The mg# values range from 0.48–0.84. Pressures for the crystallization of the MME amphiboles vary between 2.6 and 7.8 kbar and the temperatures of solidus and liquidus were estimated between 600–659 and 887–908°C, respectively. The early MME amphibole crystals with lower Si and Mg content were formed under high pressure (7.8 kbar) and temperature (908 °C). The presence of amphiboles with 862°C and 5 kbar in MME and GNS reflects that the interaction between these magmas occurred at 18 km of depth.


INTRODUCTION
Amphiboles constitute a complex group of rock-forming minerals, which present large compositional variation and occur in several mineral associations. Their structures and chemical compositions are affected by changes in conditions during or after their crystallization (Hawthorne 1983). Amphiboles occur in igneous rocks, from acidic to ultrabasic, and in medium-to high-temperature metamorphic rocks (Deer et al. 1997, Martin 2007. Additionally, the occurrence of amphibole in mafic-ultramafic enclaves associated with granites and basalts indicates that it is stable in the mantle (e.g., Ringwood 1963, Krawczynski et al. 2012).
Microgranular mafic enclaves (MME) are common in the SOS granites and some of them show high amphibole contents (Sial et al. 1998). The genesis of amphibole enclaves in the MD has been ascribed to the mixing between felsic and mafic magmas (Sial et al. 1998, Conceição et al. 2016, Lisboa et al. 2019. Interaction between magmas with distinct compositions can generate total or partial mixing, and partial corrosion textures in early formed crystals (Barbarin and Didier 1992). The minerals formed during mixing may have their chemical composition modified or rebalanced due to reactions with the hybrid magma (Şahin et al. 2010).
This work presents and discusses the result of the chemical analysis obtained on amphibole crystals of microgranular mafic enclaves of the Glória Norte Stock (GNS), their textural relations and crystallization conditions.

GEOLOGICAL CONTEXT
The SOS defines the N-NE limit of the São Francisco Craton (SFC) (Fig. 1). The formation of the SOS is attributed to the Amphibole crystallization conditions as record of interaction between ultrapotassic enclaves and monzonitic magmas in the Glória Norte Stock, South of Borborema Province neoproterozoic collision between the SFC and the Pernambuco-Alagoas Superterraine (PEAL; Davison and Santos 1989). According to Oliveira et al. (2010), the convergence between SFC and PEAL led to the deformation of passive margins and granitic plutons emplacement in the central-northern portion of the SOS, during 628-621 and 590-570 Ma. These granites have calc-alkaline and shoshonitic signatures (Silva Filho et al. 1992, Conceição et al. 2016, Lisboa et al. 2019. The GNS, which was crystallized at 588 ± 5 Ma (zircon U-Pb SHRIMP ) is the main representative of the MD post-orogenic shoshonitic magmatism (Lisboa et al. 2019). This stock has an area of 45 km 2 (Fig. 1) and is essentially composed of porphyritic quartz-monzonite. Biotite, amphibole, and diopside are the mafic minerals present in these rocks. In quartz-monzonites, the enclaves and feldspar phenocrysts mark a magmatic flow foliation. The GNS contains abundant mafic enclaves, comprising three different types: microgranular mafic, lamprophyric, and cumulatic ultramafic.
The lamprophyric enclaves occur in the midwestern portion of the GNS. They are dark colored with varied shape and their length is up to 1.3 meters, with sharp contacts with quartz-monzonite. The diopside biotite cumulate enclaves (Fontes et al. 2018) are medium-grained rocks with olive green color and rounded shape. MME are abundant in the GNS ( Fig. 2A). They have different sizes and shapes (mostly rounded and ellipsoidal), often with the longest axis orientated by the magmatic flow. The contacts of these enclaves with the host quartz monzonitic rocks are gradational or sharp and their cores are more coarse-grained than the border (Fig. 2B). The decreasing grain size toward the edge of the MME is interpreted as resulting from the rapid cooling of the mafic magma droplets, due to temperature contrast with the felsic magma. Color gradation forms darker to lighter MME types, reflecting the mafic mineral contents. This variation is interpreted with different degrees of mixing between magmas. In the felsic magma, the interaction is less pervasive, limited to the contact (a few centimeters) with the mafic magma. Lisboa et al. (2019) characterized the MME as ultrapotassic alkali feldspar syenite and monzonite rocks (44 < %SiO 2 < 47, MgO > 3%, K 2 O > 3%, and K 2 O/Na 2 O > 2). According to these authors, the MME of GNS rocks correspond to intermediate magma, while the composition of the most primitive microgranular mafic enclaves correspond to mafic magmas. The low Ta, Nb, and Ti contents, which characterize orogenic magmas, are characteristics of both GSN and MME samples.

MATERIALS AND METHODS
The investigation was executed in polished thin sections of representative samples from the host quartz monzonite and mafic enclaves. The petrographic study was carried out with the aid of a transmitted and reflected light petrographic microscope from the Microscopy Laboratory at the Geoscience Laboratories, Universidade Federal de Sergipe (CLGeo-UFS).
EDS analysis was performed with an Oxford Instrument ® X-Act model EDS attached to a Tescan ® Scanning Electron Microscope (SEM) Vega 3 LMU model at the Laboratory of Microanalysis, Universidade Federal de Sergipe (CLGeo-UFS). Potential acceleration of 20 kV and current intensity of 17 nA, generating a beam with a diameter of 400 nm; analysis time was 30 s. The CLGeo's EDS-SEM is calibrated with copper energy and the reliability of the analyses is assessed from the analysis of ASTIMEX international mineral standards (Tab. 1). The analytical uncertainties, given by the difference between the certified values and those obtained with the EDS, were less than 2%wt for the elements with content greater than 10%wt in weight and errors between 4 and 19%wt for elements with content below 5% in weight.
Green amphibole is the dominant mafic phase in the MME (up to 53% volume). Most crystals are subhedral with a length of up to 1.5 mm. Pleochroism ranges between olive green (Y'), yellowish green (Z') to light yellowish green (X'). The grain limits with most minerals are rectilinear and irregular to interpenetrating with biotite and diopside. It shows inclusions of anhedral quartz, subhedral biotite, as well as apatite and ilmenite euhedral grains. Green amphibole is replaced by colorless amphibole at the borders.
The amphibole crystals are well distributed in the MME. They occur as grains up to 0.5 mm, as subhedral to anhedral crystals in mafic agglomerates, as well as euhedral inclusions in alkali feldspar and, more rarely, in plagioclase.
Mafic minerals such as amphibole, diopside, titanite, ilmenite, and magnetite, occur as agglomerates with up to 3.0 mm in length. Diopside is anhedral and rimmed by dark green amphibole (Fig. 4A). The limits between diopside and amphibole grains are diffuse and the diopside cleavage planes contain subhedral grains of ilmenite and magnetite. The association of amphibole with biotite is more common. In this case, biotite occurs as subhedral crystals with size up to 0.8 mm, arranged between amphibole crystals (Fig. 4B) or along their cleavage planes. Lisboa et al. (2019) described abundant crystals of stubby apatite and mafic mineral agglomerates, composed by amphibole, biotite, and diopside, in the GNS porphyritic quartz monzonite. Baxter and Feely (2002) propose that agglomerates of this nature are formed from a reaction between pyroxene and magma, thus leading to crystallization of amphibole. The occurrence of these agglomerates in the studied rocks suggests that they originated from mafic magma, which was responsible for the formation of MME, and their presence in the GNS enclaves and monzonites is interpreted as due to the interaction between felsic and mafic magmas, or could be only due to equilibrium crystallization of the quartz monzonite magma.

Nomenclature and chemical composition
Chemical classification of the amphiboles and structural formula were based on 23 oxygens, assuming the general formula A 0-1 B 2 C 5 T 8 O 22 (OH, F, Cl), whose site occupation complied with the recommendations of Leake et al. (1997). In total, 60 amphibole crystals were analyzed. Representative chemical analyses of these crystals are presented in Table 2. The MME amphibole crystals are calcic, with Ca B content between 1.5-2.0 apuf (atoms per unit formula), and can be divided into two groups (Figs. 5A and 5B): • (Na + K) A > 0.5 apuf (pargasite and edenita); • (Na + K) A < 0.5 apuf (magnesium-hornblende and actinolite).
In some amphibole crystals, the increase in Si and Mg associated with the depletion in IV Al from nucleus to border, form actinolite (Fig. 5B). The existence of such zoning may be a consequence of the increased oxygen fugacity in the magma, or it may also result from the reaction between magmatic amphibole and late fluids (Kawakatsu andYamaguchi 1987, Martin 2007). Similar zoning is also described as evidence of mixing between magmas by Sato et al. (1999).
Calcium amphiboles are also important mafic minerals in the GNS monzonites, where they reach up to 14% of the volume (Lisboa et al. 2019). Analyzes from representative samples of the GNS revealed that amphibole crystals from this stock correspond to edenite (Fig. 5A) and to magnesium-hornblende (Fig. 5B). There is an overlap among samples of the GSN and MME.     Figure 5. Classification diagrams for the studied calcic amphiboles, after Leake et al. (1997). (1) The composition of the crystal core corresponds to the circle, and (2) the rim corresponds to the diamond.
(3) The grey area represents amphiboles from the Glória Norte Stock. Leake (1971Leake ( , 1978 used the relationship between Si and (Na + Ca + K), to distinguish magmatic from post-magmatic amphiboles. Using these criteria, it is clear that some compositions in the MME amphiboles evolve from the magmatic to the post-magmatic field (Fig. 6). Crystals with higher Si content (> 7.1 apuf) are also enriched in Mg, depleted in Na + Ca + K (< 2.4 apuf) and present similar compositional variation to that observed by Yamaguchi (1985) for late-crystallized crystals, which had their chemical compositions partially modified due to reactions with magma, or formed under subsolidus conditions in the presence of fluids (Chivas 1982, Hendry et al. 1985. Spear (1981) listed the most common amphibole substitutions. The compositional variations in the MME studied crystals can be described in terms of pargasite, tschermakite, and Ti-tschermakite substitutions, in addition to the simple replacement of Fe 2+ by Mg (Spear 1981).

Substitutions
The diagram in Figure 7A shows a good positive correlation between (Na A + K A ) and IV Al for MME amphiboles. The relationship shown in this diagram is close to the 2:1 ratio, implying the predominance of pargasite substitution, indicating that the partial occupation of site A by alkalis requires the replacement of Si for IV Al at the tetrahedral site. The highest vacancy rates at site [A] are observed for the crystals with higher Mg and Si contents. The IV Al versus VI Al ratio (Fig. 7B) demonstrates the importance of edenite and pargasite substitutions in the evolution of the studied MME amphibole compositions. The inverse behavior of Fe and Mg suggests important participation in the simple substitution Fe +2 ↔ Mg, and also shows the good negative correlation existent between these elements (Fig. 7C).

Conditions of crystallization
Several studies used amphibole composition to infer pressure (e.g., Schmidt 1992, Mutch et al. 2016, temperature (e.g., Holland andBlundy 1994, Putirka 2016), and oxygen fugacity (e.g., Anderson and Smith 1995) magma conditions of crystallization. In calcium amphiboles, with increased pressure and temperature, K, Na, Ti, and Al contents are generally increased, although Al is more sensitive to changes in pressure (Hammarstron and Zen 1986, Féménias et al. 2006, Nandedkar et al. 2014. The main calcium amphibole geobarometers are based on the existence of a linear relationship between total Al and crystallization pressure (Anderson 1996). The identification of the amphibole in association with biotite, titanite, magnetite (and/or ilmenite), quartz, alkaline feldspar, and plagioclase in the studied MME allowed the use of these geobarometers. Pressure values calculated for the studied amphiboles by different algorithms are presented in Table 3.
The Al total content in the analyzed crystals ranges from 1.42 to 2.22 apuf. Considering all calibrations, the pressures (P) obtained to magmatic amphiboles range from 2.6 to 7.8 kbar, corresponding to high-and low-pressure amphiboles (Fig. 8A), according to Hammarstron and Zen (1986). The identification of both high-and low-pressure amphiboles in orogenesis-associated magmas is described in several papers (e.g., Samaniego et al. 2010, Ribeiro et al. 2016. As demonstrated by Anderson and Smith (1995), the increase of Al total may also be related to the increase in temperature. In this case, these authors recommended the use of barometers that consider the effect of temperature. When applying the calibration proposed by Anderson and Smith (1995) in the studied MME amphiboles, it is seen that the pressure values range from 3.8 to 6.6 kbar, similarly or very close to those obtained using other calibrations (Tab. 3).
The MME solidus temperature estimates were obtained from the amphibole-plagioclase pair thermometry, which is based on the amount of IV Al in the amphibole coexisting with plagioclase in silica-saturated rocks. The results can be seen in Table 4.
The crystallization temperature for the studied MME amphiboles uses the calibration made by Holland and Blundy (1994) for the edenite-richterite reaction. The temperatures were calculated based on the pressures obtained by the Schmidt (1992) and Anderson and Smith (1995) barometers. The temperature values obtained from this geothermometer are situated between 600 and 659°C (Tab. 4), below the temperature of the granite solidus, which is around 700°C (Piwinskii 1968, McDowell andWillye 1971).
The composition of the cores of the MME pargasite crystals was used to estimate the temperature using the empirical geothermometer of Ridolfi et al. (2010). The temperatures obtained range from 887 to 908°C (Tab. 4).
The fO 2 has a strong chemical control over major mafic phases in igneous rocks (Anderson and Smith 1995). The studied amphiboles show Fe/(Fe + Mg) ratios below 0.6. When these ratios are correlated to the IV Al content of the MME and Core: circle; rim: diamond.   Hammarstron and Zen (1986). (B) Fe/(Fe + Mg) versus IV Al diagram, showing fields with different conditions of oxygen fugacity (ƒO 2 ), according to Anderson and Smith (1995). The composition of grain cores corresponds to the circle; the grey area represents compositions of amphibole from monzonites of the Glória Norte Stock. monzonite amphibole, they indicate crystallization at high fO 2 (Fig. 8B). The presence of the magnetite + titanite paragenesis in the MME reinforces the hypothesis of crystallization under oxidizing conditions (Wones 1989).
The mean pressure values calculated for GNS monzonite amphiboles are approximately 5 kbar, indicating crystallization at around 18 km depth. Using the Holland and Blundy (1994) and Ridolfi et al. (2010) geothermometers, the temperature of amphibole crystallization values obtained range between 615-747 and 772-862°C, respectively.

DISCUSSION
Amphiboles are very useful for the investigation of mafic magma genesis, especially the ultrapotassic ones, in which amphibole is an early crystallizing phase (Foley 1992, Conceição and Green 2004, Förster et al. 2017. Compositional variation in amphibole is strongly affected by changes in pressure, temperature, oxidation conditions, magma composition, and water activity (Scaillet and Evans 1999, Dalpé and Baker 2000, Rooney et al. 2011. Experimental studies performed on amphiboles which crystallized in equilibrium with primitive basaltic magma have shown high mg# values (0.83-0.75; Grove et al. 2003) as compared to amphiboles equilibrated with more evolved magmas, where lower values are observed (mg# ≤ 0.65; Alonso-Perez et al. 2009). The amphiboles of the MME show mg# ranging between 0.48 and 0.84; these values correspond to those observed in primitive basaltic magmas and magmatic reequilibrium. This suggests that this mineral phase is a product of the early crystallization of these magmas (Hidalgo andRooney 2010, Rooney et al. 2011). The lower mg# values (below 0.65) of the studied MME amphiboles may be related to the physicochemical changes in the magmatic chamber (i.e., P, T, fO 2 ) or crystallization in equilibrium with more evolved magmas (Ribeiro et al. 2016 (Grove et al. 2003. Magmas in equilibrium with the studied amphiboles of the mafic enclaves have Mg# ranging from 0.71 to 0.93 (mean 0.78), indicating that these amphiboles represent early crystallized phases in mantle-derived magmas.
The compositions of the studied MME amphibole crystals, in particular the Al content, indicate that there are two groups of amphibole, respectively, crystallized at high-and low-pressures (Fig. 8A).
The highest pressure values correspond to lower Si and Mg content, and higher Al values, and should represent the earlier crystals crystallized at lower crustal levels, at ~28 km (considering 1kbar = 3.7 km of continental crust; Tulloch and Challis 2000). The lowest pressure values were obtained in Si-rich and Al-poor crystals, and may represent a second generation of amphiboles or crystals reequilibrated under shallower crustal conditions.
In the Macururé Domain plutons, the interaction between mafic and felsic magmas has been pointed out by several authors as the process responsible for generating the mafic enclaves in these massifs (e.g., Conceição et al. 2016, Fontes et al. 2018, Lisboa et al. 2019). The following evidence, coming from our data, points out that the interaction between mafic and felsic magmas is physically possible: textural evidence (e.g., mafic mineral agglomerates); the compositional similarity between MME and the GNS amphiboles; similar crystallization temperature and liquidus temperature of the MME amphiboles and the GNS monzonite. Yamaguchi et al. (2003) stated that the amphiboles formed at an early stage in mafic magma continue to crystallize after the interaction between magmas. The pre-interaction conditions can be obtained from early amphiboles and the reequilibrium conditions can be obtained from those phases which crystallized after the interaction. Our results suggest that the parental magmas of the MME were stored in deeper (~28 km) pressures, and the mixing with monzonitic magma was shallower (~18 km) at crustal conditions. The interaction between these magmas occurred at temperatures ranging from 772 to 862°C as indicated by Ridolfi et al. (2010), considered by us as the best geothermometers for involving different chemical elements in the calculation.

CONCLUSION
The amphiboles of the microgranular mafic enclaves of the Glória Norte Stock are calcic and correspond to pargasite, edenite, magnesium-hornblende, and actinolite. They have varying Si contents (6.2-7.7 apuf) and mg# ratio between 0.48 and 0.84. The main important substitutions involve Al, Si, Na, Fe, and Mg (edenite and pargasite types) and reflect a decrease in temperature and an increase of fO 2 . The magmatic amphiboles of the GNS are edenite and magnesium-hornblende. They crystallize at 772 and 862°C and the mean pressure values are in the order of 5 kbar, indicating crystallization at around 18 km depth.
The identification of similar chemical compositions between amphiboles from the MME and host GSN, as well as the estimates of similar pressure and temperature obtained and the compositional similarity between amphibole from the MME and GNS monzonites all suggest chemical reequilibration between the magmas which originated the monzonites and the enclaves. The interaction between these magmas occurred under ± 5 kbar pressure (~18 km depth), with temperature ranging between 772 and 862°C.