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The Hidden Magmatic Chamber from the Ponte Nova Maﬁc–Ultramaﬁc Alkaline Massif, SE Brazil: Clues from Clinopyroxene and Olivine Antecrysts

: Clinopyroxene and olivine primocrysts in the intrusions of the Ponte Nova maﬁc–ultramaﬁc alkaline massif (SE Brazil) present several textures and zoning that indicate open-system processes. Important compositional differences were found in the clinopyroxene. Diopside relict cores (mostly partially corroded) present higher Mg, Cr and Ni and lower Ti, Na, Al, REE and Sr than Ti-augite mantling and rims. Subordinately, two types of olivine crystals were recognized, one related to very zoned crystals with high Mg (Fo up to 86 mol.%) and Ni cores (mostly with corroded rims), and other almost without clear zonation and with lower Mg contents. Relict cores of high-Mg clinopyroxene and olivine crystals are representative of antecrysts formed in deeper chamber environments. Temperature and pressure estimates based on clinopyroxene-liquid geothermobarometers indicate crystallization of the antecrysts at ~1171 ± 10 ◦ C and ~5.7 ± 0.3 kbar, pointing to a deeper hidden magmatic chamber, whereas mantling and rim compositions indicate a shallow chamber environment. hidden chamber progressive enrichments of incompatible elements the Mg# decrement and inﬂection points in Sr and REE due to the starting of co-precipitation of apatite. The evolution trend of clinopyroxene antecrysts indicates that the main intrusions in the Ponte Nova shallow chamber were fed by a single deeper hidden chamber mainly controlled by typical fractional crystallization processes. These antecrysts indicate the presence of a complex plumbing system, which is also supported by similar antecrysts found in the lamprophyre and alkali basalt dikes of this region. The preferred petrological model for the Ponte Nova massif could be summarized as repeated inﬂuxes of antecryst-laden basanite magmas that deposited most of their suspended crystals on the ﬂoor of the upper-crust magma chamber. The weighted mean values of P and T the uncertainties of 22 pairs are 5.7 ± 0.30 kbar (MSWD = 0.3) and 1171 ± 10 ◦ C (MSWD = 0.13), respectively. All the PN intrusions are included in the 22 clinopyroxene relict core–liquid pairs and the obtained individual values for P are similar and within the uncertainties; thus, a common magmatic chamber for the formation of these clinopyroxene and olivine antecrysts is indicated. Additionally, we tested the equilibrium conditions of clinopyroxene antecryst cores found in primitive alkali basalts and lamprophyre dikes from the Mantiqueira Range: in a set of 11 analyses, just one clinopyroxene–liquid pair met the condition of the multicomponent equilibrium ﬁlters from Neave et al. (Supplementary Material The resulting calculation of this pair indicate a P of kbar and T of 1180 ± 45 ◦ C, which is completely similar to the values found for PN when the uncertainties are considered. Ponte Nova massif, corrosion and/or resorption/re-equilibration of antecrysts are a common feature, with mantled (growth type) by Ti-augite. Author Contributions: Conceptualization, R.G.A.; investigation, R.G.A., M.R.A. and L.M.C.T.; data curation, all authors; writing—original draft preparation, all authors; writing—review and editing, all authors; visualization, R.G.A., L.M.C.T. and V.G.; project administration, R.G.A.; funding acquisition, R.G.A. and E.R. authors


Introduction
Magmas generated at different pressures in the mantle will ascend through the lithosphere, possibly lodge in intermediate reservoirs, and ultimately find their final emplacement sites along a network of interconnected channels, like a plumbing system [1][2][3][4][5][6][7][8]. Such a complex system allows for cogenetic and/or captured crystals (antecrysts or xenocrysts, respectively) to be recycled during the ascent of magmas through the lithosphere. As minerals record changes in the magmatic environment, the information of each chamber  [45]. (C) Simplified geological map of the Ponte Nova alkaline mafic-ultramafic massif [41]. Utilized abbreviations: CI, central intrusion; WI, western intrusion; NI, northern intrusion; EI, eastern intrusion; CP, central plug; SSI, southern satellite intrusion; ICp, ilmenite clinopyroxenites and magnetitites; Brc, magmatic breccia; SALB: Serra de Á gua Limpa Batholith. Locations of studied samples are indicated as dots on the map; numbers in italic refer to samples analyzed by electron microprobe-wavelength dispersion system (EMP-WDS) and numbers in italic and underlined were analyzed by EMP-WDS and laser ablation-inductive coupled plasma mass spectrometry (LA ICP-MS).
The PN mafic-ultramafic alkaline massif (87.6 Ma; [39]) is an alkaline gabbroic association representative of several magmatic pulses ( Figure 1C). Emplaced into Precambrian metagranites and gneiss of the Serra da Á gua Limpa Batholith (645-630 Ma, [47]), the PN consists of a main body (~5.5 km 2 ) that encompasses five different intrusions (northern, central, western, and eastern intrusions and a central plug) and a smaller satellite body (southern satellite body; ~1 km 2 ). For detailed geological and petrographic characteristics of the massif, the reader is referred to [39,41,48].
Most of the PN main body is represented by the central (CI) and western (WI) intrusions. The lower levels of CI and WI are composed of cumulatic melagabbros and clinopyroxenites (both with variable amounts of olivine), while their higher levels group monzogabbros with variable textural features (i.e., porphyritic, equigranular, massive, and banded rocks). The northern intrusion (NI) is also cumulatic, with olivine melamonzogabbros grading to olivine-bearing melamonzogabbros towards the top. The eastern intrusion (EI) is made up of more evolved types, monzodiorites with variable nepheline (inequigranular seriate and porphyritic textures). A small plug (CP) formed by porphyritic to equigranular microgabbros occupies the central part of the main body. The southern satellite intrusion (SSI) contains porphyritic nepheline-bearing melamonzonites and monzogabbros as predominant rocks, although more evolved types (i.e., nephelinebearing monzonites) are also present. Other small intrusions cropping out in the interior of the massif include cumulates of sulfide-bearing ilmenite clinopyroxenites and magnetitites (ICp), and a magmatic breccia (Brc). Numerous thin dikes of alkaline rocks of different degrees of evolution crosscut most PN intrusions, as well as the regional metagranitoids, being those widespread in the western Mantiqueira Range [38]. Especially, nearby the PN massif, porphyritic alkaline lamprophyre and alkali basalt dikes (basanite and tephrite compositions) present clinopyroxene and olivine antecrysts [37]. Simplified from [45]. (C) Simplified geological map of the Ponte Nova alkaline mafic-ultramafic massif [41]. Utilized abbreviations: CI, central intrusion; WI, western intrusion; NI, northern intrusion; EI, eastern intrusion; CP, central plug; SSI, southern satellite intrusion; ICp, ilmenite clinopyroxenites and magnetitites; Brc, magmatic breccia; SALB: Serra de Água Limpa Batholith. Locations of studied samples are indicated as dots on the map; numbers in italic refer to samples analyzed by electron microprobe-wavelength dispersion system (EMP-WDS) and numbers in italic and underlined were analyzed by EMP-WDS and laser ablation-inductive coupled plasma mass spectrometry (LA ICP-MS).
The PN mafic-ultramafic alkaline massif (87.6 Ma; [39]) is an alkaline gabbroic association representative of several magmatic pulses ( Figure 1C). Emplaced into Precambrian metagranites and gneiss of the Serra da Água Limpa Batholith (645-630 Ma, [47]), the PN consists of a main body (~5.5 km 2 ) that encompasses five different intrusions (northern, central, western, and eastern intrusions and a central plug) and a smaller satellite body (southern satellite body;~1 km 2 ). For detailed geological and petrographic characteristics of the massif, the reader is referred to [39,41,48].
Most of the PN main body is represented by the central (CI) and western (WI) intrusions. The lower levels of CI and WI are composed of cumulatic melagabbros and clinopyroxenites (both with variable amounts of olivine), while their higher levels group monzogabbros with variable textural features (i.e., porphyritic, equigranular, massive, and banded rocks). The northern intrusion (NI) is also cumulatic, with olivine melamonzogabbros grading to olivine-bearing melamonzogabbros towards the top. The eastern intrusion (EI) is made up of more evolved types, monzodiorites with variable nepheline (inequigranular seriate and porphyritic textures). A small plug (CP) formed by porphyritic to equigranular microgabbros occupies the central part of the main body. The southern satellite intrusion (SSI) contains porphyritic nepheline-bearing melamonzonites and monzogabbros as predominant rocks, although more evolved types (i.e., nepheline-bearing monzonites) are also present. Other small intrusions cropping out in the interior of the massif include cumulates of sulfide-bearing ilmenite clinopyroxenites and magnetitites (ICp), and a magmatic breccia (Brc). Numerous thin dikes of alkaline rocks of different degrees of evolution crosscut most PN intrusions, as well as the regional metagranitoids, being those widespread in the western Mantiqueira Range [38]. Especially, nearby the PN

Clinopyroxene and Olivine Paragenesis
Coarse-grained clinopyroxene (diopside cores and Ti-augite mantling and rims) and olivine primocrysts are the main cumulus phases of the CI, WI and NI and the main macrocrysts of the EI and SSI from PN ( Figure 2). Clinopyroxene, the main mafic phase of the entire PN massif, evidences complex growth histories. These are indicated by (i) completely to partially resorbed diopside cores, with dissolution surfaces (including gulf texture) and mantled (growth type) by Ti-augite, by (ii) completely to partially resorbed diopside cores, with pervasive resorption and replaced by intercumulus minerals (Ti-augite, biotite, kaersutite, magnetite, plagioclase), by (iii) normal step zoning, by (iv) crystals with oscillatory zoning with coarse banding and with reverse zonings included, and by (v) sievetextured crystals ( Figure 3; Table 1). All these types of zoning and textures are suggestive of open-system processes [9,29,31,55,56]. Olivine is found as cumulus crystals, as minor inclusions in few diopside cores, and partially involved by Ti-augite rims in the most primitive rocks of the CI, WI and NI (Figures 2 and 3; Table 1). They present normal progressive zoning or occur as almost homogeneous crystals. In more evolved samples, olivine is absent or was completely resorbed. In EI, olivine is found as rounded macrocrysts presenting normal zoning and with resorption textures followed by pyroxene and biotite coronas. In SSI, olivine is also found as macrocrysts with normal progressive zoning and included in clinopyroxenite autoliths. Coarse-grained clinopyroxene (diopside cores and Ti-augite mantling and rims) and olivine primocrysts are the main cumulus phases of the CI, WI and NI and the main macrocrysts of the EI and SSI from PN ( Figure 2). Clinopyroxene, the main mafic phase of the entire PN massif, evidences complex growth histories. These are indicated by (i) completely to partially resorbed diopside cores, with dissolution surfaces (including gulf texture) and mantled (growth type) by Ti-augite, by (ii) completely to partially resorbed diopside cores, with pervasive resorption and replaced by intercumulus minerals (Ti-augite, biotite, kaersutite, magnetite, plagioclase), by (iii) normal step zoning, by (iv) crystals with oscillatory zoning with coarse banding and with reverse zonings included, and by (v) sieve-textured crystals ( Figure 3; Table 1). All these types of zoning and textures are suggestive of open-system processes [9,29,31,55,56]. Olivine is found as cumulus crystals, as minor inclusions in few diopside cores, and partially involved by Ti-augite rims in the most primitive rocks of the CI, WI and NI (Figures 2 and 3; Table 1). They present normal progressive zoning or occur as almost homogeneous crystals. In more evolved samples, olivine is absent or was completely resorbed. In EI, olivine is found as rounded macrocrysts presenting normal zoning and with resorption textures followed by pyroxene and biotite coronas. In SSI, olivine is also found as macrocrysts with normal progressive zoning and included in clinopyroxenite autoliths. (E,F) Porphyritic and inequigranular textures found in EI and SSI, with macrocryst population formed by clinopyroxene, subordinate olivine and minor plagioclase. Corrosion and sieve textures on complex step-zoned clinopyroxene cumulus crystals, with diopside relict cores (beige) and Ti-augite (brown) mantling and rims are found in all intrusions. Utilized mineral abbreviations: cpx, clinopyroxene; ol, olivine; opc, opaque minerals; pl, plagioclase; for additional abbreviations, see Figure 1. (E,F) Porphyritic and inequigranular textures found in EI and SSI, with macrocryst population formed by clinopyroxene, subordinate olivine and minor plagioclase. Corrosion and sieve textures on complex step-zoned clinopyroxene cumulus crystals, with diopside relict cores (beige) and Tiaugite (brown) mantling and rims are found in all intrusions. Utilized mineral abbreviations: cpx, clinopyroxene; ol, olivine; opc, opaque minerals; pl, plagioclase; for additional abbreviations, see Figure 1.      Figure 5A). Especially for mantling and rims, the progressive increment of the CaTi-tschermak component is indicated by a positive correlation of IV Al with Ti 4+ of the M1 site ( Figure 5B). The Mg#cpx is positively correlated with Cr2O3 (0-1.3 mass%) and negatively correlated with Al2O3 (2.0-9.9 mass%), TiO2 (0.5-4.5 mass%) and Na2O (0.2-1.2 mass%) ( Figure 4). The relict cores present higher Mg# (mostly 0.85-0.90 to the cumulate intrusions and 0.80-0.86 to the porphyritic intrusions), Cr and lower Ti, Na and Al than Ti-augite mantling and rims. The clinopyroxene compositions indicate main substitutions of diopside for hedenbergite (Hd) and diopside for the main Ca-tschermak endmembers (Ca VI Al IV AlSiO6, CaTiAl2O6 and CaFe 3+ AlSiO6; Figure 5A). Especially for mantling and rims, the progressive increment of the CaTi-tschermak component is indicated by a positive correlation of IV Al with Ti 4+ of the M1 site ( Figure 5B). Interestingly, the KDE of Mg#cpx for PN antecrysts are very similar to that found for antecrysts from the nearby Mantiqueira Range lamprophyre and alkali basalt dikes, as well as the KDEs for PN mantling and rims are very similar to that found for Ti-augite mantling, rims and matrix crystals for the cited nearby dikes. Interestingly, the KDE of Mg# cpx for PN antecrysts are very similar to that found for antecrysts from the nearby Mantiqueira Range lamprophyre and alkali basalt dikes, as well as the KDEs for PN mantling and rims are very similar to that found for Ti-augite mantling, rims and matrix crystals for the cited nearby dikes.  Olivine. Olivine crystals from the different intrusions of the PN are within the range of Fo86-Fo51, with crystal cores from EI and CI presenting the highest values (Fo > 80 mol.%) and crystals from CP and WI having the lowest ones (Fo < 56 mol.%, mainly rims) ( Figure 6). Positive correlation is found between Fo and NiO (0-0.31 mass%) and negative correlation is found between Fo and MnO (0.14-1.14 mass%). CaO (0.01-0.67 mass%) does not present clear linear correlation with Fo although a tendency of higher CaO values for crystal cores with higher Fo is observed. Two olivine groups are identified: (i) one associated with almost unzoned, cumulatic olivine crystals, with Mg#ol [Mg/(Mg+FeT), molecular proportions] below 0.77, and (ii) another group is found for zoned olivine crystals with Mg#ol > 0.80 (gray bar in Figure 6). Olivine. Olivine crystals from the different intrusions of the PN are within the range of Fo 86 -Fo 51 , with crystal cores from EI and CI presenting the highest values (Fo > 80 mol.%) and crystals from CP and WI having the lowest ones (Fo < 56 mol.%, mainly rims) ( Figure 6). Positive correlation is found between Fo and NiO (0-0.31 mass%) and negative correlation is found between Fo and MnO (0.14-1.14 mass%). CaO (0.01-0.67 mass%) does not present clear linear correlation with Fo although a tendency of higher CaO values for crystal cores with higher Fo is observed. Two olivine groups are identified: (i) one associated with almost unzoned, cumulatic olivine crystals, with Mg# ol [Mg/(Mg + Fe T ), molecular proportions] below 0.77, and (ii) another group is found for zoned olivine crystals with Mg# ol > 0.80 (gray bar in Figure 6).

Trace Elements
Clinopyroxenes. Trace element compositions were mainly determined in pyroxene crystals from the most primitive samples of the PN intrusions (Table SC2;

Trace Elements
Clinopyroxenes. Trace element compositions were mainly determined in pyroxene crystals from the most primitive samples of the PN intrusions (Table SC2; (Table 2). Similar intervals for cores and rims are found for Sc (31-116 ppm) and Co (27-49 ppm).
Regarding chondrite-normalized [57] rare-earth element (REE) distribution patterns of clinopyroxenes from the PN (Figure 7), relict cores of each intrusion present contrasting compositions; marked lower values in the relict cores when compared to those from mantling and rims (Figure 7; WI mantling and rims were not analyzed due to the extensive exsolution features presented in those crystals). All chondrite-normalized REE distribution patterns display smooth LREE/HREE fractionation and exhibit a concave downward pattern for normalized LREEs, with slightly depleted La values relative to Ce and Pr values, and a negative slope for normalized HREEs (Gd N /Yb N > 2.5). The compositions of some Ti-augite rims present an Eu-negative anomaly, suggesting a protracted crystallization of clinopyroxene, with its final stage just occurring after extensive fractionation of plagioclase. Whole-rock compositions (compiled from [41]-Supplementary Materials B) from the same units from which crystals were analyzed, present values of REE much more enriched than diopside cores (Figure 7). The relict cores found in the most primitive samples of each intrusion (all with resorption/corrosion features) present an evolution trend, the most primitive found for CI and NI (richer in Mg# cpx , Cr, Ni) and the most evolved related to EI (richer in Ti, Al, Na, Zn, Zr, REE, V, Co) (Figures 4 and 7). This trend seems to be independent of that found for the mantling and rims of each intrusion, which present an important overlap for most element (Supplementary Materials E).

Antecrysts in the Ponte Nova Massif
The presence of complex zoned microstructures, with diopside cores and Ti-augite mantling and rims exhibiting significant compositional differences, indicates that almost no subsolidus diffusive exchange occurred in most of the selected clinopyroxene crystals [58,59]. This is not the case of most of the analyzed olivine, especially those that present almost homogeneous compositions. In fact, the samples that presented only few clinopyroxene crystals with partially corroded cores (with most of all re-equilibrated to Ti-augite compositions) are the same where olivine crystals are almost homogeneous. Thus, the relict cores of clinopyroxene crystals and of zoned olivines, both found in the most primitive samples of each intrusion, can give some clues to a complex plumbing system existing beneath the PN massif [31,55,56,[60][61][62]. To establish if clinopyroxene or olivine was the first mineral to be crystallized from the studied plumbing system, the Mg-Fe exchange coefficient [Kd Fe × X ol Mg )] between these primocrysts from different samples of PN massif was calculated. Following experimental data in similar chemical systems, a Kd cpx−ol Mg−Fe range of 1.04-1.91 tends to represent crystallization in equilibrium conditions [63]. The Kd cpx−ol Mg−Fe values indicate that most of the high-Mg clinopyroxene relict cores are more primitive than the homogeneous olivine crystals, being instead in equilibrium with cores of strongly zoned olivine crystals ( Figure 8).
These primitive diopside cores and their restricted evolution trend ( Figure 4) imply the existence of a deeper magmatic chamber that could be connected to the shallow magmatic reservoir represented by the PN massif, in a complex plumbing system [3,30,64]. These relict cores can be classified as antecrysts, crystals that did not crystallize directly from the host magma in which they are contained, although they maintain a genetic relationship with the same system [3,10,29]. In fact, the diopside relict cores and the high-Mg zoned and corroded olivine crystals are both similar in composition, respectively, to the high-Mg clinopyroxene and olivine antecrysts from the lamprophyres and alkali basalt dikes from the nearby Mantiqueira Range [37] ( Figure 8A). Otherwise, PN Ti-augite mantling and rims are in compositional equilibrium with the nearly homogeneous olivines ( Figure 8B). Possibly, olivines with lower Mg# than that of equilibrium compositions with the clinopyroxene are affected by faster Fe-Mg diffusive exchange between olivine and liquid. The Ti-augite mantling and rims are similar in composition to the phenocrystic and groundmass Ti-augite from the lamprophyres and alkali basalt dikes from the nearby Mantiqueira Range. These similarities allow the assignment of the primitive dikes from the Mantiqueira Range [37,38] as possible liquid counterparts of the PN cumulates ( Figure 8). The presence of these antecrysts suggests a hidden deeper magmatic chamber, and understanding this part of the plumbing system responsible for the trans-crustal evolution of PN alkaline magmas and estimating the intensive parameters involved (such as P and T) are fundamental.  [65]. Iron-magnesium coefficients are from compilation of [66] and [67] for clinopyroxene-liquid exchanges. Based on (C), the high-Mg diopside relict cores are in equilibrium with the high-Mg liquids (Mg# > 60) of the Mantiqueira Range, whereas the most common Ti-augite and unzoned olivines are mostly in equilibrium with dike compositions with 45 < Mg# < 50. The points plotted in graph are the pairs of clinopyroxene-liquid which attended to the filters of equilibrium conditions for the application of the geothermobarometer of [68].

Estimation of Intensive Parameters (P-T) for High-Mg Diopside Antecrysts of PN
To estimate the crystallization pressure and temperature of the clinopyroxene antecrysts of PN, we apply the thermobarometers of [66,68]. The thermometer is based on the temperature sensitivity related to the equilibrium exchange between the diopside + hedenbergite [Ca (Mg, Fe)-DiHd-liquid and jadeite (NaAlSi 2 O 6 -Jd)-liquid] [66,69] and it is also based on the strong pressure dependency for the introduction of jadeite into clinopyroxene in equilibrium with the liquid, where a high Jd component would occur at high pressures [68,69]. Both equations are co-dependent and corresponding to thermodynamic expressions calibrated from experimental database that include tholeiitic and alkaline compositions in hydrous systems, with SEE (equivalent to the standard deviation-σ) of 1.4 kbar and 45 • C for pressure and temperature, respectively [66,68,69].
For their application, the authors of [68] recommend looking for putative liquids that are in equilibrium with the clinopyroxene compositions. In this case, we choose the ultrabasic alkaline dikes of the Mantiqueira Range [37,38] that outcrop in the region. The clinopyroxene relict cores and some of the most primitive liquid (dikes) compositions of this area are inside of the experimental data range used for model calibration by [68] and, therefore, suitable for the application of this barometer. Moreover, the [66] thermometer is the most suitable for hydrous systems [66,68].
We took into consideration the following compositional filters in our database, in part according to [68]: (i) clinopyroxene relict core compositions with Mg# cpx > 0.80 and cation sums between 3.96 and 4.04, with 1% of uncertainty of the theoretical value of 4; (ii) clinopyroxene Jd-component > 0.01; (iii) clinopyroxene with Al cation > 0.11; and (iv) mass of LOI < 4.5% and Mg# liq > 60 from whole-rock analysis of Mantiqueira dikes.
The clinopyroxene relict core-liquid (dikes) pairs were also filtered according to equilibrium constant (K D ) and multicomponent equilibrium, as suggested by [67,70]. The  Figure 8C). The multicomponent equilibrium filter compares the predicted (theoretical calculated) and the observed clinopyroxene components, accepting values within the reported SEE error of ±0.06 for ∆DiHd, ±0.05 for ∆EnFs, and ±0.03 for ∆CaTs [67,71] for the predicted values. These filters improve the equilibrium condition that must exist in the selected crystal-liquid pairs [67,70].
Applying the filters, we obtained 28 pairs of clinopyroxene relict core-liquid, using three putative liquids from Mantiqueira Range, and with clinopyroxene compositions of all studied intrusions being represented (Supplementary Material F). Despite the individual values, a Kernel density estimation (KDE) diagram was applied ( Figure 9). The peak determinations of the KDEs indicate a T of 1166 ± 45 • C and a P of 5.6 ± 1.4 kbar for the crystallization conditions in the hidden chamber of PN (Figure 9). Six clinopyroxene relict core-liquid pairs present lower pressures (<4 kbar, Figure 9), far from the main peak of 5.6 kbar. These values are interpreted as a possible consequence of re-equilibrium between the clinopyroxene relict core and the surrounding liquid. Excluding these six pairs from the previous set, the peak of KDE is~1173 • C and 5.  Mantling and rims of clinopyroxenes represent the more differentiate compositions from PN (i.e., < Mg#, > Al2O3, > TiO2 and > Na2O), so the application of these thermobarometers is not recommended, will most of the data falling outside the proposed equilibrium filters. This indicates that the clinopyroxene sectors would be in disequilibrium crystallization with the host liquid, as attested by complex, oscillatory, and step zonings patterns found in these clinopyroxenes (Figures 2 and 3). The high TiO2 content (up to 4.50 mass%) and high Ca-Tschermak's components (i.e., Ca VI Al IV AlSiO6, CaTiAl2O6 and CaFe 3+ AlSiO6: 3-14 mol.%) in the mantling and rims sectors are outside the compositional calibration range used for the [68] barometer.
Although a quantitative determination of P estimates is not possible for the Ti-augite mantling and rim composition, their evolution trend suggests very low-P crystallization conditions (cf., [64,[72][73][74][75]). Ti-augite from all intrusions of PN presents a very positive correlation of Ti and the IV Al ( Figure 5) [74] showed that there are compositional differences between high and low pressure clinopyroxenes based on trends of increasing Ti and IV Al at the expense of silica, the increase in the IV Al/ VI Al ratio and the increase in TiO2 coupled with decreasing Mg#cpx. All these features are found for the PN clinopyroxenes. The experimental work of [75] with mildly alkaline lava also indicate that Ti and Al substitution and the proportion in which it occurs are very sensitive to pressure conditions. The comparison of PN clinopyroxenes with [75] experimental data suggests that high-Mg# relict cores were crystallized in different conditions from Ti-augites mantling and rims and that Ti-augite was formed in low-pressure environment due to their trend following lower Ti:Al proportions ( Figure 10). [76] also point that the coupled increase in Ti in the M1-site and Al in the T-site is a main trend controlled by the progressive decrease in pressure. Following the procedure of [77], the PN mantling and rims present high calculated values of cell volumes (VCELL = 438.9-442.0 Å 3 ; VM1= 11.6-11.9 Å 3 ), pointing to verylow crystallization pressure [77], similar to some nearby Brazilian alkaline complexes [42,[78][79][80][81]. In fact, the estimation of low-pressure conditions for the PN massif is consistent with the expected denudation estimates based on apatite fission track in the Precambrian rocks of Mantiqueira Range [82]. The exhumation through denudation of the Mantiqueira Range, associated with continental rifts developed since Early Cretaceous period, would not expose on the surface rocks generated under pressure above ~1 kbar [82][83][84]. Hence, in the studied plumbing system, Ti-augite mantling and rims of clinopyroxene were crystallized in a low-pressure upper crust environment, the PN massif, Figure 9. KDEs of temperatures and pressures calculated for the clinopyroxene antecrysts from PN using equations [66,68]. (A) Temperature KDEs is calculated with a bandwidth of 45 • C, as recommended in [66], with peak of estimations close to~1180 • C. (B) Pressure KDEs is calculated with a bandwidth of 1.4 kbar, which is comparable to the SEE (1σ) of [68] barometer, with peak of estimations of pressure of~5.6 kbar, accompanied by boxplot graph. Considering just the antecrysts with minimum re-equilibrium (n = 22), the peak of P is~5.8 kbar, values also consistent with the medians obtained using boxplot graph.
Mantling and rims of clinopyroxenes represent the more differentiate compositions from PN (i.e., <Mg#, >Al 2 O 3 , >TiO 2 and >Na 2 O), so the application of these thermobarometers is not recommended, will most of the data falling outside the proposed equilibrium filters. This indicates that the clinopyroxene sectors would be in disequilibrium crystallization with the host liquid, as attested by complex, oscillatory, and step zonings patterns found in these clinopyroxenes (Figures 2 and 3). The high TiO 2 content (up to 4.50 mass%) and high Ca-Tschermak's components (i.e., Ca VI Al IV AlSiO 6 , CaTiAl 2 O 6 and CaFe 3+ AlSiO 6 : 3-14 mol.%) in the mantling and rims sectors are outside the compositional calibration range used for the [68] barometer.
Although a quantitative determination of P estimates is not possible for the Ti-augite mantling and rim composition, their evolution trend suggests very low-P crystallization conditions (cf., [64,[72][73][74][75]). Ti-augite from all intrusions of PN presents a very positive correlation of Ti and the IV Al ( Figure 5) [74] showed that there are compositional differences between high and low pressure clinopyroxenes based on trends of increasing Ti and IV Al at the expense of silica, the increase in the IV Al/ VI Al ratio and the increase in TiO 2 coupled with decreasing Mg# cpx . All these features are found for the PN clinopyroxenes. The experimental work of [75] with mildly alkaline lava also indicate that Ti and Al substitution and the proportion in which it occurs are very sensitive to pressure conditions. The comparison of PN clinopyroxenes with [75] experimental data suggests that high-Mg# relict cores were crystallized in different conditions from Ti-augites mantling and rims and that Ti-augite was formed in low-pressure environment due to their trend following lower Ti:Al proportions ( Figure 10). [76] also point that the coupled increase in Ti in the M1-site and Al in the T-site is a main trend controlled by the progressive decrease in pressure. Following the procedure of [77], the PN mantling and rims present high calculated values of cell volumes (V CELL = 438.9-442.0 Å 3 ; V M1 = 11.6-11.9 Å 3 ), pointing to very-low crystallization pressure [77], similar to some nearby Brazilian alkaline complexes [42,[78][79][80][81]. In fact, the estimation of low-pressure conditions for the PN massif is consistent with the expected denudation estimates based on apatite fission track in the Precambrian rocks of Mantiqueira Range [82]. The exhumation through denudation of the Mantiqueira Range, associated with continental rifts developed since Early Cretaceous period, would not expose on the surface rocks generated under pressure above~1 kbar [82][83][84]. Hence, in the studied plumbing system, Ti-augite mantling and rims of clinopyroxene were crystallized in a low-pressure upper crust environment, the PN massif, whereas diopside antecrysts were formed in a hidden chamber at higher pressure,~5.8 kbar, in the middle of the regional crust.
whereas diopside antecrysts were formed in a hidden chamber at higher pressure, ~5.8 kbar, in the middle of the regional crust. Figure 10. Ti versus Al (calculated to 6 oxygens) for the clinopyroxenes from the main intrusions of the PN massif, compared with the experimental data from [75] that correlates the proportion Al/Ticpx with pressure. The size of the symbols is related to the Mg#cpx content (higher Mg#, bigger symbols). In this sense, high-Mg relict cores tend to present higher Al/Ti values than mantling and rims. Tiaugite mantling and rims in all intrusions are more compatible with low-pressure crystallization conditions, with diopside relict cores suggesting a deeper crystallization. Figure 10. Ti versus Al (calculated to 6 oxygens) for the clinopyroxenes from the main intrusions of the PN massif, compared with the experimental data from [75] that correlates the proportion Al/Ti cpx with pressure. The size of the symbols is related to the Mg# cpx content (higher Mg#, bigger symbols). In this sense, high-Mg relict cores tend to present higher Al/Ti values than mantling and rims. Ti-augite mantling and rims in all intrusions are more compatible with low-pressure crystallization conditions, with diopside relict cores suggesting a deeper crystallization.

The Evolution of Hidden Chamber from PN
The petrological model of the PN massif [41] is assigned as repeated influxes of antecryst-laden basanite magmas. The primary cumulate intrusions of the massif deposit most of their suspended crystals on the floor of the upper-crust magma chamber. Each intrusion is representative of relatively primitive olivine-and clinopyroxene-phyric basanites that had assimilated different degrees of partial melts of heterogeneous host rocks [41,48]. The antecrysts presented in this work indicate that these influxes of antecryst-laden magmas could be derived from just one deeper magmatic chamber.
Clinopyroxenes of this hidden chamber have progressive enrichments of incompatible elements with the decrease in Mg# cpx (Figure 11), indicating that this deeper chamber was mainly controlled by typical fractional crystallization processes. The inflection points for Sr and REE are related to abrupt decreases in these elements, followed by progressive increases (Figure 11). These inflections could be due to the starting of co-precipitation of some mineral phases that have high mineral-melt partition coefficients for Sr and REE, such as plagioclase or apatite (e.g., [85][86][87][88][89]). In fact, part of the plagioclase and apatite crystals of PN are also attributed to minor components of the cumulus assemblage and also classified as antecrysts based on their in situ isotopic compositions [41]. In our study, few analyses of apatite and plagioclase (high-An) were presented (Supplementary Material G) as a reference for the incorporation of Sr and REE by these phases (Figure 12). Once plagioclase holds similar REE values to that of the high-Mg diopsides, it seems that these elements are mostly controlled by apatite ( Figure 12). Otherwise, Sr contents are higher in apatite and plagioclase in relation to clinopyroxene antecrysts, and, therefore, simultaneous crystallization of these phases could be affected by the Sr concentration in clinopyroxenes. The evolution trend of antecrystic clinopyroxene indicates that the main intrusions of the shallow chamber, the PN massif, were fed by a single deeper hidden chamber mainly controlled by fractional crystallization processes.

The Evolution of Hidden Chamber from PN
The petrological model of the PN massif [41] is assigned as repeated influxes of antecryst-laden basanite magmas. The primary cumulate intrusions of the massif deposit most of their suspended crystals on the floor of the upper-crust magma chamber. Each intrusion is representative of relatively primitive olivine-and clinopyroxene-phyric basanites that had assimilated different degrees of partial melts of heterogeneous host rocks [41,48]. The antecrysts presented in this work indicate that these influxes of antecrystladen magmas could be derived from just one deeper magmatic chamber.
Clinopyroxenes of this hidden chamber have progressive enrichments of incompatible elements with the decrease in Mg#cpx (Figure 11), indicating that this deeper chamber was mainly controlled by typical fractional crystallization processes. The inflection points for Sr and REE are related to abrupt decreases in these elements, followed by progressive increases (Figure 11). These inflections could be due to the starting of co-precipitation of some mineral phases that have high mineral-melt partition coefficients for Sr and REE, such as plagioclase or apatite (e.g., [85][86][87][88][89]). In fact, part of the plagioclase and apatite crystals of PN are also attributed to minor components of the cumulus assemblage and also classified as antecrysts based on their in situ isotopic compositions [41]. In our study, few analyses of apatite and plagioclase (high-An) were presented (Supplementary Material G) as a reference for the incorporation of Sr and REE by these phases (Figure 12). Once plagioclase holds similar REE values to that of the high-Mg diopsides, it seems that these elements are mostly controlled by apatite ( Figure 12). Otherwise, Sr contents are higher in apatite and plagioclase in relation to clinopyroxene antecrysts, and, therefore, simultaneous crystallization of these phases could be affected by the Sr concentration in clinopyroxenes. The evolution trend of antecrystic clinopyroxene indicates that the main intrusions of the shallow chamber, the PN massif, were fed by a single deeper hidden chamber mainly controlled by fractional crystallization processes.   In each intrusion of PN, clinopyroxenes antecrysts representative of different degrees of magmatic differentiation of this deeper environment are found. Basic-ultrabasic dikes from the Mantiqueira Range present similar clinopyroxene and olivine antecryst population ( Figure 8; [37]). Possibly, PN antecrysts are related to a similar plumbing system, although without its predicted interaction with evolved alkaline systems, as PN do not present the green core pyroxenes found in some occurrences of Mantiqueira region [37]. The evolution trend found for the diopside antecrysts ( Figure 11) suggests that all the main intrusions of PN were possibly related to a single deeper hidden chamber that fed the intrusions of PN with crystals of different degrees of evolution ( Figure 13). In each intrusion of PN, clinopyroxenes antecrysts representative of different degrees of magmatic differentiation of this deeper environment are found. Basic-ultrabasic dikes from the Mantiqueira Range present similar clinopyroxene and olivine antecryst population ( Figure 8; [37]). Possibly, PN antecrysts are related to a similar plumbing system, although without its predicted interaction with evolved alkaline systems, as PN do not present the green core pyroxenes found in some occurrences of Mantiqueira region [37]. The evolution trend found for the diopside antecrysts ( Figure 11) suggests that all the main intrusions of PN were possibly related to a single deeper hidden chamber that fed the intrusions of PN with crystals of different degrees of evolution ( Figure 13).
On the other hand, the very similar compositional trends found for mantling and rims of clinopyroxene crystals from the different intrusions indicate similar liquid compositions that have crystallized in the shallow environment (Figures 4 and 11). In the upper crust chamber of the PN massif, corrosion and/or resorption/re-equilibration of antecrysts are common features; in fact, other differentiation processes (apart from crystal fractionation) have also acted in this upper-crust chamber, such as crustal assimilation [41,48]. Thus, the particular behavior of some elements in mantling and rims of clinopyroxenes of the EI, SSI and WI could be attributed to assimilation processes of the regional heterogeneous host rock, as evidenced by isotopic data [41]. As a result, many disequilibrium textures were found especially for the clinopyroxene. In the lower levels of each intrusion, however, better conditions for the preservation of such relict cores were found and the evidence of a hidden chamber has been presented. Figure 13. Schematic illustrating the proposed petrological model for the Ponte Nova mafic-ultramafic alkaline massif, outlined as repeated influxes of antecryst-laden basanite magma pulses. The antecryst assemblage is mainly formed by clinopyroxene (cpx), with subordinate olivine (Ol) and minor plagioclase (pl), apatite (ap) and Ti-magnetite (mgt). These minerals are the main cumulus phases found in the lower levels of the main cumulate intrusions of the massif and indicate a deeper hidden chamber for Ponte Nova massif. The ~5.8 kbar P estimation offered are relative to the calculations based on clinopyroxene-melt thermobarometry [66,68]. The dikes shown correspond to basic-ultrabasic dikes from the Mantiqueira Range, presenting similar clinopyroxene and olivine antecryst population. In the upper crust plutons of the Ponte Nova massif, corrosion and/or resorption/re-equilibration of antecrysts are a common feature, with mantled (growth type) by Ti-augite.
On the other hand, the very similar compositional trends found for mantling and rims of clinopyroxene crystals from the different intrusions indicate similar liquid compositions that have crystallized in the shallow environment (Figures 4 and 11). In the upper crust chamber of the PN massif, corrosion and/or resorption/re-equilibration of antecrysts are common features; in fact, other differentiation processes (apart from crystal fractionation) have also acted in this upper-crust chamber, such as crustal assimilation [41,48]. Thus, the particular behavior of some elements in mantling and rims of clinopyroxenes of the EI, SSI and WI could be attributed to assimilation processes of the regional heterogeneous host rock, as evidenced by isotopic data [41]. As a result, many disequilibrium textures were found especially for the clinopyroxene. In the lower levels of each intrusion, however, better conditions for the preservation of such relict cores were found and the evidence of a hidden chamber has been presented.
Finally, the PN massif represents a good example of transcrustal scale magmatic processes (i.e., plumbing systems; [6,90]), which lead to the formation of complex alkaline systems, and could improve the understanding of the evolution of mush-type reservoirs in typical anorogenic settings.

Conclusions
Clinopyroxene and olivine antecrysts are described for the first time in the plutons of the Ponte Nova alkaline mafic-ultramafic massif (SE Brazil). Diopside antecrystic cores (mostly partially corroded) present higher Mg, Cr, Ni and lower Ti, Na, Al, REE and Sr than Ti-augite mantling and rims. Subordinately, highly zoned olivine antecrysts have high Mg (Fo up to 86 mol.%) and Ni cores (mostly with corroded rims). Temperature and Figure 13. Schematic illustrating the proposed petrological model for the Ponte Nova maficultramafic alkaline massif, outlined as repeated influxes of antecryst-laden basanite magma pulses. The antecryst assemblage is mainly formed by clinopyroxene (cpx), with subordinate olivine (Ol) and minor plagioclase (pl), apatite (ap) and Ti-magnetite (mgt). These minerals are the main cumulus phases found in the lower levels of the main cumulate intrusions of the massif and indicate a deeper hidden chamber for Ponte Nova massif. The~5.8 kbar P estimation offered are relative to the calculations based on clinopyroxene-melt thermobarometry [66,68]. The dikes shown correspond to basic-ultrabasic dikes from the Mantiqueira Range, presenting similar clinopyroxene and olivine antecryst population. In the upper crust plutons of the Ponte Nova massif, corrosion and/or resorption/re-equilibration of antecrysts are a common feature, with mantled (growth type) by Ti-augite.
Finally, the PN massif represents a good example of transcrustal scale magmatic processes (i.e., plumbing systems; [6,90]), which lead to the formation of complex alkaline systems, and could improve the understanding of the evolution of mush-type reservoirs in typical anorogenic settings.

Conclusions
Clinopyroxene and olivine antecrysts are described for the first time in the plutons of the Ponte Nova alkaline mafic-ultramafic massif (SE Brazil). Diopside antecrystic cores (mostly partially corroded) present higher Mg, Cr, Ni and lower Ti, Na, Al, REE and Sr than Ti-augite mantling and rims. Subordinately, highly zoned olivine antecrysts have high Mg (Fo up to 86 mol.%) and Ni cores (mostly with corroded rims). Temperature and pressure estimates based on clinopyroxene-liquid geothermobarometers indicate crystallization of the antecrysts at~1171 ± 10 • C and~5.7 ± 0.3 kbar, pointing to a deeper hidden magmatic chamber, whereas mantling and rim compositions indicate a shallow chamber environment. Clinopyroxenes of this hidden chamber have progressive enrichments of incompatible elements with the decrease in Mg#, but with inflection points for Sr and REE mainly due to the starting of co-precipitation of apatite. The evolution trend of antecrystic clinopyroxene indicates that the main intrusions of the shallow chamber, the Ponte Nova massif, were fed by a single deeper hidden chamber mainly controlled by typical fractional crystallization processes. These antecrysts suggest the presence of a complex plumbing system, which is also supported by the similar antecrysts found in the lamprophyre and alkali basalt dikes of this region. The preferred petrological model for the Ponte Nova massif could be summarized as repeated influxes of antecryst-laden basanite magmas that deposited most of their suspended crystals mainly on the floor of the upper-crust magma chamber.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/min12060775/s1, A-Tables of major element of clinopyroxene and olivine compositions from the Ponte Nova massif (MS-Excel file). Table SA1. Analytical parameters and calibration standards used for WDS analyses of clinopyroxene and olivine. Table SA2. Major element compositions in mass % of clinopyroxene from the Ponte Nova massif. Structural formula calculated on the basis of 6 oxygens. Abbrev.: c, cores; m/r, mantling/rims. Table SA3