Abstract

During the Paleoproterozoic Era, the Brazilian cratons experienced orogenic events that modified the archean basement and sedimentary successions. In the southern São Francisco Craton, it can be recognized evidence of an orogenic event that happened between Rhyacian and Orosirian periods. It is related to the closure of an oceanic basin at this time, which led to the collision between the Archean Divinópolis and Campo Belo metamorphic complexes. Graphite schist occurs close to the cities of Formiga and Itapecerica (Minas Gerais), located between these complexes. To contribute to the understanding of the origin and metamorphism of the graphite from Formiga, petrographic studies, X-ray diffraction (XRD) and Raman spectroscopy analyses have been done. XRD and Raman methods revealed that the temperatures recorded by graphite are around 460°C. However, Raman data showed that the crystallite sizes correspond to higher metamorphic grade conditions (amphibolite to granulite facies). Temperatures of 460°C are probably associated with hydrothermal processes along faults in post-collisional stage. The presence of todorokite, a mineral typical of deep-sea Mn nodules formed by microorganisms, in association with graphite from Formiga, suggests a biogenic origin for the graphite occurrence.

KEYWORDS:
Southern São Francisco Craton; Raman spectroscopy; X-ray diffraction; graphite

INTRODUCTION

Graphite and diamond are the polymorph occurrence of native carbon on the nature (Harlow 1998Harlow G.E. 1998. The nature of Diamonds. Cambridge: Cambridge University Press.). Graphite has a growing economic value due to its modern technological use as graphene source (Simandl et al. 2015Simandl G.J., Paradis S., Akam C. 2015. Graphite deposit types, their origin, and economic significance. In: Simandl G.J., Neetz M. (Eds.), Symposium on Strategic and Critical Materials Proceedings. Victoria: British Columbia Geological Survey, p. 163-171.). Formed in several geological settings, graphite is most commonly found in metamorphic rocks, especially in orogenic belts. Graphite can be formed through maturation and metamorphism of biogenic carbonaceous material (CM); as precipitation from C-O-H fluids; mantle-derived; and through reduction of carbonates (Simandl et al. 2015Simandl G.J., Paradis S., Akam C. 2015. Graphite deposit types, their origin, and economic significance. In: Simandl G.J., Neetz M. (Eds.), Symposium on Strategic and Critical Materials Proceedings. Victoria: British Columbia Geological Survey, p. 163-171.).

There is a vast combination of variables like temperature, pressure, kinetics, composition of country rocks and presence or absence of fluids that can influence the formation of this mineral (Wintsch et al. 1981Wintsch R.P., O’Connell A.F., Ransom B.L., Wiechmann M.J. 1981. Evidence for the influence off CH4 on the crystallinity of disseminated carbon in greenschist facies rocks, Rhode Island, USA. Contributions to Mineralogy and Petrology, 77(3):207-213. https://doi.org/10.1007/BF00373536
https://doi.org/https://doi.org/10.1007/... , Luque et al. 1998Luque F.J., Pasteris J.D., Wopenka B., Rodas M., Barrenechea J.F. 1998. Natural fluid-deposited graphite: Mineralogical characteristics and mechanisms of formation. American Journal of Science, 298:471-498. https://doi.org/10.2475/ajs.298.6.471
https://doi.org/https://doi.org/10.2475/... , Galvez et al. 2013Galvez M.E., Beyssac O., Martinez I., Benzerara K., Chaduteau C., Malvosin B., Malavieille J. 2013. Graphite formation by carbonate reductionduring subduction. Nature Geoscience, 6:473-477. https://doi.org/10.1038/ngeo1827
https://doi.org/https://doi.org/10.1038/... ). The graphite formed by biogenic CM undergoes a progressive and irreversible process called graphitization (Buseck and Beyssac 2014Buseck P.R., Beyssac O. 2014. From organic matter to graphite: Graphitization. Elements, 10(6):421-426. https://doi.org/10.2113/gselements.10.6.421
https://doi.org/https://doi.org/10.2113/... ). It occurs in temperature and pressure of burial metamorphism, and decreases the hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios, transforming disordered and non-crystalline CM in crystalline graphite (Kwiecińska and Petersen 2004Kwiecińska B., Petersen H. 2004. Graphite, semi-graphite, natural coke, and natural char classification-ICCP system. International Journal of Coal Geology, 57(2):99-116. https://doi.org/10.1016/j.coal.2003.09.003
https://doi.org/https://doi.org/10.1016/... ) (Fig. 1). The process of graphitization does not depend on the metamorphic pressure, although it is influenced by the oxygen fugacity and metamorphic temperature (Tagiri and Oba 1986Tagiri M., Oba T. 1986. Hydrothermal syntheses of graphite from bituminous coal at 0.5-5 kbar water vapor pressure and 300-600°C. Journal of Mineralogy, Petrology and Economic Geology, 81(7):260-271. https://doi.org/10.2465/ganko1941.81.260
https://doi.org/https://doi.org/10.2465/... ). There are two forms to quantify the metamorphic temperature associated to the graphitization of biogenic CM:

An efficient method to analyse the origin of CM is to evaluate its isotopic composition, inasmuch as Buseck and Beyssac (2014Buseck P.R., Beyssac O. 2014. From organic matter to graphite: Graphitization. Elements, 10(6):421-426. https://doi.org/10.2113/gselements.10.6.421
https://doi.org/https://doi.org/10.2113/... ) indicate that this kind of deposit preserves the δ¹³C between -35 and -20‰ biologic signature. A variety of minerals can be formed in this condition, as is the case of todorokite ((Mn, Mg, Ca, Ba, Na, K)₂ Mn₅O₁₂.3H₂O), which is formed by the accumulation of Mn-oxides (Lowenstam 1981Lowenstam H.A. 1981. Minerals formed by organisms. Science, 211(4487):1126-1131. https://doi.org/10.1126/science.7008198
https://doi.org/https://doi.org/10.1126/... ), and can be used as evidence of biogenic activity on seafloor (Burns et al. 1983Burns R.G., Burns V.M., Stockman H.W. 1983. A review of the todorokite-buserite problem: implications to the mineralogy of marine manganese nodules. American Mineralogist, 68(9-10):972-980.).

Graphite schist occurs at southern São Francisco Craton, in the Itapecerica Supracrustal Sequence, associated with the collision between the Archean Divinópolis and Campo Belo metamorphic complexes. In Itapecerica (Minas Gerais (MG), Brazil), Miranda et al. (2019Miranda D.A., Chaves A.O., Campello M.S., Ramos S.L.L.M. 2019. Origin and thermometry of graphites from Itapecerica supracrustal succession of the southern Sao Francisco Craton by C isotopes, X-ray diffraction, and Raman spectroscopy. International Geology Review, 61(15):1864-1875. https://doi.org/10.1080/00206814.2018.1564073
https://doi.org/https://doi.org/10.1080/... ) characterized two different forms of graphite. They presented δ¹³C between -21.23 and -27.89‰, indicating biogenic source, and high-grade metamorphism associated with a syn-collisional stage, with average temperature around 729°C (metamorphic graphite), and hydrothermalism associated with post-collisional stage, around 611°C (recrystallized graphite).

In order to complement the study of Miranda et al. (2019Miranda D.A., Chaves A.O., Campello M.S., Ramos S.L.L.M. 2019. Origin and thermometry of graphites from Itapecerica supracrustal succession of the southern Sao Francisco Craton by C isotopes, X-ray diffraction, and Raman spectroscopy. International Geology Review, 61(15):1864-1875. https://doi.org/10.1080/00206814.2018.1564073
https://doi.org/https://doi.org/10.1080/... ), this paper aims to characterize the origin, crystallinity and metamorphism temperature of graphite from an area near the city of Formiga (MG), a city located about 40 km west of Itapecerica (MG) (Fig. 2), using XRD and Raman spectroscopy. The results are compared to previous studies to corroborate to the understanding of the tectonic model proposed for the Rhyacian-Orosirian orogeny in the southern São Francisco Craton.

GEOLOGICAL SETTING

The São Francisco Craton (SFC) is a tectonic domain (Almeida 1977Almeida F.F.M. 1977. O Cráton do São Francisco. Revista Brasileira de Geociências, 7(4):349-364.) (Fig. 2) surrounded by Neoproterozoic orogens. Its southern sector is composed by Archean crust, with age between 3.5 and 2.6 Ga, that is formed mostly by granite-gneisses rocks (Farina et al. 2015Farina F.A., Albert C., Lana C. 2015. The Neoarchean transition between medium and high-K granitoids: Clues from the Southern São Francisco Craton (Brazil). Precambrian Research, 266:375-394. https://doi.org/10.1016/j.precamres.2015.05.038
https://doi.org/https://doi.org/10.1016/... ) and greenstone belts (Rio das Velhas Supergroup) constituted by mafic-ultramafic, intermediate-felsic volcanic and volcanoclastic rocks (Noce et al. 1998Noce C.M., Machado N., Teixeira W. 1998. U-Pb Geochronology of gneisses and granitoids in the Quadrilatero Ferrıfero (Southern São Francisco Craton): Age constraints for Archean and Paleoproterozoic magmatism and metamorphism. Revista Brasileira de Geociências, 28:95-102.) with terrigenous sediments (Dorr 1969Dorr J.V.N. 1969. Physiographic, stratigraphic and structural development of the Quadrilatero Ferrifero, Minas Gerais, Brazil, U.S. Geological Survey Professional Paper, 641-A:110. https://doi.org/10.3133/pp641A
https://doi.org/https://doi.org/10.3133/... , Baltazar and Zucchetti 2007Baltazar O.F., Zucchetti M. 2007. Lithofacies associations and structural evolution of the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Brazil: A review of the setting of gold deposits. Ore Geology Reviews, 32:471-499. http://dx.doi.org/10.1016/j.oregeorev.2005.03.021
https://doi.org/http://dx.doi.org/10.101... ).

The basement comprises the Campo Belo, Divinópolis, Bonfim and Belo Horizonte metamorphic complexes (Fig. 2) (Machado Filho et al. 1983Machado Filho L., Ribeiro M., Gonzales S.R., Schenini C.A., Santos Neto A., Palmeira R.C., Pires J.L., Teixeira W., Castro H.E.F. 1983. Geologia das folhas Rio de Janeiro (SF 23/24) escala 1:1.000.000, mapa e texto explicativo. Rio de Janeiro: RADAM Brasil, Ministério das Minas e Energia, 780 p., Teixeira et al. 1996Teixeira W., Carneiro M.A., Noce C.M., Machado N., Sato K., Taylor P.N. 1996. Pb, Sr and Nd isotope constraints on the Archean evolution of gneissic-granitoid complexes in the southern São Francisco Craton, Brazil. Precambrian Research, 78(1-3):151-164. https://doi.org/10.1016/0301-9268(95)00075-5
https://doi.org/https://doi.org/10.1016/... ). The supracrustal sequence is formed by the Minas Supergroup, which is characterized by clastic-chemical metasedimentary rocks from the Paleoproterozoic (minimum deposition around 2.0 Ga), including the banded iron formations of the Quadrilátero Ferrífero (Machado et al. 1996Machado N., Schrank A., Noce C.M., Gauthier G. 1996. Ages of detrital zircon from Archean-Paleproterozoic sequences: Implications for greenstone belt setting and evolution of a Transamazonian foreland basin in Quadrilátero Ferrífero, southeast Brazil: Evidence from zircon ages by laser ablation ICP-MS. Earth Planetary Science Letters, 141(1-4):259-276. https://doi.org/10.1016/0012-821X(96)00054-4
https://doi.org/https://doi.org/10.1016/... , Moreira et al. 2016Moreira H., Lana C., Nalini Júnior H.A. 2016. The detrital zircon record of an Archaean convergent basin in the Southern São Francisco Craton, Brazil. Precambrian Research, 275:84-99. https://doi.org/10.1016/j.precamres.2015.12.015
https://doi.org/https://doi.org/10.1016/... ), and also the Bambuí Group, formed by pelitic-carbonate sedimentary rocks with Neoproterozoic age (Alkmim and Martins-Neto, 2001Alkmim F.F., Martins-Neto M.A. 2001. A Bacia Intracratônica do São Francisco: Arcabouço estrutural e cenários evolutivos. In: Martins-Neto M.A., Pinto C.P. (Eds.). Bacia do São Francisco. Belo Horizonte: SBG-MG, p. 9-30.).

The Mineiro Belt (Noce et al. 1998Noce C.M., Machado N., Teixeira W. 1998. U-Pb Geochronology of gneisses and granitoids in the Quadrilatero Ferrıfero (Southern São Francisco Craton): Age constraints for Archean and Paleoproterozoic magmatism and metamorphism. Revista Brasileira de Geociências, 28:95-102., Ávila et al. 2014Ávila C.A., Teixeira W., Bongiolo E.M., Dussin I.A., Vieira T.A.T. 2014. Rhyacian evolution of subvolcanic and metasedimentary rocks of the southern segment of the Mineiro belt, São Francisco Craton, Brazil. Precambrian Research, 243:221-251. https://doi.org/10.1016/j.precamres.2013.12.028
https://doi.org/https://doi.org/10.1016/... , Teixeira et al. 2015Teixeira W., Ávila C.A., Dussin I.A., Corrêa Neto A.V., Bongiolo E.M., Santos J.O.S., Barbosa N. 2015. A juvenile accretion episode (2.36-2.33 Ga) in the Mineiro belt and its role to the long-lived Minas accretionary orogeny: zircon U-Pb-Hf and geochemical evidences. Precambrian Research, 256:148-169. https://doi.org/10.1016/j.precamres.2014.11.009
https://doi.org/https://doi.org/10.1016/... ) was formed by accretionary orogeny and occurred between Rhyacian and Orosirian, which resulted in extensive reworking of regions placed at the margins of the southern SFC (Noce et al. 2007Noce C.M., Pedrosa-Soares A.C., Silva L.C.D., Alkmim F.F. 2007. O embasamento arqueano e paleoproterozóico do orógeno Araçuaí. Geonomos, 15(1):17-23. https://doi.org/10.18285/geonomos.v15i1.104
https://doi.org/https://doi.org/10.18285... ). Chaves et al. (2015Chaves A.O., Campello M.S., Pedrosa-Soares A.C. 2015. Idade U-Th-PbT de monazitas do sillimanita-cordierita-granada-biotita gnaisse de Itapecerica (MG) e a atuação da orogenia Riaciano-Orosiriana no interior do Cráton São Francisco Meridional. Geociências, 34(3):324-334.) suggested the existence of a Paleoproterozoic event in the Itapecerica region based on chemical ages found in monazites in sillimanite-cordierite-garnet-biotite gneiss (graphite-rich khondalitic rocks), which was confirmed by Carvalho et al. (2017Carvalho B.B., Janasi V.A., Sawyer E.W. 2017. Evidence for Paleoproterozoic anatexis and crustal reworking of Archean crust in the São Francisco Craton, Brazil: A dating and isotopic study of the Kinawa migmatite. Precambrian Research, 291:98-118. https://doi.org/10.1016/j.precamres.2017.01.019
https://doi.org/https://doi.org/10.1016/... ), that found isotopic ages from zircons of 2.05-2.03 Ga (U-Pb) in the interior of the basement.

Occurrences of graphite with δ¹³C between -21.23 and -27.89‰, indicating biogenic origin, (Miranda et al. 2019Miranda D.A., Chaves A.O., Campello M.S., Ramos S.L.L.M. 2019. Origin and thermometry of graphites from Itapecerica supracrustal succession of the southern Sao Francisco Craton by C isotopes, X-ray diffraction, and Raman spectroscopy. International Geology Review, 61(15):1864-1875. https://doi.org/10.1080/00206814.2018.1564073
https://doi.org/https://doi.org/10.1080/... ) are reported in this area, where Itapecerica Supracrustal Sequence has been described (Campello et al. 2015Campello M.S., Vaz B.B., Oliveira M.A.S., Ávila M.A.C. 2015. Folha Formiga (SF. 23-VB-III) Escala 1: 100.000. Projeto Fronteiras de Minas. Minas Gerais: CODEMIG/UFMG., Chaves et al. 2015Chaves A.O., Campello M.S., Pedrosa-Soares A.C. 2015. Idade U-Th-PbT de monazitas do sillimanita-cordierita-granada-biotita gnaisse de Itapecerica (MG) e a atuação da orogenia Riaciano-Orosiriana no interior do Cráton São Francisco Meridional. Geociências, 34(3):324-334., Teixeira et al. 2017Teixeira W., Oliveira E.P., Peng P., Dantas E.L., Hollanda M.H.B.M. 2017. U-Pb geochronology of the 2.0 Ga Itapecerica graphite-rich supracrustal succession in the São Francisco Craton: Tectonic matches with the North China Craton and paleogeographic inferences. Precambrian Research, 293:91-111. https://doi.org/10.1016/j.precamres.2017.02.021
https://doi.org/https://doi.org/10.1016/... ). The studied area is located near the city of Formiga, where the graphite schist occurs between the Divinópolis and Campo Belo complexes (Fig. 2).

METHODS AND RESULTS

Three samples were collected at the Formiga area (20º26’40.4”S, 40º30’04.3”W). Samples LR44A and LR44C were chipped from an abandoned trench (Fig. 3A) and sample LR44B was chipped about 5 m away. Both collection spots are formed by fine grained graphite schist composed of quartz, graphite and mica, with presence of vertical foliation. In the trench it is possible to notice the presence of quartz veins oblique to the foliation that contain manganese minerals and occur filling fractures. The host rock presents some recrystallization in the contact with the veins. Samples LR44A and LR44B (Fig. 3B) are graphite schist with vertical foliation, while sample LR44C (Fig. 3C) was taken from an associated quartz vein (Fig. 3D), with manganese minerals, oriented N40E/75NW. The visual proportion between graphite and manganese mineral in this sample is 4:1. Each sample was between 10 and 20 cm long and weighed approximately 0.5 kg.

Petrography

Thin sections of the collected samples were prepared and examined under transmitted and reflected light microscopy at the Centro de Pesquisa (Research Centre) Professor Manoel Teixeira da Costa of Intitute of Geosciences of the Universidade Federal de Minas Gerais (CPMTC-IGC-UFMG), at Belo Horizonte, MG, Brazil.

The studied rocks are fine grained and have granolepidoblastic texture (Fig. 4). They are composed of quartz (50%), graphite (35%) and phengite (15%). Quartz is the dominant mineral in the composition and occurs in anhedral fractured grains. The second main component is graphite, presented in aggregates (LR44A) and in layers parallel with foliation (LR44B). Phengite marks the foliation in both samples, but occurs in larger crystals in sample LR44B than in sample LR44A. The foliation is more evident in sample LR44B than it is in sample LR44A.

X-ray diffraction

The XRD analyses of graphite were performed in the X-ray Laboratory at CPMTC-IGC-UFMG. The mineral was previously isolated from other minerals of the graphite schist by brushing the graphite-rich portion of the rock, followed by graphite flotation in aqueous environment and drying of the floated grains. PANalytical XPert PRO diffraction instrument with theta-theta geometry was used to record the XRD data, using a Cu Kα X-ray source (40 kV and 45 mA). The step size and scan step settings were 0.02°, 2θ and 0.5 s. The high accuracy of the lattice parameter was guaranteed using Rietveld methods (Young 1993Young R.A. 1993. The rietveld method. International Union of Crystallography. New York: Oxford University Press, 298 p.) to fit the diffraction data, with starting parameters close to realistic values and equally applied to both samples LR44A and LR44B. Figure 5 shows the XRD results of sample LR44C, composed of todorokite and manganite instead of graphite. The XRD data from the graphite schist samples are organized in Table 1 and their respective diffractogram are presented in Figure 6.

To estimate the crystal size along stacking direction (L_c(002)), we used the Equation 1 provided by Baiju et al. (2005Baiju K.R., Satish-Kumar M., Kagi H., Nambiar C.G., Ravinsankar M. 2005. Mineralogical characterization of graphite deposits from Thodupuzha-Kanjirappally belt, Madurai granulite block, Southern India. Gondwana Research, 8(2):223-230. https://doi.org/10.1016/S1342-937X(05)71120-5
https://doi.org/https://doi.org/10.1016/... ).

In this equation, k means the shape constant (0.9), λ is the X-ray wavelength in angstroms (1.5406), β₍₀₀₂₎ represents the full width at half maximum of the peak in radian and θ correspond to the angle of diffraction in radians. The graphitization degree (GD) has been calculated using the Equation 2 from Tagiri (1981Tagiri M. 1981. A measurement of the graphitizing degree by the X-ray powder diffractometer. Journal of Mineralogy, Petrology and Economic Geology, 76(11):345-352. https://doi.org/10.2465/ganko1941.76.345
https://doi.org/https://doi.org/10.2465/... ), were d₍₀₀₂₎ is the interplanar spacing:

Moreover, the Equation 3 from Wada et al. (1994Wada I.H., Tomita T., Matsuura K., Tuchi K., Ito M., Morikiyo T. 1994. Graphitization of carbonaceous matter during metamorphism with reference to carbonate and pelitic rocks of contact and regional metamorphism, Japan. Contributions to Mineralogy and Petrology, 118(3):217-228. https://doi.org/10.1007/BF00306643
https://doi.org/https://doi.org/10.1007/... ) has been used to calculate the metamorphism temperature:

The average temperature found for samples LR44A and LR44B were 442°C and 449°C respectively.

The graphs in Figure 7 show correlations that could be done with the results obtained from the XRD data. Figure 7A presents the relationship between interplanar spacing (d₍₀₀₂₎) and crystallite size (L_c(002)), that, in accordance with Tagiri and Oba (1986Tagiri M., Oba T. 1986. Hydrothermal syntheses of graphite from bituminous coal at 0.5-5 kbar water vapor pressure and 300-600°C. Journal of Mineralogy, Petrology and Economic Geology, 81(7):260-271. https://doi.org/10.2465/ganko1941.81.260
https://doi.org/https://doi.org/10.2465/... ), can give information about the GD. Both samples LR44A and LR44B were classified as graphite as opposed to fully ordered graphite. The two samples presented similar XRD temperatures and are randomly arranged in the line that correlated GD with metamorphic temperature in the Figure 7B.

Raman spectroscopy

The Raman spectroscopy analyses were performed in the Technological Center of Nanomaterials (CTnano) at Technological Park of Belo Horizonte - BHTec. The equipment used was a confocal microspore Alpha 300R WITec (Wissenschaftliche Instrumente und Technologie GmbH^®, Ulm, Germany) equipped with Nd-YAG laser with double frequency (2.49 mW, λ = 532.2 nm). The same methodology as applied by Rantitsch et al. (2016Rantitsch G., Lämmerer W., Fisslthaler E., Mitsche S., Kaltenböck H. 2016. On the discrimination of semi-graphite and graphite by Raman spectroscopy. International Journal of Coal Geology, 159:48-56. https://doi.org/10.1016/j.coal.2016.04.001
https://doi.org/https://doi.org/10.1016/... ) has been used, in which the spectra were obtained with 50× lens objective. Five scans in the 1,000-3,200 cm^-1 spectra (first order = 1,000 - 2,000 cm^-1; second order = 2,200 - 3,200 cm^-1) were performed with an acquisition time of 30 s. The analyses were made using the powder extracted from both samples LR44A and LR44B.

In each sample, five fields were randomly selected (Fig. 8A, represented by squares), in which three to five analyses were done per square field (Fig. 8B, represented by crosses). The results of Raman spectroscopy are presented in Table 2, and the respective spectrum are presented in Figure 9 (sample 44A) and Figure 10 (sample 44B).

The temperature has been calculated by using the Interactive Fitting of Raman Spectra (IFORS) method, provided by Lünsdorf and Lünsdorf (2016Lünsdorf N.K., Lünsdorf J.O. 2016. Evaluating Raman spectra of carbonaceous matter by automated, iterative curve-fitting. International Journal of Coal Geology, 160-161:51-62. https://doi.org/10.1016/j.coal.2016.04.008
https://doi.org/https://doi.org/10.1016/... ). The average temperatures found for each sample were 486°C for sample LR44A and 463°C for sample LR44B. The graphs from Figure 11 classifies, according to Rantitsch et al. (2016Rantitsch G., Lämmerer W., Fisslthaler E., Mitsche S., Kaltenböck H. 2016. On the discrimination of semi-graphite and graphite by Raman spectroscopy. International Journal of Coal Geology, 159:48-56. https://doi.org/10.1016/j.coal.2016.04.001
https://doi.org/https://doi.org/10.1016/... ), the metamorphic facies based on interplanar spacing and width at half maximum of the G-band (G HWHM) or area ratio (R2). The area ratio is calculated as described as Beyssac et al. (2002Beyssac O., Goffé B., Chopin C., Rouzaud J.N. 2002. Raman spectra of carbonaceous material in metasediments: A new geothermometer. Journal of Metamorphic Geology, 20(9):859-871. https://doi.org/10.1046/j.1525-1314.2002.00408.x
https://doi.org/https://doi.org/10.1046/... ) using the first-order peaks at ~1,350 cm^-1 (D1 band), ~1,580 cm^-1 (G band), and ~1,610 cm^−-1 (D2 band), by the Equation 4:

The results, showed in Figure 11, revealed that both samples present graphite. In Figure 11A both samples are plotted in upper amphibolite facies, while in Figure 11B they are classified as granulite. The reason why this divergence occurs is explored in the discussion section.

DISCUSSION

To characterize the graphite schist from Formiga, we correlate X-ray diffraction and Raman spectroscopy from samples LR44A and LR44B. The Table 3 compares metamorphic temperatures acquired from both methods that yielded temperatures around 460°C. According to Lünsdorf (2015Lünsdorf N.K. 2015. Geothermometry by Raman Spectroscopy of Dispersed Organic Matter. PhD thesis, University of Göttingen, Göttingen, 113 p.), Raman analyses are reliable for temperatures between 330 and 600°C, so Raman data presented here are reliable. This temperature disagrees with the high metamorphic degree expected for the area (Chaves et al. 2015Chaves A.O., Campello M.S., Pedrosa-Soares A.C. 2015. Idade U-Th-PbT de monazitas do sillimanita-cordierita-granada-biotita gnaisse de Itapecerica (MG) e a atuação da orogenia Riaciano-Orosiriana no interior do Cráton São Francisco Meridional. Geociências, 34(3):324-334.). However, it agrees with the results found by Miranda et al. (2019Miranda D.A., Chaves A.O., Campello M.S., Ramos S.L.L.M. 2019. Origin and thermometry of graphites from Itapecerica supracrustal succession of the southern Sao Francisco Craton by C isotopes, X-ray diffraction, and Raman spectroscopy. International Geology Review, 61(15):1864-1875. https://doi.org/10.1080/00206814.2018.1564073
https://doi.org/https://doi.org/10.1080/... ) for hydrothermal recrystallized graphite.

The XRD data (Tab. 1) was plotted in graphs to analyze the GD accordantly to its crystallinity. Figure 7A correlates interplanar spacing (d₍₀₀₂₎A) and crystallite size (L_c(002)A), and, as suggested by Tagiri and Oba (1986Tagiri M., Oba T. 1986. Hydrothermal syntheses of graphite from bituminous coal at 0.5-5 kbar water vapor pressure and 300-600°C. Journal of Mineralogy, Petrology and Economic Geology, 81(7):260-271. https://doi.org/10.2465/ganko1941.81.260
https://doi.org/https://doi.org/10.2465/... ), shows that both samples are classified as graphite between the fields of disordered graphite and fully ordered graphite. This corresponds to the temperatures which were found in this work, as graphite becomes fully ordered around 600°C (Beyssac et al. 2002Beyssac O., Goffé B., Chopin C., Rouzaud J.N. 2002. Raman spectra of carbonaceous material in metasediments: A new geothermometer. Journal of Metamorphic Geology, 20(9):859-871. https://doi.org/10.1046/j.1525-1314.2002.00408.x
https://doi.org/https://doi.org/10.1046/... , Lünsdorf 2015Lünsdorf N.K. 2015. Geothermometry by Raman Spectroscopy of Dispersed Organic Matter. PhD thesis, University of Göttingen, Göttingen, 113 p.). The GD and XRD data temperature (T) are plotted using the equation for graphitization in pelitic rocks (Wada et al. 1994Wada I.H., Tomita T., Matsuura K., Tuchi K., Ito M., Morikiyo T. 1994. Graphitization of carbonaceous matter during metamorphism with reference to carbonate and pelitic rocks of contact and regional metamorphism, Japan. Contributions to Mineralogy and Petrology, 118(3):217-228. https://doi.org/10.1007/BF00306643
https://doi.org/https://doi.org/10.1007/... ), and data from samples LR44A and LR44B behave similarly, leading to the conclusion that they both have been metamorphized under the same conditions.

The data acquired by Raman spectroscopy (Tab. 2) gave quite controversial results when plotted in graphs that classify the metamorphic facies as proposed by Rantitsch et al. (2016Rantitsch G., Lämmerer W., Fisslthaler E., Mitsche S., Kaltenböck H. 2016. On the discrimination of semi-graphite and graphite by Raman spectroscopy. International Journal of Coal Geology, 159:48-56. https://doi.org/10.1016/j.coal.2016.04.001
https://doi.org/https://doi.org/10.1016/... ). Figure 11A correlates the interplanar spacing d₍₀₀₂₎ and half width at half maximum of the G-band (G HWHM) and classifies the sample LR44B as related to the amphibolite facies while the sample LR44A to the upper amphibolite facies. Figure 11B shows the correlation between d₍₀₀₂₎ and area ratio (R2 from Beyssac et al. 2002Beyssac O., Goffé B., Chopin C., Rouzaud J.N. 2002. Raman spectra of carbonaceous material in metasediments: A new geothermometer. Journal of Metamorphic Geology, 20(9):859-871. https://doi.org/10.1046/j.1525-1314.2002.00408.x
https://doi.org/https://doi.org/10.1046/... ) in which both samples are classified as related to the granulite facies. This result was expected assuming the metamorphism considered for the area (Chaves et al. 2015Chaves A.O., Campello M.S., Pedrosa-Soares A.C. 2015. Idade U-Th-PbT de monazitas do sillimanita-cordierita-granada-biotita gnaisse de Itapecerica (MG) e a atuação da orogenia Riaciano-Orosiriana no interior do Cráton São Francisco Meridional. Geociências, 34(3):324-334.), but it disagrees with the temperatures found by both methods. Also, the samples are classified as graphite (Kwiecińska and Petersen 2004Kwiecińska B., Petersen H. 2004. Graphite, semi-graphite, natural coke, and natural char classification-ICCP system. International Journal of Coal Geology, 57(2):99-116. https://doi.org/10.1016/j.coal.2003.09.003
https://doi.org/https://doi.org/10.1016/... ) in both graphs presented in Figure 11.

An explanation for why the temperatures found by the XRD and Raman methods (Tab. 1), for both graphite schist samples, disagrees with the syn-collisional metamorphism (Chaves et al. 2015Chaves A.O., Campello M.S., Pedrosa-Soares A.C. 2015. Idade U-Th-PbT de monazitas do sillimanita-cordierita-granada-biotita gnaisse de Itapecerica (MG) e a atuação da orogenia Riaciano-Orosiriana no interior do Cráton São Francisco Meridional. Geociências, 34(3):324-334.), and the classifications based on crystallinity (Rantitsch et al. 2016Rantitsch G., Lämmerer W., Fisslthaler E., Mitsche S., Kaltenböck H. 2016. On the discrimination of semi-graphite and graphite by Raman spectroscopy. International Journal of Coal Geology, 159:48-56. https://doi.org/10.1016/j.coal.2016.04.001
https://doi.org/https://doi.org/10.1016/... ), revealed in Figure 11, could be associated with a post-collisional stage. In this phase, the reactivation of old faults creates open space for fluid percolation (Carvalho et al. 2017Carvalho B.B., Janasi V.A., Sawyer E.W. 2017. Evidence for Paleoproterozoic anatexis and crustal reworking of Archean crust in the São Francisco Craton, Brazil: A dating and isotopic study of the Kinawa migmatite. Precambrian Research, 291:98-118. https://doi.org/10.1016/j.precamres.2017.01.019
https://doi.org/https://doi.org/10.1016/... , Miranda et al. 2019Miranda D.A., Chaves A.O., Campello M.S., Ramos S.L.L.M. 2019. Origin and thermometry of graphites from Itapecerica supracrustal succession of the southern Sao Francisco Craton by C isotopes, X-ray diffraction, and Raman spectroscopy. International Geology Review, 61(15):1864-1875. https://doi.org/10.1080/00206814.2018.1564073
https://doi.org/https://doi.org/10.1080/... ). It is possible to observe quartz veins apparently related with this event (Fig. 3D). According to Miranda et al. (2019Miranda D.A., Chaves A.O., Campello M.S., Ramos S.L.L.M. 2019. Origin and thermometry of graphites from Itapecerica supracrustal succession of the southern Sao Francisco Craton by C isotopes, X-ray diffraction, and Raman spectroscopy. International Geology Review, 61(15):1864-1875. https://doi.org/10.1080/00206814.2018.1564073
https://doi.org/https://doi.org/10.1080/... ), hydrothermal process can decrease the crystallite size, placing the samples into high-grade metamorphic fields, even when they were metamorphized in much lower temperatures, that is, around 460°C. In their analyses, samples from hydrothermal recrystallized graphites had results similar to those presented here. They are classified as amphibolite facies in the correlation of Figure 8A and granulite facies in the correlation of Figure 11B, exactly as it happens to the samples analysed here.

The graphite schist from the Itapecerica region, which is adjacent to the present area of study, has biogenic origin (Miranda et al. 2019Miranda D.A., Chaves A.O., Campello M.S., Ramos S.L.L.M. 2019. Origin and thermometry of graphites from Itapecerica supracrustal succession of the southern Sao Francisco Craton by C isotopes, X-ray diffraction, and Raman spectroscopy. International Geology Review, 61(15):1864-1875. https://doi.org/10.1080/00206814.2018.1564073
https://doi.org/https://doi.org/10.1080/... ). As in Formiga area, todorokite (Mn-oxide mineral typical of deep-sea Mn nodules formed by microorganisms, as suggested by Lowenstam 1981Lowenstam H.A. 1981. Minerals formed by organisms. Science, 211(4487):1126-1131. https://doi.org/10.1126/science.7008198
https://doi.org/https://doi.org/10.1126/... ) is also present. Given the geological and mineralogical similarities between the two areas, it is possible to assume that the graphite schist from Formiga also has biogenic origin. This carbonaceous material would have been deposited in an oceanic basin between the Campo Belo and Divinópolis complexes in the pre-collisional stage. During the Rhyacian-Orosirian orogeny, graphite schist would be metamorphized under granulite facies conditions and faults were formed. In post-collisional stage, the faults would have been reactivated turning into pathways to fluids which changed the graphite structure, decreased its crystallite size and changed the temperature signature to around 460°C.

CONCLUSION

The graphite schist from Formiga presented temperature around 460°C by the XRD and Raman analyses. The hydrothermalism associated with a post-collisional event explains the decrease in the crystallite size of the graphite mineral and its low temperature, while temperature typical of granulite facies was expected. The occurrence of todorokite in quartz veins and the presence of graphite with δ¹³C between -21.23 and -27.89‰ in the adjacent area (Miranda et al. 2019Miranda D.A., Chaves A.O., Campello M.S., Ramos S.L.L.M. 2019. Origin and thermometry of graphites from Itapecerica supracrustal succession of the southern Sao Francisco Craton by C isotopes, X-ray diffraction, and Raman spectroscopy. International Geology Review, 61(15):1864-1875. https://doi.org/10.1080/00206814.2018.1564073
https://doi.org/https://doi.org/10.1080/... ) suggest a biogenic origin for the carbonaceous material that resulted in the graphite schist from Formiga.

ACKNOWLEDGEMENTS

We are grateful to the Institute of Geosciences of UFMG that avail the CPMTC laboratories. The second author thanks Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) for research financial support through the project APQ00654-16.

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