Abstract
Collisional granites of the Araçuaí Orogen, southeastern Brazil, record petrological and geochronological evidence for multiple crustal melting during the orogeny evolution. U-Pb zircon data indicate that these granites crystallized at 586 ± 2 M.y. High-grade metamorphism (M1) involved partial melting by fluid-absent reactions that produced the first generation of garnet in temperatures of approx. 750°C. Preservation of the mineral assemblage A1 (garnet-biotite-plagioclase-K-feldspar-quartz-ilmenite-melt) indicates that most of the generated melt was lost from these rocks at or near peak metamorphic conditions. A second metamorphic event (M2) is characterized by growth of a second generation of garnet in preserved A2 assemblage (garnet-sillimanite-biotite-plagioclase-K-feldspar-quartz-ilmenite-melt). Mineral equilibria modeling constrains conditions of M2 metamorphism to 713-729 °C and 6.2-7.3 kbar. Retrograde assemblage (A3) records equilibrium conditions at 610-660 °C. The Hf isotope composition indicates significant crustal contribution to the genesis of the collisional granites. The elevated geotherms in thickened crust provide enough heat for the M1 event at 562 ± 2 M.y. Subsequent heating probably associated to the transfer of mantle heat to the crust during the extensional thinning and gravitational collapse of the orogen lead to the M2 event at 526 ± 4 M.y. This event is concomitant to the emplacement of the post-collisional magmas in the orogen.
Key words
Araçuaí Orogen; partial melting; pseudosection; U-Pb zircon; collisional granite; recycling
INTRODUCTION
Orogenic belts are elongated crustal portions characterized by intense deformation, metamorphism and granitic magmatism, resulting from the collision of continental lithospheric plates with intra-plate shortening, crustal thickening and elevation of the relief (Kearey et al. 2009KEAREY P, KLEPEIS KA & VINE FJ. 2009. Global Tectonics, 3rd ed, Singapore: Wiley-Blackwell, Singapore, 482 p.). In active orogens, such as the Himalayas, the study of their internal structure is limited by topography and depth. In contrast, deeply eroded ancient orogenic belts expose their roots, and their investigation provides important information about the development of ancient orogenic structures and are of great value for continental reconstructions.
Geothermobarometry of high-grade rocks is an important tool for the understanding of tectonic and rock forming processes and large-scale transport of matter at great depths within the continental crust. The record of changes in metamorphic conditions could be preserved as microstructures, mineral relicts, and chemical zoning. Several conventional geothermometers and geobarometers have been used to estimate the equilibrium conditions of determined minerals (e.g., Thompson 1976THOMPSON AB. 1976. Mineral reactions in pelitic rocks. II. Calculation of some P-T-X (Fe-Mg) phase relations. Am J Sci 276: 425-454., Ferry & Spear 1978FERRY JM & SPEAR FS. 1978. Experimental calibration of the partitioning of Fe and Mg between biotite and garnet. Contrib Mineral Petr 66: 113-117., Newton & Haselton 1981NEWTON RC & HASELTON HT. 1981. Thermodynamics of the garnet-plagioclase-Al2SiO5-quartz geobarometer. In: Newton RC, Navrotsky A and Wood BJ (Eds). Thermodynamics of Minerals and Melts, New York: Spring/Verlag, p. 131-147., Hodges & Spear 1982HODGES KV & SPEAR FS. 1982. Geothermometry, geobarometry and the Al2SiO5 triple point at Mt. Moosilauke, New Hampshire. Am Mineral 67: 1118-l134., Pigage & Greenwood 1982PIGAGE LC & GREENWOOD HJ. 1982. Internally consistent estimates of pressure and temperature: the staurolite problem. Am J Sci 282: 943-969., Koziol 1989KOZIOL AM. 1989. Recalibration of the garnet-plagioclase-Al2SiO5-quartz (GASP) geobarometer and applications to natural paragenesis. EOS 70: 493., Williams & Grambling 1990WILLIAMS ML & GRAMBLING JA. 1990. Manganese, ferric iron, and the equilibrium between garnet and biotite. Am Mineral 75: 886-908.). Theriak-Domino (De Capitani & Petrakakis 2010De CAPITANI C & PETRAKAKIS K. 2010. The computation of equilibrium assemblage diagrams with Theriak/Domino software. Am Mineral 95: 1006-1016.) computer program package is commonly applied in petrology to constrain P-T conditions of metamorphic rocks. Combining internally consistent thermodynamic data set with composition-activity models for critical minerals have taken the more precise estimates of metamorphic conditions. U-Pb zircon geochronology is a fundamental tool for obtaining crystallization and/or metamorphism ages for a wide range of igneous and/or metamorphic rocks. The combination of these approaches provide the reconstruction and analysis of P-T-t paths, contributing to interpreting the development of orogens with a polycyclic history.
The Araçuaí-West Congo Orogen is one of the various components of the Brasiliano/Pan-African orogenic system generated during the amalgamation of West Gondwana (Trompette 1994TROMPETTE R. 1994. Geology of western Gondwana (2000-500 Ma): Pan-Africa-Brasiliano aggregation of South America and Africa, Rotterdam: Balkema, 350 p., Alkmim et al. 2006ALKMIM FF, MARSHAK S, PEDROSA-SOARES AC, PERES GG, CRUZ SCP & WHITTINGTON A. 2006. Kinematic evolution of the Araçuaí–West Congo orogen in Brazil and Africa: Nutcracker tectonics during the Neoproterozoic assembly of Gondwana. Precambrian Res 149: 43-63.). The Araçuaí Orogen (southeastern Brazil) records a long period of granitic magmatism (ca. 150 M.y.) associated to the different stages of the orogeny, with a large production of collisional granitoids (Pedrosa-Soares et al. 2001PEDROSA-SOARES AC, NOCE CM, WIEDEMANN CM & PINTO CP. 2001. The Araçuaí-West-Congo Orogen In Brazil: An Overview of a Confined Orogen Formed During Gondwanaland Assembly. Precambrian Res 110: 307-323., 2011, Gradim et al. 2014GRADIM C, RONCATO J, PEDROSA-SOARES AC, CORDANI U, DUSSIN I, ALKMIM FF, QUEIROGA G, JACOBSSOHN T, SILVA LC & BABINSKI M. 2014. The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation. Braz J Geol 44: 155-180.). In this study, we present new petrographic evidence, geochronological data and thermobarometric estimates for samples from the Carlos Chagas Suite collisional granites, which preserve mineral assemblages of two high-grade metamorphic events (M1 and M2). Calculated phase diagrams in combination with conventional geothermobarometry and geochronological methods are used to investigate the ages and metamorphism conditions. These data provide important constraints on the P-T-t paths of these rocks and contribute to the understanding of the crustal recycling in Neoproterozoic orogens.
Geological setting
The Araçuaí Orogen, located in southeastern Brazil, represents the largest portion of the Brasiliano/Pan-African Araçuaí-West Congo Orogenic System, developed between the São Francisco and Congo cratons during the amalgamation of West Gondwana in late Proterozoic time (Alkmim et al. 2006ALKMIM FF, MARSHAK S, PEDROSA-SOARES AC, PERES GG, CRUZ SCP & WHITTINGTON A. 2006. Kinematic evolution of the Araçuaí–West Congo orogen in Brazil and Africa: Nutcracker tectonics during the Neoproterozoic assembly of Gondwana. Precambrian Res 149: 43-63., Pedrosa-Soares et al. 2011PEDROSA-SOARES ET AL. 2011. Late Neoproterozoic - Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Geol Soc London SP 350: 25-51.) (Figure 1a). Rocks exposed in Espírito Santo, Bahia and eastern of Minas Gerais states contain a record of the evolution of an active continental margin, Rio Doce Magmatic Arc-related rocks and syn- to post-collisional magmatism from the Araçuaí Orogen (De Campos et al. 2004De CAMPOS CP, MENDES JC, LUDKA IP, MEDEIROS SR, MOURA JC & WALLFASS C. 2004. A review of the Brasiliano magmatism in southern Espírito Santo, Brazil, with emphasis on post-collisional magmatism. J Virtual Explorer 17., 2016, Alkmim et al. 2006ALKMIM FF, MARSHAK S, PEDROSA-SOARES AC, PERES GG, CRUZ SCP & WHITTINGTON A. 2006. Kinematic evolution of the Araçuaí–West Congo orogen in Brazil and Africa: Nutcracker tectonics during the Neoproterozoic assembly of Gondwana. Precambrian Res 149: 43-63., Pedrosa-Soares et al. 2011PEDROSA-SOARES ET AL. 2011. Late Neoproterozoic - Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Geol Soc London SP 350: 25-51., Peixoto et al. 2015PEIXOTO E, PEDROSA-SOARES AC, ALKMIM FF & DUSSIN IA. 2015. A suture -related accretionary wedge formed in the Neoproterozoic Araçuaí orogen (SE Brazil) during Western Gondwanaland assembly. Gondwana Res 27: 878-896., Richter et al. 2016RICHTER F, LANA C, STEVENS G, BUICK I, PEDROSA-SOARES AC, ALKMIM FF & CUTTS K. 2016. Sedimentation, metamorphism and granite generation in a back-arc region: records from the Ediacaran Nova Venécia Complex (Araçuaí Orogen, Southeastern Brazil). Precambrian Res 272: 78-100., Tedeschi et al. 2016TEDESCHI M ET AL. 2016. The Ediacaran Rio Doce magmatic arc revisited (Araçuaí-Ribeira orogenic system, SE Brazil). J S Am Earth Sci 68: 167-186., Melo et al. 2017 a-b, 2020, Serrano et al. 2018SERRANO P, PEDROSA-SOARES A, MEDEIROS-JÚNIOR E, FONTE-BOA T, ARAÚJO C, DUSSIN I, QUEIROGA G & LANA C. 2018. A-type Medina batholith and post-collisional anatexis in the Araçuaí orogen (SE Brazil). Lithos 320-321: 515-536., Araújo et al. 2020ARAÚJO C, PEDROSA-SOARES A, LANA C, DUSSIN I, QUEIROGA G, SERRANO P & MEDEIROS-JUNIOR E. 2020. Zircon in emplacement borders of post-collisional plutons compared to country rocks: A study on morphology, internal texture, U-Th-Pb geochronology and Hf isotopes (Araçuaí orogen, SE Brazil). Lithos 352-353: 105252., Soares et al. 2020SOARES CCV, QUEIROGA G, PEDROSA-SOARES AC, GOUVÊA LP, VALERIANO CM, MELO MG, MARQUES RA & FREITAS RDA. 2020. The Ediacaran Rio Doce magmatic arc in the Araçuaí – Ribeira boundary sector. Southeast Brazil: Lithochemistry and isotopic (Sm-Nd and Sr) signatures. J South Am Earth Sci 104: 102880.) (Figure 1b). However, this subduction-collision model has been subject of important discussions (e.g., Cavalcante et al. 2019CAVALCANTE C, FOSSEN H, ALMEIDA RP, HOLLANDA MHBM & EGYDIO-SILVA M. 2019. Reviewing the puzzling intracontinental termination of the Araçuaí-West Congo orogenic belt and its implications for orogenic development. Precambrian Res 322: 85-98., Fossen et al. 2020FOSSEN H, CAVALCANTE C, KONOPÁSEK J, MEIRA VT, ALMEIDA RP, HOLLANDA MHBM & TROMPETTE R. 2020. A critical discussion of the subduction-collision model for the Neoproterozoic Araçuaí-West Congo Orogen. Precambrian Res 343: 105715.). The main problems presented by Fossen et al. (2020)FOSSEN H, CAVALCANTE C, KONOPÁSEK J, MEIRA VT, ALMEIDA RP, HOLLANDA MHBM & TROMPETTE R. 2020. A critical discussion of the subduction-collision model for the Neoproterozoic Araçuaí-West Congo Orogen. Precambrian Res 343: 105715. for this model are: (i) lack evidence of high-P metamorphism; (ii) and the abrupt termination of an ocean with no realistic way to transfer the large amount of oceanic opening displacement and subsequent convergence required by the model. The intracontinental hot orogen model has been proposed by several authors as an alternative model for the Araçuaí Orogen (Vauchez et al. 2007VAUCHEZ A, EGYDIO-SILVA, M, BABINSKI M, TOMMASI A, UHLEIN A & LIU D. 2007. Deformation of a pervasively molten middle crust: insights from the Neoproterozoic Ribeira-Araçuaí orogen (SE Brazil). Terra Nova 19: 278-286., 2019VAUCHEZ A, HOLLANDA MHMZ, MONIÉ P, MONDOU M & EGYDIO-SILVA M. 2019. Slow cooling and crystallization of the roots of the Neoproterozoic Araçuaí hot orogen (SE Brazil): Implications for rheology, strain distribution, and deformation analysis. Tectonophysics 766: 500-518., Cavalcante et al. 2013CAVALCANTE GCG, EGYDIO-SILVA M, VAUCHEZ A, CAMPS P & OLIVEIRA E. 2013. Strain distribution across a partially molten middle crust: insights from the AMS mapping of the Carlos Chagas Anatexite, Araçuaí belt (East Brazil). J Struct Geol 55: 79-100., 2014CAVALCANTE GCG, VAUCHEZ A, MERLET C, HOLANDA, MHBM & BOYER B. 2014. Thermal conditions during deformation of partially molten crust from Titanio geothermometry: rheological implications for the anatectic domain of the Araçuaí belt, eastern Brazil. Solid Earth 5: 1223-1242., 2019, Fossen et al. 2020FOSSEN H, CAVALCANTE C, KONOPÁSEK J, MEIRA VT, ALMEIDA RP, HOLLANDA MHBM & TROMPETTE R. 2020. A critical discussion of the subduction-collision model for the Neoproterozoic Araçuaí-West Congo Orogen. Precambrian Res 343: 105715.).
Gondwana scenario during amalgamation of the Araçuaí-West Congo belt. (a) Araçuaí-West Congo orogenic system and adjacent cratonic region in the context of the West Gondwana (after Alkmim et al. 2006ALKMIM FF, MARSHAK S, PEDROSA-SOARES AC, PERES GG, CRUZ SCP & WHITTINGTON A. 2006. Kinematic evolution of the Araçuaí–West Congo orogen in Brazil and Africa: Nutcracker tectonics during the Neoproterozoic assembly of Gondwana. Precambrian Res 149: 43-63.). (b) Simplified geological map of the Araçuaí orogen (modified from Pedrosa-Soares et al. 2011PEDROSA-SOARES ET AL. 2011. Late Neoproterozoic - Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Geol Soc London SP 350: 25-51.), showing location of the study area (dashed rectangle).
Despite the differences, both models show vast amounts of granites that range in crystallization age from 630 to 480 M.y. The first granitoids were emplaced around 630 Ma and they were formed by crustal anatexis with minor involvement of mantle magmas (Pedrosa-Soares et al. 2011PEDROSA-SOARES ET AL. 2011. Late Neoproterozoic - Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Geol Soc London SP 350: 25-51., Tedeschi et al. 2016TEDESCHI M ET AL. 2016. The Ediacaran Rio Doce magmatic arc revisited (Araçuaí-Ribeira orogenic system, SE Brazil). J S Am Earth Sci 68: 167-186.). Large amounts of peraluminous melts were produced by partial melting of metasedimentary rocks. Previous geochronological data yield of crystallization age between 600 and 540 M.y. (Pedrosa-Soares et al. 2011PEDROSA-SOARES ET AL. 2011. Late Neoproterozoic - Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Geol Soc London SP 350: 25-51., Gradim et al. 2014GRADIM C, RONCATO J, PEDROSA-SOARES AC, CORDANI U, DUSSIN I, ALKMIM FF, QUEIROGA G, JACOBSSOHN T, SILVA LC & BABINSKI M. 2014. The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation. Braz J Geol 44: 155-180., Melo et al. 2017 a-b, Cavalcante et al. 2019CAVALCANTE C, FOSSEN H, ALMEIDA RP, HOLLANDA MHBM & EGYDIO-SILVA M. 2019. Reviewing the puzzling intracontinental termination of the Araçuaí-West Congo orogenic belt and its implications for orogenic development. Precambrian Res 322: 85-98., Araújo et al. 2020ARAÚJO C, PEDROSA-SOARES A, LANA C, DUSSIN I, QUEIROGA G, SERRANO P & MEDEIROS-JUNIOR E. 2020. Zircon in emplacement borders of post-collisional plutons compared to country rocks: A study on morphology, internal texture, U-Th-Pb geochronology and Hf isotopes (Araçuaí orogen, SE Brazil). Lithos 352-353: 105252.). Paragneisses (Nova Venécia and Jequitinhonha complexes) show widespread evidence of partial melting and they are considered as main source of peraluminous melts (Gradim et al. 2014GRADIM C, RONCATO J, PEDROSA-SOARES AC, CORDANI U, DUSSIN I, ALKMIM FF, QUEIROGA G, JACOBSSOHN T, SILVA LC & BABINSKI M. 2014. The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation. Braz J Geol 44: 155-180., Pedrosa-Soares et al. 2011PEDROSA-SOARES ET AL. 2011. Late Neoproterozoic - Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Geol Soc London SP 350: 25-51., Araújo et al. 2020ARAÚJO C, PEDROSA-SOARES A, LANA C, DUSSIN I, QUEIROGA G, SERRANO P & MEDEIROS-JUNIOR E. 2020. Zircon in emplacement borders of post-collisional plutons compared to country rocks: A study on morphology, internal texture, U-Th-Pb geochronology and Hf isotopes (Araçuaí orogen, SE Brazil). Lithos 352-353: 105252.). Geothermobarometric estimates from different techniques indicate peak metamorphic conditions of 712-930 °C and 5.0-7.5 kbar (Munhá et al. 2005MUNHÁ JMU, CORDANI U, TASSINARI CCG & PALÁCIOS T. 2005. Petrologia e termocronologia de gnaisses migmatíticos da faixa de dobramentos Araçuaí (Espírito Santo, Brasil). Rev Brasil Geocienc 35: 123-134., Moraes et al. 2015MORAES R, NICOLLET C, BARBOSA JSF, FUCK RA & SAMPAIO AR. 2015. Applications and limitations of thermobarometry in migmatites and granulites using as an example rocks of the Araçuaí Orogen in southern Bahia, including a discussion on the tectonic meaning of the current results. Braz J Geol 45: 517-539., Richter et al. 2016RICHTER F, LANA C, STEVENS G, BUICK I, PEDROSA-SOARES AC, ALKMIM FF & CUTTS K. 2016. Sedimentation, metamorphism and granite generation in a back-arc region: records from the Ediacaran Nova Venécia Complex (Araçuaí Orogen, Southeastern Brazil). Precambrian Res 272: 78-100.). Emplacement of widespread, post-collisional intrusions occurred from 530 to 480 M.y. (De Campos et al. 2004De CAMPOS CP, MENDES JC, LUDKA IP, MEDEIROS SR, MOURA JC & WALLFASS C. 2004. A review of the Brasiliano magmatism in southern Espírito Santo, Brazil, with emphasis on post-collisional magmatism. J Virtual Explorer 17., 2016, Gradim et al. 2014GRADIM C, RONCATO J, PEDROSA-SOARES AC, CORDANI U, DUSSIN I, ALKMIM FF, QUEIROGA G, JACOBSSOHN T, SILVA LC & BABINSKI M. 2014. The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation. Braz J Geol 44: 155-180., Melo et al. 2020MELO MG, LANA C, STEVENS G, HARTWIG ME & PIMENTA MS. 2020. Deciphering the source of multiple U-Pb ages and complex Hf isotope composition in zircon from post-collisional charnockite-granite associations from the Araçuaí Orogen (southeastern Brazil). J South Am Earth Sci 103: 102792.) after the regional metamorphism that took place at ca. 575-550 M.y. (Gradim et al. 2014GRADIM C, RONCATO J, PEDROSA-SOARES AC, CORDANI U, DUSSIN I, ALKMIM FF, QUEIROGA G, JACOBSSOHN T, SILVA LC & BABINSKI M. 2014. The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation. Braz J Geol 44: 155-180., Richter et al. 2016RICHTER F, LANA C, STEVENS G, BUICK I, PEDROSA-SOARES AC, ALKMIM FF & CUTTS K. 2016. Sedimentation, metamorphism and granite generation in a back-arc region: records from the Ediacaran Nova Venécia Complex (Araçuaí Orogen, Southeastern Brazil). Precambrian Res 272: 78-100., Melo et al. 2017 a-b).
Carlos Chagas Suite
The Carlos Chagas Suite occurs as a large batholith (~ 14,000 km2), consisting of S-type, peraluminous, leucocratic, whitish gray to yellowish granites. Most of the suite consists of progressively deformed facies, since the weakly foliated granites to strongly stretched ultramylonites, passing through the prevalent foliated granite with well-developed augen structure. Subordinately non-deformed facies with well-preserved igneous fabrics and free from the regional ductile Sn foliation occur (Pedrosa-Soares et al. 2007PEDROSA-SOARES AC, QUEIROGA GN, GRADIM CT, RONCATO JG, NOVO TA, JACOBSOHN T & SILVA KL. 2007. Geologia Da Folha Mantena (SE-24-Y-A-VI), Belo Horizonte: Programa Geologia do Brasil/CPRM, 81 p., 2011).
On the map, the Carlos Chagas Suite is surrounded by the foliated peraluminous granites of the Ataléia suite and Al-rich paragneisses (Jequitinhonha and Nova Venécia complexes) (Figure 1). The contacts of Carlos Chagas Suite with their host rocks vary from intrusive, with the Nova Venécia and Jequitinhonha paragneisses, to gradational or sharp, with the Ataléia foliated granites, and can also be marked by ductile shear zones elsewhere (Pedrosa-Soares et al. 2007PEDROSA-SOARES AC, QUEIROGA GN, GRADIM CT, RONCATO JG, NOVO TA, JACOBSOHN T & SILVA KL. 2007. Geologia Da Folha Mantena (SE-24-Y-A-VI), Belo Horizonte: Programa Geologia do Brasil/CPRM, 81 p., 2011). Xenoliths of the host rocks are common along the borders of the batholith. The Carlos Chagas Suite was intruded by several post-collisional bodies (ca. 530-480 M.y.) during the gravitational collapse of the orogen (De Campos et al. 2004De CAMPOS CP, MENDES JC, LUDKA IP, MEDEIROS SR, MOURA JC & WALLFASS C. 2004. A review of the Brasiliano magmatism in southern Espírito Santo, Brazil, with emphasis on post-collisional magmatism. J Virtual Explorer 17., 2016, Pedrosa-Soares et al. 2011PEDROSA-SOARES ET AL. 2011. Late Neoproterozoic - Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Geol Soc London SP 350: 25-51., Gradim et al. 2014GRADIM C, RONCATO J, PEDROSA-SOARES AC, CORDANI U, DUSSIN I, ALKMIM FF, QUEIROGA G, JACOBSSOHN T, SILVA LC & BABINSKI M. 2014. The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation. Braz J Geol 44: 155-180., Melo et al. 2020MELO MG, LANA C, STEVENS G, HARTWIG ME & PIMENTA MS. 2020. Deciphering the source of multiple U-Pb ages and complex Hf isotope composition in zircon from post-collisional charnockite-granite associations from the Araçuaí Orogen (southeastern Brazil). J South Am Earth Sci 103: 102792.).
Previous geochronological studies (U-Pb zircon) have constrained the age of crystallization of the Carlos Chagas Suite to between ca. 597 and 565 M.y. (Vauchez et al. 2007VAUCHEZ A, EGYDIO-SILVA, M, BABINSKI M, TOMMASI A, UHLEIN A & LIU D. 2007. Deformation of a pervasively molten middle crust: insights from the Neoproterozoic Ribeira-Araçuaí orogen (SE Brazil). Terra Nova 19: 278-286., Gradim et al. 2014GRADIM C, RONCATO J, PEDROSA-SOARES AC, CORDANI U, DUSSIN I, ALKMIM FF, QUEIROGA G, JACOBSSOHN T, SILVA LC & BABINSKI M. 2014. The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation. Braz J Geol 44: 155-180., Melo et al. 2017 a-b, Cavalcante et al. 2019CAVALCANTE C, FOSSEN H, ALMEIDA RP, HOLLANDA MHBM & EGYDIO-SILVA M. 2019. Reviewing the puzzling intracontinental termination of the Araçuaí-West Congo orogenic belt and its implications for orogenic development. Precambrian Res 322: 85-98.). Recent geochronological investigations of the zircon and monazite texture and U-Pb dating have shown that the Carlos Chagas Suite was recorded by two metamorphic events: ca. 570-550 M.y. and ca. 535-515 M.y. (Melo et al. 2017 a-b). The oldest metamorphic event (M1) records peak P-T conditions at 790-820 °C and 9.5-10.5 kbar, whilst the youngest event (M2) was attained at 770 °C and 6.6 kbar (Melo et al. 2017aMELO MG, LANA C, STEVENS G, PEDROSA-SOARES AC, GERDES A, ALKMIN LA, NALINI JR HA & ALKMIM FF. 2017b. Assessing the isotopic evolution of S-type granites of the Carlos Chagas Batholith, SE Brazil: Clues from U–Pb, Hf isotopes, Ti geothermometry and trace element composition of zircon. Lithos 284-285: 730-750.).
MATERIALS AND METHODS
Mineral compositions were determined using the Cameca SX-50 microprobe located at the Universidade de Brasília with an acceleration voltage of 20kV, beam current of 40nA, and counting times of 15s. Whole-rock chemistry analysis was performed at the Department of Earth Sciences, Stellenbosch University (South Africa), using XRF spectrometry on a Philips 1404 Wavelength Dispersive spectrometer.
Zircons were separated from one rock sample at the Department of Geology (DEGEO), Universidade Federal de Ouro Preto, Brazil. The zircon extraction technique makes use of jaw crusher, milling, manual panning, heavy liquids separation, hand-picking under a binocular microscope and mounting on 25 mm epoxy (SpeciFix) mounts. Mounts were polished to expose the grain centers and imaged under SEM-cathodoluminescence (CL) in a Scanning Electron Microscope hosted at Universidade de São Paulo. U-Pb analyses were carried out using a Thermo-Finnigan Element 2 sector field ICP-MS coupled to a CETAC ultraviolet laser system (LA-SF-ICP-MC) housed at DEGEO. Detailed analytical procedures were those described by Melo et al. (2017b)MELO MG, STEVENS G, LANA C, PEDROSA-SOARES AC, FREI D, ALKMIM FF & ALKMIN LA. 2017a. Two cryptic anatectic events within a syn-collisional granitoid from the Araçuaí orogen (southeastern Brazil): evidence from the polymetamorphic Carlos Chagas batholith. Lithos 277: 51-71..
Lu-Hf analyses were carried out using a Thermo-Scientific Finnigan Neptune SF-ICP-MS Thermo-Scientific Neptune Plus multi-collector ICP-MS coupled to a Photon Machines 193 Excimer laser ablation system at DEGEO, Universidade Federal de Ouro Preto (Brazil), following the instrumentation and analytical methods described by Gerdes & Zeh (2006)GERDES A & ZEH A. 2006. Combined U–Pb and Hf isotope LA-(MC-)ICP-MS analyses of detrital zircons: Comparison with SHRIMP and new constraints for the provenance and age of an Armorican metasediment in Central Germany. Earth Planet Sci Lett 249: 47-61. and Moreira et al. (2016)MOREIRA H, LANA C & NALINI JÚNIOR R HA. 2016. The detrital zircon record of an Archaean convergent basin in the Southern São Francisco Craton, Brazil. Precambrian Res 275: 84-99..
RESULTS
Field relationships and petrography
The studied area (Figure 2) is included in an important production pole of ornamental rocks, with over hundred active quarries. The Carlos Chagas granites have been exploited for ornamental rocks and these quarries contains excellent exposures for petrologic studies. The rock is holo to leucocratic, coarse-to medium grained and whitish gray in hand specimen, although yellowish varieties occur locally. The predominant texture is granoblastic, although lepidogranoblastic and nematogranoblastic textures also occur (Figure 3a). Sometimes mylonitic features are observed, such as K-feldspar porphyroclasts form eye-shaped augen (Figure 3b) due to the development of pressure shadows during rotation of the crystal. In some outcrops, granites have leucosomes crystallized from anatectic melt, consisting of peritectic garnet-rich quartz-feldspathic patches (up to 30 cm long) that are free of the regional solid-state foliation (Figure 3c). Schlieren, nebulitic and stromatic structures are also observed (Figure 3d).
Simplified geologic map for study area, showing the location of samples (modified from Pedrosa-Soares et al. 2007PEDROSA-SOARES AC, QUEIROGA GN, GRADIM CT, RONCATO JG, NOVO TA, JACOBSOHN T & SILVA KL. 2007. Geologia Da Folha Mantena (SE-24-Y-A-VI), Belo Horizonte: Programa Geologia do Brasil/CPRM, 81 p.).
Macroscopic features of studied rocks. (a) Leucocratic granite showing foliation (Sn) defined by orientation of biotite. (b) Sigma shape of K-feldspar grains in granite. (c) Neoformed leucocratic patches (dashed line) with subhedral to euhedral garnet crystals. (d) Stromatic structure that shows leucocratic segregations. (e) The contact of the xenolith with host rock is sharp. Note the presence of garnet-bearing leucosome. (f) Mafic dikes (dashed line) cutting the Carlos Chagas granite.
Migmatitic paragneisses (Nova Venécia Complex) and foliated syn-collisional granites of the Ataléia suite surround and underlie the Carlos Chagas Suite (Figure 2). The contact of the studied granite with its host rocks varies from intrusive to gradational. Xenoliths of the host rocks are locally found (Figure 3e). Post-collisional bodies are intrusive in collisional granites (Figure 3f).
Eleven representative thin sections were used for a detailed petrographic study, including thin sections previously studied by Melo et al. (2017a)MELO MG, LANA C, STEVENS G, PEDROSA-SOARES AC, GERDES A, ALKMIN LA, NALINI JR HA & ALKMIM FF. 2017b. Assessing the isotopic evolution of S-type granites of the Carlos Chagas Batholith, SE Brazil: Clues from U–Pb, Hf isotopes, Ti geothermometry and trace element composition of zircon. Lithos 284-285: 730-750.. The mineralogy consists of K-feldspar (48-67 vol.%), quartz (16-28 vol.%), plagioclase (10-21 vol.%) and minor amount of garnet (3-8 vol.%) and biotite (1-6 vol.%). Accessory phases include sillimanite, hercynite, apatite and ilmenite (~ 2 vol.%). Common alteration products are carbonate and muscovite (1-4 vol.%). On the QAP ternary classification diagram (Streckeisen 1974STRECKEISEN A. 1974. Classification and nomenclature of plutonic rocks: recommendations of the IUGS subcommission on the systematics of Igneous Rocks. Geol Rundsch, 63: 773-786.), the Carlos Chagas Suite is classified mainly as syenogranite (Figure 4).
Modal QAP diagram of Streckeisen (1974)STRECKEISEN A. 1974. Classification and nomenclature of plutonic rocks: recommendations of the IUGS subcommission on the systematics of Igneous Rocks. Geol Rundsch, 63: 773-786., showing the composition of the studied granites.
The studied samples contain three preserved mineral assemblages (A1 to A3). The A1 assemblage consists of course-to medium grained plagioclase, K-feldspar, quartz, garnet, biotite and ilmenite. The A2 assemblage differs from A1 assemblage by the presence of sillimanite and by generation of a second type of garnet. The A3 assemblage is characterized by retrograde phases (biotite and hercynite), which occasionally replace garnet boundaries. The samples show granolepidoblastic texture and occasionally nematoblastic texture (Figure 5a).
Photomicrographs of microstructures observed in the studied granites. (a) Granolepidoblastic and nematoblastic textures showing the orientation of biotite (Bt2) and sillimanite. (b) Intergrowth myrmekite. (c) Films of quartz in perthitic K-feldspar boundaries in the matrix. (d) Polygonal contacts between plagioclase (Pl) grains. (e) Symplectite intergrowth between biotite and quartz in garnet boundaries. Note quartz-filled pools adjacent to garnet. (f) Quartz films along the grain boundaries of garnet in the matrix. (g) Quartz films along the grain boundaries of K-feldspar in the matrix. (h) Quartz ribbon shows intense grain boundary migration recrystallization. Note ameboid to lobate contacts between quartz and feldspar grains. Mineral abbreviations are after Whitney & Evans (2010)WHITNEY DL & EVANS BW. 2010. Abbreviations for names of rock-forming minerals. Am Mineral 95: 185-187..
Anhedral K-feldspar grains are medium-to coarse-grained and often show vermicular quartz hosted in plagioclase denotes myrmekite texture (Figure 5b). Some of them are perthitic with exsolved plagioclase lamellae (Figure 5c). Evidence of ductile deformation includes undulose extinction and recrystallized aggregates. of plagioclase with polygonal contacts (Figure 5d).
Quartz grains are anhedral with variable grain sizes, and sometimes concentrate as polygonal aggregates. Symplectic intergrowth involving biotite and quartz are common (Figure 5e). Thin films of quartz are observed along the boundaries of garnet crystal (Figure 5f) and between plagioclase and K-feldspar grains in the matrix (Figure 5g). Ribbons formed by quartz grains in mylonitic samples are common (Figure 5h). The quartz grains commonly exhibit undulose extinction (Figure 6a), subgrains and new grains. Some plagioclase grains exhibit undulose extinction (Figure 6b) and subgrain boundaries. Microfractures filled with quartz are observed cutting K-feldspar grains (Figure 6c).
Photomicrographs of microstructures observed in the studied granites. (a) Evidence of deformation in quartz grains include undulose extinction, subgrain formation and grain boundary migration. (b) Recrystallized plagioclase by subgrain rotation. (c) Microfracture filled by quartz. (d) Garnet poikiloblastic (Grt1) host quartz, biotite (Bt1) and ilmenite inclusions. (e) Sillimanite needles wrap around Grt1, forming a second stage of garnet growth (Grt2). (f) Garnet poikiloblastic (Grt2) showing inclusions of sillimanite needles. (g) Hercynite (Spl) inclusions in prismatic sillimanite associated with biotite flakes (Bt2). (h) Hercynite partially replaces the garnet boundaries.
Two textural varieties of garnet porphyroblasts are observed (Grt1 e Grt2). The first variety occurs as subhedral to anhedral poikiloblastic crystals that contain lobate to rounded biotite, quartz and ilmenite (Figure 6d). A second type of garnet commonly occurs as overgrowths in Grt1 porphyroblasts (Figure 6e) and as euhedral to anhedral crystals, both with inclusions of sillimanite needles (Figure 6f).
Biotite occurs both in the matrix and as inclusions in garnet crystals. Those in the matrix (Bt2) are brown, aligned and fine-grained, defining the foliation (Figure 5a). Rounded biotite inclusions (Bt1) are reddish brown in colour (Figures 6 d-e), implying high Ti contents. In local domains of retrograde re-equilibrium, garnet is partially replaced by biotite (Bt3) (Figures 5e and 6d).
Oriented aggregates of prismatic sillimanite are associated with biotite, whilst sillimanite needles (fibrolite) occur as inclusions in plagioclase and Grt2. Hercynite occurs with biotite and sillimanite in the matrix or partially replacing garnet boundaries. Rare hercynite occurs as inclusions within garnet porphyroblasts and prismatic sillimanite (Figures 6 g-h).
Mineral chemistry
Representative mineral analyses are presented in Tables I to V. Both generations of garnet in studied rocks are almandine-rich (Figure 7a) and individual crystals do not display any significant zoning (Figure 8). The Grt1 crystals composition is similar in all samples, with XAlm values of 0.74-0.80 (CCS-21) and 0.77-0.82 (CCS-25), XPrp values of 0.15-0.21 (CCS-21) and 0.13-0.19 (CCS-25). Both samples have low Xsps (0.02-0.03) and XGrs (0.02-0.03) contents and Mg# of 0.16-0.22 (CCS-21) and 0.13-0.20 (CCS-25). Typical compositional ranges for Grt2 crystals in the samples are XAlm 0.73-0.82, XPrp 0.13-0.21, XGrs 0.02-0.03, XSps 0.03-0.04 (CCS-21) and XAlm 0.76-0.82, XPrp 0.13-0.18, XGrs 0.02-0.04, XSps 0.02-0.03 (CCS-25). Mg# values range between 0.13 and 0.22 (CCS-21) and 0.13-0.19 (CCS-25).
Average (Av) and standard deviation (SD) of the mineral chemistry of garnets. Calculations based on 12 oxygens, following the methodology of Droop (1987).
Average (Av) and standard deviation (SD) of the mineral chemistry of feldspars in matrix. Calculations based on 8 oxygens, according to parameters of Deer et al. (2020).
Average (Av) and standard deviation (SD) of the mineral chemistry of biotite. Calculations based on 20 oxygens, following the methodology of Tindle & Webb (1990).
Average (Av) and standard deviation (SD) of the mineral chemistry of ilmenite. Calculations based on 3 oxygens, following the methodology of Droop (1987).
Average (Av) and standard deviation (SD) of the mineral chemistry of spinel. Calculations based on 4 oxygens, following the methodology of Droop (1987).
Mineral chemistry of the studied rocks. (a) Alm-Prp-Grs garnet ternary diagram. (b) Mica classification according to Tieschendorf et al. (1997). (c) Biotite compositions plotted on a Ti (a.p.f.u.) vs. Mg#. (d) An-Ab-Or feldspar classification diagram (after Deer et al. 2010DEER WA, HOWIE RA, ZUSSMAN J. 2010. Minerais Constituintes das Rochas – Uma Introdução, 2nd ed., Lisboa: Fundação Colouste Gulbenkian, 696 p.). (e) TiO2-FeO-Fe2O3 ternary diagram (after Deer et al. 2010DEER WA, HOWIE RA, ZUSSMAN J. 2010. Minerais Constituintes das Rochas – Uma Introdução, 2nd ed., Lisboa: Fundação Colouste Gulbenkian, 696 p.).
Chemical profile in garnets showing content of XAlm, XPrp and XSps in section along the grains.
According to the binary diagram of Tischendorf et al. (1997)TISCHENDORF G, GOTTESMANN B, FÖRSTER HJ & TRUMBULL RB. 1997. On Li-bearing micas: estimating Li from electron microprobe analyses and an improved diagram for graphical representation. Mineral Mag 61: 809-834. (Figure 7b), most of the biotite analyses plot into the domain of the Mg-biotite. Biotite inclusions (Bt1) in garnet show Mg# values of 0.59-0.67 (CCS-21) and 0.49-0.66 (CCS-25) and Ti values of 0.37-0.55 (CCS-21) and 0.34-0.50 (CCS-25). Two generations of biotite exist in the matrix of the granites: a predominant peak assemblage biotite (Bt2) characterized by Mg# values of 0.50-0.53 (CCS-21) and 0.46–0.49 (CCS-25) and Ti values of 0.46-0.56 (CCS-21) and 0.41-0.42 (CCS-25); and a retrograde biotite (Bt3) that partially replaced garnet boundaries, with Mg# of 0.54-0.62 (CCS-21) and 0.48-0.50 (CCS-25) and Ti values of 0.37-0.57 (CCS-21) and 0.43-0.57 (Figure 7c). Ti in the biotite inclusions is slightly higher than Ti in the biotite of the matrix but varies from sample to sample (Figure 7c).
Plagioclase of the matrix is unzoned and displays Xan values of 0.19–0.25 (CCS-21) and 0.24-0.28 (CCS-25) (Figure 7d). Matrix K-feldspar crystals have low anorthite content (~ 0.01) and variable XOr values of 0.56-0.93 (CCS-21) and 0.72-0.91 (CCS-25) (Figure 7d). Ilmenite grains are classified as Fe-Ti ilmenite (Figure 7e), with Fe2+ contents of 0.91-0.92 apfu (CCS-21) and 0.92-0.9 apfu (CCS-25) and Fe3+ values of 0.06-0.08 apfu (CCS-21) and 0.01-0.04 apfu (CCS-25) Spinel crystals have relatively high Zn-content with XGhn of 0.10-0.22 (CCS-21).
Conventional geothermobarometer
In order to establish the path of metamorphism, temperatures were calculated by using the garnet-biotite geothermometer. Peak metamorphic conditions (M1) were estimated using garnet core compositions (Grt1) together with inclusions of biotite (Bt1). For peak metamorphic temperature estimates of the M2 event, garnet core compositions (Grt2) and matrix biotite (Bt2) were used. Rim compositions of garnet and biotite (Bt3) give rough estimates of temperatures of the retrograde reactions associate to the M2 event. Considering pressures of 7-10 kbar, estimates geothermometric indicates equilibrium conditions at 633-643 °C (CCS-21) and 736-746 °C for the M1 event (Table VI). Temperatures around 757-775 °C (CCS-21) and 718-735 °C (CCS-25) were attained during the M2 event (Table VI). Estimate pressures of the peak metamorphic of the M2 event have been determined using the garnet-sillimanite-quartz-plagioclase (GASP) geobarometer. Calculated peak pressures are around 5.4-8.5 kbar (CCS-21) and 5.2-8.2 kbar (CCS-25) at temperatures between 700 and 900 °C (Table VI). Retrograde re-equilibrium indicates lower temperature of about 600-609 °C (CCS-21) and 665-664 °C (CCS-25) at pressures of 4-6 kbar (Table VI).
Phase equilibrium modeling
Phase equilibria modelling was undertaken in the chemical system MnO-Na2O-CaO-K2OFeO-MgO-Al2O3-SiO2-H2O-TiO2-O2 (MnNCKFMASHTO) using Theriak-Domino software (De Capitani & Petrakakis 2010De CAPITANI C & PETRAKAKIS K. 2010. The computation of equilibrium assemblage diagrams with Theriak/Domino software. Am Mineral 95: 1006-1016.), in combination with the updated Holland & Powell (1998)HOLLAND TJB & POWELL R. 1998. An internally consistent thermodynamic data set for phases of petrological interest. J Metamorph Geol 16: 309-343. data set that includes silicate melt. The following a-x models were used: feldspar (Holland & Powell 2003HOLLAND T & POWELL R. 2003. Activity–composition relations for phases in petrological calculations: an asymmetric multicomponent formulation. Contrib Mineral Petr 145: 492-501.); garnet and biotite (White et al. 2005WHITE RW, POMROY NE & POWELL R. 2005. An in situ metatexite-diatexite transition in upper amphibolite facies rocks from Broken Hill, Australia. J Metamorph Geol 23: 579-602); orthopyroxene (White et al. 2002WHITE RW, POWELL R & CLARKE GL. 2002. The interpretation of reaction textures in Fe rich metapelite granulites of the Musgrave Block, Central Australia: constraints from mineral equilibria calculations in the system K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3. J Metamorph Geol 20: 41-55.); ilmenite (White et al. 2000WHITE RW, POWELL R, HOLLAND TJB & WORLEY BA. 2000. The effect of TiO2 and Fe2O3 on metapelitic assemblages at greenschist and amphibolite facies conditions: mineral equilibria calculations in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3. J Metamorph Geol 18: 497-511., 2005); cordierite (Holland & Powell 1998HOLLAND TJB & POWELL R. 1998. An internally consistent thermodynamic data set for phases of petrological interest. J Metamorph Geol 16: 309-343.); muscovite (Coggon & Holland 2002COGGON R & HOLLAND TJB. 2002. Mixing properties of phengiticmicas and revised garnet–phengite thermobarometers. J Metamorph Geol 20: 683-696.); and melt (White et al. 2007WHITE RW, POWELL R & HOLLAND TJB. 2007. Progress relating to calculation of partial melting equilibria for metapelites. J Metamorph Geol 25: 511-527.).
The investigated samples (CCS-21 and CCS-25) preserve their A1 assemblage only as isolated porphyroblasts in a foliated matrix that consists of the A2 assemblage (Kfs+Pl+Qz+Grt2+Bt2+Sil+Ilm+melt). This mineral assemblage was used to estimate P-T conditions during the peak metamorphic of the M2 event. Table VII presents the bulk composition used for metamorphic modeling. H2O in the bulk composition were set by using T-X pseudosections and the bulk rock H2O content was chosen based on the methodology of White & Powell (2002)WHITE RW & POWELL R. 2002. Melt loss and the preservation of granulite facies mineral assemblages. J Metamorph Geol 20: 621-632., such that the solidus was located just below the lowest temperature of the interpreted peak assemblage. The H2O values were of 0.14 wt.% (CCS-21) and 0.18 wt.% (CCS-25). The Fe2O3 content were determined using P-X pseudosection and the values chosen for Fe3+ concentration in each bulk composition (CCS-21 = 0.05 wt.% and CCS-25 = 0.09 wt.%) results in isopleths of Fe3+ in garnet, within the field of the preserved assemblage.
The peak metamorphic assemblage of the M2 event is stable over a wide range of pressures (4.2 to >8 kbar) and relatively restricted temperatures (690 to 760 ˚C) (Figure 9a). The stability of biotite places an upper temperature constraint of < 760 °C (Figure 9a). The lower pressure limit of the A2 assemblage is bounded by stability of cordierite. The absence of A2 cordierite constrains pressures to < 4.2 kbar at 700 °C and < 5.3 kbar at > 700 °C (Figure 9a). XAn values for modeled plagioclase range from 0.24 to 0.28 in in the preserved assemblage field (Figure 9b). Modeled garnet composition shows Mg# of 0.11–0.25, XPrp of 0.10–0.23, XAlm of 0.70-0.81 and XSps of 0.02–0.05 in the preserved assemblage field (Figures 9 c-d-e). The mode of melt at preserved metamorphic assemblage is less than 3 vol% (Figure 9f). These values encompass those of measured in plagioclase and Grt2, showing XAn, Grt Mg#, XPrp, XAlm and XSps values of 0.19-0.25, 0.13-0.22, 0.13-0.21, 0.73-0.82 and 0.029-0.04, respectively. The use of XAn, Grt Mg#, XPrp, XAlm and XSps values help to more tightly constrain the P–T conditions of the preserved (A2) assemblage to 713–719 °C and 6.4-7.1 kbar (Figure 9f).
P–T pseudosection calculated using the bulk composition from sample CCS-21 (A2 assemblage). (a) P–T fields showing the preserved assemblage Kfs+Pl+Qz+Grt2+Bt2+Sil+Ilm+melt (bold line). (b) Plots of XAn. (c) Plots of Mg# in garnet. (d) Plots of XPrp. (e) Plots of XAlm and XSps. (f) P–T conditions of equilibrium based on the use of overlapping ranges of phase composition.
In sample CCS-25, pseudosection modelling constrains the preserved (A2) metamorphic assemblage of Kfs+Pl+Qz+Grt2+Bt2+Sil+Ilm+melt to a field of stability between 700-760 ˚C and pressures ranging from 4.3 to >8 kbar (Figure 10a). These P-T conditions are like those of the sample CCS-21 (Figure 10a). Modeled plagioclase, biotite, garnet compositions show XAn of 0.28-0.31, Grt Mg# of 0.12-0.26, XPrp of 0.10–0.23, XAlm of 0.69-0.80 and XSps of 0.02–0.04 in the preserved assemblage field (Figures 10 b-c-d-e). Within this field, the mode of melt is less than 3 vol% (Figure 10f). Plagioclase, biotite and garnet compositional isopleths show that the measured compositions these mineral phases intersect the preserved A2 assemblage field between 719 ˚C and 729 °C and 6.1- 7.3 kbar (Figure 10f).
P–T pseudosection calculated using the bulk composition from sample CCS-25 (A2 assemblage). (a) P–T fields showing the preserved assemblage Kfs+Pl+Qz+Grt2+Bt2+Sil+Ilm+melt (bold line). (b) Plots of XAn and Mg# in biotite. (c) Plots of Mg# in garnet. (d) Plots of XPrp. (e) Plots of XAlm and XSps. (f) P–T conditions of equilibrium based on the use of overlapping ranges of phase composition.
Zircon U-Pb and Lu-Hf data
In order to evaluate the metamorphic history from Carlos Chagas Suite, we investigated the internal structure of the zircon grains (sample CCS-35a) through cathodoluminescence (CL) imaging (Figure 11). Spot analyses were carried on zircon core and/or rim domains via LA-SF-ICP-MS. The analytical results are presented in Table VIII and ages presented in figure 12.
Representative cathodoluminescence (CL) images showing LA-ICP-MS spots for Hf isotopes. U-Pb error reported as 2σ.
U-Pb ages and Hf isotopic composition for the Carlos Chagas Suite. (a and b) Concordia diagrams and weighted mean 206Pb/238U age plots containing the results of zircon U-Pb dating of the sample CCS-35b. (c) Initial 176Hf/177Hf ratios of zircon populations from the studied sample plotted against their age (Ma). The error bars are at 2σ level. (d) εHf(t) vs. age diagrams for zircon grains this study. Data compiled from Gonçalves et al. (2016), Tedeschi et al. (2016)TEDESCHI M ET AL. 2016. The Ediacaran Rio Doce magmatic arc revisited (Araçuaí-Ribeira orogenic system, SE Brazil). J S Am Earth Sci 68: 167-186., Melo et al. (2017b)MELO MG, STEVENS G, LANA C, PEDROSA-SOARES AC, FREI D, ALKMIM FF & ALKMIN LA. 2017a. Two cryptic anatectic events within a syn-collisional granitoid from the Araçuaí orogen (southeastern Brazil): evidence from the polymetamorphic Carlos Chagas batholith. Lithos 277: 51-71. and Araújo et al. (2020)ARAÚJO C, PEDROSA-SOARES A, LANA C, DUSSIN I, QUEIROGA G, SERRANO P & MEDEIROS-JUNIOR E. 2020. Zircon in emplacement borders of post-collisional plutons compared to country rocks: A study on morphology, internal texture, U-Th-Pb geochronology and Hf isotopes (Araçuaí orogen, SE Brazil). Lithos 352-353: 105252..
Zircon grains are subhedral to euhedral prismatic, light pink, translucent, ranging from 80 to 500 µm, with length/width ratios in average of 4:1. CL images of the internal structure of the zircons show that they have a core with oscillatory-zoned patterns or irregular zoning, which are generally surrounded by low luminescence and structureless rim (Figure 11). Some rounded and unzoned cores appear luminescent in CL images. Thirty-seven analytical spots yield a range of concordant ages that spread along the Concordia diagram from ca. 631 to 520 M.y. (Figure 12a). Four populations have been distinguished for zircon 206Pb/238U ages (groups I to IV). The oldest population (Group I; n = 8) is represented by rounded cores with bright luminescence with ages ranging from ca. 631 to 604 M.y. and Th/U ratios of 0.16-0.98 (Table VIII). Other thirteen spot analyses in core domains (Group II) give weighted mean 206Pb/238U age of 586 ± 2 M.y. (MSWD = 1.02; Figures 12 a-b), with Th/U ratio ranging from 0.09 to 1.11 (Figure 12a). Nine spot analyses in rims (overgrowths) and unzoned domains (Group III) yield a weighted mean of 562 ± 2 M.y. (MSWD = 0.72; Figures 12 a-b) and Th/U ratio between 0.12 and 0.37 (Table VIII). Other seven spot analyses in zircon rims (Group IV) give a weighted mean of 526 ± 4 M.y. (MSWD = 1.5; Figures 12 a-b) and Th/U ratio between 0.04 and 0.65 (Table VIII).
Five analyses on the oldest cores (Group I) yield initial 176Hf/177Hf ratios ranging from 0.28227 to 0.28229, with ɛHf(t) values between -4.3 and -5.1. The distribution of model age (TDM) spans from 1490 to 1530 M.y. (Figures 12 c-d and Table IX). Six analyses performed on zircon cores (Group II) gives initial 176Hf/177Hf ratios ranging from 0.28226 to 0.28230, ɛHf(t) between -5.9 and -4.5 and TDM age varying from 1490 to 1540 M.y. (Figure 12c and Table IX). Three analyses performed on unzoned rims (Group III) yield initial 176Hf/177Hf ratios between 0.28227 and 0.28233, with ɛHf(t) values ranging from -6.0 to -4.1 and Hf TDM model ages from 1440 to 1540 M.y. (Figures 12 c-d and Table IX). Seven analyses performed on youngest zircon rims (Group IV) gives initial 176Hf/177Hf ratios ranging from 0.28225 to 0.28232 and ɛHf(t) values between -7.1 and -4.8 and Hf TDM model ages from 1460 to 1560 M.y. (Figures 12 c-d and Table IX).
DISCUSSION
P-T conditions of metamorphism
The P-T history of the Carlos Chagas granites can be summarized in a number of stages (M1 to M2) based on textural evidence, mineral chemistry (Figures 5 to 8) and geothermobarometric data presented above (Figures 9, 10 and Table VI). Two main preserved metamorphic assemblages are recognized: A1, given by Grt1 + Bt1 + Pl + Kfs + melt; and A2, given by Grt2 + Bt2 + Pl + Kfs ± Ilm ± Sil ± melt.
Estimates geothermometric indicates that the garnet and biotite (mineral assemblage A1) reached equilibrium conditions at 740-750 °C during the peak metamorphic M1 event. The presence of biotite, plagioclase and quartz as inclusions in garnet (Figures 6 d-e) indicates that garnet growth in granite at the expense of these minerals according to the dehydration reaction: Bt1 + Pl + Qz = Grt1 ± Kfs + melt. However, experimental studies have shown that this reaction only occurs at temperatures between 780 and 830 °C (Patiño Douce & Johnston 1991PATIÑO DOUCE AE & JOHNSTON AD. 1991. Phase equilibria and melt productivity in the pelitic system: implications for the origin of peraluminous granitoids and aluminous granulites. Contrib Mineral Petrol 107: 202-218., Vielzeuf & Montel 1994VIELZEUF D & MONTEL JM. 1994. Partial melting of metagreywackes. Part I. Fluid-absent experiments and phase relationships. Contrib Mineral Petr 117: 375-393., Stevens et al. 1997STEVENS G, CLEMENS JD & DROOP GTR. 1997. Melt production during granulite-facies anatexis: experimental data from “primitive” metasedimentary protholiths. Contrib Mineral Petr 128: 352-370.), implying that the values found here are significantly lower than expected. Taken into consideration the uncertainties of the Grt-Bt thermometer (~ 60 °C), our estimates of temperature are like the recent results of Melo et al. (2017a)MELO MG, LANA C, STEVENS G, PEDROSA-SOARES AC, GERDES A, ALKMIN LA, NALINI JR HA & ALKMIM FF. 2017b. Assessing the isotopic evolution of S-type granites of the Carlos Chagas Batholith, SE Brazil: Clues from U–Pb, Hf isotopes, Ti geothermometry and trace element composition of zircon. Lithos 284-285: 730-750.. The peak pressure was attained at 9.5-10.5 kbar (Melo et al. 2017aMELO MG, LANA C, STEVENS G, PEDROSA-SOARES AC, GERDES A, ALKMIN LA, NALINI JR HA & ALKMIM FF. 2017b. Assessing the isotopic evolution of S-type granites of the Carlos Chagas Batholith, SE Brazil: Clues from U–Pb, Hf isotopes, Ti geothermometry and trace element composition of zircon. Lithos 284-285: 730-750.). The lack of extensive post-peak retrogression is consistent with progressive melt loss and dehydration during metamorphism (White & Powell 2002WHITE RW & POWELL R. 2002. Melt loss and the preservation of granulite facies mineral assemblages. J Metamorph Geol 20: 621-632.), producing an almost completely anhydrous and refractory rock.
For the subsequent anatectic process, these rocks probably underwent rehydration after M1 melt loss and prior to M2 event. The development of muscovite-biotite fabric is interpreted as a strong evidence of this rehydration process (Melo et al. 2017aMELO MG, LANA C, STEVENS G, PEDROSA-SOARES AC, GERDES A, ALKMIN LA, NALINI JR HA & ALKMIM FF. 2017b. Assessing the isotopic evolution of S-type granites of the Carlos Chagas Batholith, SE Brazil: Clues from U–Pb, Hf isotopes, Ti geothermometry and trace element composition of zircon. Lithos 284-285: 730-750.). Shear zones are considered tectonic corridors for fluid percolation. In the field, such structures are characterized by mylonitic rocks that show evidence of ductile deformation. The rehydration of the sheared rocks must be occurred through hydration of K-feldspar and some minor fraction of garnet (Grt1), producing a muscovite-biotite fabric (Melo et al. 2017aMELO MG, LANA C, STEVENS G, PEDROSA-SOARES AC, GERDES A, ALKMIN LA, NALINI JR HA & ALKMIM FF. 2017b. Assessing the isotopic evolution of S-type granites of the Carlos Chagas Batholith, SE Brazil: Clues from U–Pb, Hf isotopes, Ti geothermometry and trace element composition of zircon. Lithos 284-285: 730-750.). Hydrothermal gem deposits from districts of the Eastern Pegmatite Province in Araçuaí Orogen attest fluid movement through the crust (Pedrosa-Soares et al. 2011PEDROSA-SOARES ET AL. 2011. Late Neoproterozoic - Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Geol Soc London SP 350: 25-51.).
For the peak metamorphic of the M2 event, the thermodynamic modeling (Figures 9 and 10) of the preserved mineral assemblage A2 suggests that all samples (CCS-21 and CCS-25) depict a similar P-T conditions, recording temperatures between 713 and 729 °C and pressures from 6.3 to 7.2 kbar. According to conventional geothermobarometry data, both samples were formed within temperature and pressure ranges corresponding to medium pressure (5.2-5.5 kbar) and relatively high temperature (710°C). Grt2 crystals have sillimanite inclusions (Figs. 6e and 6f), indicating that the garnet was produced by dehydration reaction: Pl + Qz + Sil + Bt2 = Grt2 ± Kfs + melt. According to the experimental studies of Vielzeuf & Holloway (1988)VIELZEUF D & HOLLOWAY JR. 1988. Experimental determination of the fluid-absent melting relations in the pelitic system – consequences for crustal differentiation. Contrib Mineral Petrol 98: 257-276., this reaction occurs at temperatures of approximately 750-800 °C. Pseudomorphed melt films and Bt-Qz symplectites are observed surrounding Grt2 crystals (Figure 5e), corroborating that garnet grew in the presence of melt in studied samples. This remain residuum is confirmed by thermodynamic modelling, which shows < 2 vol.% of melt preserved in mineral assemblage A2 (Figures 9 and 10). The studied granites contain a few leucocratic patches with diffuse margins which may be considered as leucosomes produced by a second metamorphic event (Figure 3c). In fact, they are free of the regional solid-state foliation and occur as autochthonous patches, indicating that the granites have undergone renewed melting. Considering the average density of the crust to be 2.75 g/cm3 and the barometric gradient to be 270 bar/km, we estimated depths of 23-27 km with a geothermal gradient up to 27-31 °C/km for the M2 event that generated the mineral assemblage A2 (Figure 13).
P–T path for collisional rocks from the Carlos Chagas Suite. Compilation of the metamorphic conditions from paragneisses (Nova Venécia Complex) and pre-collisional and collisional granitoids (Richter et al. 2016RICHTER F, LANA C, STEVENS G, BUICK I, PEDROSA-SOARES AC, ALKMIM FF & CUTTS K. 2016. Sedimentation, metamorphism and granite generation in a back-arc region: records from the Ediacaran Nova Venécia Complex (Araçuaí Orogen, Southeastern Brazil). Precambrian Res 272: 78-100., Melo et al. 2017aMELO MG, LANA C, STEVENS G, PEDROSA-SOARES AC, GERDES A, ALKMIN LA, NALINI JR HA & ALKMIM FF. 2017b. Assessing the isotopic evolution of S-type granites of the Carlos Chagas Batholith, SE Brazil: Clues from U–Pb, Hf isotopes, Ti geothermometry and trace element composition of zircon. Lithos 284-285: 730-750., Gouvêa et al. 2020GOUVÊA LP, MEDEIROS SR, MENDES JC, SOARES C, MARQUES R & MELO, M. 2020. Magmatic activity period and estimation of P-T metamorphic conditions of pre-collisional opx-metatonalite from Araçuaí-Ribeira orogens boundary, SE Brazil. J S Am Earth Sci 99: 102506.). Reaction curves after partial melting experiments (Patiño Douce & Johnston 1991PATIÑO DOUCE AE & JOHNSTON AD. 1991. Phase equilibria and melt productivity in the pelitic system: implications for the origin of peraluminous granitoids and aluminous granulites. Contrib Mineral Petrol 107: 202-218., Stevens et al. 1997STEVENS G, CLEMENS JD & DROOP GTR. 1997. Melt production during granulite-facies anatexis: experimental data from “primitive” metasedimentary protholiths. Contrib Mineral Petr 128: 352-370., Vielzeuf & Montel 1994VIELZEUF D & MONTEL JM. 1994. Partial melting of metagreywackes. Part I. Fluid-absent experiments and phase relationships. Contrib Mineral Petr 117: 375-393.).
If the melt is not removed via an open system, H2O amounts of the system will favor significant retrogression of the mineral assemblage. The occurrence of retrograde assemblages led us to the infer that the studied rocks must have been rehydrated after M2 metamorphic peak. Some garnet crystals (Grt2) are partially replaced by biotite (Bt3), suggesting retaining enough H2O from crystallizing melt to develop the observed texture (Stevens et al. 1997STEVENS G, CLEMENS JD & DROOP GTR. 1997. Melt production during granulite-facies anatexis: experimental data from “primitive” metasedimentary protholiths. Contrib Mineral Petr 128: 352-370.). The application of Grt2-Bt3 geothermometer indicates low temperature (600-660 °C – Table IX, Figure 13), resulting from retrograde Fe-Mg exchange between garnet and biotite (Ferry & Spear 1978FERRY JM & SPEAR FS. 1978. Experimental calibration of the partitioning of Fe and Mg between biotite and garnet. Contrib Mineral Petr 66: 113-117.). This temperature is consistent with the hercynite formation in garnet rims at temperatures below 700 °C (Spear 1993SPEAR FS. 1993. Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Washington D. C.: Mineralogical Society of America, 799 p.).
The occurrence of two generations of garnet in combination with microstructures evidencing the former presence melt in studied samples may suggest temperatures at > 750 °C (Vielzeuf & Holloway 1988VIELZEUF D & HOLLOWAY JR. 1988. Experimental determination of the fluid-absent melting relations in the pelitic system – consequences for crustal differentiation. Contrib Mineral Petrol 98: 257-276., Patiño Douce & Johnston 1991PATIÑO DOUCE AE & JOHNSTON AD. 1991. Phase equilibria and melt productivity in the pelitic system: implications for the origin of peraluminous granitoids and aluminous granulites. Contrib Mineral Petrol 107: 202-218., Vielzeuf & Montel 1994VIELZEUF D & MONTEL JM. 1994. Partial melting of metagreywackes. Part I. Fluid-absent experiments and phase relationships. Contrib Mineral Petr 117: 375-393., Stevens et al. 1997STEVENS G, CLEMENS JD & DROOP GTR. 1997. Melt production during granulite-facies anatexis: experimental data from “primitive” metasedimentary protholiths. Contrib Mineral Petr 128: 352-370.). In fact, the lack of major element zoning in garnet (Figure 8) indicates that temperatures of metamorphism exceeded 650 °C, the temperature at which growth zoning is typically homogenized by volume diffusion (Tracy 1982TRACY RJ. 1982. Compositional zoning and inclusions in metamorphic minerals. Rev Mineral Geochem 10: 355-397.). Therefore, we interpreted that the metamorphic peak in all samples was attained at granulite-facies conditions and the mineral assemblages equilibrated close to the transition between upper amphibolite to granulite conditions (Figure 13).
Interpretation of scattered ages and source rocks
The investigation of zircon textures via CL images associated with U-Pb data suggest a complex magmatic/metamorphic history. The U-Pb data from this study reveal a wide range of zircon ages (ca. 631-520 M.y.) (Figures 12 a-b). The oldest rounded zircon grains (Group I: ca. 631-604 M.y.) show resorbed domains and are interpreted as inherited. Oscillatory to weakly zoned zircon cores (Group II) indicate the magmatic crystallization age of the granite at 586 ± 2 M.y. (Figures 12 a-b). Zircon grains commonly show structureless rims and low luminescence in CL images, which may be produced from dissolution-reprecipitation and overgrowth processes in response to interactions with silicate melts. The youngest zircon populations with ages peaks at 562 ± 2 M.y. (Group III) and 526 ± 4 M.y. (Group IV) represent overgrowth rims, formed during the regional metamorphism and later thermal episodes, respectively. Previous geochronological studies (U-Pb zircon and monazite) have constrained the age of the regional metamorphism at ca. 570-550 M.y. (Gradim et al. 2014GRADIM C, RONCATO J, PEDROSA-SOARES AC, CORDANI U, DUSSIN I, ALKMIM FF, QUEIROGA G, JACOBSSOHN T, SILVA LC & BABINSKI M. 2014. The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation. Braz J Geol 44: 155-180., Richter et al. 2016RICHTER F, LANA C, STEVENS G, BUICK I, PEDROSA-SOARES AC, ALKMIM FF & CUTTS K. 2016. Sedimentation, metamorphism and granite generation in a back-arc region: records from the Ediacaran Nova Venécia Complex (Araçuaí Orogen, Southeastern Brazil). Precambrian Res 272: 78-100., Melo et al. 2017 a-b, Araújo et al. 2020ARAÚJO C, PEDROSA-SOARES A, LANA C, DUSSIN I, QUEIROGA G, SERRANO P & MEDEIROS-JUNIOR E. 2020. Zircon in emplacement borders of post-collisional plutons compared to country rocks: A study on morphology, internal texture, U-Th-Pb geochronology and Hf isotopes (Araçuaí orogen, SE Brazil). Lithos 352-353: 105252.). Several studies have reported the record of a later thermal event (ca. 530- 515 M.y.), coeval with the emplacement of post-collisional melts (Richter et al. 2016RICHTER F, LANA C, STEVENS G, BUICK I, PEDROSA-SOARES AC, ALKMIM FF & CUTTS K. 2016. Sedimentation, metamorphism and granite generation in a back-arc region: records from the Ediacaran Nova Venécia Complex (Araçuaí Orogen, Southeastern Brazil). Precambrian Res 272: 78-100., Melo et al. 2017aMELO MG, LANA C, STEVENS G, PEDROSA-SOARES AC, GERDES A, ALKMIN LA, NALINI JR HA & ALKMIM FF. 2017b. Assessing the isotopic evolution of S-type granites of the Carlos Chagas Batholith, SE Brazil: Clues from U–Pb, Hf isotopes, Ti geothermometry and trace element composition of zircon. Lithos 284-285: 730-750., b, Peixoto et al. 2018PEIXOTO E, ALKMIM FF, PEDROSA-SOARES AC, LANA C & CHAVES AO. 2018. Metamorphic record of collision and collapse in the Ediacaran-Cambrian Araçuaí orogen, SE-Brazil: Insights from P-T pseudosections and monazite dating. J Metamorph Geol 36: 147-172., Araújo et al. 2020ARAÚJO C, PEDROSA-SOARES A, LANA C, DUSSIN I, QUEIROGA G, SERRANO P & MEDEIROS-JUNIOR E. 2020. Zircon in emplacement borders of post-collisional plutons compared to country rocks: A study on morphology, internal texture, U-Th-Pb geochronology and Hf isotopes (Araçuaí orogen, SE Brazil). Lithos 352-353: 105252., Melo et al. 2020MELO MG, LANA C, STEVENS G, HARTWIG ME & PIMENTA MS. 2020. Deciphering the source of multiple U-Pb ages and complex Hf isotope composition in zircon from post-collisional charnockite-granite associations from the Araçuaí Orogen (southeastern Brazil). J South Am Earth Sci 103: 102792.). The Lu-Hf data from zircon grains of the studied rock show significant crustal contribution to the genesis of the collisional granites. As can see in Figures 12b and 12c, the protholiths involved during crustal melting for Carlos Chagas granites generation could be the metasedimentary rocks like the Nova Venécia complex.
Solutions of thermobarometric and geochronological data
The anatectic episodes recorded in Carlos Chagas Suite require a sustained heat source to maintain high temperatures. High crustal heat production can be associated with elevated amounts of heat-producing elements in a thickened crust during the collisional stage. Fertile paragneisses of the Nova Venécia complex have been interpreted as the main source of S-type granites of the Araçuaí Orogen (Gradim et al. 2014GRADIM C, RONCATO J, PEDROSA-SOARES AC, CORDANI U, DUSSIN I, ALKMIM FF, QUEIROGA G, JACOBSSOHN T, SILVA LC & BABINSKI M. 2014. The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation. Braz J Geol 44: 155-180., Pedrosa-Soares et al. 2011PEDROSA-SOARES ET AL. 2011. Late Neoproterozoic - Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the Eastern Brazilian Pegmatite Province and related mineral resources. Geol Soc London SP 350: 25-51., Araújo et al. 2020ARAÚJO C, PEDROSA-SOARES A, LANA C, DUSSIN I, QUEIROGA G, SERRANO P & MEDEIROS-JUNIOR E. 2020. Zircon in emplacement borders of post-collisional plutons compared to country rocks: A study on morphology, internal texture, U-Th-Pb geochronology and Hf isotopes (Araçuaí orogen, SE Brazil). Lithos 352-353: 105252.). However, the oldest Carlos Chagas granites crystallized before (ca. 600-590 M.y.: Melo et al. 2017bMELO MG, STEVENS G, LANA C, PEDROSA-SOARES AC, FREI D, ALKMIM FF & ALKMIN LA. 2017a. Two cryptic anatectic events within a syn-collisional granitoid from the Araçuaí orogen (southeastern Brazil): evidence from the polymetamorphic Carlos Chagas batholith. Lithos 277: 51-71., Cavalcante et al. 2019CAVALCANTE C, FOSSEN H, ALMEIDA RP, HOLLANDA MHBM & EGYDIO-SILVA M. 2019. Reviewing the puzzling intracontinental termination of the Araçuaí-West Congo orogenic belt and its implications for orogenic development. Precambrian Res 322: 85-98.) the regional metamorphic event (ca. 570-550 M.y.: Gradim et al. 2014GRADIM C, RONCATO J, PEDROSA-SOARES AC, CORDANI U, DUSSIN I, ALKMIM FF, QUEIROGA G, JACOBSSOHN T, SILVA LC & BABINSKI M. 2014. The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation. Braz J Geol 44: 155-180., Richter et al. 2016RICHTER F, LANA C, STEVENS G, BUICK I, PEDROSA-SOARES AC, ALKMIM FF & CUTTS K. 2016. Sedimentation, metamorphism and granite generation in a back-arc region: records from the Ediacaran Nova Venécia Complex (Araçuaí Orogen, Southeastern Brazil). Precambrian Res 272: 78-100., Melo et al. 2017 a-b, Peixoto et al. 2018PEIXOTO E, ALKMIM FF, PEDROSA-SOARES AC, LANA C & CHAVES AO. 2018. Metamorphic record of collision and collapse in the Ediacaran-Cambrian Araçuaí orogen, SE-Brazil: Insights from P-T pseudosections and monazite dating. J Metamorph Geol 36: 147-172., Araújo et al. 2020ARAÚJO C, PEDROSA-SOARES A, LANA C, DUSSIN I, QUEIROGA G, SERRANO P & MEDEIROS-JUNIOR E. 2020. Zircon in emplacement borders of post-collisional plutons compared to country rocks: A study on morphology, internal texture, U-Th-Pb geochronology and Hf isotopes (Araçuaí orogen, SE Brazil). Lithos 352-353: 105252.. Gouvêa et al. 2020GOUVÊA LP, MEDEIROS SR, MENDES JC, SOARES C, MARQUES R & MELO, M. 2020. Magmatic activity period and estimation of P-T metamorphic conditions of pre-collisional opx-metatonalite from Araçuaí-Ribeira orogens boundary, SE Brazil. J S Am Earth Sci 99: 102506.), implying that collision must have started before 600 M.y. (Cavalcante et al. 2019CAVALCANTE C, FOSSEN H, ALMEIDA RP, HOLLANDA MHBM & EGYDIO-SILVA M. 2019. Reviewing the puzzling intracontinental termination of the Araçuaí-West Congo orogenic belt and its implications for orogenic development. Precambrian Res 322: 85-98.; Fossen et al. 2020FOSSEN H, CAVALCANTE C, KONOPÁSEK J, MEIRA VT, ALMEIDA RP, HOLLANDA MHBM & TROMPETTE R. 2020. A critical discussion of the subduction-collision model for the Neoproterozoic Araçuaí-West Congo Orogen. Precambrian Res 343: 105715.).
As a consequence of crustal thickening and gravitational overloading, the Araçuaí Orogen underwent extensional collapse. A huge number of post-collisional plutons (felsic and mafic) were emplaced between 530 and 480 M.y. (Gradim et al. 2014GRADIM C, RONCATO J, PEDROSA-SOARES AC, CORDANI U, DUSSIN I, ALKMIM FF, QUEIROGA G, JACOBSSOHN T, SILVA LC & BABINSKI M. 2014. The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation. Braz J Geol 44: 155-180., De Campos et al. 2016De CAMPOS CP, MEDEIROS SR, MENDES JC, PEDROSA-SOARES AC, DUSSIN I, LUDKA IP & DANTAS EL. 2016. Cambro-Ordovician magmatism in the Araçuaí Belt (SE Brazil): snapshots from a post-collisional event. J S Am Earth Sci 68: 248-268., Aranda et al. 2020ARANDA RO, CHAVES AO, MEDEIROS-JUNIOR EB & VENTURINI-JUNIOR R. 2020. Petrology of the Afonso Cláudio Intrusive Complex: New insights for the Cambro-Ordovician post-collisional magmatism in the Araçuaí-West Congo Orogen, Southeast Brazil. J S Am Earth Sci 98: 102465., Araújo et al. 2020ARAÚJO C, PEDROSA-SOARES A, LANA C, DUSSIN I, QUEIROGA G, SERRANO P & MEDEIROS-JUNIOR E. 2020. Zircon in emplacement borders of post-collisional plutons compared to country rocks: A study on morphology, internal texture, U-Th-Pb geochronology and Hf isotopes (Araçuaí orogen, SE Brazil). Lithos 352-353: 105252., Melo et al. 2020MELO MG, LANA C, STEVENS G, HARTWIG ME & PIMENTA MS. 2020. Deciphering the source of multiple U-Pb ages and complex Hf isotope composition in zircon from post-collisional charnockite-granite associations from the Araçuaí Orogen (southeastern Brazil). J South Am Earth Sci 103: 102792.). The heat source to generate the post-collisional rocks can be associated with asthenospheric mantle ascent probably related to slab break-off followed by delamination of lithosphere mantle during the extensional thinning and gravitational collapse of the orogen (Gradim et al. 2014GRADIM C, RONCATO J, PEDROSA-SOARES AC, CORDANI U, DUSSIN I, ALKMIM FF, QUEIROGA G, JACOBSSOHN T, SILVA LC & BABINSKI M. 2014. The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation. Braz J Geol 44: 155-180.) or the onset of a hotspot due to the destabilization of the asthenospheric mantle (De Campos et al. 2016De CAMPOS CP, MEDEIROS SR, MENDES JC, PEDROSA-SOARES AC, DUSSIN I, LUDKA IP & DANTAS EL. 2016. Cambro-Ordovician magmatism in the Araçuaí Belt (SE Brazil): snapshots from a post-collisional event. J S Am Earth Sci 68: 248-268.). Another hypothesis is the tectonic switching that is induced by changes in the angle of the subducting slab (Moraes et al. 2015MORAES R, NICOLLET C, BARBOSA JSF, FUCK RA & SAMPAIO AR. 2015. Applications and limitations of thermobarometry in migmatites and granulites using as an example rocks of the Araçuaí Orogen in southern Bahia, including a discussion on the tectonic meaning of the current results. Braz J Geol 45: 517-539.). This must have driven the high heat flux into the crust, raising the geotherms, and contributed to the generation of post-collisional rocks, as well as a second metamorphism in the studied area. In fact, the CL-images in combination with U-Pb data clearly show two different ages of overgrowth formation zircon (Figs. 11 and 12), being the younger age (M2 event) concomitant to the emplacement of the post-collisional magmas in the orogen. Petrological and geochronological evidence of this late metamorphism event have been recorded in several units from the Araçuaí orogen (e.g., Melo et al. 2017 a-b, Peixoto et al. 2018PEIXOTO E, ALKMIM FF, PEDROSA-SOARES AC, LANA C & CHAVES AO. 2018. Metamorphic record of collision and collapse in the Ediacaran-Cambrian Araçuaí orogen, SE-Brazil: Insights from P-T pseudosections and monazite dating. J Metamorph Geol 36: 147-172., Serrano et al. 2018SERRANO P, PEDROSA-SOARES A, MEDEIROS-JÚNIOR E, FONTE-BOA T, ARAÚJO C, DUSSIN I, QUEIROGA G & LANA C. 2018. A-type Medina batholith and post-collisional anatexis in the Araçuaí orogen (SE Brazil). Lithos 320-321: 515-536.).
ACKNOWLEDGMENTS
This work is part of the Project nº 9486/2019 registered in PRPPG (UFES). Sandro L.M. Ferreira acknowledges a scholarship from PIIC/UFES (Edital 2019/2020). We appreciate financial support provided by Conselho Nacional de Desenvolvimento Científico (CNPq 402852/2012-5 and 401334/2012-0). We would like to thank associate editor and anonymous reviewer for the detailed and constructive reviews and Editor-in-Chief Claudio Riccomini for the editorial handling.
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Publication Dates
-
Publication in this collection
29 Oct 2021 -
Date of issue
2021
History
-
Received
27 Apr 2020 -
Accepted
18 Feb 2021