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

Abstract

ABSTRACT:In southern Bahia, there are outcrops of migmatites and granulites in the Jequitinhonha Complex, which is part of the northern portion of the Araçuaí Orogen. Migmatites (garnet-cordierite diatexite) dominate the metamorphic rocks and host lenses and layers of felsic garnet granulite. The conditions of temperature and pressure of metamorphism were calculated using conventional thermobarometry and the software THERMOCALC. Values around 850 °C and 7 kbar were obtained with THERMOCALC. The calculations for the garnet-cordierite diatexite were made considering a H2O equal to 1, but the best results of calculations for the granulites are obtained with a H2O values of 0.3. Pressure values obtained with GAPES resulted in consistent values with THERMOCALC, but the pair garnet-orthopyroxene always produces low values for temperature and high ones for pressure. The results are consistent with the presence of the pair garnet and cordierite in diatexite and orthopyroxene in felsic granulite. From the tectonic point of view, the setting in which metamorphism of these rocks occurred requires high heat flow with a thermal anomaly in mid continental crust, as indicated by values of 7 kbar. Recent studies have favored the closure of a back-arc basin for this tectonic setting, but it does not solve the problem that the time span between metamorphic peak and the end of granite intrusions, involving large bodies of charnockite, is more than 80 million years. RESUMO: INTRODUCTIONTo calculate the conditions of pressure and temperature for rocks that underwent granulite facies metamorphism should be simple and straightforward, as the formation conditions overlap those used in most experiments to calibrate thermometers and barometers. However, several factors linked to the genesis of granulites and migmatites contribute negatively to make this task simple and easy. After the metamorphic peak temperature is achieved, often greater than 800°C, granulites and migmatites cool down slowly in the middle to lower portions of the continental crust, a situation that leads to their re-equilibration. Furthermore, the presence of a melt phase is common in granulite facies conditions and can enhance the rate of diffusion or even destroy anhydrous metamorphic peak mineralogy during cooling (Moraes et al. 2002;White & Powell 2002, 2010. For the effective calculation of P-T conditions, either corrections on mineral compositions or thermodynamic modeling must be done. The loss of simplicity makes the task complex, challenging, attractive and, particularly, an interesting subject.To illustrate some of the problems related to the P-T calculation in migmatites and granulites, rocks of the Araçuaí Orogen, in southern Bahia, will be used as an example. Once the results are obtained, an evaluation of their geological, metamorphic and tectonic meaning is undertaken. Thermobarometry applied to granulites and migmatites: problems and restrictionsIn continental crust granulite facies metamorphism, in addition to recrystallization of the protolith, condi...

Hot Versus Cold Orogenic Behavior: Comparing the Araçuaí‐West Congo and the Caledonian Orogens

“…In most of the internal part temperatures were higher during deformation and peak metamorphism, up to 850°C and locally even higher (Figure b). This has been documented by a variety of geothermometers applied to stable mineral parageneses in granulites and migmatites of the orogen, as discussed in detail by Moraes et al (), who found temperature values around 850°C in the northern part of the hot internal domain. This is consistent with other results, which generally show peak metamorphic temperatures in excess of 750°C for this domain, as illustrated in Figure c (Belém, ; Dias et al, ; Gradim et al, ; Gonçalves et al, ; Moraes et al, ; Munhá et al, ; Uhlein et al, ).…”

“…This has been documented by a variety of geothermometers applied to stable mineral parageneses in granulites and migmatites of the orogen, as discussed in detail by Moraes et al (), who found temperature values around 850°C in the northern part of the hot internal domain. This is consistent with other results, which generally show peak metamorphic temperatures in excess of 750°C for this domain, as illustrated in Figure c (Belém, ; Dias et al, ; Gradim et al, ; Gonçalves et al, ; Moraes et al, ; Munhá et al, ; Uhlein et al, ). A rather well‐studied portion of this internal hot part of the orogen is the Carlos Chagas anatectic domain (CC in Figure ), where vast amounts of crustal melt have been produced and deformed in the nonsolid state at midcrustal levels during the orogeny (Cavalcante et al, , ).…”

“…TitaniQ temperatures from Cavalcante et al (). Geobarothermometric data from Garcia et al (), Schmitt et al () Munhá et al (), Belém (), Petitgirard et al (), Uhlein et al (), Bento dos Santos et al (), Gradim et al (), Moraes et al (), and Degler et al (). Kinematic data mainly from Alkmim et al ().…”

“…Several estimates of paleopressures associated with the metamorphic peak in the internal part of the orogen have been presented in the literature, generally varying between 0.5 and 0.7 GPa. (Belém, ; Dias et al, ; Gradim et al, ; Gonçalves et al, ; Moraes et al, ; Munhá et al, ; Petitgirard et al, ; Uhlein et al, ). The highest pressure (0.8 GPa) for the main Araçuaí‐West Congo orogeny was found in the southernmost part of the Araçuaí belt (Bento dos Santos et al, ).…”

Age, provenance and tectonic setting of the high-grade Jequitinhonha Complex, Araçuaí Orogen, eastern Brazil

“…These data, with the metamorphic mineral assemblages, indicate regional metamorphism in the amphibolite-granulite facies transition, accompanied by abundant partial melting of the kinzigitic gneiss (see also Uhlein et al 1998). New multi-equilibrium thermobarometry data presented by Moraes et al (2015) on migmatites and granulites correlative to the Jequitinhonha Complex in southern Bahia indicate metamorphic peak conditions of 850ºC and 7 kbar.…”