Electron microprobe Th-U-Pb monazite dating and metamorphic evolution of the Acaiaca Granulite Complex, Minas Gerais, Brazil

Abstract

The Acaiaca Complex (AC) is located in southeastern Minas Gerais state, and comprises felsic, mafic, ultramafic, and aluminous granulites as well as lower grade gneisses and mylonites. The complex is distributed over an area of ca. 36 km by 6 km, surrounded by amphibolite facies gneisses of the Mantiqueira Complex (MC). The discrepancy in the metamorphic grade between both complexes led to the present study aiming to understand the metamorphic history of the AC by means of geothermobarometric calculations and electron microprobe Th-U-Pb monazite dating. Estimates of the metamorphic conditions of the granulites based on conventional geothermobarometry and THERMOCALC resulted in temperatures around 800 ºC and pressures between of 5.0 and 9.9 kbar and a retrometamorphic path characterized by near-isobaric cooling. Part of the granulites was affected by anatexis. The melting of felsic granulites resulted in the generation of pegmatites and two aluminous lithotypes. These are: i) garnet-sillimanite granulite with euhedral plagioclase and cordierite that show straight faces against quartz, and is the crystallization product of an anatectic melt, and ii) garnet-kyanite-cordierite granulite, which is probably the restite of anatexis, as indicated by textures and high magnesium contents. Th-U-Pb monazite geochronology of two granulite samples resulted in a metamorphic age around 2060 Ma, which is similar to the age of the MC registered in the literature. The similar Paleoproterozoic metamorphic ages of both complexes lead to the conclusion that the Acaiaca Complex may be the high grade metamorphic unit geochronologically related to the lower grade Mantiqueira Complex.

Zircon U-Pb and Lu-Hf record from high-grade complexes within the Mantiqueira Complex: First evidence of juvenile crustal input at 2.4–2.2 Ga and implications for the Palaeoproterozoic evolution of the São Francisco Craton

“…The rocks are highly deformed with mafic, felsic and aluminous granulites frequently occurring interbanded on a cm to mm scale. The mafic and felsic granulites have previously been interpreted as orthogneisses, with the aluminous granulites being interpreted as metasediments (Medeiros Júnior et al, 2016). The AC occurs as a NS-trending body in the hanging wall of a shallow dipping and W-verging thrust affecting the Mantiqueira Complex.…”

“…2060 Ma for estimated peak-metamorphic P-T conditions of ca. 800 °C and 5-10 kbar (Medeiros Júnior, 2016;Medeiros Júnior et al, 2016).…”

“…2015 Ma (Cutts et al, 2018) or ca. 2060Ma (Medeiros Júnior, 2016Medeiros Júnior et al, 2016). The Mantiqueira Complex is also suggested to have experienced a metamorphic event at ca.…”

“…These units both consist of granulite facies meta-igneous and metasedimentary rocks and both outcrop within the amphibolite facies, exclusively orthogneiss Mantiqueira Complex. However, the two complexes have been mapped as different units due to their higher metamorphic grade and the presence of aluminous rock types (Marinho et al, 2016;Medeiros Júnior et al, 2016).…”

“…The Mantiqueira Complex has been investigated using zircon U-Pb and whole-rock Sm-Nd (Noce et al, 2007), and more recently, zircon U-Pb combined with Lu-Hf (Kuribara et al, 2019). The Acaiaca complex has only been the target of metamorphic studies (Cutts et al, 2018;Medeiros Júnior et al, 2016), whereas some minor detrital zircon work has been conducted on the Pedra Dourada Granulite (Marinho, 2014). Both the Mineiro Belt and Juiz de Fora Complex have been the target of combined zircon U-Pb and Lu-Hf studies (MB -Moreira et al, 2018;JFC -Degler et al, 2018;Kuribara et al, 2019).…”