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

Edgar Batista Medeiros Júnior,Hanna Jordt-Evangelista,Gláucia Nascimento Queiroga,Rodson Abreu Marques,Reik Degler,Bernhard Schulz

doi:10.1590/0370-44672015690139

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 ofca. 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.

Highlights

The Acaiaca Complex (AC) is located near the town of Acaiaca in the central-southeastern region of Minas Gerais state (Figure 1)
This is a consequence of the retrogressive process which accompanied the exhumation of the Acaiaca Complex that was followed by partial mylonitization and gneissification of the granulites
The Acaiaca Complex records a metamorphic event of granulite facies characterized by temperatures near to 800°C and pressure conditions between 5.0 and 9.9 kbar that generated felsic, mafic, ultramafic and aluminous granulites

Summary

Introduction

The Acaiaca Complex (AC) is located near the town of Acaiaca in the central-southeastern region of Minas Gerais state (Figure 1). The granulites of the AC are distributed along a narrow strip within the amphibolite facies gneisses of the Mantiqueira Complex. This strip (Figure 1) extends for at least 36 km in the north-south direction and has a width of about 6 km. The strip occurs in the central portion of the complex (Medeiros Júnior, 2009; Medeiros Júnior and Jordt-Evangelista, 2010). Medeiros Junior and Jordt-Evangelista (2010) described para-derived granulites with garnet + sillimanite + quartz + plagioclase + biotite ± cordierite ± potassic feldspar, and a metaultramafic granulite composed of olivine and orthopyroxene. The only age published for the AC to date is a Rb-Sr isochron of 1991 ± 42 Ma (Teixeira et al, 1987), attributed to the granulite facies metamorphism of a felsic granulite

Methods

Findings

Discussion

Conclusion