Petrography, geochemistry and geochronology of the potassic granitoids of the Rio Itacambiruçu Supersuite: implications for the Meso- to Neoarchean evolution of the Itacambira-Monte Azul block

Samuel Moreira Bersan,Cristiano Lana,Francisco Robério De Abreu,André Danderfer Filho

doi:10.1590/2317-4889201820170131

Samuel Moreira Bersan, Cristiano Lana + Show 2 more

Open Access

https://doi.org/10.1590/2317-4889201820170131

Copy DOI

Abstract

ABSTRACT: In the Itacambira-Monte Azul block (BIMA) the Archean rocks are represented by tonalite-trondhjemite-granodiorite (TTGs) of the Porteirinha Complex and by potassic granitoids of the Rio Itacambirucu Suite that crop out at the Pedra do Urubu (PUp), Rio Gorutuba (RGp), Barrocao (BAp) and Lagoa Nova (LNp) plutons. This work presents new data for the PUp, RGp and BAp. The petrographic and lithochemical analyses allowed the subdivision of the Rio Itacambirucu Suite in two groups of granitoids: a medium potassium group, represented by the PUp and RGp, and a high potassium group, in which the BAp rocks are inserted. The geochronological data showed different ages for the medium and high potassium groups, with crystallization ages at ca. 2.92 Ga and 2.65 Ga, respectively. Despite the differences observed between these two groups, the chemical signatures of these rocks are compatible with the signature of biotite-granites generated by the reworking of ancient continental crust. Thereby, the data obtained in this work, added to some previously published results, allowed to the identification of two Archean crustal stabilization events recorded in BIMA, one of Mesoarchean age and the other of Neoarchean age. In addition, we propose the lithodemic reclassification of the Rio Itacambirucu Suite to Rio Itacambirucu Supersuite.

Highlights

A considerable portion of the present-day continental crust was built early in the evolution of the planet; it is widely accepted that approximately 50% of the crust was consolidated before 2.5 Ga (Armstrong 1981, Belousova et al 2010, Dhuime et al 2011, Guitreau et al 2012)
A considerable decrease in the modal proportions of biotite, magmatic epidote, allanite, titanite and zircon is observed between the Pedra do Urubu pluton (PUp) and Rio Gorutuba pluton (RGp) rocks, with the latter being dominated by leucocratic phases
The studied rocks can be separated into two groups based on their K2O/Na2O ratios (Tab. 1): the rocks of the Barrocão pluton (BAp) are classified in a group that is highly enriched in potassium, whereas the PUp and RGp rocks are classified in a medium-potassium group with K2O/Na2O ratios that are close to one (Fig. 6E)

Summary

Introduction

A considerable portion of the present-day continental crust was built early in the evolution of the planet; it is widely accepted that approximately 50% of the crust was consolidated before 2.5 Ga (Armstrong 1981, Belousova et al 2010, Dhuime et al 2011, Guitreau et al 2012). The dominantly sodic crust of Paleoarchean tonalite-trondhjemite-granodiorite (TTG) complexes began to exhibit a granitic and predominantly potassic signature (Martin et al 2005, Moyen et al 2003, Laurent et al 2014, Halla et al 2017) These potassic-rich rocks represent at least 20% of the total exposed Archean rocks and consist of different types of granitoids (e.g., sanukitoids, biotite and two-mica granites, peralkaline granitoids) that mark the thermal stabilization of the Archean lithosphere (Moyen et al 2003, Laurent et al 2014).

Methods

Results

Discussion

Conclusion