Ion microprobe (SHRIMP) dates complex granulite from Santa Catarina, southern Brazil

Léo A Hartmann,João O S Santos,Luiz C Dasilva,Neal J Mcnaughton,Marcos A Z Vasconcellos

doi:10.1590/s0001-37652000000400007

Abstract

Complex polymetamorphic granulites have been dated in the Santa Catarina granulite complex of southern Brazil through SHRIMP study of zircon. This complex is dominated by intermediate-acid plutonic rocks and contains small volumes of mafic and ultramafic rocks, and minor quartzite and banded iron formation. Porphyroblasts of orthopyroxene, clinopyroxene and plagioclase in mafic and acid rocks are interpreted as magmatic remnants in a volumetrically dominant granoblastic aggregate (M1) of the same minerals and hornblende. Hornblende formed during a later M2 metamorphic event constitutes rims around pyroxene, but the hornblende is also rimmed by granoblastic simplectites of orthopyroxene, clinopyroxene, hornblende and plagioclase in a second granulite facies event (M3). Chlorite and epidote occur in shear zones (M4). This granulite terrain is part of a Neoproterozoic craton, because it was little affected by the Brasiliano Cycle. The two granulite-facies events (M1 and M3) are dated by U/Pb zircon SHRIMP at about 2.68 and 2.17 Ga, while the magmatic protoliths formed at about 2.72 Ga. The amphibolite facies event (M2) probably occurred close to the 2.17 Ga granulitic metamorphism.

Highlights

Precise dating of the initial magmatic crystallization and subsequent metamorphism of granulite facies rocks is a difficult task, because the intense deformation and extensive geochemical reorganization of such rocks tend to partially reset and blurr isotopic
The complex internal structures of granulite facies zircon crystals in the Santa Catarina granulite complex of southern Brazil provide an opportunity to test the capability of these techniques to unravel the evolution of continental crust, in a region composed of complex granulites
The age of the metamorphic events can only be estimated, because of the small number of sputtered sensitive high-mass resolution ion microprobe (SHRIMP) analytical spots.The youngest age (2168 ± 18 Ma) is considered the time of M3 high-grade granulite facies metamorphism, because it was obtained on rims of zircon crystals

Summary

Introduction

Precise dating of the initial magmatic crystallization and subsequent metamorphism of granulite facies rocks is a difficult task, because the intense deformation and extensive geochemical reorganization of such rocks tend to partially reset and blurr isotopicLÉO A. The second generation is metamorphic (M1), and corresponds to the strong granulite facies event observed in the complex. The age of the metamorphic events can only be estimated, because of the small number of sputtered SHRIMP analytical spots.The youngest age (2168 ± 18 Ma) is considered the time of M3 high-grade granulite facies metamorphism, because it was obtained on rims of zircon crystals.

Results

Conclusion