Date of Award

Winter 1992

Project Type

Dissertation

Program or Major

Earth Sciences

Degree Name

Doctor of Philosophy

First Advisor

Henri E Gaudette

Abstract

Orthogneisses representing the basement rocks of the Araguaia belt (AB), Brazil, are exposed in the core of dome-like structures. Gneisses of trondhjemitic, tonalitic, and granodioritic (TTG) composition constitute the Colmeia Complex, while augen gneiss, of granitic composition, is termed Cantao Gneiss (CG). An Archean age is suggested for the TTG gneisses, although the available Rb-Sr ages for these rocks range between 1.8 and 2.6 Ga. A Rb-Sr age of 1.77 Ga for the CG is interpreted either as a crystallization age or a resetting age.

Geochronological and geochemical studies of the orthogneisses of five of these dome-like structures of the AB were made to investigate the age(s) of crystallization of the orthogneisses, and their geochemical evolution. This study also provides geochronological and geochemical parameters for comparison of these orthogneisses with rocks of the adjacent Amazonian Craton (AC). Finally, the age of structural development of the AB is constrained by dating syntectonic granitic rocks associated with the supracrustal sequences of this belt.

U-Pb ages in zircon and single zircon Pb-evaporation (SZPE) ages indicate that the TTG gneisses of the AB are Archean in age. SZPE data suggest a minimum age of emplacement of the igneous protoliths of the Archean orthogneisses 2.85 Ga ago. SZPE data also indicate a Middle Proterozoic age for the CG, and suggest a minimum age of emplacement of the igneous protolith 1.85 Ga ago.

Sm-Nd model ages suggest the source of the TTG parental magma was extracted from the mantle around 3.0 Ga. The Cantao Gneiss protolith was generated by partial melting of Archean rocks, however with lower 87Sr/86Sr than the Archean TTG of AB.

Geochronological and geochemical similarities between the basement orthogneisses of the AB and the rocks of the AC are suggested. This supports structural and geophysical interpretations suggesting that the supracrustal sequences of this belt were deposited on the rocks of AC.

The singe zircon ages of the syntectonic granitic bodies associated with the supracrustal rocks of the AB indicate the Brasiliano thermo-tectonic cycle (700-450 Ma) was responsible for the structural evolution of this belt.

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