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.

Abstract

The Mantiqueira Complex together with the Mineiro Belt and Juiz de Fora Complex, characterize a long-lived system of oceanic and continental magmatic arcs. The proposed accretion of these terranes in the Rhyacian resulted in the closure of the Minas Basin, a fundamental turning point for the geology of this area. Understanding this event is critical for determining the evolution of the South American continent. Despite its importance, there is a paucity of zircon U-Pb and Lu-Hf data available for the Mantiqueira Complex and no data from two complexes within the later.

U-Pb zircon data were obtained from interpreted metasediments of those two complexes, the granulite facies Acaiaca Complex and Pedra Dourada Granulite. Four targeted samples yield cores with ages between 3.5 and 2.2 Ga and a large proportion of metamorphic grains and rims at ca. 2.1–2.0 Ga. The similarity between the detrital zircon signatures of the Acaiaca Complex and Pedra Dourada Granulite, and the Archaean inheritance of the Mantiqueira Complex, indicates that these three units are related. Potentially the Acaiaca Complex and Pedra Dourada Granulite either sourced the Mantiqueira Complex or the Mantiqueira intrusive suites reworked the protoliths of the Acaiaca Complex and Pedra Dourada Granulite.

Hf-in-zircon data indicate two discrete sources for both the Acaiaca Complex and Pedra Dourada Granulite. An Archaean source (model ages of 3.2–2.6 Ga), which is consistent with the suggested origin of the Mantiqueira Complex as recycled Archaean crust sourced from the São Francisco Craton and reworked during the ca. 2.1 Ga Transamazonian orogeny. A second more juvenile source (model ages of 2.4–2.2 Ga) recorded exclusively within the more mafic samples is identical in age and Hf to that found in the Juiz de Fora Complex. This could indicate a close link between the Mantiqueira and Juiz de Fora Complexes in the period 2.2 to 2.0 Ga. Alternatively, these juvenile zircons may represent an as yet unidentified juvenile portion of the Mantiqueira arc.

The results of this study are consistent with the prevailing theories for formation of the Mantiqueira Complex as a continental arc with cumulative proportion curves indicating an active-margin type depositional setting.