Wed 24th July@ noon, Rm 312.222
Abstract:
The Araçuaí orogen (SE Brazil) is one of the largest (350,000 km2) and long-lived (ca. 630 – 480 Ma) granitic province in the world. It was formed by the convergence between the Congo (Africa) and São Francisco (Brazil) cratons during the assembly of West Gondwana. Currently, two models are used to explain its origin. The first one suggests a long-lived pre-collisional subduction-related magmatism (630 – 580 Ma) followed by a collisional stage. The other model proposes a hot orogeny with a limited pre-collisional subduction. These models are mostly based on the investigation of the Galiléia batholith (15.000 km2), a weakly deformed, medium- to high-K calc-alkaline metaluminous body (0.97 < A/CNK < 1.07) characterized mostly by tonalites and granodiorites, and representing the core of this orogeny. Although these rocks have been extensively studied, the widespread coexistence of grossular-rich garnet and epidote, uncommon minerals in these type of granites, have never been investigated. A similar fate has been reserved for the geochronological and isotopic character of these rocks. In this talk I will present field, geochemical and isotopic results, these lasts obtained from garnets, zircons, titanite and apatite crystals. Garnet and epidote of magmatic origin suggest crystallization in a deep crust (≥ 25 km). Garnet crystals have δ18O values resembling a meta-igneous source and are characterized by a positive Eu/Eu* anomaly, in contrast to their granitic host. High-precision CA-ID-TIMS and LA-MC-ICPMS U-Pb zircon dating showed that for each sample, the age variability of magmatic zircon reach up to 50 Ma, spanning the entire lifetime of the batholith (ca. 630 – 580 Ma), with emplacement age at 581 ± 0.4 Ma. Together with their rather crustal εHf signature, we suggest that these granitoids were likely originated through a petrological cannibalism where each new magma batch was able to re-melt the former one up to batholith crystallization at 581 Ma. Titanites having ages of 576 ± 2 Ma suggest further 5 Myrs to cross the solidus while apatites showing different ages between the centre of the batholith (532 ± 17 Ma) and its easternmost part (510 ± 26 Ma) indicate different cooling histories. Overall, we suggest that the Galiléia batholith was assembled in the lower crust during the orogenic stages of the West Gondwana and experienced a slow cooling history (ca. 4 °C/Ma) consistent with models indicating that the Araçuaí orogen is a hot orogeny which experienced limited pre-collisional subduction.
Short bio:
Francesco Narduzzi was born in Italy in 1985. He obtained his BSc (2009) and MSc (2012) in mantle petrology at the Department of Geoscience of the University of Trieste. In February 2018, through a joint PhD agreement between the Department of Geology of the Universidade Federal de Ouro Preto, Minas Gerais, Brazil and the Department of Earth Science of the University of Stellenbosch, South Africa he obtained a double doctorate degree in Crustal Evolution and Natural Resources. Since November 2018 is a post doctorate fellow at the Instituto de Astronomia, Geofísica e Ciências Atmosféricas of the University of São Paulo, Brazil. Currently his main research regards the time constraints, redox conditions and sediment sources attending the Proterozoic rise of atmospheric oxygen in Brazil. Of his particular interest is the possible relationship between Proterozoic subaerial Large Igneous Provinces, Snowball Earth’s periods and the rise of atmospheric oxygen.
