EM - Escola de Minas

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Metamorphism and exhumation of basement gneiss domes in the Quadrilátero Ferrífero : two stage dome-and-keel evolution?
(2019) Cutts, Kathryn Ann; Lana, Cristiano de Carvalho; Alkmim, Fernando Flecha de; Farina, Federico; Moreira, Hugo Souza; Coelho, Viviane Viana
The presence of dome-and-keel provinces in Archean cratons has been connected with the initiation of plate tectonics on Earth as these features are most commonly observed in Archean rocks. The Quadrilátero Ferrífero in Brazil has been identified as a Paleoproterozoic dome-and-keel province for more than three decades. The prevailing model suggests that it formed during the Rhyacian Transamazonian orogeny, making it unique among dome-and-keel provinces. However, a lack of appropriate lithologies, datable minerals and the metamorphic overprint of later orogenesis has resulted in a cryptic metamorphic record for the formation of this dome-and-keel province. A clinopyroxene-bearing migmatite from the core of the Bação dome has peak PeT conditions of 5e7 kbar and 700e750 C and a published age of ca. 2730 Ma based on UePb ages of zircon from leucosomes, suggesting that this age represents the migmatisation event. A fine-grained epidote-albite-titanite assemblage overprints the coarse-grained clinopyroxene and amphibole, giving PeT conditions of 8e9 kbar and 550 C with an associated titanite age of ca. 2050 Ma. A garnet-bearing amphibolite sample also from the core of the dome has peak PeT conditions of 7e8 kbar and 650e700 C, and texturally late titanite from this sample produces an age of ca. 2060 Ma. Three additional samples were collected from the edges of the dome. A garnet-gedrite bearing felsic schist produces peak PeT conditions of 8e9 kbar and 650e700 C on a clockwise PeT evolution. This sample has a UePb zircon age of ca. 2775 Ma, which could date metamorphism or be the age of its volcaniclastic protolith. Texturally unconstrained titanite from the sample gives an age of ca. 2040 Ma. A garnet-bearing amphibolite that occurs as a boudin within the felsic schist gives both zircon and titanite ages of ca. 2050 Ma and has peak PeT conditions of 5e6 kbar and 650e700 C on a near isobaric PeT path. An amphibolite dike, observed to cross-cut the felsic schist produces a zircon UePb age of ca. 2760 Ma. Altogether this data suggests that the samples were metamorphosed in the Archean (ca. 2775e2730 Ma) and again during the Transamazonian event. The most plausible explanation for this data is that dome-and-keel formation occurred in the Archean with migmatisation and high-temperature metamorphism occurring at this time. The Paleoproterozoic event is interpreted as a reactivation of the dome-and-keel formation structures, with Paleoproterozoic keels crosscutting Archean keels and producing metamorphic aureoles. The high radiogenic heat production and the presence of dense sedimentary successions in Archean terranes make dome-and-keel provinces a uniquely Archean feature, but they are susceptible to reworking, resulting in an enigmatic record of formation.
Metamorphic imprints on units of the southern Araçuaí belt, SE Brazil : the history of superimposed Transamazonian and Brasiliano orogenesis.
(2018) Cutts, Kathryn Ann; Lana, Cristiano de Carvalho; Alkmim, Fernando Flecha de; Peres, Guilherme Gravina
The extensive Neoproterozoic Brasiliano belts that surround the São Francisco craton in southern Brazil are a consequence of tectonic processes that resulted in the formation of the supercontinent Gondwana in the Ediacaran to Cambrian. One of these Brasiliano-age orogens, the Araçuaí belt, overprints an earlier orogen developed during the so-called Transamazonian event at the transition between the Rhyacian and Orosirian periods. Archean and Paleoproterozoic basement units in the southern Araçuaí belt have been shown to have experienced both the Transamazonian and Brasiliano orogenies and have the potential to have preserved the P-T conditions of both events. This study investigates a series of granulite- and amphibolite-facies samples from the southern Araçuaí belt. Granulite-facies samples from the Acaiaca Complex and Pedra Dourada Granulite give monazite U-Pb ages in the range: 2015–1960 Ma. Both the granulite samples preserve two metamorphic textures. Coarse-grained garnet + biotite + orthopyroxene + K-feldspar + plagioclase + quartz + ilmenite + inferred melt and later, fine-grained garnet + biotite + hornblende + quartz that grows at the expense of orthopyroxene. Peak conditions for the earlier event were c. 750–800 °C and 6 kbar, whereas those of the later event are poorly constrained. A garnet-bearing amphibolite-facies sample from the Mantiqueira Complex contains monazite included within garnet, which gives U-Pb ages of c. 1940 Ma, whereas matrix monazite grains give ages of c. 590–540 Ma. Garnet core compositional zonation indicates P-T conditions of 6 kbar and 700 °C for the c. 1940 Ma metamorphism, while the matrix assemblage records a P-T path culminating in peak conditions of 8–9 kbar at 650 °C. A second amphibolite-facies sample was obtained from the Dom Silvério Group, whose depositional age post-dates the Transamazonian event. This sample gives two monazite U-Pb age populations, 597 ± 3 Ma and 547 ± 5 Ma. Garnet core zonation indicates a prograde P-T evolution with peak conditions of c. 8–9 kbar and 650 °C, although it is uncertain at this time what age population this event corresponds to. The results of this study provide better constraints on the chronology and metamorphic regimes of both the Transamazonian and Brasiliano events in the southern Araçuaí belt.

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