Metamorphism and exhumation of basement gneiss domes in the Quadrilátero Ferrífero: Two stage dome-and-keel evolution?

摘要

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 Ba??o dome has peakP–Tconditions of 5–7?kbar and 700–750?°C and a published age of ca. 2730?Ma based on U–Pb 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, givingP–Tconditions of 8–9?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 peakP–Tconditions of 7–8?kbar and 650–700?°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 peakP–Tconditions of 8–9?kbar and 650–700?°C on a clockwiseP–Tevolution. This sample has a U–Pb 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 peakP–Tconditions of 5–6?kbar and 650–700?°C on a near isobaricP–Tpath. An amphibolite dike, observed to cross-cut the felsic schist produces a zircon U–Pb age of ca. 2760?Ma. Altogether this data suggests that the samples were metamorphosed in the Archean (ca. 2775–2730?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.