Chronology of the Torngat orogeny: duration and extent of crustal melting from combined U-Pb zircon and monazite geochronology and Lu-Hf and Sm-Nd garnet geochronology

English Abstract:

The Torngat Orogen is a transpressional, hot, narrow, doubly-vergent orogenic belt part of the South-Eastern Churchill Province (SECP), located between the Superior and Nain cratons in north-eastern Canada. Exhibited as a tripartite structure, the SECP is composed of an Archean-Paleoproterozoic gneissic core, the Core Zone, surrounded by two NNW-trending orogens on its eastern and western margins, namely the Torngat (1.87-1.82 Ga) and the New-Quebec (1.82-1.77 Ga) orogens. Produced by the collision of the Core Zone and the Nain Province, the deeply eroded Torngat Orogen exposes granulitic roots dominated by ortho- and paragneisses and metaplutonic rocks, and lower-grade continental margin metasediments and orthogneisses. Although previous studies highlighted three tectonometamorphic phases associated with distinct regimes (i.e. crustal thickening, transpression, and exhumation-cooling), the understanding of the prograde to retrograde evolution of the Torngat orogeny in terms of dominant processes in the middle and lower crust ‒ such as partial melting, melt extraction, crustal differentiation or crustal flow ‒ is still embryonic. In this contribution, we present preliminary results from the application of multiple geochronology methods (i.e. U-Pb, Lu-Hf and Sm-Nd) to accessory and major metamorphic minerals (i.e. zircon, monazite and garnet) combined with microstructural analysis, mineral chemistry and multi-equilibria thermobarometry. Because of the different closure temperatures of the investigated isotopic systems and since metamorphic reactions producing these minerals occur at various phases through the orogeny, the new data provide insights in the evolution of the Torngat, from the onset of thickening to anatexis to the stabilisation and relaxation of the orogen. Preliminary results suggest that crustal thickening started as early as 1887 Ma, and that granulitic/anatectic conditions might have been sustained for tens of Ma.