Petrologic modelling and geochronology of Paleoproterozoic migmatites in the Zen... A. López-Carmona скачать в хорошем качестве

Petrologic modelling and geochronology of Paleoproterozoic migmatites in the Zen... A. López-Carmona

Petrologic modelling and geochronology of Paleoproterozoic migmatites in the Zenaga inlier (Anti-Atlas, Morocco) López-Carmona A., Abati J., Gutiérrez-Alonso G. and Ennih N. Understanding the nature, physicochemical conditions and timing of metamorphic processes is of paramount importance in deciphering the Earth history and, when happening during the Precambrian, they can also shed light into the now standing hot debate on the age of the onset of plate tectonics and related processes [1, 2]. One of the best suited areas to study the Paleoproterozoic (2.25-2.00 Ga) metamorphic events is the Eburnean of the West African Craton (WAC) and its extension on the basement to the Pan-African terranes in the Anti-Atlas belt of Morocco. In this region the controversy between the Archean type hot-orogen evolution is expanded to the Paleoproterozoic, as the WAC constitutes one of the youngest cratonic provinces on Earth [3]. The Paleoproterozoic rocks studied comprise a metamorphic complex (Zenaga Complex or Zenaga Group) intruded by numerous plutonic rocks of the Tazenakht Suite, predominantly granites to granodiorites, together with dolerite dykes and sill swarms. In this work, we study the metamorphic evolution of g+bi+sill+pl+q bearing migmatite samples from the northern part of the Zenaga inlier basement using petrological modelling (pseudosections), combined with U-Pb zircon geochronology. Predicted models in the MnNCKFMASHTOZr system through P/T−X (Fe2O3; ZrO) and P−T pseudosections provides migmatisation conditions at 8.5 kbar and 750 °C. The obtained conditions are part of the cooling history of the inferred clockwise P−T−t path of the studied rocks and, according to phase diagrams, peak conditions could have been in the range of 10-14 kbar. U-Pb LA-ICP MS analyses performed on ca. 150 zircon grains (average Th/U=0.51) have provided a single age population with a narrow range of ages from 2020 to 1980 Ma. Using this dataset an estimated migmatisation age of 2076.0 ±4.6 Ma is calculated. The obtained P-T paths and time constraints allows us to propose an evolutionary model for the oldest known rocks in the Anti-Atlas Belt. The new data would presumably permit better constraints for palaeographic reconstructions, geodynamic models, and a better knowledge of the behaviour of old continental dynamics in the WAC.