Sandstone petrography and geochemistry of late proterozoic sediments of the armorican massif (France) — A key to basin development during the cadomian orogeny

Abstract

Brioverian (Late Proterozoic) sediments of the Armorican Massif in Northwest France are contained within isolated basins whose development is linked to Cadomian orogenesis. These basins were formed between c. 650 and 530 Ma ago, mainly by gravitational flow processes. Petrographical study and geochemical analysis of major and trace elements show that the sandstone formations of North Brittany are characterized by great immaturity (10–20% feldspar; 10–20% lithic fragments) and high concentrations of Na 2O, Ba, Sr, V and Cr. The formations from Western and Central Brittany, as well as the southern part of Normandy, are compositionally more mature (< 15% feldspar + lithic fragments; K 2O/Na 2O > 1). Petrographical and geochemical data for North Brittany formations yield plots which suggest a mixed source region composed of a continental active margin (island arc) and ancient basement material; sedimentation of the Brioverian Group was influenced by a distensive tectonic regime. Towards the south, sedimentary formations are characterized by continental source areas and were laid down in a passive tectonic environment. However, REE distribution patterns for sediments of this southern domain preserve evidence for the contribution of magmatic arc material; such formations could have been deposited in a marginal basin.