Mise en évidence de cordiérite héritée des terrains traversés dans le pluton granitique des Oulad Ouaslam (Jebilet, Maroc)

Abstract

Cordierite, a quite common mineral in granitic rocks, originates from various processes; many types of crystals can be found in a given massif. The high-level Oulad Ouaslam pluton, in Morocco, displays cordierites of three different origins. Primary magmatic crystals, as well as later ones, grown in equilibrium with a water-rich fluid phase, are readily distinguishable from inherited metamorphic cordierites. The latter, which are characterized by numerous spinel (Zn-poor hercynite) inclusions, are the most interesting. They originated from the disaggregation of peraluminous enclaves, which either represent refractory assemblages or solid products of incongruent melting reactions. Thermobarometric data indicate that these rocks equilibrated at about 750 °C and 3,5 kbar (350 MPa). These values are believed to represent the conditions under which the rocks were enclaved, and of the resulting partial melting event. Peraluminous enclaves, and thus inherited cordierite, are not derived from the source region of the granite. They are partly digested pelitic xenoliths, picked up by the ascending magma. Therefore, they represent true relicts of assimilated rocks. Among the possible mechanisms of cordierite formation, contamination of the magma by aluminous rocks is a classical one. However, no clear textural evidence of the process has ever been presented in the literature. The Oulad Ouaslam pluton provides such evidence and thus confirms, at least locally, the validity of this hypothesis.