2.46Ga kalsilite and nepheline syenites from the Awsard pluton, Reguibat Rise of the West African Craton, Morocco. Generation of extremely K-rich magmas at the Archean-Proterozoic transition

摘要

The Awsard pluton in the Moroccan part of the Reguibat Rise, West African Craton, contains the oldest kalsilite-bearing rocks discovered to date, with a SHRIMP zircon U/Pb age of 2.46±0.01Ga. The pluton is composed of nepheline syenites, kalsilite syenites and minor silica-saturated syenites, all with the same primitive Sr and Nd initial isotope compositions and Nd model age that cluster around ~(87)Sr/~(86)Sr_(2.46Ga)≈0.7029, e{open}Nd_(2.46Ga)≈-1.4, and T_(DM)≈2.75Ga. Silica-saturated syenites are in fact contact fenites that grade into nepheline syenites, but the two feldspathoidal syenites are true magmatic rocks that crystallized from two coeval highly fractionated K-rich magmas with sharply different chemical compositions. Chemical, isotopic and textural evidence suggests that the two magmas originated by liquid immiscibility within an already fractionated alkaline potassic magma of asthenospheric origin that split in two phases, a nephelinitic melt rich in HFSE+REE, and a kalsilitic melt poor in HFSE+REE. The Awsard pluton, spatially associated to carbonatites and other alkaline rocks, does not mark a Late Archean fossil subduction zone but represents the first manifestation of a Paleoproterozoic alkaline province located in the Reguibat Rise, the full extent and importance of which is yet to be determined.