The origin and emplacement of the Freetown Intrusion, Sierra Leone

Abstract

The Freetown Intrusion in Sierra Leone is a large, layered mafic intrusion. Only a small part of the intrusion appears on land to form the Freetown Peninsula, and the larger part lies beneath the Atlantic Ocean. This is demonstrated by gravity and geomagnetic measurements, which also indicate that intrusion occurred over at least 1–2 Ma. The intrusion was formed at 196–201 Ma and is, therefore, concordant with the Central Atlantic Magmatic Province (CAMP) magmatism. At this time, the region was undergoing crustal extension prior to the later opening of the Central Atlantic Ocean. Initially, the magma intruded the Pan-African 2 suture and used this as a conduit for magma ascent, interdigitating with the highly sheared and metamorphosed rocks of the suture. The regional tensile stress allowed the conduit to open further and permitted the continued ascent of magma with subsequent crystal deposition and formation of the layers. Crystallisation commenced at depth to form a crystal mush. The layering was caused by flow, with variations in the supply of the crystal mush and varying proportions of olivine, pyroxene, and plagioclase. The silicates do not show a progressive change in composition indicative of mineral fractionation in a closed chamber. Locally, the intrusion of a plagioclase-rich crystal mush led to the presence of thick anorthosite layers. The initial layering followed the dip of the suture, and the layering steepened as the conduit opened. Sr–Nd isotopic and geochemical analyses both indicate that there was a significant contribution to the intrusion from Pan-African and older host rocks. These were assimilated into the magma. Variations in this contribution led to the formation of both gabbroic and noritic layers. Fragments of the host rocks were entrained during magma ascent and are now present as xenoliths.