Gabbroic and ultramafic nodules: High level intracrustal nodular occurrences in alkalic basalts and associated volcanics from Kenya, described and compared with those of Hawaii

Abstract

Nodules in alkalic olivine basalt and associated volcanic rocks from Kenya include gabbroic, dioritic, syenitic and eclogitic material. Nodules of ijolitic and uncompahgritic material are found in the nephelinite volcanics. All but the eclogitic nodules of the Chyulu Hills are considered to be of high level, intracrustal provenance: the eclogitic nodules described by Saggerson may be of upper mantle provenance. These occurrences are compared with gabbroic and peridotite/dunite nodule occurrences of Hualalai and Mauna Kea, Hawaii, which are probably largely of intracrustal cumulate derivation. The mechanism of gas-coring that is believed to be responsible for the phenomenon of nodular inclusions is tentatively related to an immiscible carbonate fraction, separated in alkalic and alkaline magmas, but not in tholeiitic magmas. A brief discussion of the whole spectrum of nodular occurrences, with special reference to their level of provenance, concludes this account.