Identification of ancient mantle plumes

Abstract

Ancient mantle plumes can be recognized in the geologic record from uplift prior to volcanism, the orientation of dikes that feed the volcanism, the physical characteristics of the volcanism, the presence of definite age progressions along volcanic chains, and the chemistry of the magmas that mantle plumes produce. The thermal anomaly introduced by the arrival of a plume head beneath continental crust leads to widespread uplift prior to volcanism, which is followed by gradual subsidence as the mantle thermal anomaly decays over a period of ∼1 b.y. The domal uplift associated with emplacement of a plume head leads to extension around the circumference of the dome and to the formation of radial dike swarms, provided there is no external force acting on the region. If there is a pre-existing external force, the dikes will tend to partially reorient perpendicular to that force to produce a pattern of curved divergent dikes. The physical volcanology of plume-derived basalts is characterized by massive flows that can be correlated over large distances. They contrast with island-arc basalts, which dominantly form pyroclastic deposits that are difficult to correlate over large distances, and with mid-oceanic-ridge basalts, which are found in the ancient record only in ophiolite sequences. Chains of volcanoes, which are produced by plume tailrelated melts, show systematic age progressions that are diagnostic of mantle plumes. The chemistry of plume-derived magmas reflects their origin in anomalously hot zones in the mantle. High-temperature magmas, such as picrites and komatiites, and high-pressure magmas, such as alkali basalts, nephelinites, and type-1 kimberlites, are characteristic of plumes.