Differentiated Gabbro Intrusives

Abstract

The extrusion of large volumes of tholeiite basalt on the continents discussed in Chap. 5 has been an important phenomenon throughout most of geologic time. Remnants of Precambrian basalt plateaus still exist in scattered locations on the Precambrian shields of the major continents (e.g. the Coppermine River basalt, Northwest Territories, Canada). However, the basalt flows have been eroded in most cases, leaving only swarms of feeder dikes or large differentiated gabbroic intrusives which formed in the magma reservoirs that supported the volcanic activity at the surface. The Skaergaard Intrusion of East Greenland (Sect. 5.4.6), the Freetown Complex of Liberia (Sect. 5.9.4), and the Dufek Intrusion of Antarctica (Sect. 5.12.5) have already been mentioned. The differentiated gabbroic intrusives to be considered in this chapter range in age from Archean to Cenozoic and will be examined to detect evidence of crustal contamination of the magmas, to draw attention to mineral deposits associated with these kinds of rocks, and to understand the origin and evolution of the magmas in large crustal magma chambers.