The nature and significance of magma chamber margins in ophiolites: examples from the Norwegian Caledonides

Abstract

Large-scale intrusive contacts with associated marginal series have been encountered within Norwegian ophiolite complexes at Karmøy, Solund and Leka. The contacts limit individual magma chambers and are found at different structural levels of the plutonic suites. Examples of magma chamber margins adjacent to interlayered ultramafic and gabbroic rocks, modally-layered gabbros, high-level gabbros and sheeted dykes, are described. The nature of the intrusive boundaries and the presence of partially resorbed xenoliths in the vicinity of the intrusive margins suggest that stoping and assimilation have been important mechanisms during the development of the magma chambers. Characteristic marginal series are developed along the intrusive boundaries. The thicknesses and appearance of these series vary with depth in the complexes. Whereas the marginal series are well developed within the uppermost levels of the plutonic complexes (exhibiting rock types such as microgabbro, massive gabbro and magnetite gabbro), the marginal series observed at lower levels are thinner and also devoid of chilled facies rocks and magnetite gabbros. The marginal series may be subdivided into border and roof series. The latter are characterized by an intimate relationship with sheeted dykes, which comprise dyke swarms formed both prior to, during, and subsequent to crystallization of the roof series. Based on these relationships the dykes can be subdivided into rooted and rootless dykes. A multiple magma chamber model, with magma chambers migrating from a low to a high level within the oceanic crust, is proposed on the basis of the observed features.