The role of crustal contamination in magma evolution through geological time

Abstract

The temperatures of the mantle-derived magmas that have ascended through an intruded continental crust have decreased through geological time. A possible consequence of this change is that crustal contamination could have been a more significant factor in magma genesis in the Precambrian than in the modern Earth. Calculations suggest that a peridotitic komatiite magma (28% MgO) has the potential to assimilate more than three times the amount of crust that can be assimilated by basalt. The greater potential for assimilation by komatiite is a consequence of the large crystallization interval ( ∼ 400°C) and the high heat of fusion of olivine. In favourable circumstances thermal constraints allow basaltic and andesitic magmas containing more than 50% crust to be derived by an assimilation and fractional crystallization (AFC) process from a komatiite parent. AFC in komatiites can generate derivative magmas that would not be expected in the modern Earth. Substantial contamination of komatiite with average Archean upper crust or tonalite-trondjhemite gneiss can generate silica-rich Mg-rich basaltic komatiites and Mg-rich andesites. Contamination may explain why many basaltic komatiite lavas cannot be generated by olivine fractionation of the komatiites with which they are closely associated. Highly contaminated magmas crystallize in the order ol-opx-plag-cpx and are appropriate parental magmas for the ultramafic zones of large Precambrian layered intrusions such as the Bushveld and Stillwater Complexes. Whereas modern basalts are unlikely to be substantially contaminated by mafic or ultramafic rocks, komatiites are also capable of assimilating large amounts of gabbro, mafic granulite and some ultramafic rocks such as pyroxenite. If the Archean lower crust were made of such rocks AFC processes with a komatiite parental magma can generate iron-rich and alumina-rich basaltic magmas. Such magma compositions would crystallize in the order ol-plag-cpx/opx at low pressure and bear some resemblance to the magmas required to form the anorthositic parts of intrusions such as Bushveld and Stillwater. A similar process could also have had a role in generating the parental magmas to the massive Proterozoic anorthosite massifs, by assimilation of lower crust into picritic parent magmas.