Noritic magmatism

Abstract

Over the past two decades much effort among petrologists has been devoted to the problems of magnesium-rich basic magmatic suites. In the early Precambrian these suites comprise komatiites, komatiitic basalts (previously known as basaltic komatiites), siliceous high-Mg basalts (SHMB), norites and the parental magmas of the noritic portions of major layered, Bushveld-type intrusions (’U’-type magmas). However, of these, the petrology of norites has not been given a great deal of attention. Indeed, norites are mentioned only fleetingly, or not at all, in most petrology texts. Norites are mafic rocks consisting essentially of orthopyroxene and plagioclase, with varying amounts of olivine (olivine-norite) and clinopyroxene (augite-norite, gabbronorite). Thus, they are distinctive and easily recognised. The relative paucity of petrological data for norites results principally from the fact that they occur most obviously as cumulates in large layered intrusions. However, the abundance of early orthopyroxene suggests that they are not derived simply from tholeiitic magmas, in which orthopyroxene is not a significant low pressure liquidus phase. Norites warrant a little more emphasis here than they have been afforded previously because they represent a considerable volume of mafic magmatism during the early Precambrian, and they are distributed world wide.