Ultramafic and Related Rocks

Abstract

The ultramafic rocks are composed mainly of dark-colored mafic or ferro-magnesian minerals such as olivine, pyroxene, amphibole, serpentine, garnet, biotite, and opaque oxides. They are usually, but not always, ultra-basic (having less than 45 percent SiO₂). Ultramafic rocks have been comprehensively reviewed in a volume edited by Wyllie (1967), who recognizes 10 ultramafic rock associations. ⁸⁷Sr/⁸⁶Sr ratio measurements are available for the following: (1) the layered gabbro-norite-peridotite association in major intrusions, (2) the alpine-type peridotite-serpentinite association,(3) ultrmafic rocks in differentiated basic sills and in minor intrusions,(4) kimberlites, (5) ultrmafic nodules, (6) alkali ultrabasic rocks in ring complexes, and (7) ultrabasic lavas. In the following sections of this chapter we shall review the strontium isotopic data for the first five of these associations; the last two have been covered in Chapter VI.