Ophitic Texture and Basaltic Crystallization

Abstract

A micrometric study of several hundred thin slices of intrusive dolerites and tholeiites throws some light on the crystallization history of basaltic minerals and, in particular, on the origin of ophitic texture. Special attention is given to the phenocrysts of chilled contacts. It is found that the texture bears only a rough relationship to the mineralogy but that the more extreme phases of ophitic texture are found in olivine-dolerites with a low pyroxene/feldspar ratio. Evidence gained from chilled contacts and from fresh tholeiites shows that in tholeiitic suites both pyroxene and plagioclase started to form at a very early stage and continued to crystallize in ophitic relationship and in fairly constant proportions throughout the cooling history. In olivine-basalt providences, however, olivine or plagioclase is the first mineral to form, being followed much later by pyroxene.