ORIGIN OF CLINOPYROXENITES AND MELANOCRATIC GABBROIDS IN CONCENTRICALLY ZONED ULTRAMAFIC PLUTONS

Abstract

To elucidate the origin of clinopyroxenites and melanocratic gabbros, which generally form annular rims around the dunite 'cores' of the plutons, and of their paragenetic association with the dunites, it is important to understand the origin of the silicate glasses that occur as films up to 100 mm thick among the silicate-iron oxide spherules which we isolated from the pulverized dunites of the Konderay, Inagli, and certain other plutons. They are X-ray amorphous, as indicated by the absence of any traces of X-ray diffraction in the powder pattern (other than those resulting from scattering by the air). A similar conclusion is indicated by the isotropic pattern of the material obtained on immersion. In particular, microprobe analyses of silicate glasses in the spherules from mafic rocks of the Kondera pluton indicate that these glasses differ somewhat in composition from the clinopyroxenites and melanocratic gabbros of the same pluton body and can only partially be reduced to normative components corresponding to the abovementioned rocks.