Exsolution features in pyroxene phenocrysts from an anorthosite massif in northern Kerala, South India

Peculiar exsolution features occur in pyroxene phenocrysts from the Perinthatta anorthosite massif in northern Kerala, South India. Optical microscopy, combined with color imaging methods using energy dispersive X-ray studies of both clinopyroxene and orthopyroxene phenocrysts, indicate that they show exsolution intergrowth of plagioclase together with Fe-Ti oxides. Substantial amounts of exsolved orthopyroxene in clinopyroxene and clinopyroxene in orthopyroxene were also observed. The occurrence of exsolution (rather inversion) features like inverted pigeonite, herring-bone type of exsolution, and diallage were noticed in these pyroxene phenocrysts. Oxide inclusions, usually found as separate grains scattered through the host pyroxene, show prominent occurrence as small arrays of particles exsolved in orientations that are semi-coherent with the pyroxene. Biotite occurs with pyroxene phenocrysts, locally as an integral portion of exsolution lamellae, as a probable exsolution phase (?). The widths of exsolution lamellae vary from micrometer to nanometer scale, with the lamellae of the same generation having nearly the same width. All phases in lamellae are crystallographically oriented in the pyroxene hosts and are essentially those of the planes of best fit at nucleation. The various silicate exsolution features are consistent with slow cooling of the pyroxene, resulting in coherent exsolution by heterogeneous nucleation and growth of the precipitate, with the oxide particles interpreted as the homogeneously nucleated analogue of the first. The association of oxide inclusions suggests that oxidizing conditions played an important role in controlling the properties and compositions of these phenocrysts. The various exsolution features documented here are indicative of a high initial crystallization temperature and also a high degree of cooling history for the anorthosite mass.