Mechanism of phenocryst formation in alkaline ultrabasic and basic volcanic rocks: Evidence from Makhtesh Ramon, southern Israel

Petrographic study of the alkaline ultrabasic and basic volcanic rocks of Makhtesh Ramon raises the possibility that growth of phenocrysts occurred by aggregation of very small crystals and their reconstitution into large crystals. The process is accompanied by optical homogenization and formation of euhedral crystal boundaries. The texture of volcanic rocks studied is seriate porphyritic. The proposed mechanism of phenocryst growth was observed for pyroxene crystals. At an early stage, one sees initiation of a euhedral crystal outline enclosing numerous small crystals with random optical orientation. Subsequently this aggregate is transformed into a single phenocryst. In some cases, when transformation is incomplete (intermediate phase), a cluster of two or more crystals may be enclosed within the boundary of the phenocryst.