megaphenocrysts

Megaphenocrysts are unusually large phenocrysts found in some igneous rocks. They are crystals that began to crystallize from a magma and are conspicuously larger than the surrounding groundmass and the average phenocryst population, producing a distinct porphyritic texture. The term focuses on size relative to other crystals in the same rock, rather than on the mineral type.

Megaphenocrysts occur in a variety of lithologies, including mafic and felsic systems, and can be composed of common igneous minerals such as olivine, pyroxene, feldspars, and quartz. Their sizes can range from a few millimeters to several centimeters, making them visually prominent in hand samples and thin sections.

Formation and interpretation: the presence of megaphenocrysts indicates episodes of relatively slow cooling or localized pockets of slower crystallization within the evolving magma. Such conditions allow certain crystals to grow far larger than the surrounding crystals before the magma fully solidifies. The study of megaphenocrysts can provide insights into the thermal history, crystallization sequence, and potential magma mixing or crustal residence times that affected the rock’s formation.

Notes: the term megaphenocryst describes size relations within the rock’s texture and does not by itself specify the crystal’s origin. In most cases, megaphenocrysts are considered prominent phenocrysts, but extraordinarily large crystals could in some instances be xenocrytic, reflecting incorporation of pre-existing crystals from surrounding rocks.