南アフリカ、サザーランド・グリーンストン帯、クレイン・レタバ鉱山産泥質片岩中のざくろ石にみられる組成累構造

The Sutherland Greenstone Belt, located near the northeastern edge of the Kaapvaal Craton in South Africa, is composed mainly of Archean metasedimentary and metavolcanic rocks of amphibolite grade. Sample of pelitic schist collected from Klein Letaba Mine contains mineral assemblage of quartz, plagioclase, garnet, biotite, anthophyllite, gedrite, kyanite, staurolite, and tourmaline. Compositional profile of euhedral garnet shows growth zoning pattern (Fe+Mg increases and Ca+Mn decreases continuously from core to rim) which was produced during prograde metamorphism. In contrast, edge of the garnet is characterized by reverse trend; Fe+Mg decreases and Ca+Mn increases from rim to edge. The garnet is considered to have suffered subsequent overgrowth or modification of composition near the edge of the crystal after peak metamorphism. The retrograde event may be associated with thrusting of the high-grade Limpopo Belt onto the low-grade Kaapvaal Craton.