Metamorphic P-T-t evolution of ultrahigh-temperature granulite facies metapelites from the Lundazi terrane, Irumide Belt, Eastern Zambia

Abstract

The Lundazi terrane is part of the Mesoproterozoic Irumide Belt of Central Africa which together with other terranes of similar ages are interpreted to record collisional events relation to the amalgamation of the Rodinia Supercontinent. Pelitic granulites from the area preserve evidence of ultrahigh-temperature metamorphism and partial melting with complex textures involving garnet + sillimanite + spinel + quartz ± cordierite ± orthopyroxene, rare corundum, and ternary feldspars. Garnet porphyroblasts preserve inclusions of spinel + quartz and sillimanite + feldspar which indicate high temperatures were reached during the metamorphic peak. The metamorphic evolution of these rocks has been investigated using phase equilibria modelling and complemented by ternary feldspar thermometry and garnet-orthopyroxene thermobarometry which indicate peak conditions of up to ∼900–1000 °C and 6–9.5 kbar. The preservation of peak assemblages and textural features of these rocks suggest substantial melt loss during their prograde evolution. Melt-reintegrated pseudosections indicate isobaric heating prior to exhumation and subsequent cooling. Monazite EPMA dating identifies the thermal peak occurred at 1056–1028 ± 8 Ma, followed by retrograde recrystallization at 1003 ± 8–975 ± 7 Ma. The high-grade metamorphism is interpreted in terms of clockwise P-T-t paths with retrograde recrystallization marked by the partial resorption of garnet rims to form plagioclase, the growth of late cordierite around garnet and sillimanite, and alteration of monazite. The absence of a Neoproterozoic overprint in the area indicates the Lundazi terrane is part of the Irumide Belt s.s rather than the Southern Irumide Belt, and a revision of the boundary between the two regions is proposed.