Sapphirine-quartz-orthopyroxene symplectites after cordierite in the Archaean Napier Complex, Antarctica: evidence for a counterclockwise P-T path ?
Motoyoshi, Yoichi; Hensen, Bastiaan J.
European Journal of Mineralogy Volume 1 Number 3 (1989), p. 467 - 472
published: Jul 27, 1989
manuscript accepted: Feb 22, 1989
manuscript received: Jan 31, 1989
ArtNo. ESP147050103003, Price: 29.00 €
Abstract Sapphirine-quartz-orthopyroxene symplectites in granulites from Mt. Riiser-Larsen, Enderby Land, Antarctica, have equant to rectangular shapes and have bulk composition identical to or approximating cordierite composition. It is suggested that these symplectites represent pseudomorphs after cordierite. The high Al2O3 content of orthopyroxene (up to 11.5 wt %) in the symplectites indicates that they were formed at near peak metamorphic conditions. In view of the shallow dP/dT slope for the reaction cordierite = sapphirine + quartz + orthopyroxene, it is likely that the symplectites formed in response to increasing pressure as the rocks approached their thermal maximum. If taken in conjunction with the well-established isobaric cooling path for these granulites, our observations imply a counterclockwise P-T path. This interpretation is at odds with crustal thickening (collisional) models for the formation of these granulites and rather indicates extensional tectonics, accompanied by massive heat input from mantle-derived magmas.