The significance of prograde and retrograde quartz-bearing intergrowth microstructures in partially melted granulite-facies rocks

Pelitic and semi-pelitic granulites from Namaqualand, South Africa underwent dehydration melting and developed leucosome domains in which large garnets are intimately associated with coarse-grained quartz in lobate embayments and inclusions. Local retrograde replacement of leucosome anhydrous phases by coarse biotite and/or sillimanite is not uncommon, but is far from universally developed. Also developed are distinctive skeletal or symplectic intergrowths of quartz with biotite, sillimanite or, rarely, garnet, which partly replace feldspar and cordierite. Mass balance calculations indicate an approximate but not exact conservation of Si and Al in the quartz-bearing intergrowths. Microstructural criteria and kinetic arguments are used to infer that the coarse garnet–quartz association develops during prograde dehydration melting because of the relatively low mobility of Al and Si compared with reaction rate, and that direct retrograde reaction with melt produced coarse radiating clusters of biotite and sillimanite. The skeletal and symplectic quartz-bearing intergrowths post-date the coarse clusters, and, although they probably represent the final stage of back-reaction, are solid-state replacements, which involved neither melt nor aqueous fluid as a direct local participant. These microstructures can be distinguished from more general retrogression, which forms polycrystalline pseudomorphs. It follows from the limited development of retrograde products that the water transferred into silicate melt during dehydration melting largely succeeds in leaving the local systems, and that migmatisation functions as an effective mechanism for dehydrating high grade terrains.