The origin of Archaean gneisses in the Fyfe Hills Region, Enderby Land; field occurrence, petrography and geochemistry

Abstract

Sandiford, M. and Wilson, C.J.L., 1986. The origin of Archaean gneisses in the Fyfe Hills region, Enderby Land: field occurrence, petrography and geochemistry. Precambrian Res., 31: 37--68. Granulite and amphibolite facies gnelsses at Fyfe Hills in Enderby Land, East Antarctica, form part of an early Archaean (c. 3.8 Ga) terrain termed the Napier Complex. The gneissic sequence at Fyfe Hills is composed of both supracrustal and intrusive rocks. The oldest lithological association, the Layered Gnelssic Series, is defined by interlayered felsic and basic metavolcanics and metasediments derived from psammitic, pelitic, ferruginous and calcareous precursors. Felsic gneisses in the Layered Gneissic Series have low Y/Ce, low Ga/A1 and are low in incompatible elements and, therefore, correspond to Sheraton and Black's depleted granulites. The Layered Gneissic Series has been intruded by ultrabasic rocks and by subconcordant sheets of charnockite, enderbite and pyroxene granulite which form the Massive Gneissic Series. The 'undepleted' felsic gneiases of the Massive Gneissic Series probably derive from crustal anatexis associated with granulite facies metamorphism of the Napier Complex. Dehydration during this unusually high grade metamorphism was achieved by the removal of partial melts, rather than by 'CO2-flushing', as metamorphism occurred either during vapour-absent conditions or, if vapour was present, then the composition of the vapour phase was internally buffered. Anatectic pegmatites generated during granulite facies metamorphism differ little in chemistry from the felsic granulites from which they derive. Chemical modification accompanied rehydration during waning granulite facies metamorphism and during the development of retrograde shear zones, the latter are associated with a magmatic episode in which numerous pegmatites were intruded into the Napier Complex.