Enderby land, Antarctica—an unusual Precambrian high‐grade metamorphic terrain

Abstract

The high‐grade metamorphic rocks of Enderby Land, which form part of the East Antarctic Precambrian Shield, have been subdivided into two major metamorphic complexes—the Archaean Napier Complex and the Proterozoic Rayner Complex. The Napier Complex consists predominantly of pyroxene‐quartz‐feldspar gneiss and garnet‐quartz‐feldspar gneiss, with subordinate mafic granulite, pyroxenite, and a variety of siliceous, aluminous, and ferruginous metasediments. The gneisses are intruded by several types of mafic dyke, including an abundant suite of dolerites, the Amundsen Dykes, and locally by granitic rocks and pegmatite. Much of the Rayner Complex probably represents re‐metamorphosed Napier Complex rocks, and mafic dykes occur only as metamorphosed relics. The Rayner Complex is correlated with the Proterozoic metamorphics of the MacRobertson Land coast and northern Prince Charles Mountains, for which Rb‐Sr dates of 800–1100 m.y. have been obtained. Temperatures of at least 950°C, at pressures of 8–10 kb, corresponding to intermediate‐pressure granulite facies, were reached in parts of the Napier Complex. The presence in metapelites of the rare associations sapphirine + quartz, orthopyroxene + sillimanite, and osumilite, as well as the common occurrence of calcic mesoper‐thite, is compatible with very low water pressures (less than 500 bars) during meta‐morphism. The metamorphic grade of the Rayner Complex is generally rather lower (upper amphibolite to granulite facies) than that of the Napier Complex, although high‐pressure granulites occur locally. Water pressures were higher, as evidenced by the relative abundance of migmatitic gneisses and hydrous minerals (biotite and hornblende), and by the absence of mesoperthite. Within the Napier Complex, the effects of the metamorphism which led to the formation of Rayner Complex are mostly confined to localized shear zones and areas of retrogression.