Robert Duncan-Enzmann
Avco Research and Development
Published in the Fourth Western National Meeting Program, 1964, Transactions, AGU 1964
Orbicular Bodies of Namaqualand, Southwest Africa
A belt of orbicular textured granites, syenites, diorites, charnockites, and anorthositic copper deposits in Namaqualand (29031’S to 29050’S latitude and 17020’ to 18010’E long) with the granite-like rock, paragneisses, metavolcanic, metasedimentary, and sedimentary extensions of the system (northward in S. W. Africa southward about van Rhynsdorp, westward about Prieska, and westward under the Atlantic Ocean) are known as the Namaqualand batholith or granite-gneiss massif.
This complex is a greatly altered Geosynclinal system approximately 2250 million years old. Orbicular-textured and non-orbicular schists, charnockites, and anorthosites from only a small fraction of the total volume of the system, however, they show vast extents along the strike. Orbicular textures are only, though not always, found when the following conditions are fulfilled: 1 – stratigraphic restriction to schists and calcic-granulites; 2 – structural restriction to rick volumes on the north flanks and toward the crests of discordant, singly or doubly-plunging folds; metamorphic restriction to physical conditions of the granulite facies. The several hundred orbicular bodies examined have gross shapes which may be placed in a sequence starting with 1) stratigraphically layered orbs, which are essentially flat-lying and concordant, and which seem to alter to 2) comma-shaped bodies essentially discordant though influenced by bedding planes. The series terminates in 3) migmatic diapirs, shaped like exclamation marks and completely transgressive. Individual orbicules display characteristic shapes in each type of body: 1) schistose, agmatitic clots in layered bodies, 2) migmatic ptygmatically folded orbs in comma-shaped bodies, 3) magmatic and near magmatic anatectically schliered orbs in the migmatic diapirs.
Single orbs vary greatly in size, shape, and complexity, both as a function of the above generalizations and of position in the gross orbicular-textured bodies. The Kheis geosyncline, or Namaqualand granite-gneiss massif, is described as a sequence of three cyclothemes each of which contains five metasediments: white gneiss, granulite, silexite-quartzite, schists, and calcic granulites. These three cyclothemes are capped by a thick suite of gray gneisses, and the gray gneisses by a metavolcanic suite. The association of orbicular textured bodies with minor overfolds in the vastly thicker white gneisses of the three cyclothemes and the relationship of these large folds in the white gneisses to major geosynclinal events are outlined.
