FIELD RELATIONS AND PETROGRAPHY OF THE VREDEFORT GRANOPHYRE

A unique melt rock, the Vredefort Granophyre, occurs in the Vredefort structure, South Africa, as a series of vertical dykes either extendin g northwest- southeast and northeast- southwest within the granitic co re or straddling along the contact between the Proterozoic collar and the core of Archaean gneisses. A field study of the Vredefort Granophy re was completed for all exposed dykes. The observations demonstrate t hat there are textural and dimensional distinctions between the Granop hyre dykes occurring in the core and those at the core-collar boundary of the Vredefort structure. Four major textural types are observed: ( 1) very fine-grained spherulitic granophyre confined to narrow zones c lose to the centre of some core dykes (referred to as spherulitic A gr anophyre), (2) fine- to medium-grained spherulitic granophyre, with 1 to 4 cm diameter spherules (spherulitic B granophyre), (3) fine-graine d spherulitic granophyre, with spherules less than 2 cm in diameter, b ut characterized by a micro-ophitic texture (spherulitic C granophyre) , and (4) fine- to medium-grained granular granophyre dominated by hyp idiomorphic textures and predominantly occurring in core-collar dykes (granular granophyre). The major mineral phases of all textural types are hypersthene, plagioclase, orthoclase, quartz, biotite, magnetite, and ilmenite. In addition, augite and pigeonite are also major mineral phases in the granular granophyre. The Granophyre samples studied sho w a generally higher modal abundance of biotite and opaque grains in c ore dykes, a higher modal abundance of orthopyroxene in relation to cl inopyroxene in the core dykes and within the extreme eastern and weste rn part of core-collar dykes, and a higher or equal modal abundance of clinopyroxene in relation to orthopyroxene in the core-collar dykes. The mineral and rock textures observed indicate moderate to high super cooling, with a lower degree of supercooling for spherulitic A and B g ranophyres compared to spherulitic C and granular granophyres. Abundan t clasts, consisting of fragments of the major country rocks, are foun d in the Vredefort Granophyre. Small clasts (< 2 cm in diameter) have virtually the same distribution in all dykes and vary in abundance bet ween 10 and 20 vol.%. Large clasts (2 - 80 cm in diameter) are less ad undant than small clasts and have a larger volume content in core dyke s compared to core-collar dykes. These clasts show recrystallization, and to explain their distribution it is necessary to infer that the Vr edefort Granophyre melt intruded downward. The dyke size was constrain ed by the fracture system available at the time of emplacement. It is concluded that the Vredefort Granophyre represents the remainder of an impact melt sheet, from which material filled fractures produced duri ng the formation of the Vredefort structure by meteoritic impact at ab out 2 Ga ago.