Regional and detailed field and geochemical studies of Archean trondhjemitic gneisses, migmatites and greenstone xenoliths in the southern part of the Barberton Mountain Land, South Africa

Abstract

Abstract A wide variety of Archean granites and greenstones occupy the extensive tract of country southwest of the Barberton Greenstone Belt. The greenstones, which occur as xenoliths wedged between numerous trondhjemitic gneiss diapirs, contain lithologies that are indisputably equatable with assemblages found in the lower formations of the Onverwacht Group. Intrusive into this environment are a complex suite of potash-rich granites of batholitic dimensions, as well as several smaller granitic and syenitic bodies. Mafic dykes of various ages also transect the region. Field investigations show that complex areas of migmatite in the area are not randomly distributed but are intimately linked with the greenstone remnants and are probably the result of granite—greenstone interaction. It can furthermore be demonstrated on a variety of scales that, in certain places, lit-par-lit intrusion of the greenstones by the granites has taken place. The mapping, in detail, of selected platform exposures has led to the recognition of a variety of granitic phases hitherto unrecognized in the district. At one locality two phases of trondhjemitic gneiss are recorded and the initial observations suggest that the earlier trondhjemite gneiss may predate the greenstone xenolith with which it is juxtaposed. A second exposure shows trondhjemite intrusive into an older felsic migmatite—gneiss unit which, it is suggested, may represent the migmatized equivalent of an acid tuffaceous rock unit from a typical greenstone assemblage. This paper reports some of the findings to date and draws attention to some of the anomalies that exist in the region. Work still in progress should provide a greater understanding of the interactive processes between granites and greenstone and will result in a much refined model for the nature and development of the Archean crust in the Barberton region.