The geology of the acid phase of the Bushveld complex, north of Pretoria: a geochemical/statistical approach

[摘要] A petrographical and geochemical study of the acid phase of the BushveldComplex, north of Pretoria, was undertaken with the aim to identify thedifferent rock units, to determine their interrelationships and to classifythe rocks as well as describing their geochemistry.The oldest geological formation in the area is the Rooiberg Group which issubdivided into two units, namely the Kwaggasnek (lower) and Schrikkloof(upper) Formations. Petrographical, mineralogical and geochemical dataare submitted for the different units. From the data it can be deducedthat these units formed as products of a single parental magma, while statisticalmanipulation of the geochemical data indicates that these formationsdiffer significantly from the Damwal Formation farther to the east. Thegradational contact relationships between the felsites and underlying granophyreare described and explained in the text.The various granophyre occurrences of .the Rashoop Granophyre Suite areclassifiedand described. The mineralogical, petrographical and geochemicaldata indicate a limited differentiation trend from the felsites into the granophyre.This may indicate that the granophyre in part resulted from. the rapid crystallization of the parental magma of the Rooiberg Group.A model for the origin and formation of the Rashoop Granophyre Suite,based on petrographical and-qeochernical evidence, is proposed.The granites are subdivided according to age and field relationships, as wellas mineralogical, petrographical and geochemical characteristics into theSekhukhuni, Verena, Makhutso, Klipvoor and Klipkloof granites. The modeof intrusion as well as the mineralogical, petrological and geochemical compositionof each type are discussed. A petrochemical investigation of thegranites indicate that the various granites, with the exception of the Klipkloofgranite, represent the differentiation products of a single parentalmagma. A similar study on the Makhutso and.Verena granites indicates thatleptite assimilation influenced the final differentiation trend in these granites,causing enrichment in certain elements and depletion in others.

[发布机构] University of the Free State