The Rb/Sr radiometric ages of the Richtersveld complex and neighbouring bostonite dykes

Abstract

Abstract provided by author:

It is established that the syenites and alaskitic granite members of the Richtersveld Igneous Complex were emplaced approximately 850 million years ago. The intrusion of the porphyritic microgranite 751 ± 8 million years ago, signifies a younger and separate event, which, on account, of the one hundred million year time interval, can not be related to the formation of the Complex

It is probable that the quartz-bostonite dikes invaded the country rock approximately 655 ± 70 million years ago as a homogeneous magma. Evidence points against an alternate theory attempting to explain the observed variations of the initial Sr 87/Sr 86 ratios, by which the dikes are proposed to have been emplaced in the form of ash-flows rather than as a uniform magma. On account of the one hundred million year time interval between the intrusion, of the porphyritic microgranite and the quartz-bostonite dikes, and on account of the two hundred million year interval between the emplacement of the syenite and alaskite and the quartz-bostonite dikes, the latter also can not be regarded as part of the Complex. However, a mineralogical relationship between the parent Richtersveld magma, the porphyritic microgranite and the quartz-bostonite dikes has been recorded by Middlemost and others

The Nama System was deposited under shallow water conditions during late Precambrian times at a maximum age of approximately 650 million years and a minimum age of 550 million years. The latter age refers to a post-Nama granite stock of the Bremen pluton in South-West-Africa

An average metamorphic age of 470 million years was determined for the quartz-bostonite dikes. At this time the syenites, granites and dikes in the Richtersveld behaved as open systems with respect to rubidium and/or strontium. This 470 million year event appears to be synchronous with the Nama folding and is also contemporaneous with a wide-spread period of igneous activity in the western regions of southern Africa. This folding episode caused medium grade metamorphism of the dike rocks and resulted in Sr 87* migration in all samples investigated. Whether the adamellitic gneiss was affected by this event is not known with certainty

In the Appendix a detailed mineralogical study of most of the rock samples is given, as well as a brief explanation of the theory and background underlying the equipment and methods employed