Episodic granitoid emplacement in the Archaean Amalia–Kraaipan terrane, South Africa: confirmation from single zircon U–Pb geochronology

Abstract

The Amalia–Kraaipan granite–greenstone terrane, located in the western part of the Kaapvaal Craton, South Africa consists of metamorphosed mafic volcanic rocks and interlayered ferruginous and siliceous metasediments (mainly banded iron formations), intruded by a variety of granitoid rocks comprising tonalitic and trondhjemitic gneisses, granodiorites and adamellites. This study presents new single zircon dating for several granitoid rocks in order to define the chronology of magmatic events influencing this terrane. New TIMS and SHRIMP U–Pb data demonstrate episodic granitoid emplacement events, which occurred over a time-span of approximately 250 Ma. The oldest granitoid rocks so far recognized are dated at ca. 3008 Ma and are represented by biotite–trondhjemite gneisses located southwest of the town of Amalia. Homogeneous leuco-trondhjemite dykes intruding these gneisses yield an age of ca. 2940 Ma. Two granodiorite samples from the central part of the Amalia–Kraaipan terrane were dated at 2913±17 and 2915±12 Ma, respectively, while a further two granodiorite samples from the central and northern parts of the study area yielded ages of 2879±9 and 2879±11 Ma. The youngest granitoid body, the Mosita adamellite, which is exposed in the northwest sector of the Amalia–Kraaipan terrane, was dated at 2791±8 Ma. Since the 3008 Ma biotite–trondhjemite gneisses contain conformably interlayered amphibolite xenoliths, at least some of the Kraaipan Group volcano-sedimentary rocks are probably older than 3008 Ma. Based on available age data the 3010–2920 Ma Amalia–Kraaipan granitoids described in this paper could represent a possible source for some of the Witwatersrand sediments 100 km to the east and their contained gold. The ca. 2791 Ma age obtained for the Mosita adamellite permits a possible genetic link between this granitoid body and the ca. 2781 Ma Gaborone Granite Complex exposed approximately 120 km north in neighbouring Botswana.