Age and tectonic significance of the Banana Beach Gneiss, KwaZulu-Natal South Coast, South Africa

摘要

A new ion probe U-Pb zircon date of 1065 +/- 10 Ma has been obtained on the Banana Beach gneiss, Margate terrane, Natal sector of the Namaqua-Natal Metamorphic Province. The date is considered to represent a magmatic crystallization age. Zircon fin-is have been dated at 1021 24 Ma, reflecting the regional high-grade metamorphic event. The magmatic date is younger than anticipated, as the gneiss was previously thought to be related to early arc-related magmatism during the early development of the Margate terrane, equivalent to the oldest similar to 1.24 to similar to 1.21 Ga arc-related intrusive and supracrustal rocks of the Mzumbe terrane to the north. The new date has rendered this interpretation untenable, as it shows that the gneiss is approximately coeval with granitoids and charnockites of the Oribi Gorge Suite, which have a within-plate signature. Published whole-rock geochemical analyses from the Banana Beach gneiss have been revisited and shown to have a mixed geochemical signature with respect to some elements. Thus the Banana Beach gneiss is now reinterpreted as a syn- to late-tectonic intrusion, possibly related to the within- plate Munster Metabasite Suite of the southern part of the Margate terrane. The differing geochemical signatures of the Banana Beach gneiss, Munster and Oribi Gorge Suites probably reflect different proportions of mantle versus crustal material derived from the thick lithosphere formed during the accretion of the province.