Petrography and geochemistry of granitoid and metamorphite pebbles from the early Archaean Moodies Group, Barberton Mountainland/South Africa

Abstract

Pebbles of potassic granitoids and metamorphites constitute up to 5% of the basal conglomerate of the Moodies Group in a ratio of 2 : 1. The granitoid pebbles frequently show micrographic quartz–feldspar intergrowth, whereas the metamorphites—of a modal composition similar to that of the granitoids—are characterized by large quartz grains which could represent original quartz phenocrysts in felsic volcanic precursors. The granitoids show high K 2O, Sr, K 2O/Na 2O, and K/Rb, small enrichment of light REE, large negative Eu-anomalies, and slightly depleted and fractionated heavy REE. Compared to the granitoids the metamorphites show higher Fe 2O 3, TiO 2, and Cr concentrations, greater enrichment of light REE, and also large negative Eu-anomalies. There is little similarity between the Moodies pebbles and the majority of the rocks of the Ancient Gneiss Complex of Swaziland (AGC). There is only some similarity of the REE distribution patterns between the pebbles and the Mkhondo Metamorphic Suite, possibly an areally restricted phase of the AGC. The geochemical data, and especially the large negative Eu-anomalies suggest that the Moodies pebbles were derived from granites which represent residual magmas from which much plagioclase had been removed. The granites crystallized at depths of < 7 km from magmas with low H 2O-pressures in a rather thick sialic crust. It appears possible that the pre-Moodies granitoids originated through partial melting of low-Al 2O 3 siliceous gneisses of the AGC. A chronologic connection of the formation of the granitoids with the late Onverwacht Group volcanicity is possible.