Calc-alkaline volcanism along the eastern margin of the namaqua mobile belt, south Africa - A possible middle proterozoic volcanic arc

Abstract

Abstract The Boksputs and Upington sequences of the Areachap Group, present along the eastern margin of the Namaqua mobile belt in southern Africa, contain amphibolites with chemical properties which suggest that they are the metamorphic products of arc related precursors. The amphibolites, which form the predominant rock type of the Jannelsepan Formation of the Areachap Group, are subdivided on mineralogical grounds into massive and pyroxene amphibolite. The massive amphibolite of the Boksputs sequence chemically resembles low-K arc tholeiite, while the pyroxene amphibolite is slightly more calc-alkaline in character. The massive amphibolite of the Upington sequence is predominantly calc-alkaline, while the pyroxene amphibolite shows a strong shoshonitic affinity. The chemical differences between the two metavolcanic sequences are interpreted in terms of arc polarity and arc maturity. The present position of the associated granite plutons of the Keimoes Suite indicates that the two sequences most probably represent fragments of an originally northwest-trending arc system with a southwest-northeast polarity. The possibility of a north-south polarity, however, cannot be ruled out. The chemical characteristics may be indicative of arc maturity: the Upington sequence thus represents a more mature and the Boksputs sequence a less mature portion of a middle Proterozoic arc system. The Areachap Group is interpreted as an accretion feature with characteristics generated in a subduction environment.