The distribition and diagnostic features of deformed plutonic rocks in two terranes of the Namaque mobile belt along the Orange (Gariep) River, South Africa

Abstract

Two periods of deformation have been documented by detailed mapping in a strip of 3000 km 2 along the Orange (Gariep) River. The mapping has enabled field discrimination and regional correlation of well-exposed plutonic tectonites that volumetrically dominate the western part of the Namaqua mobile belt. The associated metasedimentary and metavolcanic successions that supplied tectonostratigraphic keys for correlation of rock units, were described in previous publications. Deformation took place during an orogenesis that may be termed the Orange River Event (ORO, ~2.0 to ~1.6 Ga, with associated calcalkaline magmatism) and the later Namaqua Event (NMO, ~1.2 to ~1.1 Ga, with associated granitic magmatism). The ORO magmatism includes the Vioolsdrif Suite, represented by outcrops of deformed leucogranite, adamellite, granodiorite and mafic rocks that were later intruded by small mafic complexes. The products of the NMO magmatism include various members correlated with the Little Namaqualand Suite (the Kabis, Pipeline, Swartmodder, Koeipoort, Mission, Coboop and Eendoorn gneisses). Textural properties of the metaplutonic rocks vary from very coarsely porphyroblastic ( e.g. Eendoorn gneiss, megacrysts >10 cm) to fine even-grained (e.g. some Kabis gneiss). The megacrysts, invariably composite on microscopic scale, are deformed relicts of phenocrysts. Variation in strain is well illustrated by megacrysts that grade from nearly equidimensional to flattened, visible as pencil-shaped streaks on exposed surfaces. Several gneisses that appear to be unrelated because of different textural properties, merely record variation in strain intensity ( e.g. the little deformed granodioritic occurrences at Vioolsdrif are coeval with severely deformed rocks at Pella, 150 km furthur to the east). Detailed structural-stratigraphic mapping and petrographic study helped to prevent unnecessary proliferation of names. Compressional tectonics were responsible for the general style of both orogeneses reflecting the effects of progressive ductile shearing rather than consecutive discrete deformational events. ORO intrusives belong to specific tectonic terranes and tend to be batholithic (in the Pofadder terrane). The grade of metamorphism was greenschist to amphibolite. The rapid tectonism of the subsequent NMO cycle led to terrane accretion and to granulite/upper amphibolite grade metamorphism. Associated granitic intrusives are not terrane-specific. The predominant sheet-like to phacolithic habits reflect emplacement during horizontal tectonism.