A REEVALUATION OF ARCHEAN INTRACRATONIC TERRANE BOUNDARIES ON THE KAAPVAAL CRATON, SOUTH AFRICA: COLLISIONAL SUTURE ZONES?

Abstract

Increasing support for the adoption of plate-tectonic processes to account for Archean geotectonic events has led to the reevaluation of selected terranes in southern Africa in order to determine the viability of plate tectonics as a mechanism for terrane accretion and the growth of continents. Geotectonic reconstructions indicate that a number of Archean ultramafic complexes and associated volcanic rocks are developed in three linear zones on the Kaapvaal Craton that may represent the fossil traces of Archean sutures or oceanic crustal collisional zones. The ultramafic complexes consist primarily of massive and schistose serpentinitic and pyroxenitic rocks derived from dunite, harzburgite, lherzolite, pyroxenite (ortho- and clinopyroxenite), and lesser gabbroic bodies believed to have been derived from high-Mg, komatiitic-type parent magmas. The ultramafic complexes have previously been interpreted to have formed as a result of differentiation processes and were emplaced either magmatically or tectonically into the early Archean crust. An alternative view suggests that the ultramafic bodies may be likened to oceanic crust or "layered series" assemblages of Phanerozoic ophiolites. Serpentinized ultramafic components of ophiolites are, in turn, commonly encountered in orogenic belts where they mark the positions of suture zones. The three areas where extraordinary developments of serpentinized ultramafic rocks are to be found on the Kaapvaal Craton include: (1) the northern flank of the Barberton greenstone belt; (2) the area south of the Murchison greenstone belt; and (3) the region extending across the southern portion of the Johannesburg Dome. The age of the ultramafic bodies has not been determined directly, but intrusive granitoid rocks suggest they are mostly older than ca. 3200 Ma and in some cases may be as old as ca. 3600 Ma. In each of the three regions discussed the proposed suture zones mark the position of collisional convergent plate boundaries, now represented by intracratonic terrane boundaries. The suture zones separate protocontinental crustal blocks that formed as a result of the amalgamation of multicomponent, continental basement rocks, which initially developed as volcanic arcs and which are today preserved on the Kaapvaal Craton as Archean granite-greenstone complexes ranging in age between ca. 3600 and 2700 Ma. The suggested linear suture zones, which have not yet been examined in detail geophysically, are shown to be part of more extensive intracratonic linear features, such as the Thabazimbi-Murchison and Barberton Lineaments, and are believed to represent the only physical expression of terrane boundaries on the Kaapvaal Craton. © 2006 Geological Society of America.