Chapter 5 Archaean Plates and Greenstone Belts

Abstract

Archaean greenstone belts lie in granite-greenstone terrains, contain mafic volcanic rocks and feature low- to medium-grade metamorphic assemblages, yet otherwise display great diversity in size, form, lithology, stratigraphy, age, basement-cover relations and structural deformation as illustrated by consideration of four main granite-greenstone terrains respectively in Peninsular India, southern Africa, western Australia and the southern Canadian Shield. Archaean greenstone belt diversity demands equally flexible tectonic development. Consideration of Archaean relations and constraints on a global scale leads to a preferred rift-and-sag model involving repeated attenuation and fissuring of pre-existing sialic crust within a time span of at least 1200 Ma with both ensialic and ensimatic accumulation of assorted greenstone assemblages. Greenstone belt development is attributed to an Archaean plate-tectonic process that involved limited movement of numerous small plates operating under a high geo-thermal gradient and thereby lacking conventional subduction (Benioff) zones. Thus the Archaean plate-tectonic process differed in significant respects from modern plate tectonics, yet produced similar volcanic and plutonic products. It is contended that during earth history the plate-tectonic process has taken several forms, all involving at least some moving plates, of which the current is but the latest.