Crustal heat production and style of metamorphism: a comparison between two Archean high grade provinces in the Limpopo Belt, southern Africa

Different characteristics of late Archean meta morphism in the Northern and Southern Marginal Zones of the Limpopo Belt in southern Africa are examine d in the light of differences between these provinces in contents of heat g enerating elements. In the Northern Marginal Zone intrusion of charnoenderb itic magmas lasted from 2.75 to 2.58 Ga, accompanied by repeated low to med ium pressure granulite facies metamorphism with some anticlockwise P-T path s, In contrast, the medium pressure granulites of the Southern Marginal Zon e experienced peak metamorphism at 2.69 Ga without much evidence of plutoni sm at that time, followed by near isobaric cooling at approximately 6.5 kba r. As a basis for our approach we used steady state geotherms, calculated f or the deeply eroded Marginal Zones for late Archean times on the basis of their average K, Th and U concentrations, and assuming low basal heat flow values of 20 mW m(-2). The Southern Marginal Zone and the adjacent northern Kaapvaal Craton are both characterised by low average Th and U concentrati ons (Th: 1.1 and 1.5 ppm, U: 0.3 and 0.65 ppm), which are not a consequence of high grade metamorphism. The average Th and U concentrations in the Zim babwe Craton (8.8 and 2.2 ppm) are much higher than those in the Kaapvaal C raton, and those found in the Northern Marginal Zone are, peak metamorphic temperatures are much above even higher (13.8 and 2.6 ppm). In the Southern Marginal zone. reasonable steady state geotherms and peak conditions could only have been attained through transient very high basal heat flux, which , in the context of a subduction zone, might have resulted from slab breako ff or slab windows. The Northern Marginal Zone in contrast has such a large internal heat generation that high grade metamorphic conditions plot close to steady state geotherms even with basal heat flux values of 20 mW m(-2), and minor crustal thickening or basal heat flux increase would have suffic ed to exceed fluid absent tonalite solidus temperatures in the lower crust. The repeated high grade metamorphism and charnoenderbite intrusions over a period of 1.7 x 10(8) years in the province can be explained by this obser vation. Both marginal zones of the Limpopo Belt can be understood as Archea n active continental margins (of the Kaapvaal and Zimbabwe Craton, respecti vely). While the problems of explaining the metamorphism in the Southern Ma rginal Zone (i.e. the source of the required heat) are similar to those fac ed in many present day margins or orogens, the Northern Marginal Zone is no n-uniformitarian in the sense that its extreme heat generation rate could n ot be a property of any geological province today. (C) 2001 Elsevier Scienc e B.V. All rights reserved.