Abstract
Different characteristics of late Archean metamorphism in the Northern and Southern Marginal Zones of the Limpopo Belt in southern Africa are examined in the light of differences between these provinces in contents of heat generating elements. In the Northern Marginal Zone intrusion of charnoenderbitic magmas lasted from 2.75 to 2.58 Ga, accompanied by repeated low to medium pressure granulite facies metamorphism with some anticlockwise P-T paths. In contrast, the medium pressure granulites of the Southern Marginal Zone experienced peak metamorphism at 2.69 Ga without much evidence of plutonism at that time, followed by near isobaric cooling at approximately 6.5 kbar. As a basis for our approach we used steady state geotherms, calculated for 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 concentrations (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 Zimbabwe Craton (8.8 and 2.2 ppm) are much higher than those in the Kaapvaal Craton, and those found in the Northern Marginal Zone are even higher (13.8 and 2.6 ppm). In the Southern Marginal zone, peak metamorphic temperatures are much above 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 breakoff 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 sufficed 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 × 108 years in the province can be explained by this observation. Both marginal zones of the Limpopo Belt can be understood as Archean active continental margins (of the Kaapvaal and Zimbabwe Craton, respectively). While the problems of explaining the metamorphism in the Southern Marginal Zone (i.e. the source of the required heat) are similar to those faced in many present day margins or orogens, the Northern Marginal Zone is non-uniformitarian in the sense that its extreme heat generation rate could not be a property of any geological province today.
Original language | English |
---|---|
Pages (from-to) | 149-163 |
Number of pages | 15 |
Journal | Precambrian Research |
Volume | 112 |
Issue number | 1-2 |
DOIs | |
Publication status | Published - 15 Nov 2001 |
Externally published | Yes |
Keywords
- Archean
- Geotherms
- Heat flow
- Limpopo Belt
- Southern Africa
- Tectono-metamorphism
ASJC Scopus subject areas
- Geology
- Geochemistry and Petrology