Mathematics – Logic
Scientific paper
Jun 1982
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982e%26psl..59..177r&link_type=abstract
Earth and Planetary Science Letters, Volume 59, Issue 1, p. 177-191.
Mathematics
Logic
2
Scientific paper
Supracrustal rocks in the Archaean of Sierra Leone can be divided into two distinct geographical provinces which are characterised by high and low grades of metamorphism. In the west of the craton greenstone belts up to 130 km long with thick successions of mafic lavas and clastic sediments are metamorphosed to amphibolite facies. In the east of the craton there are smaller schist relics metamorphosed to granulite facies. The dominant lithologies are banded iron formation and mafic granulites. Mineral equilibria indicate that the metamorphic climax in the greenstone belts was 595+/-50°C, 5.5+/-0.5 kbar (Nimini Hills), 565+/-50°C, 4.9+/-2.5 kbar (Gori Hills) and in the granulites was 770+/-50°C, 7.5+/-1.5 kbar. The difference in P-T conditions for the metamorphic maximum in the greenstone belt and granulites can be explained by four possible models for crustal thickening and subsequent uplift and erosion. It is possible that the crust was thickened by tectonic processes, so that the greenstone belts and granulites formed part of the same vertical pile. In this case the difference in recorded P-T conditions results from variable erosion rates or from a variably thickened crust. On the other hand the crust may have been thickened by the addition of magmas and similarly, the recorded difference in P-T conditions results from variable erosion rates or non-uniform thickening. Geological evidence favours, although does not prove, a magmatic model for crustal thickening. The recorded P-T conditions reflect a difference in erosion level between the greenstone belts and granulites, but it is not possible to show whether this is the product of variable erosion rates or a variably thickened crust.
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