Skip to Main Content

Although granulite facies rocks are present in zones of orogenesis ranging in age from 300 to 3,000 m.y. in Africa, only two distinct ages of granulite facies metamorphism, at about 1,200 and 3,000 m.y., have thus far been recognized. Granulite facies suites in regions of orogenesis of other ages — 1,850 ± 250 m.y., 600 ± 100 m.y., and Phanerozoic — represent older remnants since affected by regional retrograde metamorphism. The retrogression is particularly widespread in the 600 ± 100 m.y. orogenic zone, an important part of which is interpreted in terms of a former zone of granulite facies rocks, remnants of which are now preserved as isolated masses in amphibolite facies rocks.

In most African examples, hypersthene granulites of silicic to mafic and ultramafic composition are associated with undoubted metasedimentary rocks and a variety of other rock types, including anorthosite; intrusive chamockite also occurs in a number of regions. The alkali-silica plot for 240 analyses of pyroxene granulite and chamockite suggests a chemical affinity with high-alumina basalt and its calc-alkaline derivatives; the AFM trends are similar to those of calc-alkaline and alkaline igneous suites.

With few exceptions, rocks of appropriate composition in the granulite facies contain hypersthene + diopside + plagioclase ± garnet, reflecting medium to high pressure. Available Sr-isotopic data for granulite-chamockite suites of Africa and elsewhere in the world yield a crude “evolution path” that may reflect Sr-isotopic development in the deeper parts of the Earth’s crust or in the mantle beneath the continental crust.

You do not currently have access to this chapter.

Figures & Tables

Contents

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal