WHERE WERE THE PAN-AFRICAN MOUNTAINS - NO EVIDENCE OF 500 MY DETRITAL ZIRCONS

Evidence for or against the collision hypothesis for the origin of intracontinental mobile belts, such as the Pan-African, is difficult to devise in view of the complexities of collision sutures. We propose that the simple criterion of elevation, with subsequent deep erosion of crystalline rocks which shed isotopically dateable zircons into adjacent sediments, such as in the case of the Appalachians, may provide such evidence. Zircons from the Phanerozoic sedimentary deposits in northern Africa, collected in Tunisia, Morocco, Sicily and Egypt, have been found to contain radiogenic lead which in all cases lies along primary chords of about 1750 m.y. in a Concordia plot. Evidence in the data points to the absence of any significant contribution of Pan African zircons (about 500 m.y.) in any of the samples, which cover a range in time of deposition from Ordovician to the present. From this evidence we conclude that the extensive Pan-African mobile belts which should have developed great chains of mountains over a large proportion of Africa if they resulted from subduction or collision, were not of that origin. © 1979.