Recognition of Early and Late Neoproterozoic supracrustal units in West Africa and North-East Brazil from detrital zircon geochronology

Abstract

The 207Pb/206Pb age distributions of detrital zircons from eight samples of high-grade Neoproterozoic metasedimentary rocks from Nigeria, Cameroon, and NE Brazil have been studied by LA-ICP-MS, and the results have been compared with published data for non-metamorphic sediments from the Volta basin, Ghana, and associated rocks. Apart from detrital zircons, several samples from Nigeria, Cameroon and NE Brazil contain 630–640Ma metamorphic zircon, formed during an event of high-grade Neoproterozoic (Pan-African – Brasiliano) metamorphism. This young zircon occurs mainly as rims around older zircon, and can often be recognized by its low Th/U ratios. The detrital zircons vary widely in age: Palaeoproterozoic zircons are present in all samples; Archaean, Mesoproterozoic and Neoproterozoic are present in some. Five samples (one from Nigeria, two from Cameroon and two from NE Brazil) lack detrital zircons younger than ca. 950Ma, and are argued to represent units of early Neoproterozoic sedimentary rocks. Three samples (one from Nigeria, Cameroon and NE Brazil each) contain numerous zircons with ages of 650–950Ma, and have been deposited during the late Neoproterozoic. An age difference of several hundred millions of years between the two groups is implied. These results are similar to those earlier obtained for the lower and upper parts, respectively, of the Volta basin in Ghana.