By Cyril Ponnamperuma (Eds.)

ISBN-10: 0125613601

ISBN-13: 9780125613606

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Extra resources for Chemical Evolution of the Early Precambrian

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Hunter, 1974a,b). Thus, the composition of the 3000- to 3500-million year old rocks that presumably com­ posed the basement source for most Archean greenstones may be more mafic than the average of the present exposed crust, with the present composition controlled partly by intrusion and met­ asomatism after greenstone formation. Krupicka (1975) discuss-' es the problem of the amount of true granite in the earliest Archean crusts. The studies of shield evolution mentioned earlier may lend some support to the concept of a relatively mafic early Archean crust, particularly one with a very low K 2 0/Na 2 0 ratio.

Our knowledge of this time period in this area will always be at best, very incomplete. IV. EVIDENCE OF ANCIENT LIFE IN THE ARCHEAN OF THE SLAVE PRO­ VINCE Evidence of ancient life in the Archean of the Slave Pro­ vince is present, but sparse, due to the metamorphic and tec­ tonic disruption of the rock, the easily degraded nature of such material, and the fact that much of the province has only been explored on a reconnaissance scale. One possible indicator of the existence of biologic acti­ vity is the carbon-rich sedimentary deposits which are present throughout the province.

First, some continental shield areas appear to have formed in Proterozoic time and do not show evidence of reworking of earlier Archean continental crust. Examples of these shields may be the Llano uplift of central Texas and the Eastern Desert of Egypt. Second, Phanerozoic orogenic belts now incorporated within continents can be shown to have evolved from initially oceanic areas. An example is the Cordilleran belt of western Nevada and eastern California, where volcanic and associated sedimentary features show westward growth of the North American continent through much of Late Paleozoic and Third, graywackes and associated sediments of Mesozoic time.

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Chemical Evolution of the Early Precambrian by Cyril Ponnamperuma (Eds.)

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