Abstract

Isotopic data for Swaziland Precambrian gneissic rocks, with Rb-Sr whole-rock ages in the range from 3,400 to 3,100 m.y., lie on a steep initial 87Sr/86Sr ratio versus time development line that intersects the basalt development line (or region) at about 3500 m.y. B.P. The preferred interpretation is that, locally, extensive sialic crust did not begin forming much before 3,500 m.y. ago and that the various gneissic rocks defining this steep development line attained their present state by repeated metamorphism and Sr isotopic homogenization within this early sialic crust. Constraints are noted for alternative models that involve much older separation of sialic material. At ∼3000 m.y. B.P., large-scale granite formation took place, and there is evidence that this event effectively stabilized the crust. Younger granites, intruding the earlier gneissic rocks, have initial 87Sr/86Sr ratios lying mainly outside the basalt field. The isotopic data may be interpreted in terms of a two-stage model as a first approximation to the mode of evolution of the crust.

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