Chapter 21 The Rare Earth Element Evidence in Precambrian Sedimentary Rocks: Implications for Crustal Evolution

Abstract

Abstract The chemical composition of Phanerozoic igneous rocks has been utilized in revealing plate-tectonic histories, but application of such procedures to Precambrian (particularly Archaean) rocks is equivocal. This is well demonstrated by the composition of the Archaean Marda Complex which mimics modern island-arc chemistry, but was clearly formed under a different tectonic environment. Application of geochemistry to the evolution of the continental crust is more soundly based. The composition of the present-day upper continental crust can be characterized by the rare earth element (REE) patterns in sedimentary rocks, since these elements are not easily fractionated by most sedimentary or metamorphic processes. Sedimentary REE patterns indicate that the present upper continental crust approximates to granodiorite. Such a composition cannot be representative of the entire continental crust due to geochemical balance and heat flow arguments. Intracrustal melting of andesite (representing the total crust) at depths Archaean sedimentary rocks have differing REE patterns, resembling island-arc volcanic rocks, and indicate a much less evolved upper crust at that time. The change in REE patterns, as suggested by the Huronian and Pine Creek Geosyncline data, was related to large-scale emplacement of K-rich granites, into the upper crust, near the Archaean—Proterozoic boundary. Modelling of the Archaean sedimentary data is most consistent with an upper crust dominated by mafic volcanics and Na-rich granitic rocks (tonalites, trondhjemites). The contribution of K-rich granitic rocks with negative Eu anomalies to the upper Archaean crust is restricted to 30%) of exposed sialic material, in the form of Na-rich granitic rocks, were present. This is in conflict with some popular models of the nature of the early crust. Archaean REE patterns are observable as far back as the sedimentary record is available (> 3.7 Ga) indicating the crust of the earth was chemically and lithologically complex as far back as the geologic record is available. Firm evidence for modern subduction-like processes is only clear back to about 1000 Ma ago, although initiation of such phenomena may have been related to the massive growth of the continents near the Archaean—Proterozoic boundary. The chemical evidence from sedimentary rocks, consistent with tholeiitic-trondhjemitic bimodal igneous activity, implies that tectonic conditions in the Archaean did not resemble present-day plate-tectonic subduction regimes.