SEDIMENTARY-ROCKS AND CRUSTAL EVOLUTION - TECTONIC SETTING AND SECULAR TRENDS

A significant change in the composition of turbidites deposited at active tectonic settings is seen at the Archean/post-Archean transition. In comparison to post-Archean active margin turbidites, Archean greenstone turbidites exhibit more uniform Th/Sc ratios (but with a similar average), greater HREE-depletion (Gd(N)/Yb(N) > 2), less Eu-depletion (Eu/Eu* mostly > 0.85) and an absence of low Th/U ratios (Th/U > 3). These data indicate that differing mantle sources and/or conditions of mantle melting existed during the Archean and that intracrustally differentiated rocks (older cratons or young differentiated arcs) were relatively unimportant as provenance components for Archean turbidites. In contrast, some sedimentary rocks preserved in Archean platformal sequences of high-grade terranes display significant negative Eu-anomalies similar to the majority of post-Archean shales, indicating that the general process of cratonization, including intracrustal differentiation, has taken place since at least 3.8 Ga. However, several lines of evidence indicate that the overall extent of such regions was minor. On balance, the sedimentary data are consistent with a change in upper crustal composition at the end of the Archean, representing a first-order feature in the geological record. It is generally agreed that a major episode of continental growth also occurred at the end of the Archean and that crustal growth appears episodic throughout earth history. Crustal growth also appears to be early, with most models suggesting > 50% of the crust in place by about 2.5 Ga. Early crustal growth is consistent with a hotter Archean earth, but discontinuous aspects of crustal evolution are not so readily understood for a continually cooling earth. It may be possible to reconcile these features by considering the effects of supercontinental cycles, beginning in the late Archean, superimposed on a framework of secular earth cooling.