Petrogenesis of Archean Granitoids and Implications for the Chemical Evolution of Cratonic/Lithosphere

Suny At Stony Brook, Stony Brook NY

Investigators

Abstract

Rapp EAR-0003638 Despite comprehensive field, petrologic, isotopic, geochemical, and structural studies in a number of well-exposed early- to late-Archean terrains (2.7-3.8 Ga old), some relatively basic issues regarding the origin and early growth of the continental crust remain unresolved. These include the tectonic setting in which early continental growth took place, the ultimate source of the granitoid magmas that helped stabilize continental masses in the Archean, and the nature of the relationship between granitoid magmatism within the evolving Archean continents and the stabilization of deep cratonic roots (keels) in the underlying mantle. The proposal seeks funding to conduct high-pressure experiments in the multi-anvil apparatus aimed at constraining the origin of Archean granitoids, and designed to provide insight into the extent to which the mantle was involved in their origin. Previous experimental studies have shown that low degrees of melting of hydrous basalt at 1-3 GPa produces liquids with the general chemical composition of Archean tonalite-trondhjemite-granodiorite suites (TTG), the dominant granitoid rock in Archean high- and low-grade terrains, but with some minor but perhaps significant discrepancies. Late-Archean (2.6-2.9 Ga old) granite-greenstone terrains in the Superior and Slave Provinces of the Canadian Shield (members of the so-called "sanukitoid suite"), possess chemical characteristics (such as high Cr and Ni contents) that are inconsistent with a simple origin by partial melting of metabasalt, and that suggest a mantle lineage. Yet in terms of most geochemical parameters, TTG granitoids, derived from partial melting of mafic crust, appear to be compositionally continuous with presumably mantle-derived granitoids of the sanukitoid suite. A number of explanations, which will be tested experimentally, could account for these observations: (1) assimilation of mantle peridotite by TTG melts, (2) the basaltic source for Archean TTG was more magnesian than the starting materials used in the basalt melting experiments at1-3 GPa, or (3) sanukitoids result from partial melting of mantle that had been previously modified by TTG melts. Possibilities (1) and (3), of course, involve intimate chemical interaction between TTG melts and peridotitic mantle, with potentially profound implications for the chemical evolution of the sub-continental mantle.