The Origin and Contamination of Magmas in the Lowermost Crust

University Of Massachusetts Amherst, Amherst MA

Investigators

Abstract

Granites are unique to Earth's continental crust, and granitic magma is the source of the planet's largest explosive volcanic eruptions. Granitic magmas are generally considered to originate in the middle or shallow crust, where water and appropriate source rocks that partially melt to produce granitic magma are abundant. However, the Athabasca Granulite Terrane of northern Saskatchewan, one of the largest exposures of lower crustal rocks on Earth, provides evidence showing that large volumes of granitic magmas form in the lowermost crust (approximately 30 km beneath the Earth's surface). The rocks presently on the surface in the Athabasca Granulite Terrane were part of the lower crust approximately 2.6 billion years ago. This location provides the opportunity to document the magma-producing processes now occurring at the crust-mantle boundary in zones of collision of tectonic plates. Rocks in the Athabasca Granulite Terrane were invaded by mafic magmas (the 2800-km-long Chipman dike swarm) that caused the lower crustal rocks to partially melt. The partial melts mingled and mixed with the mafic dike magmas, showing that hybridization processes commonly identified in the shallowest magma chambers are common in the lowermost crust as well. In this project, the lineage of granitic magmas of the lower crust will be traced, from the origin of the parent rock from which they partially melted, through the contaminating magmas that they encountered on their journey from the lower crustal site of their origin. The goal of this project is to test the hypothesis that the lowermost crust is a common site of granitic magma production and contamination, and of contamination of basalts that began as continental underplates or interplates. Major, trace, and rare earth element geochemistry and Nd and Hf isotopic analyses both on whole rock samples and on zircon, garnet, and plagioclase crystals in situ will be used to identify: 1) the source material characteristics of felsic and mafic end member magmas in the near-Moho setting; 2) the compositional range of hybrids between Chipman dikes and melts produced by two distinct melting reactions in the 2.6-Ga Fehr granite, 3) the degree to which the Chipman dikes retain their near-Moho compositional characteristics after migrating to the middle crust, and 4) the isotopic and compositional characteristics of mantle partial melt that underplated and interplated the lower crust during both the Archean (2.6 Ga Fon du Lac basaltic sills) and the Proterozoic (1.9 Ga Chipman dikes). This study will quantitatively evaluate the degree to which the fingerprint of heterogeneity in both granites and basalts of the middle and shallow crust is a remnant of the lithologically diverse magma environment of the hot, partially liquid mantle/crust transition zone.