Origin of Granite and Mafic-felsic Magma Interaction in the Lower Crust: Athabasca Granulite Terrane, CA

University Of Massachusetts Amherst, Amherst MA

Investigators

Abstract

"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)." Intellectual Merit. The Athabasca granulite terrane, Saskatchewan, Canada is one of Earth's largest exposures of intact lower continental crust, and offers the opportunity to see first-hand the complexity of magmatic processes that contributed to formation of the deep crust. In this terrane are unique exposures of heterogeneous lower crust comprising igneous rocks ranging from mafic to felsic compositions. Contrary to common belief that the lower crust is very poor in H2O, many of these rocks host abundant hydrous phases. The eastern part of the terrane is dominated by the 3.3 Ga Chipman tonalite and the 2.6 Ga Fehr granite/granodiorite. Prior to 1.9 Ga, these rocks made up the stable deep crust of the Rae Province. At 1.9 Ga, the Chipman mafic dike swarm intruded both the tonalite and the granite. The tonalite and some of the already crystallized mafic dikes were melted to a small extent. The Fehr granite underwent extensive partial melting, producing true granitic magmas. Over an area of tens of km2, brush-fired cleaned pavement exposures record the partial melting process and a stunning array of mixing and mingling textures between partial melt of Fehr granite and the Chipman dike mafic magma. It is hypothesized that the heterogeneity of the deep crust makes it a fertile source for a variety of melt compositions and that magma interaction processes so well documented in shallow crustal exposures elsewhere are also common at near-mantle depths as well. Funds are requested to carry out comprehensive petrologic characterization of these rocks. Field relationships, textures, and fabrics will be described along a north to south gradient/transect from minimal to extensive partial melting. Major, trace and isotopic analyses will establish the origin of end-member and hybrid rocks as well as source rocks and mid-crustal equivalents of melts identified in the lower crust. Broader Impacts. This grant will support the training of a Ph.D. student and the enrichment of the undergraduate mineralogy, petrology, and structural geology courses. In addition, the field work will provide an opportunity for outreach, through presentations and discussions, to a people who live in a very remote area, but whose livelihoods are generally based on the natural resources of the area. The work will help to lay the groundwork for a planned field forum for geologists and geophysicists to discuss the implications of the heterogeneous lower crust, and finally the project will involve the development of web resources for teachers and researchers about this special exposure of deep continental crust.