Granite genesis and mafic-felsic magma interaction in the lower crust

Abstract

Field observations from an exposed section of deep continental crust, the Athabasca granulite terrane (AGT), Saskatchewan, Canada, provide a view of granite genesis and a mechanism for deep-seated contamination of felsic and mafi c magmas. The 1.9 Ga Chipman mafi c dike swarm was emplaced into the Chipman Tonalite (ca. 3.3 Ga) and the megacrystic Fehr granite (ca. 2.6 Ga) at a crustal depth of ~40 km. The Fehr granite shows evidence for extensive partial melting and generation of granitic leucosome. Mafi c dikes and granitic leucosome display magma mingling and mixing textures similar to those widely described from shallow crustal exposures. The AGT provides a view of a dynamic, heterogeneous, and locally fertile deep crust. Mantle-derived mafi c magma promotes extensive partial melting of fertile granitoids, which in turn fi lter and entrap later mafi c dikes and sills. The result is almost inevitable mingling and hybridization (i.e., contamination) of mafi c and felsic end members. This interaction of magmas in the deep crustal environment may account for the isotopic and compositional signatures of igneous rocks at shallower crustal levels that typically record contamination of crustal melts by mantle material and vice versa.

The Cora Lake shear zone, Athabasca granulite terrane, an intraplate response to far-field orogenic processes during the amalgamation of Laurentia

“…Their composition ranges from a nearly pristine primary assemblage of hornblende and plagioclase, to garnet clinopyroxene granulites, and migmatitic members Flowers et al 2006aFlowers et al , 2008. The dykes almost exclusively intrude Chipman tonalite and further to the southeast, the Fehr granite (Koteas et al 2010). Dykes are commonly slightly oblique to the NE-striking host fabric (Fig.…”

Neoarchean arc magmatism and subsequent collisional orogenesis along the eastern Rae domain, western Churchill Province: Implications for the early growth of Laurentia

“…Province of the Canadian Shield, partly because of the vast size and remoteness of the region and partly because of the complex overprinting and interleaving of Proterozoic and Archean components (Mahan et al, 2006a;Williams et al, 2009;Dumond et al, 2010;Bethune et al, 2013). The Churchill Province has typically been subdivided into two large subprovinces, Rae and Hearne domains, separated by the geophysically defined Snowbird Tectonic Zone (STZ; Hoffman, 1988).…”

“…Plutonic rocks of the Northwestern subdomain range in composition from felsic to intermediate, with subordinate mafic lithologies (see below). The largest, and best studied component is the Mary granodiorite (Hanmer, 1994;1997;Williams et al, 2000;2014;Dumond et al, 2010). Felsic granulite (classically the "Reeve diatexite" after Hanmer, 1994) commonly forms concordant, semi-continuous screens completely engulfed by the meta-plutonic rocks.…”

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“…Several studies have documented evidence for hybridization of mafi c magmas with lowercrustal rocks/magmas (e.g., Hildreth and Moorbath, 1988;Williams et al, 1995;Voshage et al, 1990;Koteas et al, 2010;Jagoutz et al, 2011). In some cases, Moho-depth magmatism has resulted in formation of dense, garnet-bearing cumulates.…”