Geochemistry and evolution of late Archean plutonism and its significance to the tectonic development of the Slave craton

Abstract

The intrusion of voluminous Late Archean plutonic rocks is the final event leading to the stabilization of the Slave craton. Three groups of plutonic rocks are recognized in the central part of the province: Group 1 (∼2689–2650 Ma) includes high-Al 2O 3 (Olga Suite) and low-Al 2O 3 (Gondor and Wishbone Suites) trondhjemites and diorite (Central Belt Suite) all of which are temporally or spatially associated with volcanic rocks; Group 2 (∼2610–600 Ma) includes syn- to late deformation hornblende-biotite monzodiorite through granodiorite (Concession Suite) and trondhjemites (Siege Suite); and Group 3 (∼2599–2580 Ma) consists of post-deformational muscovite-biotite granites (Contwoyto Suite) and biotite granites (Yamba Suite). Volcanic and associated mafic plutonic rocks are calc-alkaline and have trace element characteristics diagnostic of rocks formed in modern subduction zone settings. The high-A1 20 3 and low-A1 20 3 trondhjemite suites may be derived by high and low pressure partial melting of mafic crust, respectively. Rocks of the Concession Suite are calc-alkaline, metaluminous and have high contents of Sr, Ba and LREE, high (Ce/Yb) Nratios and negative Nb anomalies. Parental magmas are derived from melting enriched mantle sources and related rocks were produced through assimilation-fractional crystallization and mixing processes in the crust. The younger granites are derived from mixing and homogenization of mantle- and crust-derived materials (Yamba Suite) and the partial melting of metasedimentary protoliths (Contwoyto Suite). Volcanic and Group 1 rocks are interpreted to be remnants of an allochthonous island arc terrane which was accreted to a continental block. The secular evolution in the composition of Group 2 and 3 plutonic rocks reflects a change from dominantly mantle- to crustal-derived magmatism during and following collision. Thickening of the lithosphere during collision, followed by lithospheric detachment caused melting of previously subduction-modified mantle beneath the accreted island arc terranes and generation of magmas parental to the Concession Suite. Intrusion of mantle-derived magmas, in combination with crustal thickening resulted in high T, low P metamorphism, crustal melting and the generation of the two Group 3 plutonic suites.