Paleoproterozoic arc and ophiolitic rocks on the northwest-margin of the Trans-Hudson Orogen, Saskatchewan, Canada: their contribution to a revised tectonic framework for the orogen

Abstract

A new lithotectonic framework for the northwestern Reindeer Zone of the Trans-Hudson Orogen divides rocks into five northwest- to north-dipping volcano-sedimentary assemblages: (1) at the structural base, the 1.92–1.87 Ga largely sedimentary Levesque Bay Assemblage (partly equivalent to former ‘MacLean Lake gneisses’), which lies within the confines of the Kisseynew Domain and is tectonically imbricated with metasedimentary rocks of the <1.85 Ga McLennan and Burntwood groups; (2) the turbiditic Duck Lake Assemblage, also located along the northern edge of the Kisseynew Domain; it contains detrital zircons ranging in age between 1.92 and 1.87 Ga; (3) the ?1.92 Ga mafic–ultramafic volcano-plutonic Lawrence Point Assemblage of the La Ronge Domain; (4) the ≥1.88 Ga felsic to intermediate volcano-plutonic Reed Lake Assemblage of the La Ronge Domain; and (5) the turbiditic Milton Island Assemblage of the Rottenstone Domain, which contains detrital zircons ranging in age between 2.83 and 1.86 Ga. The assemblages are intruded by a variety of 1.91–1.78 Ga mafic to felsic plutons. The Lawrence Point Assemblage is interpreted as a dismembered supra-subduction zone ophiolite. High-MgO refractory harzburgite (‘Group 1’ ultramafic rocks), at the structural base of the assemblage, is geochemically identical to the upper mantle section of selected supra-subduction zone ophiolites and mantle tectonites. Chromite and olivine compositions of the ‘Group 1’ ultramafic rocks are also comparable to those of ophiolitic harzburgite and mantle tectonite. Mafic metavolcanic rocks of the assemblage are classified as subalkaline tholeiitic basalts. Their trace element patterns and Hf, Ta, Th, Y, Nb, and La element ratios resemble those of modern back-arc basin basalts. The Reed Lake Assemblage represents a subduction-generated arc complex that was built on top of the Lawrence Point Assemblage; its mafic metavolcanic rocks are subalkaline basalts, with calc-alkaline trends, and elevated Th and Ce concentrations and negative Nb anomalies. Feldspar porphyry dykes intruding the Lawrence Point and Duck Lake assemblages constrain timing of Lawrence Point ophiolite emplacement onto the Duck Lake Assemblage to 1.86–1.84 Ga. The trace element geochemistry of the dykes suggests continued arc volcanism after ophiolite emplacement. Mafic metavolcanic rocks of the Levesque Bay Assemblage are geochemically similar to those of the Lawrence Point Assemblage. Other ultramafic rocks (peridotite to pyroxenite) are abundant in the Lawrence Point Assemblage, but have similar geochemistry to small ultramafic bodies intruding the Reed Lake, Duck Lake and Levesque Bay Assemblages. They represent a separate, later phase (?1.86 Ga) of ultramafic plutonism, which post-dates ophiolite emplacement. Timing of Lawrence Point ophiolite emplacement (between 1.86 and 1.84 Ga) and geochemistry of later felsic and mafic/ultramafic volcanism suggest that the Lawrence Point ophiolite and overlying Reed Lake arc assemblage were not accreted to the Hearne Craton prior to 1.86 Ga, but were first accreted to the Flin Flon–Glennie Complex after 1.86 Ga.