Timing, geochemistry, and depositional setting of lower Paleozoic alkaline mafic volcanic rocks in the Yukon Territory and adjacent Northwest Territories: Implications for the rift and post-rift evolution of the western Laurentian margin

The breakup of the supercontinent Rodinia and development of the western Laurentian passive margin are recorded by Neoproterozoic to mid-Paleozoic igneous and sedimentary rock successions in the Canadian Cordillera. These rocks include platformal carbonate and coeval basinal clastic strata that are offset along margin-perpendicular rift-transfer faults and spatially associated with Cryogenian to Late Ordovician alkaline mafic volcanic rocks present along the ancient continental margin from Yukon Territory, Canada to Nevada, USA. This study contributes new field mapping with facies analysis, U-Pb zircon geochronology, fossil dating, and major element, trace element and Nd-Hf radiogenic isotope geochemistry of some of these alkaline mafic igneous rocks to understand the timing of emplacement, the depositional environment during eruptions, the melt sources of rocks, and their relationships to the tectonic development of the western Laurentian margin. The Dawson fault is a prominent east-west-striking structure in central Yukon that is interpreted herein to have been active as a rift-transfer fault in the late Cambrian. This is supported by new zircon chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb dates from Miaolingian alkaline mafic volcanic rocks concentrated along the Dawson fault, which erupted or were emplaced immediately before the development of a regional sub-Jiangshanian (~ 494 Ma) unconformity and indicate final continental breakup occurred around ca. 501-497 Ma in the northern Canadian Cordillera. Post-rift, Upper Ordovician (453-445 Ma) alkaline mafic volcanic and plutonic rocks occur along a northwest-southeast striking segment of the Dawson fault. New bedrock mapping, geochronological, paleontological, and petrological data from lower Paleozoic rocks exposed near this fault segment indicate that localized Late Ordovician pull-apart basin development and associated volcanism was potentially associated with dextral strike-slip reactivation of the antecedent Dawson fault. New bedrock mapping and volcanic facies analysis of Lower Ordovician mafic rocks assigned to the Menzie Creek Formation in central Yukon suggest these deposits represent ancient seamounts that formed in a margin-parallel graben in a post-rift setting along the western Laurentian margin. The Early Ordovician age of the seamount eruptions is based on two samples from the volcanic successions that yielded high-precision CA-ID-TIMS zircon U-Pb dates of ca. 484 Ma (Tremadocian). Menzie Creek Formation volcanic rocks are interlayered with continental slope strata and contain several kilometers of hyaloclastite breccia and pillow basalt with rare sedimentary rocks. Menzie Creek Formation seamounts form a linear array parallel to the Twopete fault, an ancient extensional or strike-slip fault in central Yukon that localized mag-matism along the nascent western Laurentian margin. The whole-rock trace element and Nd-Hf isotope compositions indicate that the volcanic rocks are predominantly alkali basalt with ocean-island-like signatures consistent with the partial melting of subcontinental lithospheric mantle at ~75–100 km depth. Alkaline igneous rocks from three different areas in southeastern Yukon and adjacent Northwest Territories occur within the Cambrian to Devonian Haywire Formation, Cambrian to Ordovician Broken Skull Formation, and Cambrian to Ordovician Crow Formation. The age of the Broken Skull Formation volcanic rocks is constrained by two high precision U-Pb zircon dates of ca. 494-493 Ma near Brintnell River in western Northwest Territories. Mafic volcanic rocks from the Haywire Formation near Island Lakes, Northwest Territories, are constrained to the Middle Ordovician by conodont microfossils from limestone and gastropod-bearing dolostone interbedded with the volcanic rocks. Crow Formation mafic volcanic rocks and gabbro are ca. 486 Ma from the Camp Creek area in southeast Yukon. Trace element geochemical results from these three areas indicate that the melts formed from the low degree partial melting of an enriched mantle source from > 75 km depth. The whole-rock Nd and Hf isotopic compositions of the mafic rocks yield initial epsilon Nd and Hf values of +2.8 to +4.1 and +4.2 to +9.2, respectively, which are consistent with melting of the subcontinental lithospheric mantle. Alkaline igneous rocks along the northern Cordilleran passive margin coincide with facies belt transitions from shallow-water rocks of the Mackenzie platform to deep-water rocks of the Selwyn basin. Major extensional fault systems that formed during rifting, which accommodated platform-to-basin transitions, were reactivated during passive margin development in the late Cambrian and Middle Ordovician and resulted in low-volume decompressional melts. Aside from syn-rift, Miaolingian magmatism along the Dawson fault, all of the alkaline mafic igneous rocks investigated during this study erupted after lithospheric breakup during the early development of the Laurentian passive margin. The early to mid-Paleozoic counterclockwise rotation of the Laurentian craton, as indicated by published paleomagnetic studies, likely reactivated rift structures via the transfer of stress to the base of the lithosphere from mantle drag forces. It is proposed in this dissertation that normal and strike-slip fault reactivation caused periodic, post-breakup extension which resulted in small-volume, decompression partial melting of the subcontinental lithospheric mantle along western Laurentia.