Volcanic stratigraphy of the Cambrian-Ordovician Kechika group, Pelly Mountains, south-central Yukon R.W. Campbell and L.P. Beranek* Department of Earth Sciences, Memorial University of Newfoundland Campbell, R.W. and Beranek, L.P., 2017. Volcanic stratigraphy of the Cambrian-Ordovician Kechika group, Pelly Mountains, south-central Yukon. In: Yukon Exploration and Geology 2016, K.E. MacFarlane and L.H. Weston (eds.), Yukon Geological Survey, p. 25-45. ABSTRACT Volcanic rocks occur throughout the lower Paleozoic passive margin successions of western Canada. The tectonic significance of post-breakup magmatism is uncertain, however, some volcanic rocks are spatially associated with margin-parallel normal faults. At the plate-scale, such magmatism is consistent with asymmetric rift models for passive margins, including those with lineaments or transform-transfer zones that form at high angles to the rifted margin. A two-year project was conducted to define the stratigraphy of post-breakup volcanism in the Pelly Mountains, south-central Yukon, and test genetic relationships with the adjacent Liard Line lineament. Field studies targeted Cambrian-Ordovician volcanic strata of the Kechika group in the Quiet Lake map area (NTS 105F). Observed lithofacies are indicative of submarine volcanic edifices and sediment-sill complexes that develop during continental extension. Analogous margin-parallel extension is recognized along the length of the Canadian Cordillera, but the influence of the Liard Line on Cambrian-Ordovician magmatism requires further testing. * [email protected] YUKON EXPLORATION AND GEOLOGY 2016 25 YUKON GEOLOGICAL RESEARCH INTRODUCTION others) and probably related to local faulting (e.g., Pigage, 2004; Cobbett, 2016). Early Paleozoic faults allowed The ancestral Pacific margin of western North America volumetrically small, incompatible element-enriched, low (Fig. 1) was created during the protracted rifting of Rodinia degree partial mantle melts to erupt onto the surface or (e.g., Bond et al., 1984, 1985). An initial Neoproterozoic crystallize in the upper crust (Goodfellow et al., 1995; rifting event in the Canadian Cordillera is in part recorded Millonig et al., 2012). by the Franklin LIP and Windermere Supergroup (Heaman et al., 1992; Colpron et al., 2002). The Hamill and Gog The inferred volume of early Paleozoic magmatism is groups are the inferred products of the final stage of minor compared to that associated with continental continental break-up within the southern Canadian breakup and no other major tectonic event is recorded Rocky Mountains and the Selkirk and Purcell mountains, within Cambrian-Ordovician rocks along the ancient southeastern British Columbia (Fig. 1; e.g., Bond et al., Pacific margin (e.g., Bond et al., 1984; Devlin and Bond, 1985; Devlin and Bond, 1988; Nelson et al., 2013). 1988; Cecile and Norford, 1993). Periodic, Late Cambrian In this region, the timing of final rifting is constrained to Ordovician volcanism is therefore unlikely to result by late Ediacaran (ca. 570 Ma) volcanic rocks of the from subduction or plume-related magmatism. Instead, Hamill Group (Colpron et al., 2002). The rift to post-rift post-breakup magmatism is consistent with asymmetric transition did not occur until at least the Early Cambrian rift models for passive continental margins (e.g., Lister et based on Tommotian to Atdabanian (Terreneuvian al., 1986, 1991). Such models combine lithospheric-scale to Cambrian Series 2) fossils in the Gog Group and detachment faults and related low-angle shear zones with McNaughton Formation (Bond et al., 1985; Magwood crustal thinning to achieve continental breakup. In most and Pemberton, 1988; Hein and McMechan, 1994) and versions of this model, the hanging wall of the detachment thermal subsidence trends (e.g., Bond and Kominz, 1984). fault, termed the upper-plate, undergoes less extension Ediacaran to Lower Cambrian strata similarly record the and is dominated by upper crust. Conversely the footwall, establishment of shallow-water Mackenzie platform and termed the lower-plate, undergoes a greater degree of deep-water Selwyn basin in eastern Yukon and adjacent extension and is more dominated by lower and middle Northwest Territories (Fig. 1; e.g., Gordey and Anderson, crust (Fig. 3; e.g., Lister et al., 1986). Along the length 1993; Moynihan, 2014). of a rifted margin, upper and lower-plate segments are separated by lithospheric-scale transform-transfer faults Magmatism along the western Laurentian margin did (e.g., Lister et al., 1986). Such structural zones may provide not cease with continental break-up and remains an a pathway for mantle-derived melts (e.g., Corti et al., outstanding problem in Cordilleran geology. For example, 2002). Asymmetric rifting processes have been applied Upper Cambrian-Ordovician volcanic rocks observed to the Paleozoic Cordilleran margin to explain the control along the length of the North American Cordillera of lithospheric-scale structures on extension and regional (e.g., Goodfellow et al., 1995; Lund et al., 2010; Millonig paleogeography (Fig. 2; e.g., Cecile et al., 1997; Lund, et al., 2012; Pigage et al., 2012) are seemingly inconsistent 2008). with simplistic models for post-breakup, passive margin sedimentation (e.g., Aitken, 1993). Goodfellow et al. (1995) In order to understand the implications of post-breakup suggested a spatial association between lower Paleozoic magmatism along the Cordilleran margin, a field project volcanic rocks and margin-parallel normal faults that define was designed to characterize the physical stratigraphy the western extent of the basin to the basin-platform of Cambrian-Ordovician volcanic rocks in the Pelly transition zone (Fig. 2; Hayward, 2015). Early Paleozoic Mountains, south-central Yukon. These rocks form part volcanism in the region has therefore been linked to of the Cassiar terrane, a parautochthonous fragment of periodic extension (e.g., Fritz et al., 1991; MacIntyre, 1998; ancestral North America (Fig. 1) that underwent at least Pyle and Barnes, 2003). For example, Marmot Formation 430 km of dextral displacement along the Tintina fault rocks in the Misty Creek Embayment, Northwest (Gabrielse et al., 2006). Our fieldwork targeted outcrops Territories (Fig. 1) are associated with a “steer’s head” rift of the Kechika group (informal) in the Pass Peak (105F/9) profile (Cecile et al., 1982,1997; Leslie, 2009). Menzie and Cloutier Creek (105F/10) map areas of the Quiet Creek formation volcanic rocks in the Anvil district, central Lake 1:250 000 sheet, south of the Tintina Trench and Yukon (Fig. 1), are adjacent to sedimentary exhalative community of Ross River. Cecile et al. (1997) interpreted (SEDEX) base-metal deposits (Faro, Grum, Vangorda and that a northeast-trending, transform-transfer zone named 26 YUKON EXPLORATION AND GEOLOGY 2016 CAMPBELL AND BERANEK - VOLCANIC STRATIGRAPHY KECHIKA GROUP, PELLY MTNS 140°W 132°W 124°W 116°W 108°W 70°N TERRANES COL Outboard SH UM TI B YA AA I I Yakutat R A B CG Chugach PR AA G AG Y Inuvik E O E V CR Crescent L R OG U ICAL S Insular Alaska AX Alexander 66°N WR Wrangellia KS Coast complex AG NAb NAp N Alaska 66°N NAb AA Arctic Alaska NAp Angayucham, AG Tozitna YT Intermontane BR Bridge River CC Cache Creek NAb eastern MT Methow NAb Misty Creek YT NAp Embayment NAc CD Cadwallader Mackenzie HA Harrison Lake Selwyn basin platform CK Chilliwack 62°N SM WR OK Okanagan Anvil Figure 5 ST district Stikinia 62°N QN Quesnellia SM l YT i Yukon-Tanana CG KS m i AX Whitehorse Pelly t SM Slide Mountain Mtns. Ancestral North America YA CA Cassiar Yukon NWT NAb Kootenay CG NAp North America - YT BC platform Cassiar NAc North America - CC craton & cover 58°N Mtns. Juneau of AX CA 58°N YT Kechik a A t l r b CG ough e r t a NAc Alaska NAp AX Cordilleran ST QN 54°N Prince Rupert 54°N Edmonton WR deformation Coast SM CA plutonic Pacific CC Ocean complex NAb NAp Calgary 50°N CD MT Selkirk and BR QN Purcell Mtns. 50°N WR OK Vancouver MT CK BR USA Copyright © 2011 0 100 200 300 Yukon Geological Survey / PR Victoria British Columbia Geological Survey CR km 132°W 124°W 116°W Figure 1. Terrane map of the Canadian Cordillera (modified from Colpron and Nelson, 2011). YUKON EXPLORATION AND GEOLOGY 2016 27 YUKON GEOLOGICAL RESEARCH W 140° W a lask AlaskaA ukonukon YY Northwest Territories T in tina f fault e Liard Line t -pla te wer - Fort Norman Line Lower-plateLo -pla wer Lo te Upper pla Upper-plate - Uppetre pla Liard Line Cassiar terrane Great Lake Shear Zone 0 200 km km British Columbia Alberta 50° NN Figure 2. Distribution of major faults, PacificPacific Ocean Ocean lower Paleozoic igneous rocks and Ordovician-Silurian paleogeography in the Canadian Cordillera (modified USA from Goodfellow et al., 1995; Cecile Explanation et al., 1997). The division between the northern lower-plate and the Marmot Ordovician-Silurian southern upper-plate is an inferred Kechika gp Fm platformal facies ancestral transfer fault, the Liard Line, Ordovician-Silurian lower Paleozoic Menzie which is offset in the west due to basin facies magmatic rocks Creek fm dextral movement along the Tintina fault. 28 YUKON EXPLORATION AND GEOLOGY 2016 CAMPBELL AND BERANEK - VOLCANIC STRATIGRAPHY KECHIKA GROUP, PELLY MTNS Lower-plate margin Upper-plate margin narrow arch sedimentary sag basin continental shelf Detachment fault Moho Moho underplated asthenosphere igneous rocks Figure 3. Schematic cross section of an asymmetric rift system (modified from Lister et al., 1991; Lund, 2008). the Liard Line is present in the Pelly Mountains, placing A similar mid-Cambrian unconformity has been inferred most of the southern Cassiar terrane within an upper- within the Cassiar terrane of the Cassiar Mountains and plate setting (Fig. 2). The Liard Line is likely a reactivated the Kechika trough of the northern Canadian Rocky ancient basement structure (Hayward, 2015) that also Mountains (Fig.