T h e C o a s t - Insular Boundary Revisited: Mid-Jurassic ductile deformation on Quadra Island, British Columbia Sandra Johnstone [email protected]; Vancouver Island University, 900 Fifth Street, Nanaimo British Columbia, V9R 5S5 Project Overview Relationship to Strongly deformed basalt dikes are boudined and transposed parallel Sample Description Ductile Strongly Deformed Dike to bedding. Mineralogy of OB-14-26 is dominated by subhedral laths 35 Post-Deformation Intrusions The boundary between the Insular and Coast geomorphological belts of the North American Cordillera is well exposed at Open Deformation of strongly sericitized plagioclase averaging 0.5mm (70%), with 20% Samples OB-14-24 and OB-14-27 are both intrusive phases that cross-cut ductile deformation struc- N Bay, on Quadra Island, British Columbia. On the southwestern side of the boundary at Open Bay is the upper Triassic-aged actinolite that may be an alteration product of hornblende. Unfortu- N Basaltic Pre– or early syn- 83 tures. OB-14-24 is an ~5m wide unfoliated plagioclase-phyric dacite dike. OB-14-24 zircons are zoned Quatsino limestone and Karmutsen basalts of the Wrangellia terrane. On the northeastern side of the boundary are intrusive OB-14-26 nately only one zircon was recovered from Sample OB-14-26. This andesite pod deformation 65 single zircon yielded a concordant age of 157.6 ± 8.9 Ma. euhedral to subhedral crystals and some fragments; nineteen crystals yield an age of 158.7 ± 0.8 Ma. phases of the Coast Batholith. At Open Bay the Quatsino limestone (Upper Triassic) has been subjected to strong ductile defor- N Folded mation resulting in re-folded transposed bedding. The goal of this project is to constrain the age of the ductile deformation at the OB-14-23 Syn-deformation OB-14-27 is an unfoliated hornblende quartz diorite intrusion that cuts across bedding and folds of basalt dike boundary between the Wrangellia terrane and the Coast Batholith using U-Pb zircon age dating in intrusive phases with clear OB-14-26 bedding in the limestone, as well as cutting basaltic andesite pods. OB-14-27 zircons are zoned subhe- Hornblende Post–dates ductile dral crystals and some fragments. Eighteen crystals yielded a concordant age of 160.1 ± 1.6 Ma. relative age relationships to the deformation. Four intrusions were selected for dating: 1) a strongly deformed and boudined in- OB-14-27 3 quartz diorite deformation 88 157.6 ± 8.9 Ma trusion interpreted to be pre– or early syn-deformational; 2) a basalt dike that cuts isoclinal folds, but is itself folded, interpreted Only one zircon crystal Plagioclase-phyric Post-dates ductile OB-14-27 as syn-deformational ; 3) an undeformed hornblende quartz diorite intrusion, believed to post-date ductile deformation; and 4) an OB-14-24 undeformed plagioclase-phyric dacite dike, also believed to post-date ductile deformation. dacite dike deformation 60 The Coast-Insular Boundary on Quadra Island 60 32 OB-14-24: Undeformed plag-phyric dacite contains a diorite xenolith 500um Photomicrograph shows typical mineralogy OB-14-27: Undeformed hornblende quartz diorite OB-14-24 18 OB-14-26 35 15 0 OB-14-24: Undeformed plag-phyric dacite cuts transposed limestone OB-14-27 25 2m 2m Pods are transposed with bedding are locally OB-14-24 OB-14-27 Lithologies observed to cut bedding. Symbols 500um 158.7 ± 0.8 Ma 160.1 ± 1.6 Ma Legend N=18, MSWD=1.6 N=18, MSWD=1.15 Hornblende pyroxene diorite 30 5 L e g e n d Lithologies Symbols F1 fold hinge Lithologies Symbols Coarse crystalline calcite Plagioclase-phyric dacite Trace of bedding on Trace of bedding on out- 30 dike outcrop surface Legend crop surface Undeformed mafic dike 30 The Insular-Coast boundary is typically obscured by water below the Strait of Georgia or covered by a mantle of sediment, so F1 fold hinge Basaltic andesite dike it has have been the subject of only a few previous studies (Carlisle & Susuki, 1965; Nelson, 1976; Monger, 1991). At Open Coarse crystalline cal- 30 30 F1 fold hinge U-Pb sample OB-14-27 Coarse crystalline calcite Orientation of trans- Bay on Quadra Island this boundary is exposed as a magmatic front; the supracrustal rocks of the upper Triassic Quatsino 60 cite posed bedding Well-bedded grey limestone Limestone are cut by numerous intrusions, ranging in texture from aphanitic to porphyritic to phaneritic, and ranging in com- Moderately-deformed basaltic position from mafic to felsic (Carlisle & Susuki, 1965; Roddick & Woodsworth, 2006). The limestone of the Quatsino for- 25 andesite dike 60 Orientation of bed- 60 Trace of bedding on Basaltic andesite dike mation has been subjected to strong ductile deformation, and some of the intrusive phases are deformed to variable de- ding Strongly deformed basaltic an- outcrop surface grees. Major oxide and trace element geochemistry from some of the deformed mafic aphanitic intrusions suggests an arc desite dike affinity (i.e. Bonanza or Coast; Johnstone, 2014). Additionally, previously published U-Pb isotopic data from a quartz diorite Well-bedded grey lime- U-Pb sample OB-14-24 Well-bedded grey limestone U-Pb sample OB-14-26 stone collected on NW Quadra Island yields a crystallization age of 163.8 ± 0.4 Ma, interpreted to mean that the plutonic rocks on 2m Quadra Island are likely early phases of the Coast Batholith (Monger and McNicoll, 1993). Conclusions Note: Exposures of Wrangellia north of the Coast—Insular boundary are rare, and pre- dominantly comprise blocks of marble and amphibolite interpreted to be metamorphosed Ductile deformation was well underway by the time the basalt dikes (OB- equivalents of Wrangellian volcanics and carbonates. (i.e. Roddick & Woodsworth, 2006) 14-23) intruded ca. 164.8 ± 1.6 Ma. ST Study Area at Open Bay Folded Dike Ductile deformation of the Quatsino limestone at Open Bay was finished 341400.000000 341600.000000 341800.000000 342000.000000 342200.000000 342400.000000 342600.000000 342800.000000 343000.000000 343200.000000 343400.000000 by 160.1 ± 1.6 Ma (OB-14-27). OB-14-23 is a folded basalt N 1 .000000 .000000 dike that cuts both isoclinal The ages of all of the intrusive phases at Open Bay are roughly synchro- folds (F1) and the bedding- nous with the transition from Island Plutonic suite magmatism to Coast 2 5556600 5556600 parallel S foliation. The dike 60 1 50 45 plutonism ca. 166 – 167 Ma (Cui & Russell, 1995; Bustin et al, 2013; De- Terranes has a rusty red-brown weath- 3 10 Bari et al, 1999; Canil et al, 2013). BR – Bridge River ered colour and medium grey CC – Cache Creek 4 CD – Cadwaller .000000 .000000 fresh colour, with finely dis- The Coast – Insular boundary appears to be coincident with occurrences CH – Chilliwack CPC – Coast Plutonic Complex seminated pyrite. Grain size CR – Crescent 70 OB-14-23 of strong deformation along a linear trend within the Georgia Basin. This H – Harrison PR – Pacific Rim 5556400 5556400 is aphanitic, and the dike is 10 trend connects deformed rocks on West Thurlow Island (Nelson, 1976), M – Methow SH – Shukan WR – Wrangellia OB-14-24 80 weakly magnetic. ST – Stikinia Q – Quesnellia Quadra Island (this study), Hernando Island (Carlisle & Susuki, 1965), OB-14-26 Twenty zoned euhedral to Texada Island (Weber & Ray, 1990) and Bowen Island (Monger, 1991) .000000 .000000 subhedral zircons and frag- that are all deformed ca. Mid—Late Jurassic. This distribution and timing Terrane map of southwestern British Columbia (after BC Geological Survey, 2017). Lo- ments were selected from OB OB-14-23 of deformation suggests a linear structural corridor created during Mid– cations where mid-Jurassic ductile deformation is observed are labelled in the white diamonds: 5556200 5556200 -14-23, yielding a concordant 80 Late Jurassic NW-SE directed shortening. 1 – Hardwicke and West Thurlow Islands, 2 – Hernando Island, 3 – Texada Island, and 4 – Bowen 71 age of 164.8 ± 1.6 Ma. Island. Red star indicates location of the study area. 90 .000000 .000000 12 OB-14-23 Johnstone, S. 2014. Petrogenesis of deformed intrusive pods in the Quatsino For- 5556000 5556000 References mation, Open Bay, Quadra Island, British Columbia. GSA Abstracts with Programs. Vol. Acknowledgements Bedrock geology of Open Bay, Quadra Island, British Columbia 164.8 ± 1.6 Ma Bustin, A.M.M., Clowes, R.M., Monger, J.W.H., Journeay, J.M., 2013. The southern Coast 46, No. 6. Vancouver BC. (October 2014) N=20, MSWD=1.3 Intrusions Stratigraphy Mountains, British Columbia: New interpretations from geological, seismic reflection, and Monger, J.W.H., 1991. Georgia Basin Project: structural evolution of parts of southern Funding for this project was provided by the Andy gravity data. Canadian Journal of Earth Sciences. Vol. 50, p. 1033-1050 Insular and southwestern Coast belts, British Columbia. In: Current Research, Part A, Geological Survey of Canada, Paper 91-1A, p. 219-228. Spencer Bamfield Research Grant, awarded an- .000000 Plag-phyric dacite dikes .000000 Canil, D., Johnston, S.T., Larocque, J., Friedman, R., and Heaman, L.M., 2013. Age, construc- Well-bedded and locally metamorphosed limestone 2m tion, and exhumation of the midcrust of the Bonanza arc, Vancouver Island, Canada, LITHO- Monger, J.W.H., and McNicoll, V.J., 1993. New U-Pb dates from southwestern Coast nually to faculty members in the Faculty of Science SPHERE, Vol. 5, No. 1, p. 82-91. Belt, British Columbia; in Radiogenic Age and Isotopic Studies: Report 7; Geological Diorite—granodiorite and Technology at Vancouver Island University. Rich- 5555800 Survey of Canada, Paper 93-2, p. 119-126. Massive and plag-phyric basalt, locally pillowed 5555800 Carlisle D. and Susuki, T.