Lead- and strontium-isotope geochemistry of the Karmutsen Formation, Vancouver Island, British Columbia

Abstract

Lead-isotope whole-rock ratios of the Triassic flood basalts of the Karmutsen Formation are heterogeneous, with '06Pbl 2"Pb = 18.72 -21.51, 207Pb/2"Pb = 15.56 -15.77, and 208Pb/2"Pb = 38. 16 -40.15. Whole-rock lead initial ratios are coincident with galena analyses having 206Pb/204Pb = 18.1 -19.0, 2 0 7~b / 2 0 4~b = 15.53 -15.60, and 208Pb/204Pb = 38.2 -38.6. Lead, uranium, and thorium concentrations are greater than for mid-ocean-ridge basalts, less than for island arcs, but similar to those in ocean-island and continental flood basalts. Isotopic similarities exist between the Columbia River -Chilcotin continental flood basalts and the Karmutsen Formation, suggesting similar origins for these basalts. The heterogeneity in wholerock ratios can be explained in terms of isotopic mixing between a lead-rich average crustal lead type and an ocean-island basalt-type mantle source. Thus the Karmutsen Formation displays evidence for sediment contamination of the mantle.Les rapports isotopiques du plomb sur roche totale des plateaux de basalte de la Formation de Karmutsen, d'lge triasique, varient considCrablement : 206Pb/2MPb = 18,72-21,51, 207Pb/204Pb = 15,56-15,77 et 208Pb/204Pb = 38,16-40,15. Les rapports initiaux des isotopes du plomb sur roche totale coincident avec ceux des analyses du plomb de la galBne dont les valeurs obtenues sont 206Pb/204Pb = 18,l-19,0, 207Pb/204Pb = 1533 -15,60 et 208~b/2"Pb = 38,2 -38,6. Les concentrations en plomb, uranium et thorium sont plus ClevCes que celles des basaltes de c&te mCdio-ockanique, inferieures B celles des arcs insulaires mais analogues B celles des basaltes de plateaux continentaux et d'iles oc6aniques. Des ressemblances isotopiques existent entre les basaltes des plateaux continentaux de Columbia River -Chilcotin et de la Formation de Karmutsen, suggCrant une origine commune pour ces basaltes. La variabilitk des rapports sur roche totale peut Ctre expliquke en termes d'un mClange isotopique entre un type de plomb crustal, dont la teneur moyenne en plomb est ClevCe, et une source mantellique foumissant un type de basalte d'ile odanique. La Formation de Karmutsen pksente donc des indices de contamination du manteau par assimilation de skdiments.[Traduit par la revue]Can. I. Ehth Sci. 26, 908-919 (1989)

Lead isotope systematics of sulfide minerals in the Middle Valley hydrothermal system, northern Juan de Fuca Ridge

“…The sources of the turbiditic sediments to Middle Valley are from the north, including detritus mostly from Vancouver Island. Pb isotopic studies of the Mesozoic rocks and sulfide mineral deposits from central Vancouver Island show that these rocks are much too radiogenic to be the old sedimentary component seen in Middle Valley sulfides (Figure 6) [ Andrew et al , 1991; Andrew and Godwin , 1989a; Andrew and Godwin , 1989c].…”

“…Comparison of the Pb isotopic composition of massive sulfides from Middle Valley with those of major Besshi‐type mining districts [ Andrew et al , 1991; Andrew and Godwin , 1989a; Andrew and Godwin , 1989b; Andrew and Godwin , 1989c; LeHuray , 1984; McCutcheon et al , 1993; Peter and Scott , 1997; Sato and Sasaki , 1976; Swinden and Thorpe , 1984] and other modern seafloor hydrothermal vent deposits [J. Peter, unpublished data, 1998; Hegner and Tatsumoto , 1987; LeHuray et al , 1988]. Stacey‐Kramers average continental crust evolution curve is shown with crosses every 0.5 Ga. NHRL, Northern Hemisphere Reference Line [ Hart , 1984].…”

“…composition of the nonbasaltic component ranges from that of modern surface sediments to a continental crust-like component with a model age of $1.5 Ga. This old component cannot be derived from the mantle (via basalts) and cannot be derived from deep Pacific seawater [von Blanckenburg et al, 1996] Figure 6) [Andrew et al, 1991;Andrew and Godwin, 1989a;Andrew and Godwin, 1989c].…”

“…Recent U-Pb studies of the Monashee Complex, southern British Columbia, show that some of these metamorphic rocks include zircon grains that are as old as 2.3 Ga (J. Crowley, personal Figure 6. Comparison of the Pb isotopic composition of massive sulfides from Middle Valley with those of major Besshi-type mining districts [Andrew et al, 1991;Andrew and Godwin, 1989a;Andrew and Godwin, 1989b;Andrew and Godwin, 1989c;LeHuray, 1984;McCutcheon et al, 1993;Peter and Scott, 1997;Sato and Sasaki, 1976;Swinden and Thorpe, 1984] and other modern seafloor hydrothermal vent deposits [J. Peter, unpublished data, 1998;Hegner and Tatsumoto, 1987;LeHuray et al, 1988].…”