Lead-isotope whole-rock ratios of the Triassic flood basalts of the Karmutsen Formation are heterogeneous, with 206Pb/2MPb = 18.72–21.51, 207Pb/204Pb = 15.56–15.77, and 208Pb/204Pb = 38.16–40.15. Whole-rock lead initial ratios are coincident with galena analyses having 206Pb/204Pb = 18.1–19.0, 207Pb/204Pb = 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 whole-rock 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.
Geological significance of the newly discovered Middle Permian ocean island basalt‐type gabbros in Ewulang,
Nianqing‐Sumdo
area, Tibet
