Northern Hemisphere Reference Line Research Articles

Geochemical characterization of mid-Cretaceous granitoids of the Kootenay Arc in the southern Canadian Cordillera

Within southeast British Columbia, mid-Cretaceous granitoid batholiths are exposed in the Omineca Belt, a north–south-trending metamorphic and plutonic orogenic belt. The Bugaboo, Horsethief Creek, and Fry Creek batholiths are post-kinematic with respect to regional metamorphism that affected their host rocks, and are composed of hornblende and biotite granodiorites and granites in all three batholiths, and two-mica granites in Fry Creek. The biotite granites are weakly peraluminous, have initial εSr ranging from + 36 to + 56 and initial εNd ranging from −4.8 to −7.5, and overlap the range of Nd–Sr isotopic compositions for Precambrian basement gneisses and Proterozoic metasediments found in southeast British Columbia. The initial 206Pb/204Pb versus 208Pb/204Pb and 207Pb/204Pb for these granitoids lie in bands between model lower and upper crustal Pb isotopic compositions above the Northern Hemisphere Reference Line for oceanic basalts. These granitoids have high Nb and Rb and low Ba abundances relative to granites found in volcanic arcs. The two-mica granites are strongly peraluminous and have εSr ranging from + 170 to + 470, εNd ranging from −10 to−21, and more radiogenic initial Pb-isotope ratios than the biotite granites. The two-mica granites have trace element compositions similar to those of granites found in within-plate and collisional tectonic settings.We favor a model for crustal anatexis of Precambrian basement gneisses and Proterozoic metapelites to produce the mid-Cretaceous biotite granites and two-mica granites, respectively. Crustal anatexis was likely in response to crustal thickening that occurred during the Mesozoic in southeast British Columbia as exotic terranes collided with and were accreted to the western edge of the North American continent.

Isotopic characteristics of Hannuoba basalts, eastern China: Implications for their petrogenesis and the composition of subcontinental mantle

Alkalic and tholeiitic basalts from the Hannuoba region of eastern China have 87 Sr 86 Sr and 143 Nd 144 Nd ranging from the prevalent mantle (PREMA) field to inferred bulk Earth ratios. Ratios of 206 Pb 204 Pb range from 17.10 to 17.94 and all samples have 207 Pb 204 Pb and 208 Pb 204 Pb ratios significantly above the Northern Hemisphere reference line (NHRL). Although the oldest Hannuoba lavas are alkalic basalts, alkalic and tholeiitic lavas are intercalated, and they are isotopically distinct. Relative to the alkalic lavas, tholeiitic lavas have higher 87Sr 86 Sr , lower 143 Nd 144 Nd and less radiogenic Pb isotope ratios. The isotopic ratios and incompatible-element trends defined by all Hannuoba lavas are consistent with mixing of isotopically and compositionally distinct components. We infer that the alkalic basalts were generated by small degrees of melting of an asthenospheric source, similar to that required for some oceanic basalts. However, the tholeiitic basalts contain an enriched mantle component, similar to EMI in the oceanic mantle, which may have resided in the upwelling asthenosphere or subcontinental lithospheric mantle. Mixing between these mantle components may have occurred during the rifting event that caused extension in northeast China ∼ 40 Ma ago. Specifically, we propose that melting within the upwelling asthenospheric created the Hannuoba alkalic lavas whereas the dominantly younger tholeiitic lavas resulted from partial melting of continental lithosphere that was heated by the ascending asthenosphere. Similar models have been proposed for continental basalts erupted in the western U.S.A. and British Tertiary Province.