EarthRef.org Reference Database (ERR) -- Tepper et al. 1993

Calc-alkaline granitoid rocks of the Oligocene-Pliocene Chilliwack batholith, North Cascades, range from quartz diorites to granites (57-78% SiO2), and are coeval with small gabbroic stocks. Modeling of major element, trace element, and isotopic data for granitoid and mafic rocks suggests that: (1) the granitoids were derived from amphibolitic lower crust having REE (rare-earth-element) and Sr-Nd isotopic characteristics of the exposed gabbros; (2) lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting. The main effect of f(H2O) variation is to change the relative proportions of plagioclase and amphibole in the residuum. The REE data for intermediate granitoids (quartz diorite-granodiorite; Eu/Eu* = 0.84 0.50) are modeled by melting with f(H2O) < 1 kbar, leaving a plagioclase + pyroxene residuum. In contrast, data for leucocratic granitoids (leuco-granodiorites and granites; Eu/Eu* = 1.0-0.54) require residual amphibole in the source and are modeled by melting with f(H2O) = 2-3 kbar. Consistent with this model, isotopic data for the granitoids show no systematic variation with rock type (Sr-87/Sr-86i = 0.7033-0.7043; epsilonNd(0) = + 3.3 to + 5.5) and overlap significantly with data for the gabbroic rocks (Sr-87/Sr-86, = 0.7034-0.7040; epsilonNd(0) = + 3.3 to + 6.9). The f(H2O) variations during melting may reflect additions of H2O to the lower crust from crystallizing basaltic magmas having a range of H2O contents; Chillwack gabbros document the existence of such basalts. One-dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for largescale melting of amphibolitic lower crust, provided that ambient wallrock temperatures exceed 800-degrees-C. Based on lithologic and geochemical similarities, this model may be applicable to other Cordilleran batholiths.