Mafic rocks in the Chilliwack are divided into a medium-K series (MKS) and a low-K series (LKS). The former contain 0.7 - 2.4 wt.% K2O and are similar in composition to calc-alkaline basalts and basaltic andesites. Inverse REE modeling supports derivation of the MKS by 9 - 27% melting of a garnet-free, LREE-enriched source (La/YbN approx. equal to 2). Chilliwack LKS gabbros have chemical characteristics of low-K olivine tholeiites, including low K2O (0.3 - 0.5 wt.%) and La/YbN (1.7-3.4), and high CaO (8.8 - 11.3 wt.%) and Na2O/K2O (6-22). These traits suggest a source with more clinopyroxene and lower La/YbN than the MKS source. Differences in eNd(0) between MKS and LKS gabbros suggest lower Nd/Sm is a long-lived LKS source characteristic. Lithologic variation within composite plutons of both series resulted primarily from multiple intrusion of related magmas, in some cases differentiates of a common parent. Two contrasting examples were studied in detail. At Mt. Sefrit, MKS variation (gabbronorite - quartz diorite) is modeled by low-pressure fractionation (ol + plag + cpx), accompanied by ~10% wallrock assimilation. In contrast, chemical and Sr-Nd isotopic variation among LKS gabbro - quartz diorite at Copper Lake points to crystallization dominated by clinopyroxene + plagioclase + Cr-spinel, indicative of differentiation at pressures > 10 kb, although the assimilant in this case is poorly constrained.
Chemical and isotopic similarities between these mafic plutons and Quaternary Cascade lavas indicate that mafic magmas present during the production of Chilliwack granitoids were low- and medium-K arc basalts.
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