Olivine and Chromian Spinel in Primitive and Near-Primitive Garibaldi Belt lavas: Implications for Sub-arc Mantle Sources in the 'Hot' Northern Cascadia Subduction System

Low-volume mafic lavas of the Garibaldi volcanic belt exhibit a systematic along-strike change from high-alumina olivine tholeiite, LILE- and LREE-enriched, HFSE-depleted calc-alkaline basalts and magnesian andesites in northwestern Washington through transitional basalts to alkali-olivine basalts and trachybasalts in southwestern British Columbia. Northward variation in basalt type accompanies a decrease (approximately 7 m.y.) in age of young (<22 m.y.), relatively hot Juan de Fuca oceanic crust subducted beneath the volcanic front. Arc-parallel variations in both major- and trace element basalt chemistries, which have been related to impact of slab thermal structure on magma generation in sub-arc mantle, have suggested that mantle heterogeneity had only a limited influence on GVB basalt compositions, which were controlled largely by different degrees of mantle melting and inputs of slab-derived fluxes. Electron-microprobe analyses of olivine-spinel pairs in primitive and near-primitive GVB basalts (MgO >6.0, up to 11.28 wt %; >100 ppm Ni and Cr) are used to constrain further the nature and extent of source heterogeneity beneath the volcanic belt. The lavas are aphyric or sparsely porphyritic (<5% phenocrysts), with olivine as the only or dominant phenocryst and microphenocryst phase; plagioclase and extremely rare clinopyroxene are solely present as microphenocrysts. Olivines in all lavas show considerable compositional range, but most phenocryst cores exhibit compositions between Fo₈₂ and Fo₈₉, contain up to 0.35 wt % NiO and 0.1 wt % CoO, and crystallized at high temperatures (> 1235° C). Chromian spinels included in olivines display a large range in Cr # (22-69) that correlate negatively with Fo content of host olivine. Ti contents (<2 wt %) in spinels decrease with increasing Al (10-42 wt %), Fe²⁺/Fe^Total (0.53-0.74 based on stoichiometry) and Mg # (42-65). Considering most primitive olivine-spinel pairs, (1) Cr # ranges from 33 to 49 with no systematic along-strike compositional variation in spinels of forearc basalts from Glacier Peak northward to Meager Mountain; (2) spinels in Meager Mountain trachybasalt and behind-the volcanic-front trachyandesites/magnesian andesites in the central GVB have elevated Cr # (57-62), suggesting more depleted source regions than associated basalts; and (3) spinels in alkalic basalts of northernmost GVB suites (Bridge River and Salal Glacier) have the lowest Cr # (22-28) and highest Mg # (54-64), consistent with slightly more enriched, possible deeper, source regions in the subarc mantle. Spinel compositions suggest that geochemical diversity of GVB basalts may reflect local, but not along-strike variability in fertility of mantle sources.