HIGH-SR MAGNESIAN ANDESITES OF THE RAFFUSE CREEK-MAMQUAM RIVER AREA, SW BRITISH COLUMBIA: EVIDENCE FOR MULTISTAGE EVOLUTION OF HYDROUS MANTLE MELTS

Late Cenozoic high-Sr magnesian andesites and basaltic andesites (3.8-5.2 wt. % MgO; Mg#=0.48-0.62; 49-105 ppm Ni; 41-102 ppm Cr; 750-2030 ppm Sr) erupted in southwestern British Columbia. The andesitic lavas form both early and late subglacial domes on valley slopes adjacent to the Holocene Ring Creek dacite flow, 10-15 km east of Squamish. These Garibaldi belt eruptive suites are associated with subduction of the young (<15 Ma) and relatively ‘hot’ Juan de Fuca plate beneath North America. Weakly porphyritic basaltic andesites have plagioclase (An_80-32), augite (Wo_32-46En_39-50Fs_11-22), hypersthene (En_81-68), and rare olivine (Fo_83-67), resorbed amphibole and magnetite microphenocrysts. Mafic andesites contain plagioclase (An_78-23), decomposed amphibole (Mg#=0.71-0.57), orthopyroxene and/or augite microphenocrysts. Ring Creek dacites are characterized by sieve-texture to moth-eaten plagioclase (An_72-27) phenocrysts and orthopyroxene (En_75-68) and magnetite microphenocrysts; lava samples also contain phenocrysts and microphenocrysts of hornblende (Mg#=0.72-0.50) alone or hornblende plus biotite (Mg#=0.84-0.70). Many lavas contain small cognate xenoliths and/or fragments of partially fused basement rock.

The lavas are characterized by overall enrichments in K, Ba, Rb, Rb/Sr, Ba/Sr, and Ba/Nb and depletions in Ti, total iron, Mn, Mg, Ca, Cr, Ni, V, Y, Zn, and K/Rb with increasing silica. Intrasuite variations are compatible with crystal fractionation along either hydrous or anhydrous paths. High-Sr, medium-K lavas have low Rb (7-13 ppm), Nb (1.7-4.5 ppm), Ta (0.08-0.24 ppm) and Th (1.0-2.6 ppm) contents; those containing hornblende show greatest Sr-enrichment (Sr/Y ratios=40-100), but at somewhat higher Y contents (10-19 ppm) than most presumed slab melts. These calc-alkaline lavas, which exhibit LREE-enrichment (La/Sm=4.0-6.3), flat to slightly concave-upward HREE patterns and Yb_N=7-11, are unlikely to represent melts of eclogitic or amphibolitic source materials. Observed major and trace element variations suggest the multistage evolution of a hydrous, high degree (10-20%) partial melt of mantle peridotite modified by possible interaction with lower crustal (granulitic) melts and subsequent fractional crystallization.