Origin of Basalts in a Hot Subduction Setting: Petrological and Geochemical Insights from Mt. Baker, Northern Cascade Arc

Abstract

The northern Cascade arc is an end-member ‘hot’ subduction zone where slab dehydration may be essentially complete prior to reaching sub-arc depths, presenting a potential problem for a flux melting origin for arc basalts. Nevertheless, mafic lavas from the Mt. Baker volcanic field, the most magmatically productive volcano in the northern Cascade arc, record subduction input and compositions typical of calc-alkaline magmas. Relative to normal mid-ocean ridge basalt, the most primitive lavas at Mt. Baker show elevated abundances of large ion lithophile elements (LILE), Pb and light rare earth elements (LREE). High field strength elements (HFSE) and heavy REE (HREE) are depleted relative to LILE. Reconstructed primary magmas define three groups: Group I calcalkaline basalts (Sulphur Creek, Lake Shannon, Hogback) have lower SiO2, (La/Yb)N and LILE/HFSE, and are olivineand plagioclase-phyric; Group II high-Mg basaltic andesites (Tarn Plateau, Cathedral Crag) also contain clinopyroxene phenocrysts and have higher H2O, SiO2, (La/Yb)N and LILE/HFSE; Group III low-K olivine tholeiite (Park Butte) has the lowest (La/Yb)N but highest LILE/HFSE.The redox states of all groups lie between QFM (quartz^fayalite^magnetite) and QFM þ1·4. Pb, Sr and Nd isotope compositions are similar among all the analysed samples and are consistent with a depleted mantle source modified by input from the subducting slab. Trace element^isotope modeling indicates a subduction component composed predominantly of metabasaltderived fluid with lesser amounts of sediment melt and metabasalt melt. Group I basalts record the smallest melt fractions ( 5^7%), lowest water contents (1·5^2·1wt %), and highest temperatures and pressures of mantle segregation (up to 13548C, 1·5 GPa) from lherzolitic ( spinel) residues. Group II basaltic andesites show the greatest extents of mantle metasomatism, the highest water contents (2·7^3·7 wt %) and partial melt fractions (10^12%), and segregated from harzburgite at 12708C, 1GPa, consistent with pooling of melts at the Moho. Group III records P^Tconditions similar to Group I ( 1·4 GPa, 13268C) but melt fractions (12%) and mantle residues (harzburgite) are more similar to Group II, and H2O contents ( 2·1wt %) are intermediate. Melting beneath Mt. Baker was initiated by dehydration melting of amphibole peridotite at 95 km and 10208C, within the stability field of garnet lherzolite. Initial melt fractions were small ( 1^2%) and near water-saturation. Phase equilibria and trace element modeling show no evidence for garnet-bearing mantle residues, indicating that progressive melting during ascent of diapirs through the hot core of the convecting mantle wedge reduced H2O contents and erased any residual garnet signature. Because fluid release from the slab is restricted to the forearc, mantle hydrated at shallow depths in the serpentine and/or chlorite stability fields must be down-dragged to the region of amphibole stability to initiate dehydration melting.