Influence of slab thermal structure on basalt source regions and melting conditions:REE and HFSE constraints from the Garibaldi volcanic belt,northern Cascadia subduction system

摘要

Garibaldi volcanic belt (GVB)basalts were erupted above the relatively young (<24 Ma)Juan de Fuca plate,which comprises the subducted oceanic lithosphere that becomes progressively younger (22-13 Ma),and presumably hotter,northward along the northern Cascadia convergent margin.Primitive and near-primitive mafic lavas of the 15-km-wide volcanic belt change from high-alumina olivine tholeiites and magnesian andesites near Glacier Peak,northwestern Washington,through transitional basalts to alkali-olivine basalts and basanites in the Bridge River-Salal Glacier areas,southwestern British Columbia.The distribution of different basalt types is consistent with varied source conditions imposed by differences in the thermal structure of the underlying subducted plate.Significant arc-parallel variations characterize REE and HFSE contents in GVB basalts and suggest that source enrichment processes and melting conditions vary within the mantle wedge as the age and thermal state of the underlying subducted plate changes.More northerly GVB basaltic suites tend to have higher TiO_2,Nb,Ta,total REE,La,Sm/Yb,Nb/ Yb,Ti/V,Y/Sc and Zr/Yb and lower Th/U,Zr/Ti and Zr/Nb than their southern counterparts.The basalts have sub-chondritic to chondritic Nb/Ta (6-21)and super-chondritic Zr/Hf (up to 55.90)ratios that exhibit positive correlation.Only Mount Baker and Glacier Peak basalts exhibit the distinctive negative Nb-Ta anomalies associated with arc lavas.Inter-HFSE and REE fractionations (including La/Yb,La/Nb and Ce/Pb)show significant correlations with the inferred age of the underlying subducted plate.Proportions of slab-derived HFSE-REE components (SC)transferred to basalt sources in the Cascadia mantle wedge appear to vary from negligible (Ti,Nb,Ta,Zr,Hf,Y,Sm,Eu and Tb:less than 15% SC)to perceptible (Nd:up to 35% SC)through moderate (La:up to 75% SC)to substantial (U,Th and Pb:up to 95% SC).Arc-parallel HFSE-REE variations in primitive GVB basalts cannot be explained by variable degrees of depletion produced during prior episodes of melt generation in the mantle wedge.Instead,these differences in basalt chemistry probably reflect different extents of melting of a regionally homogeneous,locally heterogeneous,mantle wedge under conditions influenced by the thermal structure of the underlying subduction zone.Phase relationships and REE systematics of the primitive to near-primitive basalts argue that conditions of magma generation beneath the Bridge River,Salal Glacier,Meager Mountain and Cheakamus Valley lava fields involved lower degrees of melting,higher pressures,and mantle sources richer in garnet than those beneath Mount Baker and Glacier Peak.In addition,Nb/Ta in the Glacier Peak basalts exhibits slight positive correlations with Nb,Ta,La/Yb,and Th/Yb but not Nb/La or Nb/Th,implying that a residual mineral,most likely rutile,controlled extremely low HFSE partitioning into subduction-related fluids that equilibrated with mantle source regions above older,colder portions of the subducted plate.