HR: 0830h
AN: V31C-0948    [PDF]
TI: Sr-Pb isotopic studies of primitive and near-primitive basaltic magmas, Garibaldi volcanic belt, northern Cascadia subduction system
AU: * Green, N L
EM: ngreen@wgs.geo.ua.edu
AF: University of Alabama, 202 Bevill Building, Tuscaloosa, AL 35487 United States
AU: Sinha, A K
EM: pitlab@vt.edu
AF: Virginia Polytechnical Institute and State University, 4044 Derring Hall, Blacksburg, VA 24061 United States
AB: The northern Cascadia subduction system is intimately associated with aseismic subduction of extremely young and presumably `hot' oceanic lithosphere beneath northwestern Washington and southwestern British Columbia. Sr and Pb isotopic analyses are presented for primitive and near-primitive ($>$6.0 wt. % MgO) basalts from a southeast-northwest transect along the Garibaldi volcanic belt (GVB), which overlies subducted oceanic lithosphere that decreases in age from ca. 22 m.y. below Glacier Peak at its southern end to about 13 m.y. beneath the northernmost eruptive centers in the Mosaic (Meager Mountain) and Salal Glacier-Bridge River areas. The basaltic rocks have initial $^{87}$Sr/$^{86}$Sr ratios of 0.70317-0.70426, with minimum observed values in individual lava suites decreasing northward. Values of Pb isotopic ratios for GVB basaltic suites range from 18.22 to 18.97 for $^{206}$Pb/$^{204}$Pb, from 15.51 to 15.59 for $^{207}$Pb/$^{204}$Pb, and from 37.73 to 38.49 for $^{208}$Pb/$^{204}$Pb. Sr isotopic compositions, unsupported by lava Rb contents, show positive correlations with Cs/Rb, La/Nb, Ba/La, Ba/Nb, Ba/Ta, B/La, B/Zr, Sr/Nd, and primitive mantle normalized Sr/P; and negative correlations with high field strength elements (HFSE: Nb, and Ta), FeO and other transition metals (Co and Zn), Ce/Pb,Cr/Ni, Sm/Yb, Ta/Yb, Hf/Yb, K/Ba, K/Sr, and La/Yb. Moderate to strong correlations between $^{87}$Sr/$^{86}$Sr and ratios involving fluid-mobile and less-fluid-mobile elements are compatible with decreased slab input northward along the volcanic front as the subducted plate becomes hotter. The Pb isotopic compositions exhibit only limited variations when examined against position along the volcanic arc, but show uniform to extremely weak positive correlations of $^{206}$Pb/$^{204}$Pb, $^{207}$Pb/$^{204}$Pb and $^{208}$Pb/$^{204}$Pb with $^{87}$Sr/$^{86}$Sr, Ba/La, Sr/Nd, Ba/Nb, Zr/Nb, Mg-Number and SiO2; and similarly weak negative correlation of Ce/Pb with $^{206}$Pb/$^{204}$Pb and $^{207}$Pb/$^{204}$Pb. The isotopic data suggest that GVB basaltic magmas have experienced with little or no involvement of old continental material, but may be consistent with the combined effects of melting of mantle and subordinate anatexis of isotopically juvenile terranes (e.g., Wrangellia). When plotted against corresponding $^{206}$Pb/$^{204}$Pb, the $^{207}$Pb/$^{204}$Pb values of GVB basalts plot above whereas $^{208}$Pb/$^{204}$Pb values plot below the Northern Hemisphere Reference Line (Hart, 1984); similar variations apparently characterize plutonic rocks of the underlying Coast Plutonic Complex. The basalt values therefore may reflect either similar extents of crustal input or possibly a long-lived U-enriched, Th-depleted character of mantle sources above the northern Cascadia subduction system.
DE: 1040 Isotopic composition/chemistry
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 9350 North America
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting