PB ISOTOPES FROM MT. BAKER VOLCANIC FIELD AND CHILLIWACK BATHOLITH, CASCADE ARC: INSIGHTS ON THE INTERMONTANE-INSULAR TERRANE BOUNDARY

The Mt. Baker volcanic field (MB: 1.3 – 0 Ma) in the northern Cascade arc may represent the continuation of magmatism in the adjacent Chilliwack batholith (CB: 34 – 2.75 Ma). However, new high resolution Pb isotopic data for whole rock samples ranging from 50-75 wt.% silica show no overlap between MB and CB. On plots of initial ²⁰⁸Pb/²⁰⁴Pb or ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb, MB and CB display linear trends. The MB data are isotopically more primitive and define a steeper trend. Samples ranging from 9.8 ka basalt to 3.72 Ma rhyolite define the MB trend, whereas samples ranging from 34 Ma gabbro to ~4 Ma granite define the CB trend. Initial Pb isotopic ratios are not correlated with sample age. Both suites show increasing Pb isotopic ratios with silica content, implying compositional diversity was controlled by mixing and/or AFC. Isotopic compositions of adjacent country rocks preclude upper crustal assimilation. Pb isotopic ratios are also correlated with sample location; the CB trend is defined by samples east of Hannegan caldera. Although the MB-CB region is underlain by two distinct crustal blocks (NW Cascades thrust system and Cascades core) separated by the Straight Cr. fault, the fault does not coincide with the isotopic boundary between the MB and CB suites. Lower crustal processes appear to control the Pb isotopic compositions. The lower crust beneath MB must be fundamentally different than beneath CB, probably due to a deep boundary between accreted terranes. To identify the terranes, we generated a ~500 km long W-E isotopic profile of the lower crust across the Insular, Coast, and Intermontane belts of southeastern B.C. using data from Mesozoic/Tertiary magmatic rocks as probes of the lower crust. The transect is underlain by terranes that may form the deep crust of the MB-CB area. Pb data are sparse so more abundant Sr and Nd data were used to construct profiles. The Nd profile shows an abrupt decrease of ~2.5 εNd within the Coast Belt across the suture between the Insular and Intermontane superterranes. Sr and Nd data from MB and CB exhibit a similar transition, implying that the MB/CB isotopic distinction (best observed in the Pb data) is a result of Wrangellian lower crust beneath MB and Quesnellian lower crust beneath CB. The diversity in the Pb data is due to interaction of mantle melts with the different lower crustal compositions.