Sr, Nd, Pb and Os Isotopic Compositions of Lavas From the Mount Baker Volcanic Field, Cascade Arc

We present the results of a trace element and Sr, Nd, Pb and Os isotopic study of the Mt. Baker volcanic field (MBVF), part of the northern segment of the Cascade magmatic arc known as the Garibaldi Belt. To date, only 4 Sr isotopic ratios (all from the Sulphur Creek flow) have been published. The Mount Baker volcanic field extends to 3.72 Ma and a case can be made for continuous magmatic activity in this region extending from 34 Ma to present. Our goal is to use isotope ratios to characterize the mantle source regions that underlie the Garibaldi Belt, to document the chemical inputs of slab fluid/melt, sediment, and lower crust, and to assess temporal and spatial variations in these factors. We measured 29 Sr and Nd isotopic ratios, 8 Pb isotopic ratios, and 9 Os isotopic ratios, representing the full age range and compositional diversity (calc- alkaline basalt through rhyolite) of the MBVF, including all known MBVF basalts. A 22.86-Ma gabbronorite from the adjacent Chilliwack batholith was analyzed as an analog for the modern mafic lower crust. All Mt. Baker lavas are calc-alkaline with the arc-characteristic signatures of HFSE depletion and LILE enrichment. MBVF 87Sr/86Sr values (0.703932 to 0.703057) and ɛNd (+4.71 to +7.79) are well correlated and lie within the mantle array. Mt. Baker Sr and Nd data are indistinguishable from other Garibaldi belt lavas (Green & Harry 1999, Green & Sinha 2005), and also overlap data from the neighboring Chilliwack batholith (Tepper 1996; Tepper et al. 1993). In contrast, central and southern Cascade arc lavas with similar Sr ratios have corresponding ɛNd values that are lower by ~2 epsilon units. The Garibaldi Belt and Chilliwack magmas are tapping a mantle source distinct from that of the rest of the Cascade arc. 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of MBVF basalts plot close to the NHRL, in a linear trend between Juan de Fuca MORB and Pacific sediment, indicating a sediment contribution to the MBVF magmas. With the exception of the Sulphur Creek flow, Pb isotopic values are well correlated with Nd and Sr. (Sr/P)N, commonly used as an indicator of a slab fluid-derived component, ranges from 1.4 to 5 but correlates poorly with Sr, Nd and Pb isotopic ratios. The similarity of isotopic and trace element data among Mt. Baker and Chilliwack magmas is consistent with the hypothesis that magmas of the MBVF represent a modern continuation of the Chilliwack magmatic system. Mafic and felsic MBVF rocks are indistinguishable in their Sr and Nd isotopic compositions, most likely because the lower crust beneath Mt. Baker is primarily Cenozoic in age and mafic in composition (the root of a long-lived arc system). In an attempt to 'see through' this problem, we measured Os isotopic ratios on whole rocks and mineral separates as only a few million years are required for crustal 187Os/188Os ratios to evolve from mantle values. MBVF basalt γOs values range from +70 to +522. The most primitive sample is Table Mountain andesite (γOs 65). The γOs values and Os concentrations (1.56 to 17.7 ppt) do not correlate with any major or trace element trends or with Sr, Nd, or Pb isotope ratios. Given that primitive arc mantle-derived magmas have γOs values of ~10, we hypothesize that Cenozoic lower crust has contributed substantially to the modern Mount Baker magmas via assimilation, melting and mixing. It is likely that Cenozoic lower crust would have variable Os isotopic compositions but would be virtually homogeneous in terms of Sr, Nd, and Pb isotopic ratios.