PETROLOGY AND GEOCHEMISTRY OF LATE PLEISTOCENE ANDESITE LAVA FLOWS FROM MT. BAKER, WASHINGTON

Mt. Baker, an andesitic stratovolcano located in the Garibaldi volcanic belt of the Cascade magmatic arc, has erupted a wide range of compositions since 0.99 Ma, with 92% by volume as andesite. Physical and chemical processes responsible for generating these andesitic lavas are unknown, as no geochemical data apart from major elements have been available. Presented here are major oxide concentrations, and for the first time, trace and REE concentrations for three Late Pleistocene and Holocene lava flows: the basaltic andesite of Sulphur Creek (52.5 to 57.6 wt.% SiO ₂), the andesite of Glacier Creek (59.5 to 63.3 wt.% SiO ₂), and the andesite and dacite of Boulder Glacier (60.5 to 64.1 wt.% SiO₂). The goal of this work is to characterize the relationship between these flows, if any, and to evaluate their petrogenesis.

The three lava flows are classified as medium K, and to a lesser degree, high K, calc-alkaline basalts through dacites. Major oxide concentrations for Sulphur Creek and Boulder Glacier lavas form curvilinear trends with increasing SiO₂. Relative to the other lavas, the andesite of Glacier Creek has distinct major oxide chemistry, with higher wt.% MgO and CaO, and lower wt.% Na₂O, P₂O₅, and K₂O for a given wt.% SiO₂. Glacier Creek lavas also show enrichment of Ni, Cr, and Sr and depletion in La, Nb, Ta, Zr and Y relative to the other lavas.

The more evolved lavas have lower REE abundances relative to the mafic Sulphur Creek lavas, but the abundances cannot be correlated with degree of differentiation. However, LREE depletion is correlated with increasing La/Yb ratio in the Sulphur Creek and Glacier Creek lavas, with LREE abundances of 73.9 and 67.1 times chondrite, respectively, and La/Yb of 4.5 and 6.7, respectively. The Boulder Glacier lavas have enriched LREE abundances relative to the other lavas at 85.0 times chondrite with intermediate La/Yb of 6.4.

These data indicate that there is not a simple petrologic relationship between the above lavas. Crystal fractionation from the basaltic andesite of Sulphur Creek cannot account for higher MgO in the more differentiated Glacier Creek lavas. This requires multiple mantle sources for these lavas. However, the andesite and dacite of Boulder Glacier may be related to the basaltic andesite of Sulphur Creek through complicated crystal fractionation and/or magma mixing processes.