ORIGIN AND GEOCHEMICAL EVOLUTION OF MAFIC MAGMAS FROM MOUNT BAKER IN THE NORTHERN CASCADE ARC, WASHINGTON: PROBES INTO MANTLE AND CRUSTAL PROCESSES

Five mafic lava flows located on the southern flank of Mount Baker are the focus of this study. These lavas are among the most primitive in the volcanic field. A comprehensive whole rock and mineral chemistry dataset reveals the diversity between these mafic lavas, which come from distinct sources and have been variably affected by ascent through the crust.

In evaluating possible mantle sources, at least three endmember lava types are represented. These include modified low-K tholeiitic basalt (LKOT), typical calcalkaline lavas, and high-Mg andesite. The first type, the basalt of Park Butte (49.3-50.3 wt.% SiO₂, Mg# 64-65), is classified as low-K, and has major element chemistry similar to LKOT found elsewhere in the Cascades. Park Butte also has the lowest abundances of Zr and middle REE among the flows, indicating it is either derived from the most depleted mantle source or has undergone the largest degree of partial melting. However, unlike other Cascade LKOT, it also displays enrichment of LILE and depletion of Nb and Ta typical of calcalkaline lavas. A second lava type is represented by the basalts of Lake Shannon (50.7-52.6 wt.% SiO₂, Mg# 58-62) and Sulphur Creek (51.2-54.6 wt.% SiO₂, Mg# 56-57). These two lavas are comparable to calcalkaline rocks found in arcs worldwide, but display different degrees of source depletion (Lake Shannon samples have lower Zr and middle REE than Sulphur Creek). The third lava type is represented by the basaltic andesites of Tarn Plateau (51.8-54.0 wt.% SiO₂, Mg# 68-70) and Cathedral Crag (52.2-52.6 wt.% SiO₂, Mg# 55-58). They are more silicic than the other lavas, and are strongly depleted in HREE, with intermediate Zr and middle REE. The Tarn Plateau basaltic andesite can be described as a typical high-Mg andesite, in that it has higher Mg# than the basalts of this study.

Pervasive disequilibrium textures present in all of the lavas indicate that crustal processes have modified the geochemical composition of the magmas. The evidence cannot clearly point to one process or another (i.e. accumulation of olivine, magma mixing), but some combination of processes must be invoked to explain the disequilibrium in the lavas. However, lack of within flow variation in many of the most highly incompatible elements and other trace element signatures still allow for interpretation of mantle sources.