PETROGENESIS OF INTERMEDIATE MAGMAS AT MOUNT BAKER VOLCANIC FIELD, NORTHERN CASCADE ARC

The Mount Baker volcanic field, located in the Garibaldi volcanic belt (GVB) of the northern Cascade arc, has been continuously active since 1.3 Ma. Andesitic lavas are the most voluminous component of the Mount Baker volcanic field, however, only limited studies have thus far been aimed at understanding their origin. We show that open-system processes are dominant in the petrogenesis of four andesitic lava flows of the Mount Baker volcanic field: andesites of Dobbs Cleaver (adb) (58-59% SiO₂, Mg# 49-52), Dobbs Creek (ado) (56-57% SiO₂, Mg# 57-58), Coleman Pinnacle (acp) (57-65% SiO₂, Mg# 40-50) and Swift Creek (asw) (55-56% SiO₂, Mg# 60-61). All lavas are calc-alkaline and med-K, with the exception of the high-K, hornblende-bearing acp. Trace element and REE patterns of three of the lava flows suggest a cogenetic relationship, but the same is not true of the high-K acp. The acp samples are the most enriched in LREEs and most depleted in HREEs. They display large variations in Sr concentrations that cannot be explained by fractional crystallization. The andesitic to dacitic acp, with scarce pyroxene and olivine, may be parental to a previously described, enigmatic hornblende-bearing dacite of Nooksack Falls that shares similar characteristics. With the exception of acp, plagioclase, augite, orthopyroxene and olivine are the dominant mineral phases observed. These minerals display complex zoning patterns and reaction and disequilibrium textures, which are indicative of open-system processes. Furthermore, the abundance of several populations of cumulophyric clots in the four flows suggests an even more complex history of open-system processes, which no study has previously characterized. Scanning electron microscope analyses provide insight into the geochemical variation of the cumulophyric clots and their role in petrogenesis of the andesites at the Mount Baker volcanic field.