|Institution:||Western Washington University|
|Keywords:||Andesite – Washington (State) – Baker; Mount – Analysis; Petrogenesis – Washington (State) – Baker; Mount; Mineralogy – Washington (State) – Baker; Mount; Phenocrysts – Washington (State) – Baker; Mount; Magmas – Washington (State) – Baker; Mount; Analytical geochemistry – Washington (State) – Baker; Mount; Geology; Baker, Mount (Wash.); Academic theses|
|Full text PDF:||http://cedar.wwu.edu/wwuet/471|
To better understand the complex history of open system differentiation in intermediate subduction zone magmas, complex crystal populations from andesites in the Mount Baker volcanic field (MBVF) in the northern Cascade arc were analyzed. Previous studies have suggested that open-system processes play a dominant role in the petrogenesis of these andesites; however, the studies relied heavily on bulk rock compositions and overlooked complex mineral textures and compositions. This study focuses on establishing mineral and crystal clot populations in four andesite flow units, from which co-crystallizing assemblages were identified. The flow units are the medium-K andesites of Swift Creek (asw; 55-56% SiO2), Dobbs Creek (ado; 56-57% SiO2), and Dobbs Cleaver (adb; 58-60% SiO2), and the high-K andesite of Coleman Pinnacle (acp; 58-65% SiO2). Mineral compositions of co-crystallizing assemblages were used to identify their likely parental compositions and are labeled as follows: B (basalt to high-magnesium basaltic-andesite), BA (basaltic andesitic magmas), A (andesitic magmas) and D (dacitic magmas). The andesites of Swift Creek (~48 ka) and Dobbs Creek (~119 ka) both contain a mafic crystal assemblage (B1) that is nearest in equilibrium with the bulk rock Mg#, suggesting that the other, more differentiated, crystal assemblages (BA1, A1 and D1) come from incorporation of liquid-poor crystal mushes. The asw flow unit contains an additional mafic assemblage (B2), which is not present in the other flow units, and is interpreted to have been crystallized from the same or similar magma as the B1 assemblage; however, with a distinct crystallization history. The andesite of Dobbs Cleaver (~105 ka) contains the same B1, A1 and D1 crystal assemblages present in asw and ado; however, an additional, more liquid-rich magmatic component (BA2) is responsible for the higher SiO2-content of the adb flow unit (evidenced from microscopic magma mingling textures). As a result, the native B1 assemblage in adb is slightly out of Mg-equilibrium with the bulk rock composition. The BA2 assemblage of adb is distinct from the BA1 assemblage of asw and ado in that it contains megacrystic phenocrysts and crystal clots. Across all the medium-K flow units, a B1, A1 and D1 assemblage are identified. The high-K, hornblende-bearing andesite of Coleman Pinnacle (~305 ka) hosts the BA3 crystal assemblage, unique to this flow unit, which is nearest in Mg-equilibrium to the bulk rock. The compositions of augites in the BA3 assemblage are more calcic-rich than all augites of the medium-K andesites, suggesting they crystallized from a distinct parental component. Despite a distinct parental component, the same felsic D1 crystal assemblage described above is also observed in the acp flow unit. I interpret the crystal clots, present in all flow units, to represent cumulate… Advisors/Committee Members: DeBari, Susan M., Teasdale, Rachel, Caplan-Auerbach, Jackie.