title: Mineral Complexities as Evidence for Open-system Processes in Formation of Intermediate Magmas of the Mount Baker Volcanic Field, Northern Cascade Arc
Creators: name: Escobar-Burciaga, Ricardo D.
Publisher: name: Western Washington University
Year: 2016

description: <p>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 (<em>asw</em>; 55-56% SiO<sub>2</sub>), Dobbs Creek (<em>ado</em>; 56-57% SiO<sub>2</sub>), and Dobbs Cleaver (<em>adb</em>; 58-60% SiO<sub>2</sub>), and the high-K andesite of Coleman Pinnacle (<em>acp</em>; 58-65% SiO<sub>2</sub>).</p>
<p>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 <em>asw</em> 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 <em>asw</em> and <em>ado</em>; however, an additional, more liquid-rich magmatic component (BA2) is responsible for the higher SiO<sub>2</sub>-content of the <em>adb</em> flow unit (evidenced from microscopic magma mingling textures). As a result, the native B1 assemblage in <em>adb</em> is slightly out of Mg-equilibrium with the bulk rock composition. The BA2 assemblage of <em>adb</em> is distinct from the BA1 assemblage of <em>asw</em> and <em>ado </em>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 <em>acp</em> flow unit.</p>
<p>I interpret the crystal clots, present in all flow units, to represent cumulate material entrained in the erupting host magma and that related phenocrysts are disaggregates of crystal clots. The existence of common, multiple phenocryst and crystal clot populations in each flow unit of different age and SiO<sub>2</sub> content provides strong evidence that intermediate magmas of MBVF are a result of complex, open system processes involving passage through multiple crystal mushes. They are more than just the end product of mixing between two magmas. Furthermore, we suggest that most phenocrysts do <em>not</em> represent equilibrium products of their host liquid, evident from wide compositional ranges of ferromagnesian minerals (e.g., augite core Mg# 69-85). The most mafic phenocryst populations (B1 and B2 assemblages) show the least amount of disequilibrium textures. Given the fact that the SiO<sub>2</sub> content of the magmas are in the basaltic-andesite to andesite range, the B1 and B2 assemblages are interpreted to have fractionated from a high-magnesium basaltic andesitic magma, as opposed to a basaltic magma.</p>
<p>Identification of parental magmas for these medium K andesites utilizes their distinctive, most Mg-rich mineral compositions. The olivine (Fo<sub>85-87</sub>) and augite (Mg# 81-85) core compositions of the B1 and B2 assemblage are some of the most mafic crystal assemblages observed at MBVF. The only near-primary flow unit at MBVF that carries such mafic equilibrium olivine and augite is the high-Mg basaltic andesite of Tarn Plateau. Similar whole rock trace element patterns (e.g., steep REE patterns) of the medium-K flow units and the high-Mg basaltic andesite of Tarn Plateau supports their co-genetic relationship. The high-K <em>acp</em> flow unit must be derived from a distinct parental component, as the BA3 assemblage lacks olivine and contains augite that is more calcic in composition than any augite population in the medium-K flow units, and has a distinctive REE pattern. Despite the wide age range among the four flow units of this study, mineral compositions and textures observed in all flow units suggest that at least two types of parental magmas to the andesites in this study tapped a similar range of cumulate material on their way up to the surface.</p>