title: High-Mg Andesites from the Northern Cascade Arc: Using Mineral Chemistry to Distinguish Between Hypotheses for Petrogenesis
Creators: name: Sas, May
Publisher: name: Western Washington University
Year: 2015

description: "<p>A fundamental question in the Earth Sciences is whether, in certain situations, subducting lithospheric plates can get hot enough to generate melt that consequently contributes to magmatic output in volcanic arcs. Because the subducting plate beneath the Cascade arc is relatively young, and therefore understood to be hotter than older subducting slabs, slab melt generation is considered possible. Recent work has shown that slab melt is involved in magma petrogenesis in the southern Cascade arc (Walowski et al., 2015). To better understand the role of slab melt in north Cascades magmas, this study focused on petrogenesis of high-Mg lavas from the two northernmost active volcanoes in Washington.</p>
<p>High-Mg andesites (HMA) and basaltic andesites (HMBA) are magmas that have unexpectedly high Mg# (molar Mg/(Mg+Fe)) relative to their SiO<sub>2</sub> contents. HMA also exhibit Ni and Cr enrichment, as well as elevated La/Yb ratios that are due to notable depletion in heavy REE. Their steep REE is interpreted to result from separation of melt from a garnet-bearing residuum because of the high compatibility of heavy REE in garnet. A major debate centers on the garnet’s origin, as it could be present in mineral assemblages from the subducting slab, deep mantle, thick lower crust, or basalts that undergo high pressure crystallization.</p>
<p>The HMA examined in this study include the Tarn Plateau basaltic andesite (51.8–54.0 wt.% SiO<sub>2</sub>, Mg# 68–70) and Glacier Creek andesite (58.3–58.7 wt.% SiO<sub>2</sub>, Mg# 63–64) from the Mount Baker Volcanic Field, and the Lightning Creek basaltic andesite (54.8–57.9 SiO<sub>2</sub>, Mg# 69–72) from Glacier Peak. This study combined whole rock and mineral geochemical analyses of these HMA to test several petrogenesis hypotheses, and determine if any are applicable to north Cascades HMA. In Tarn Plateau HMBA, high La/Yb that positively correlates with Mg# in equilibrium liquids calculated from clinopyroxenes suggests an origin similar to that of Aleutian adakites. This petrogenetic hypothesis calls on slab-derived melts that, upon interaction with the overlying mantle, become Mg-rich, and subsequently mix with mantle-derived basalts. The source for steep REE in Glacier Creek HMA is more ambiguous, and could have originated from the garnet-bearing mantle. However, since Sr is fluid mobile, and P is fluid immobile, high whole rock Sr/P imply origin from a mantle that was hydrated by a slab component (fluid ± melt) that was enriched in fluid-mobile elements and resulted in the observe REE signature. The Lightning Creek HMBA has three identifiable magmatic components, two of which share the HMA traits and are derived from a similar source. Cr and Mg contents in Cr-spinel and olivine pairs suggest this source to be a depleted mantle, and, similarly to the Glacier Creek HMA, high whole rock Sr/P values indicate mantle melting that was induced through hydration. Therefore this HMBA is interpreted have originated from a refractory mantle source that underwent melting through interaction with an incompatible element-rich slab component (fluid ± melt). In summary, results indicate that in addition to slab-derived fluids, slab-derived melts also have an important role in the production of HMA in the north Cascade arc.</p>"