ELEMENTAL CHEMISTRY OF SILICIC GLASSES FROM ICELAND AND MOUNT ST HELENS VOLCANIC CENTERS REVEALS DISTINCT TECTONOMAGMATIC SIGNATURES AND CONFIRMS AN ANOMALOUS ICELANDIC SUBDUCTION-LIKE VOLCANO

Glass compositions from silicic lavas and pumice from Iceland and Mount St. Helens (MSH) reveal expected strong contrasts between magmas at a mid-ocean spreading center/hotspot and a continental margin subduction zone, with one glaring exception: Kambur dome in Iceland’s Króksfjörður volcanic center. We analyzed 10 whole rock Iceland samples ranging in age from <1 ka to 12 Ma (dacites and rhyolites, SiO₂ 68-74 wt%) and four Mount St. Helens samples (3-250 ka; dacites, SiO₂ 65-68%). Glasses were all rhyolitic (Iceland 72-79% SiO₂, MSH 75-77% SiO₂). MSH samples are characterized by plag and amph phenocrysts, ± bio and qtz. Iceland samples have plag + px ± fayalite, with the notable exception of Kambur (plag + amph + qtz + px).

Silicic glasses retain elemental patterns of whole rocks, with trace elements displaced in predictable ways–-higher incompatible and lower compatible elements.. MSH glasses exhibit very strong Nb-Ta depletion, relatively minor Sr depletion and very small negative Eu anomalies, relatively low total REE, very low HREE and negatively-sloping MREE-HREE patterns. Iceland glasses, except Kambur, are undepleted in Nb-Ta and extremely depleted in Sr and Eu, and have high total REE, very high HREE, and flat MREE-HREE patterns. Kambur glasses have elemental patterns that differ strikingly from other Iceland samples, but that are almost identical to MSH elemental patterns. Typical Iceland glasses have far higher Zr and zircon saturation temperatures (average 880°C) than MSH (average 700°C) or Kambur (average 680°C, Boehnke et al 2013; all samples contain zircon).

Our data demonstrate that, even when highly fractionated, rhyolitic melts (glasses) capture the fundamental trace element characteristics of parent magmas and tectonic settings. For Iceland, they provide a geochemical fingerprint for silicic magmatism, and they support and amplify previous suggestions that Króksfjörður volcanic center includes volumetrically minor calc-alkaline magma with distinctly different petrogenesis from the rest of the island. Magma genesis at Króksfjörður, in some way, may have resembled that in a typical subduction zone (cf. Jónasson et al. 1992; Willbold et al., 2009).

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Session No. 25--Booth# 22

Igneous and Metamorphic Petrology (Posters)

Wednesday, 6 April 2016: 8:30 AM-12:30 PM

Ballroom C (Ontario Convention Center)

Geological Society of America Abstracts with Programs. Vol. 48, No. 4
doi: 10.1130/abs/2016CD-274234

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