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Fournelle, J.H., 1988

The geology and petrology of Shishaldin Volcano, Unimak Island, Aleutian arc, Alaska

Bibliographic Reference

Fournelle, J.H., 1988, The geology and petrology of Shishaldin Volcano, Unimak Island, Aleutian arc, Alaska: Baltimore, Maryland, The Johns Hopkins University, Ph.D. dissertation, 507 p., illust., maps.


Shishaldin is a large, active stratovolcano on Unimak Island. It consists of a post-glacial edifice atop an older glaciated Somma and shield. More than 24 monogenetic cones cover the northwest flank. Basalts predominate, and three varieties are present: (1) high magnesian, (2) high alumina, and (3) FeTi enriched. All have elevated TiO2 (≤ 2.5 wt%). Coexisting FeTi oxides and Fe2O3/FeO ratios indicate fO2 was buffered near NNO. Geothermometry suggests the low Mg basalts crystallized at 1,150-1,250 degrees C and the dacites at ~200 degrees lower. H2O in the basaltic magmas was low (≤0.5 wt%), whereas the dacites contained up to 4.5 wt%. The high magnesian basalts have ~20 vol% olivine and diopsidic clinopyroxene, and are clearly not equilibrium systems. Two distinct olivine populations are present, FO 93-94 and FO 72-74; neither is in equilibrium with either the whole rock or the groundmass. Chromite-olivine (Fo94) thermometry gives subsolidus temperatures, 820-1,060 degrees C. Textures suggest that the clinopyroxenes are products of chemical interaction between Fo-rich olivine and low MgO liquid. Chromite and clinopyroxene compositions resemble those of the nearby Border Ranges Ultramafic Complex. The high Mg lavas appear to be hybrids from interaction of arc magma with this sub-arc layered intrusion. Porphyritic high alumina basalts contain 30-45 vol% plagioclase, ≤ 5 vol% olivine and rare clinopyroxene. Plagioclase cores (~An75-85) appear to be in equilibrium with a lava of the whole-rock composition, while rims (~An50-65) are in equilibrium with the groundmass. Olivine is Fo70-75 and in Fe-Mg exchange equilibrium with the lava. These phases suggest low pressure crystallization (P ≤ 3 kbar). The whole-rock composition, however, could have been derived at pressures ≤ 20 kbar. Plagioclase accumulation was probably not significant during petrogenesis. The ~aphyric FeTi basalts have < 20 vol% plagioclase and rare olivine. Plagioclase microphenocrysts (~An60) are abundant, with occassional large phenocrysts (~An80). Zr reaches 230 ppm and LaN = 75. They were apparently derived by ~8 kbar fractionation of high alumina basalt. Dacites may be lower pressure derivatives. Partial melting of the subducted slab may have produced the high alumina basalt. DSDP 183, directly south on the Pacific plate, found ~FeTi basalt with 87Sr/86Sr of 0.70297, identical to that of all Shishaldin basalt types.

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