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Nicolaysen, K.E., 1994

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Nicolaysen, K.E., 1994, Stratigraphic and geochemical constraints on the magmatic evolution of Coats Caldera, Yunaska Island, central Aleutian Arc, Alaska: University of Wyoming, Laramie, M.S. thesis, 57 p.


The Quaternary composite volcano on eastern Yunaska Island, Aleutian island arc, consists of two nested calderas. The younger of these, Coats Caldera, exposes more than thirty tholeiitic basalt and basaltic andesite flows. These flows, sampled stratigraphically, and related syn-caldera dikes and post-caldera flows are a basalt to dacite suite (51.5 64.5% SiO2 ). These anhydrous lavas are generally phenocryst poor, containing assemblages of pl±ol+cpx±opx±mt+ilm. Petrographic and geochemical breaks within the suite group the samples into lower (LCS), middle (MCS), and upper (UCS) caldera sections and dike/post-caldera units. Process identification, Pearce element diagrams and mass balance calculations indicate the importance of plagioclase and olivine fractionation for the entire suite but also suggest that open system processes, such as recharge, mixing and contamination all played an important role. A simple model for the formation of the Coats Caldera suite calls for a previously modified parental magma to undergo fractionation with intermittent eruption and recharge. A pulse of partly differentiated magma (MCS) mixes with the parental (LCS) magma to form the first UCS magma which continues to fractionate and erupt. Sustained high heat flow stimulates local crustal melting/assimilation, represented by the most evolved dacite, which mixed in varying proportions with remnant UCS magma to form the dikes and post-caldera lavas.

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