Nye, Christopher J.2024-11-142024-11-141987-12http://hdl.handle.net/11122/15631UAG R-311; with an appendix describing geothermal fluid chemistry by Roman J. Motyka and Christopher J. Nye.The Spurr Volcanic Complex (SVC) is a calcalkaline, medium-K, sequence of andesites erupted over the last quarter of a million years by the easternmost currently active volcanic center in the Aleutian Arc. The ancestral Mt. Spurr was built mostly of andesites of uniform composition (58-60% SiO₂), although andesite production was episodically interrupted by the introduction of new batches of more mafic magma. Near the end of the Pleistocene the ancestral Mt. Spurr underwent Bezymianny-type avalanche caldera formation, resulting in the production of a volcanic debris avalanche with overlying ashflows. Immediately afterward, a large dome (the present Mt. Spurr) was emplaced in the caldera. Both the ashflows and dome are made of acid andesite more silicic than any analyzed lavas from the ancestral Mt. Spurr (60-63% SiO₂), yet contain olivine and amphibole xenocrysts derived from more mafic magma. The mafic magma (53-57% SiO₂) erupted during and after dome emplacement, forming proto-Crater Peak and Crater Peak. Hybrid pyroclastic flows and lavas were also produced. Proto-Crater Peak underwent glacial dissection prior to the formation of Crater Peak in approximately the same location. The vents for the silicic and mafic lavas are in the center and in the breach of the 5 by 6 km horseshoe shaped caldera, respectively, and are less than 4 km apart. Late Holocene eruptive activity is restricted to Crater Peak, and magmas continue to be relatively mafic and derived from deep within the crust. SVC lavas are plag ± ol + cpx ± opx + mt bearing. All post-caldera units contain small amounts of high AL₂O₃, high alkali pargasite, and Proto-Crater Peak and Crater Peak lavas contain abundant pyroxenite and anorthosite clots presumably derived from an immediately pre-existing magma chamber. Ranges of mineral chemistries within individual samples are often nearly as large as ranges of mineral chemistries throughout the SVC suite, suggesting that magma mixing is common. SVC lavas are unlike experimentally produced cotectic liquids and are thus unlikely to be related to each other by fractional crystallization. Magmatic evolution must instead be controlled in large part by crustal assimilation. Flat Y-SiO₂ and Nb-SiO₂ trends and Rb enrichment beyond that which can be reasonably modeled by fractional crystallization also suggest extensive assimilation of lower crust, bulk upper crust, or partial melts of local batholithic material. Since at least the mid-Holocene there has been no shallow, silicic magma chamber at the SVC. This increases the expectation that the low resistivity layer described by Turner and Wescott (1986) is a highly conductive layer of bedrock, such as a thick, altered tuff.Abstract – Introduction – Tectonic and geologic setting – Stratigraphy and geology – Geochemistry – Major elements – Trace elements – Petrology – Introduction – Plagioclase – Olivine – Clinopyroxene – Orthopyroxene – Amphibole – Spinel – Crystal clots – Synthesis and interpretation – Implications for geothermal resources – Acknowledgements – References – Appendix I. Whole rock geochemistry and point count data – Appendix II. Summary of mineral compositions – Appendix III. Plagioclase compositions – Appendix IV. Olivine compositions – Appendix V. Clinopyroxene compositions – Appendix VI. Orthopyroxene compositions – Appendix VII. Hornblende compositions – Appendix VIII. Spinel compositions – Appendix IX. Chemistry of geothermal fluids.en-USPetrologyGeologyVolcanismSpurr, Mount, RegionReport