Conrad, W.K., 1983, Petrology and geochemistry of igneous rocks from the McDermitt caldera complex, Nevada-Oregon, and Adak Island, Alaska; evidence for crustal development: Ithaca, New York, Cornell University, Ph.D. dissertation, 325 p., illust., maps.
(1) The McDermitt caldera complex, Nevada-Oregon, is a Miocene collapse structure formed following eruption of at least 3 large volume (> 100 km3) ash-flow tuffs. Early-erupted portions of the tuffs are mildly peralkaline, high-Si comendite. Later erupted portions of two tuff sheets are compositionally-zoned to metaluminous, low-Si rhyolite, containing highly-zoned phenocryst assemblages: ternary-alkali feldspar+ ferroaugite- ferrohedenbergite+ fayalite+ magnetite+ ilmenite+ quartz+ apatite. Systematic variations in mineral compositions and whole-rock chemistry record evolution of the rhyolite tuffs through fractional crystallization of parental dacite-rhyodacite magmas. The caldera complex developed near the southern margin of an earlier Miocene rift structure (Orevada Rift). Rift-related basaltic-intermediate volcanic rocks are divided into older and younger basalt series, and high-Fe andesites. Time-progressive variations in levels of U, Th, K, Rb and Ba relative to Hf, Zr and LREE indicate that older basalts were mantle-derived, having undergone little fractionation or crustal contamination; younger basaltic, andesitic and dacitic magmas were derived or modified by partial melting in a pyroxene granulite facies lower crust similar in composition to the older basalts. (2) Xenolithic inclusions in calc-alkaline andesite from Mt. Moffett volcano, Adak Island, Aleutian arc, reveal a nearly continuous record of crystallization of basaltic magmas in the crust and possibly upper mantle of the arc. Cumulate xenoliths form a progressive series in modal mineralogy from ultramafic hornblende-bearing clinopyroxenite to both hornblende-bearing and hornblende-free gabbros. Xenolithic inclusions without cumulate textures, termed composite xenoliths, are characterized by forsteritic olivine, zoned Cr-diopside and hornblende. Variations of Ni in olivine in both types of xenoliths indicate that their parent magmas had minimum MgO contents of 9 wt %. Successive injections of batches of primary melt into a magma chamber fractionating olivine and clinopyroxene can reproduce observed compatible element (Ni, Cr) depletion trends in the xenoliths. This open-system process can also yield the abundant high-Al basalt type in the Aleutian arc. Continued crystallization of high-Al basalt in lower crustal magma chambers, recorded in pyroxenite to hornblende gabbro xenoliths, can yield basalts and andesites of the Aleutian calc-alkaline series.
Theses and Dissertations