Subalklaine Volcanism in the Eastern Basin and Range Province: Examples from the Miocene McCullough Range Volcanic Field and the Pliocene to Holocene Black Rock Desert Volcanic Field

This dissertation is composed of three papers that apply geologic mapping, whole rock geochemical and isotopic analysis, and mineral chemistry to describe two large, long-lived (~6 million years), and compositionally heterogeneous volcanic fields and posit sources of the rock types present in each. These fields are the McCullough Range in southern Nevada, which was a locus of volcanic activity from ~19 - 12 Ma and the Black Rock Desert volcanic field in west-central Utah, which began forming at ~6 Ma and is still active today, with the most recent eruption occurring ~720 years ago. Chapter Two is composed of a paper summarizing the physical and chemical properties of the Black Rock Desert (BRD) and Markagunt Plateau (MP) volcanic fields in Utah. Both the BRD and MP volcanic fields have been active for about 6 million years and are composed of similar rock types. In the last 2.5 million years, volcanoes in both the BRD and MP fields erupted tholeiitic basalts and calc-alkaline intermediate rocks. Comparing and contrasting the chemical attributes of the two volcanic fields led to the conclusion that despite differences in the amount of crustal extension, subalkaline rocks in both volcanic fields are both common and share common petrogenetic histories. Chapter Three is a detailed study of a subfield of the BRD volcanic field: the ca. 2.7 - 2.1 Ma Twin Peaks volcanic field. The paper presents the results of detailed mapping and extensive sampling, and utilizes both whole rock and mineral chemistry to propose a basic petrogenetic model for the formation and evolution of the Twin Peaks caldera. Two major conclusions came from this study: 1) the Twin Peaks caldera was produced in a similar fashion to the 1912 Novarupta-Katmai eruption in Alaska, in that the earliest eruptions, including those that formed the Twin Peaks caldera, occurred ~10 km west of the actual location of the caldera itself. 2) The range in compositions and order of eruptions for the first episode was the product of partially emptying a zoned magma chamber with rhyolite at the top and basalt at the bottom of the erupted portion. Chapter Four is focused on the lower to middle Miocene strata of the McCullough Range volcanic field. Located along the western margin of the northern Colorado River extensional corridor (NCREC), the McCullough Range volcanic field has two attributes which make it ideally suited for detailed geochemical study: 1) it was active prior to and during large magnitude extension in the central Basin and Range Province; and, 2) it is tilted but not as highly extended or altered as volcanic fields within the NCREC itself. The major findings of this study include the discovery of the pre-extensional Enigma stratovolcano. The Enigma Volcano is comprised of a chemically enriched lower portion and of a calc-alkaline upper portion. Detailed mapping and chemistry suggested that volcanic rocks formed during early magmatic activity in the NCREC obtained their chemical enrichments (high volatiles, P, Zr, Sr, Ba, and LREE) from partial melting of a melt-metasomatized and subduction-modified sub-continental lithospheric mantle. In contrast to the early activity, later pre-extensional NCREC volcanism, typified by the upper Enigma Volcano, was the typical calc-alkaline intermediate composition associated with the early phases of extensional activity in the rest of the BRP