Theses and Dissertations at Montana State University (MSU)
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Item Sedimentary tectonics of the Mesoproterozoic Lahood Formation, southwest Montana(Montana State University - Bozeman, College of Letters & Science, 2017) Fox, Nicholas Reid; Chairperson, Graduate Committee: David W. MogkThe LaHood Formation plays a critical role in constraining the sources and tectonic setting during the initial stages of formation of the Mesoproterozoic Belt Basin, particularly in the Helena Embayment. In the Bridger Range the LaHood Formation is composed of coarse conglomerates, lithic arkoses, calcareous shales and siliceous shales. Twelve measured stratigraphic sections show a general fining to the northeast with calcareous shales exposed only in the northern half of the Range. Lithology provides the basis for the LaHood Formation to be divided into three Members: A) conglomerate; B) lithic arkose and sandy micaceous shale; C) interbedded Member B with calcareous and siliceous shales that are interpreted as correlative rocks of the upper Newland Formation. U-Pb ages of detrital zircons (LA-ICP-MS) from four sandstones show: 1) The basal conglomerate and coarse arkosic facies in the lower and southern two sections have a primary age distribution at ~3.2 Ga with a minor ~3.5 Ga component that corresponds to the major TTG crust-forming event in the northern Wyoming Province; 2) zircons from an arkosic sandstone inter-fingered with calcareous shales have a dominant ~3.2 Ga signal, but includes a significant secondary concentration at 2.8 Ga, which corresponds to the age of the Beartooth-Bighorn Magmatic Zone; 3) the northernmost section exhibits a concentration near 3.2 Ga, a more significant ~2.8 Ga signal, and the first occurrence of ~1.8 Ga grains, indicative of the Great Falls Tectonic Zone. These detrital zircon ages confirm the results of Guerrero et al. (2015) in the Bridger Range and from LaHood exposures in the Horseshoe Hills to the west, but differ significantly from the dominant 3.4-3.5 Ga signal from LaHood exposures in Jefferson Canyon and north of Cardwell, MT (Ross and Villeneuve, 2003; Mueller et al., 2016). Collectively, the detrital zircon ages demonstrate a) occurrences of the LaHood Formation across the Belt Basin received sediment from temporally distinct sources that do not necessarily include proximal Archean basement, and b) the stratigraphic succession reflects contributions from progressively younger source areas. The significant variety of ages and abrupt facies changes in the LaHood Formation support a series of compartmentalized sub basins.Item The petrogenesis of the alkaline rocks of the Judith Mountains, central Montana(Montana State University - Bozeman, College of Letters & Science, 1982) Barrick, Paula JeanItem Timing of deformation along the leading edge of the Montana Disturbed Belt, northern Crazy Mountains Basin, Montana(Montana State University - Bozeman, College of Letters & Science, 1986) Harlan, Stephen ScottItem Characterization of subglacial till from Robertson Glacier, Alberta, Canada : implications for biogeochemical weathering(Montana State University - Bozeman, College of Letters & Science, 2013) Griggs, Russell Kelly; Chairperson, Graduate Committee: Mark L. SkidmorePhysical and biogeochemical weathering occurring during transport in the subglacial traction zone impacts the lithology and petrology of sediment in tills. Grain size distribution, particulate organic carbon (POC) content, and bulk/clay mineralogy of recently exposed tills from the terminus of Robertson Glacier, Alberta, Canada were characterized via sieving, laser diffractometry, loss-on-ignition, and X-ray diffraction/scanning electron microscopy (SEM) in order to describe physical and mineralogical properties of these tills. The matrix material of all tills exhibit a grain size distribution biased toward medium to coarse sand-sized particles and the dominant minerals are calcite, dolomite, quartz, and K-feldspar with lesser muscovite, pyrite, phlogopite, and chlorite. POC abundance ranges from 1.1 to 3.5 weight percent and is negatively correlated with grain size for grains from 125 - 2000 microns. POC abundance is also positively correlated with calcite abundance, especially for grains from 125 - 2000 microns. Pyrite is present at < 1% bulk abundance, with a significant portion of the pyrite observed via SEM as ~5-15 micron grains. The modal particle size of the till matrix of medium to coarse sand is consistent with limited physical weathering due to the short (< 3 km) subglacial transport distance and is comparable to that from a metasedimentary glaciated catchment in the Swiss Alps. The small grain size of pyrite produces a large reactive surface area, which supports its potential importance as a key chemical energy source for microbially-mediated chemical weathering.Item Stable isotope (18 O/16 O and D/H) studies of cascade volcanic arc magmatism(Montana State University - Bozeman, College of Letters & Science, 2009) Underwood, Sandra Jean; Chairperson, Graduate Committee: Todd FeeleyStable isotope ratios (18 O/16 O and D/H) potentially constrain origins of magma and volatile sources in igneous rocks. Modification of magmas by closed system processes (fractional crystallization and closed system devolatilization) or open-system processes (assimilation and fractional crystallization, magma mixing, degassing) affect 18 O/16 O and D/H ratios in known ways. Magma degassing during volcanic eruptions and subsequent rehydration or alteration of groundmass glass reduces accuracy of whole rock stable isotope measurements, the traditional method of measuring glassy volcanic rocks. However, phenocrysts separated from fresh volcanic rocks may retain magmatic 18 O/16 O and D/H values. Accordingly, oxygen isotopes in olivine, clinopyroxene, and plagioclase crystals were measured to determine whether Quaternary little-evolved mafic lavas of the Southernmost Cascades (SMC) represent only melts of heterogeneously contaminated mantle sources. In variably degassed silicic volcanic rocks from the 1980-1986 eruptions at Mount St. Helens (MSH) and the 1915 eruptions at Lassen Volcanic Center (LVC), hydrogen isotopes in amphibole and biotite phenocrysts were measured to evaluate shallow subvolcano magmatic processes. Magmas in each study are strongly influenced by crustal stress fields associated with each tectonic setting. The mantle source of the SMC little-evolved mafic lavas is heterogeneous, but these continental arc magmas acquire crustal contamination that reflects vent location across a region of extended heterogeneous crust. Hydrous phenocrysts in comparatively shallow, sill-like LVC silicic magma bodies recorded heating and devolatilization associated with the periodic injections of mafic magma. The broadly distributed crustal extension enables crystallizing silicic magmas to devolatilize as they are variably remobilized by heat and volatiles of recharging mafic magmas. At MSH, numerous small dacite magma pulses crowded into the narrow extensional volume below the vent. This focusing of all magmas and exsolved volatiles from a deeper main magma body produced the explosive May 18, 1980, eruption that also extensively damaged at least the upper 5 km of subvolcano plumbing. Through 1986, pulses of volatile-rich dacite magma degassed in a structurally and thermally evolving plumbing system. [Mineral data are located in separate Supplemental Data Files.].