Theses and Dissertations at Montana State University (MSU)
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Item Syn-orogenic magmatism, mid-crust exhumation, and placer gold deposition: the Anaconda metamorphic core complex of western Montana(Montana State University - Bozeman, College of Letters & Science, 2020) Howlett, Caden James; Chairperson, Graduate Committee: Andrew K. Laskowski; Andrew K. Laskowski was a co-author of the article, 'Determining the source of placer gold in the Anaconda metamorphic core complex supradetachment basin using detrital zircon U-PB geochronology, pioneer district, western Montana' submitted to the journal 'Geosphere' which is contained within this thesis.; Aislin N. Reynolds and Andrew K. Laskowski were co-authors of the article, 'Farallon slab-removal as a driving force of metamorphic core complex formation in the western USA: details from the Anaconda metamorphic core complex of western Montana' submitted to the journal 'Tectonics' which is contained within this thesis.Since their initial discovery in the late 1960's, metamorphic core complexes have remained of high interest in tectonics research. Early uncertainty regarding the mechanics of slip along low-angle normal (detachment) faults is now accompanied by controversy surrounding the relationship between magmatism and large-magnitude extension. As deeply exhumed geologic structures that record lithospheric thermomechanical processes, investigating core complex formation is crucial to understanding how the mid-crust behaves in extensional tectonic settings. In some regions, the exhumation of these structures is also linked to the formation of economically valuable mineral deposits, making them of notable societal importance. This thesis is a two-part investigation of core complex evolution that addresses the concepts above, formatted with introductory and concluding chapters that bound two main chapters prepared for publication. Chapter two consists of a study that tests the utility of using detrital zircon (DZ) U-Pb geochronology and DZ unmixing models to determine the source of placer gold. New zircon U-Pb (n=1,058) and Lu-Hf (n=61) isotopic data are presented from four placer deposit samples extracted from the Pioneer District of western Montana. Geochronology and DZ unmixing modeling suggest that gold from the placer deposits was derived from vein and skarn lode sources in northern footwall of the Anaconda metamorphic core complex (AMCC). Our data offers the first DZ-based support for previous interpretations that the Late Cretaceous Royal Stock pluton precipitated gold along its contact with overlying Proterozoic through Mesozoic supracrustal rock, and was subsequently weathered, transported, and deposited in the AMCC supradetachment basin during the Late Oligocene-Early Miocene. Chapter three consists of an integrated geologic, geochronologic, thermochronologic, and isotopic investigation of the AMCC footwall. Results suggest that the AMCC is an example of a core complex that was primed for large-magnitude extension through crustal thickening and voluminous magmatism. It is proposed that buckling of the Farallon slab, marked by the onset of 'ignimbrite flare up' volcanism, was responsible for the initiation of AMCC extension. Furthermore, a compilation of MCC cooling ages and ages of Cenozoic volcanics across the western USA suggest that removal of the Farallon Plate was a primary driver of Cordilleran core complex formation.Item Deformation and metasomatism of the Qomolangma Formation: a geochemical and microstructural analysis of the summit limestone, Mount Everest, Nepal(Montana State University - Bozeman, College of Letters & Science, 2015) Corthouts, Travis Leo; Chairperson, Graduate Committee: David R. LagesonThis study is based on a suite of new samples that represent a structural transect across the Qomolangma Formation, an Ordovician limestone that comprises the uppermost ~125 meters of Mount Everest. Past studies posit Everest's summit limestone to be mildly sheared and unmetamorphosed; however, this study shows that the Qomolangma Formation has endured more strain and metamorphism than previously thought. Ti-in-biotite and Ti-in-quartz geothermometry, electron backscattered diffraction analysis, and geochemical analysis of tourmaline has been used to infer the metamorphic and deformational history of this formation. Results show that the Qomolangma Formation has experienced significant ductile shear throughout, though samples preserve a stronger shear fabric toward the top of the formation. Furthermore, our data show a gradation in the degree of metamorphism across the Qomolangma Formation, increasing toward the base of the unit. Samples collected from the structural top of the formation (Summit samples) have a penetrative foliation with significant grain size reduction of calcite (~8 microns) and yield temperature estimates of > or = 250°C. In contrast, samples from the base (South Summit samples) are distinguished by an increase in recrystallized grain size of calcite (~66 microns) and yield temperatures estimates of 500-600°C. Another important difference between Summit and South Summit samples is aggregates of the coexisting minerals muscovite, chlorite, tourmaline, rutile and biotite found throughout South Summit samples. Geochemical analysis of tourmaline suggests these minerals crystallized from metasomatic fluids (hydrothermal fluids) released from leucogranite bodies emplaced into rocks structurally subjacent to the summit limestone. As a result, metasomatic fluids caused localized metamorphism at the base of the Qomolangma Formation, producing the significant increase in metamorphic grade observed in South Summit samples. It is interpreted that the fabric preserved in Summit samples was ingrained during initial thrust faulting in the Eocene to Oligocene, whereas the fabric and metamorphic grade observed in South Summit samples is the result of detachment faulting and leucogranite emplacement in the Early Miocene. Lastly, microstructural observations suggest that metasomatic fluids may have promoted faulting on the Qomolangma detachment, a splay of the South Tibetan Detachment believed to be at the base of the Qomolangma Formation.Item Amphibolite to granulite facies metamorphism and deformation in the Jerome Rock Lakes area, Spanish Peaks, Montana(Montana State University - Bozeman, College of Letters & Science, 2015) Chadwick, Jesse Socorro; Chairperson, Graduate Committee: David W. MogkStructural analysis, bedrock mapping, and quantitative thermobarometry reveal a protracted metamorphic history for the multiple lithologies present in the Jerome Rock Lakes area of the Spanish Peaks, located in the northern Madison Range, southwest Montana. Volumetrically-dominant quartzofeldspathic gneisses are primarily of the tonalite-trondhjemite-granodiorite (TTG) suite and record multiple episodes of high-temperature deformation. Coarse-grained hornblende in association with K feldspar-bearing leucosome indicates locally vapor-present melting. Clinopyroxene-bearing metabasic enclaves in the quartzofeldspathic gneisses record the subsolidus hydration of clinopyroxene porphyroclasts to hornblende and subsequent dehydration melting of hornblende in the transitional granulite facies to produce garnet coronae and clinopyroxene neoblasts under vapor-absent conditions. Variably migmatized garnet-sillimanite schists record amphibolite facies metamorphism at ca. 5.5 kbar and 640 °C, with extensive melt production via melting of biotite. Inclusions of staurolite in kyanite-bearing metaquartzite are inferred to indicate the presence of modern thicknesses of continental crust and a geothermal gradient similar to Phanerozoic values. Infrequent layers containing tschermakitic amphibole layers may represent the residuum of vapor-present melting of biotite and indicate local variation in the activity of water. Migmatized garnet amphibolites record peak upper-amphibolite to transitional granulite facies metamorphism at ca. 10 kbar and 800 °C. A clockwise P-T path of Alpine character is proposed for the lithologies of the Jerome Rock Lakes area, with peak pressures in the kyanite field attained before peak temperatures, followed by rapid, steeply adiabatic to isothermal decompression. Early isoclinal folds developed during prograde metamorphism and were rotated into the regional foliation. Progressive deformation resulted in the formation of later isoclinal folds coplanar with foliation. Peak temperatures were attained after cessation of deformation, indicated by undeformed leucosomes. A second episode of deformation produced outcrop-scale parallel folds trending north-south. The Precambrian tectonic history of the northern Madison Range is inferred to have terminated in a continental collision with northwest-southeast directed shortening that produced steeply-dipping, NNE-trending regional foliation. This style is generally consistent with that observed elsewhere in the Spanish Peaks and in the Tobacco Root Range, generated by the ca. 1.7 Ga collision of western North America with the Medicine Hat craton.