Characterisation of mafic crustal xenoliths from the Wyoming Craton, Montana (USA), using accessory mineral geochronology and geochemistry, with implications for lower crustal evolution

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dc.contributor.authorThakurdin, Yashirvad
dc.date.accessioned2020-09-14T10:58:46Z
dc.date.available2020-09-14T10:58:46Z
dc.date.issued2019
dc.descriptionA thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy, School of Geosciences, Johannesburg, 2019en_ZA
dc.description.abstractCharacterization of mafic crustal xenoliths from the Wyoming Craton, Montana (USA), using accessory mineral geochronology and geochemistry, with implications for lower crustal evolution
dc.description.librarianPH2020en_ZA
dc.description.sponsorshipLower-middle crustal xenoliths from the Bearpaw Mountains, Montana, USA provide a valuable opportunity to investigate deeper crustal processes along the northern margin of the Wyoming Craton. In this study, combined geochemistry and geochronology on zircon and apatite from a suite of amphibolite and granulite xenoliths are used to constrain the processes of xenolith formation, and the large-scale geological implications for the deeper continental lithosphere in the region. Whole-rock geochemistry and thermobarometry, combined with U-Pb (SIMS) geochronology were conducted on a single amphibolite and four granulite xenoliths. The xenoliths are hosted within Eocene alkaline volcanic rocks (“minette”). Thermobarometric calculations indicate temperatures and pressures of 642-817 °C and 3.5-7.5 kbar confirming the middle-lower crustal nature of these xenoliths. Mantle-normalised trace element patterns for all xenoliths show a subduction-type signature, with extreme enrichments in large-ion lithophile (LILE) and fluid mobile elements (Cs, Rb, Ba, Pb, Sr) relative to high-field strength elements, together with relative Zr-Hf and Nb-Ta-Ti depletions. SEM-CL imaging of zircons reveals complex internal microstructures characteristic of both magmatic and metamorphic zircon. Magmatic zircon is recognised by weakly preserved oscillatory zoning and high Th/U ratios (��̅ = 1.2). Metamorphic zircon is present as overgrowths and newly formed grains, and is devoid of internal microstructures together with low Th/U (0.08-0.2). In more complex cases, magmatic zircon is recrystallised in the presence of a melt phase to create a variety convolute internal textures. U-Pb geochronology within magmatic zircon yield dominantly Proterozoic populations between 1834±19 Ma and 1874±8 Ma. Ages within metamorphic domains constrain a slightly younger population between 1772±5 Ma and 1788±4 Ma. A series of dispersed ages within complex magmatic zircon yield dates between 2004±17 and 2534±9 Ma. Proterozoic ages are correlated with lithospheric processes along the cratonic north-western margin (Great Falls Tectonic Zone, Medicine Hat Block and Montana Metasedimentary Terrane), involving arc-related melting and collisional metamorphism. The protolith to xenoliths containing mid-Proterozoic (ca. 1800 Ma) magmatic zircon ages were formed by subduction-induced melting of the overlying mantle wedge. Xenoliths containing older magmatic zircon ages (ca. 2534 Ma) sample protoliths produced by melting of a subduction-related metasomatised mantle in the Late Archaean or earlier. Both Proterozoic and Late Archaean protolith magmas are envisaged as new additions to the Wyoming Craton that resided within middle to lower crustal levels of the continental lithosphere before being subjected to metamorphism and recrystallisation at ca. 1770 Ma. Trace elements (REE) together with oxygen and hafnium isotopes were obtained using LA-ICP-MS on previously dated and imaged zircon grains. REE in zircon for all xenoliths are steep and positively sloping without discernible HREE depletion, implying zircon (re)crystallisation in the absence of garnet. Negative Eu anomalies signify concomitant zircon and feldspar crystallisation. Solid-state recrystallisation may be responsible for variations in LREE, Eu and Ce. Xenoliths containing magmatic zircon (1834±19 Ma) with mantle-like δ18O (4.4-5.5 ‰) and radiogenic initial εHf (-2.3 to +3.7) likely formed through crystallisation of melts derived from a mantle source that incorporated minor amounts of subducted sedimentary/supracrustal material. Proterozoic (1874±8 Ma) xenoliths with elevated δ18O (6.0-7.0 ‰) and unradiogenic εHf (-8.2 to -9.6) within magmatic zircon represent melt products of subduction-induced melting and metasomatism in the mantle wedge. Zircons containing non-mantle like δ18O (6.4-7.2‰) and unradiogenic εHf (+1.4 to -17.6) in older (ca. 2534 Ma) xenoliths represent protolith magmas derived from a subduction-metasomatised mantle prior to crystallisation in the Late Archaean or earlier. U-Pb geochronology and trace element chemistry (LA-ICP-MS), together with major element and halogen geochemistry was then conducted on apatite grains in order to provide insight into lower temperature processes operating within amphibolite and granulite xenoliths. Apatite in all xenoliths are metamorphic and show light rare earth (LREE) enrichments relative to heavy rare earth elements (HREE), negative Eu anomalies and generally high concentrations of trace elements. Halogen compositions are uniform and allow classification of the apatites as fluorapatite. Trace element patterns in metamorphic apatite signify progressive prograde metamorphism in which apatite grains continuously exchanged ions with minerals in the system, and acquired trace element concentrations redistributed by newly forming and previously crystallised metamorphic phases. Specifically, as metamorphic grade increases, the break-down of monazite and crystallisation of metamorphic feldspar resulted in LREE enrichments and Eu depletions in metamorphic apatite respectively. Growth of garnet near the metamorphic peak depleted apatite in HREE. Due to the high metamorphic grade of these rocks, the rock volume is likely to have been in large-scale chemical equilibrium, resulting in uniform apatite trace element and halogen concentrations in each xenolith. Geochronology on granulite xenoliths yield Neoproterozoic to Palaeozoic (ca. 300-500 Ma) lower-intercept ages that are not correlated with known thermal or metasomatic events within the Wyoming Craton that could have induced isotopic resetting at these times. Rather, these young ages are interpreted to represent protracted and slow cooling in the middle to lower crust following a thermal maximum in the mid-Proterozoic (ca. 1770 Ma). Integration of geochronological and geochemical data on apatite and zircon indicate the formation of new melts added by subduction-induced melting in the Proterozic (ca. 1800 Ma). These new magmatic additions form the protolith to the xenoliths, and resided at lower to middle portions of the continental lithosphere before being recrystallised to amphibolite/granulite facies during collision-related metamorphism at 1770 Ma. A period of tectonic quiescence subsequently occurred from ca. 1770 Ma and 55 Ma in this region of the Wyoming Craton.en_ZA
dc.facultyFaculty of Scienceen_ZA
dc.format.extentOnline resource (147 leaves)
dc.identifier.citationThakurdin, Yashirvad, Characterisation of mafic crustal xenoliths from the Wyoming Craton, Montana (USA), using accessory mineral geochronology and geochemistry, with implications for lower crustal evolution, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/29630>
dc.identifier.urihttps://hdl.handle.net/10539/29630
dc.language.isoenen_ZA
dc.phd.titlePhDen_ZA
dc.schoolSchool of Geosciencesen_ZA
dc.subject.lcshGeological time
dc.subject.lcshGeochemistry
dc.titleCharacterisation of mafic crustal xenoliths from the Wyoming Craton, Montana (USA), using accessory mineral geochronology and geochemistry, with implications for lower crustal evolutionen_ZA
dc.typeThesisen_ZA

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