No abstract

The origin of Large Igneous Provinces (LIPs) associated with continental breakup and the reconstruction of continents older than ca. 320 million years (pre-Pangea) are contentious research problems. Here we study the petrology of a 615-590 Ma dolerite dyke complex that intruded rift basins of the magma-rich margin of Baltica and now is exposed in the Scandinavian Caledonides. These dykes are part of the Central Iapetus Magmatic Province (CIMP), a LIP emplaced in Baltica and Laurentia during opening of the Caledonian Wilson Cycle. The >1,000-km-long dyke complex displays lateral geochemical zonation from enriched to depleted basaltic compositions from south to north. Geochemical modelling of major and trace elements shows these compositions are best explained by melting hot mantle 75-250°C above ambient mantle. Although the trace element modelling solutions are nonunique, the best explanation involves melting a laterally zoned mantle plume with enriched and depleted peridotite lithologies, similar to present-day Iceland and to the North Atlantic Igneous Province. The origin of CIMP appears to have involved several mantle plumes. This is best explained if rifting and breakup magmatism coincided with plume generation zones at the margins of a Large Low Shear-wave Velocity Province (LLSVP) at the core mantle boundary. If the LLSVPs are quasi-stationary back in time as suggested in recent geodynamic models, the CIMP provides a guide for reconstructing the paleogeography of Baltica and Laurentia 615 million years ago to the LLSVP now positioned under the Pacific Ocean. Our results provide a stimulus for using LIPs as piercing points for plate reconstructions.

Granitoids associated with the Neoproterozoic-early Paleozoic Ross orogeny are extensively exposed in the Dry Valleys region of southern Victoria Land, Antarctica, affording an exceptional opportunity to gain insight into the temporal and spatial scales of continental arc magmatism. Samples spanning 150 km along strike and 50 km across strike were selected for isotopic and geochemical analysis. Zircon U-Pb geochronology and the first Hf isotope data for Dry Valleys granitoids, coupled with whole-rock elemental data, reveal mixing between enriched lithospheric mantle and Precambrian crustal components and indicate that the principal phase of magmatism in the Dry Valleys area was restricted to a period of 23 m.y., from ca. 515 to 492 Ma. This relatively short period of magmatism contrasts with other segments of the Ross orogen, in which magmatism spanned greater than 100 m.y. Most calc-alkaline intrusions spanned 515-500 Ma, while postkinematic granitoids with alkali-calcic geochemical signatures spanned 505-492 Ma, indicating a transitional shift to an overall extensional tectonic regime. Zircon e Hf(i) values range between -0.3 and -7.2, with two-stage depleted-mantle model ages ranging from 1.5 to 1.9 Ga. Low e Hf(i) values in mafic samples are consistent with derivation from an enriched subcontinental lithospheric mantle source, while the large-volume granitic intrusions show evidence for increasing assimilation of old crust over time. A broadening of the e Hf(i) range to more negative values in the younger intrusions may reflect crustal thickening or underplating of fertile continental material into the source region of the arc.

iv to think about the geochemical data, and assisted with making interpretations that are reasonable given his deep knowledge of magma mixing, petrology, and thermodynamics. I would like to thank the Earth Science graduate students at UCSB, who are generally incredibly helpful, supportive, positive, and constructive. Pet Food seminars were helpful when first formulating the ideas that are presented in this thesis, and informal discussions with many students were incredibly helpful. Andrew Reinhard deserves special recognition for his help thinking about isotope geochemistry and grappling with intra-lava heterogeneity. And perhaps most importantly, I would like to thank my partner Elissa McBride, who was incredibly supportive throughout my graduate career as I struggled with technical challenges, traveled for months at a time for fieldwork and mass spectrometry, or shirked social obligations to grade papers or make a deadline.

We present in situ laser ablation-multicollector-inductively coupled plasma-mass spectrometry Sr isotope data for plagioclase from a reference stratigraphic profile of the entire Layered Series and Upper Border Series of the Skaergaard intrusion (East Greenland). Plagioclase Sr isotope compositions and anorthite contents vary systematically from the margins of the intrusion inwards. The lowest 87 Sr/ 86 Sr i (calculated at 56 Ma) values (~0.7041) occur near the base and top of the intrusion and systematically increase over several lithostratigraphic zones to a value of ~0.7044, which is uniform throughout the middle ~2000 m of intrusion. Across this same profile, anorthite content of plagioclase varies smoothly from An 65-70 at the base and top to ~An 25 approaching the purported "Sandwich Horizon." Plagioclase near the roof and proximal to rafts of partially assimilated basement gneiss are markedly more radiogenic ( 87 Sr/ 86 Sr i up to ~0.7046). We explain the stratigraphic relationships by progressive contamination of the magma during early stages of differentiation by basement gneiss rafts entrained during emplacement and accumulated near the top of the chamber. Contamination was transient, ceasing once the entrained gneiss was consumed or isolated from the main magma reservoir as the solidification front advanced. Modeling of fractionation-assimilation processes accounts for the observed isotopic trends with only a few percent assimilation (relative to the original magma mass). The record of contamination revealed by Sr in plagioclase supports the view that the bulk of the Skaergaard intrusion formed by closed-system differentiation with only minor in situ contamination and no magma recharge. Comparing plagioclase and bulk-rock Sr suggests that the latter may have witnessed late-stage metasomatic overprinting of phases other than plagioclase.