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Zircon and apatite in the Skaergaard intrusion, East Greenland : trace element, isotopic, and geochronological constraints on crystallization of a closed-system magma reservoir with implications for magmatism in the North Atlantic Igneous Province Moerhuis, Nichole

Abstract

The compositional diversity of igneous rocks on Earth results from differentiation processes that magma undergoes during transport, cooling, and crystallization within the crust. The Eocene Skaergaard mafic layered intrusion in East Greenland, a concentrically zoned ~280 km³ box-shaped fossil magma chamber, preserves a remarkable sequence of gabbroic to melanogranophyric rocks produced through closed-system crystallization of an iron-rich tholeiitic magma. The competing physiochemical processes operating at near-solidus conditions in mafic cumulates and magmatic differentiation timescales within a subvolcanic system are investigated using the accessory minerals zircon and apatite. The exceptional morphological and geochemical diversity of zircon in the Skaergaard intrusion formed due to both magmatic processes (e.g., fractionation, silicate immiscibility) and external influences (e.g., seismic fracturing, sill intrusion). Apatite chemistry records variable growth conditions from near-liquidus to near-solidus crystallization in the floor, roof, and center of the intrusion. Preservation and geochemical biases identified within trace element datasets from mineral separates attest to the importance of in situ analyses for maintaining petrographic context when correlating mineral textures to geochemical interpretations. High-precision uranium-lead zircon geochronology (chemical abrasion-isotope dilution-thermal ionization mass spectrometry) and thermal modelling indicate that the Skaergaard magma was emplaced 55.971 million years ago (Ma) and cooled through near-solidus conditions (750°C) between 55.969 Ma and 55.808 Ma over a 160,000-year interval. The dating results reveal that the crystallization rate increased from 1.4 km³/ka in the first 120,000 years of cooling to 2.3 km³/km during the final 15,000 years of solidification. Dating of Skaergaard cumulates and post-Skaergaard intrusions provides critical timing constraints on the subvolcanic magmatic architecture and flood basalt volcanism on the East Greenland margin within the North Atlantic Igneous Province. The Skaergaard intrusion is coeval with the Paleocene-Eocene thermal maximum (PETM), the most significant non-anthropogenic global warming event in the Cenozoic, and its accelerated late-stage cooling was likely influenced by topography-driven fluid flow and extreme PETM-induced weather conditions across the Arctic region. This dissertation provides new insights into magmatic differentiation processes, establishes the first detailed geochronologic framework for closed-system crystallization of a layered intrusion, and highlights the reciprocal feedback between shallow intrusions in large igneous provinces and global climate.

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Attribution-NonCommercial-NoDerivatives 4.0 International