The geology of the Arctic is greatly influenced by a period of widespread Cretaceous magmatic activity, the High Arctic Large Igneous Province (HALIP). Two major tholeiitic magmatic pulses characterize HALIP: an initial 120 -130 Ma pulse that affected Arctic Canada and formally adjacent regions of Svalbard (Norway) and Franz Josef Land (Russia). In Canada, this pulse fed lava flows of the Isachsen Formation. A second 90-100 Ma pulse that apparently only affected the Canadian side of the Arctic, fed flood basalts of the Strand Fiord Formation. The goal of this thesis is to improve understanding of Arctic magmatism of the enigmatic HALIP through field, remote sensing, geochemical and geochronology investigations of mafic intrusive rocks collected in the South Fiord area of Axel Heiberg Island, Nunavut, and comparison with mafic lavas of the Isachsen and Strand Fiord Formations collected from other localities on the Island. Ground-based and remote sensing observations of the South Fiord area reveal a complex network of mafic sills and mainly SSE-trending dykes. Two new U-Pb baddeleyite ages of 95.18 ± 0.35 Ma and 95.56 ± 0.24 Ma from South Fiord intrusions along with geochemical similarity confirm these intrusions (including the SSE-trending dykes) are feeders for the Strand Fiord Formation lavas. The ages constrain a 3 Ma transition between the two pulses of older tholeiitic and the younger stage of alkaline magmatism that begins with the 92 Ma Wootton Igneous Complex. Based on trace element and Sm-Nd isotopic data, this 95 Ma pulse of South Fiord intrusives and Strand Fiord Formation lavas are derived from a homogenous upper mantle source that was crustally contaminated by sedimentary rocks of the Sverdrup Basin. In contrast, lavas of the older c. 120-130 Ma Isachsen Formation are derived from a heterogeneous mantle source and experienced significantly less crustal contamination. The mantle heterogeneity of lavas in the Isachsen Formation is probably defined in part by mantle melting that incorporated sediments from paleosubduction fronts ancestral to the Aleutian subduction zone. Furthermore, major element concentrations may also suggest contamination by evaporite diapirs from the Carboniferous Otto Fiord Formation.