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New data from Bjarnasson Island and Bals Fiord on western Axel Heiberg Island provide insight into the physical volcanology, petrology, and petrogenesis of Cretaceous Isachsen Formation magmatism. The Bjarnasson Island section is the oldest volcanogenic sequence of the Isachsen Formation. The lower stratigraphic position of the Bjarnasson Island section may be the result of erosional incision of faulting. The Bals Fiord section contains a thick basal sequence of interbedded pyroclastic (hydrovolcanic), distal epiclastic (mass-flow), and thin sedimentary (fluvial) deposits, that mark an initial explosive phase of volcanism in a fluvial-deltaic setting. Volcanic deposits at Bals Fiord change abruptly from pyroclastics to a thick flow sequence with interbedded fluvial-deltaic sedimentary units. Petrographically the Bals Fiord and Bjarnasson Island samples are tholeiites or olivine-tholeiites, and occur in two distinct textural varieties: (i) fine-grained prophyritic and glomeroporphyritic flows from Bjarnasson Island, and (ii) seriate-textured flows from Bals Fiord. Three geochemically distinct populations of Isachsen Formation samples exist, each population consisting mainly of Bjarnasson Island, Bals Fiord, or Bunde Fiord samples. Petrogenetic evaluation of all available Isachsen Formation volcanic samples reveals that at least two magmas, which shared a common parental composition similar to the Bjarnasson Island basalts (primitive flows from an early eruptive event) and distinct fractionation histories, contributed to Walker Island Member volcanism. Variations in the proportion of clinopyroxene fractionation, or the introduction of magnetite fractionation in the Bals Fiord source, may account for the compositional variation between Walker Island Member sources. The Bals Fiord section is the thickest, most proximal, and western-most Walker Island Member volcanogenic sequence, all of which suggests that the source region lies off the west coast of present-day Axel Heiberg Island.

Keywords: Walker Island Member, tholeiite, Bals Fiord, Bjarnasson Island,
volcaniclastics, fractionation
Pages: 121
Supervisor: Gunter Muecke