AR-40/AR-39 AGE STUDIES OF MAFIC VOLCANICS FROM THE SVERDRUP BASIN, ARCTIC CANADA
Avison, Heather A.
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Cretaceous extrusive volcanism in the Sverdrup Basin is widespread and appears to have occurred in four major episodes (Valanginian, late Barremian, late Albian, and late Cenomanian/early Turonian). In addition to these flows, the basin is cut by abundant dykes and sills which have been dated by various researchers to range from Jurassic to Upper Cretaceous in age. Some correlation between the intrusive and extrusive activity may be expected, and therefore the reliability of the intrusive ages has been questioned. Ar-40/Ar-39 step-heating analyses have been carried out on 10 fractions from four igneous bodies in the Sverdrup Basin. The rock samples were chosen so that in addition to discovering the ages of the units sampled, the rock components best suited for dating these mafic bodies could be determined. The results of this study show that high-K deuteric phases from body interiors and chilled margin samples containing high-K phases produce the most reliable ages; provided they have not experienced a subsequent thermal disturbance above their argon blocking temperatures (generally <200°C). It is also shown that low-K separates such as mafics or plagioclase yield unreliable ages, even when the samples are apparently undisturbed. In every case, the age of the unit was found to coincide with one of the major extrusive volcanic episodes. A sill at Buchanan Lake, Axel Heiberg Island, yields an age of 126±2 Ma, and was apparently emplaced during the Valanginian episode. A dyke which cross-cuts this sill was dated to be 113±6 Ma, concurrent with volcanics of the Barremian episode outcropping at Camp Five Creek near Bunde Fjord. Both a flow and a sill near Lake Hazen, Ellesmere Island, were apparently emplaced during the Turonian episode (92±2 Ma and 91 ±2 Ma, respectively). Flows from this fourth episode are also found near the basin margin at Hanson Point, northern Ellesmere Island. Keywords: Pages: 112 Supervisors: Peter Reynolds / Gunter Muecke