Understanding Volcanic Margin Evolution through the Lens of Norway's Youngest Granite discovered by IODP Expedition 39

Abstract

Three boreholes drilled during the International Ocean Discovery Program (IODP) Expedition 396 have yielded unexpected findings of altered granitic rocks covered by basalt flows, interbedded sediments, and glacial mud on the Kolga High situated near the continent-ocean transition on the mid-Norwegian margin.  To assess basin and basement structures near Kolga High in relation to the broader regional setting, a potential field forward modelling study was conducted. One specific goal was to evaluate the density distribution beneath the Kolga granite. The necessity of low-density crustal material beneath the Kolga High challenges the hypothesis of an old, thick, dense, and inherited basement high directly beneath the basalt, given the low gravity signal observed. In our potential field model, the rock density underneath the basalt remains relatively low (2.4 g.cm-3 in average). Based on onshore measurements, Caledonian or Precambrian ‘fresh’ granitoids and other inherited basement rocks typically exhibit bulk densities usually exceeding 2.65-2.75 g.cm-3. The gravity signal observed on Kolga High, along with the low-density necessary to fit it, suggests that the inherited basement should be situated at a considerably greater depth (~up to 10 km), which is approximately 5-7 km deeper than the drilled Kolga granite/basalt interface. To unravel the weathering chronology for this enigmatic granite, the K-Ar method was selected to date fine-grained clay minerals. X-ray diffraction was performed on different grain size fractions to identify both protolithic and authigenically formed K-bearing minerals derived from the IODP rock samples (Holes U1565A and U1566A). K-Ar geochronology was then performed on five grain size fractions (<0.1, 0.1-0.4, 0.4-2, 2–6, and 6–10 µm).  Finally, the crystallisation age of the granite was verified by conducting mineral analysis on 104 zircons using laser ablation inductively coupled with mass spectrometry (LA-ICP-MS). The K-Ar dating indicates that the alteration of the Kolga granite occurred between 54.7 ± 1 and 37.1 ± 1 Ma suggesting a long period of near surface exposure after the breakup. Based on U-Pb dating of zircon, the granite’s crystallization age is determined at 56.3 ± 0.2 Ma, which aligns with the Paleocene-Eocene Thermal Maximum (around 56 Ma). Collectively, insights from the gravity model and geochronology indicate that the Kolga granite is a Paleocene intrusion, likely emplaced under exceptionally shallow conditions, possibly preceding the breakup and opening of the Norwegian-Greenland Sea. The geochronological results indicate a remarkably short period of time between the granite emplacement, its near surface weathering, and the basaltic lava flows emplacement above the paleosurface. Incidentally, this intrusion also represents the most distal and youngest granite discovered in Norway. This study provide crucial paleogeographic constraints and helps to refine the mode of breakup of a nascent volcanic margin.