Timing of break-up and thermal evolution of a pre-Caledonian Neoproterozoic exhumed magma-rich rifted margin

During the terminal stages of Wilson cycles, remnants of magma-poor margins may be incorporated into the orogens, whereas the magma-rich margins often are lost in subduction due to low buoyancy. The understanding of magma-rich margins is therefore mostly based on drill holes and geophysical observations. In this contribution, we explore the temporal evolution and the ambient conditions of a magma-rich rifted margin preserved within the Scandinavian Caledonides. The Scandinavian Dike Complex was emplaced into a sedimentary basin during the initial break-up and opening of the Iapetus Ocean 615 to 590 million years ago. The dike complex constitutes 70-90% of the magma-rich, syn-rift basins and is locally well-preserved despite the complex Caledonian history. This contribution provides new observations about the geometry, relative timing, and development of the margin. Jadeite-in-clinopyroxene geothermobarometry, titanium-in-biotite geothermometry, and garnet isopleth modeling show that the ambient pressure and temperature conditions were similar for the entire dike complex at 0.25 to 0.45 GPa, with contact metamorphic temperatures up to c. 700°C. In the northernmost part of the study area U-Pb dating of magmatic zircon shows that partial melting of the sedimentary host-rock, at relatively shallow levels, occurred at 612 Ma. This shows that the crust was molten already 6 million years before the northernmost dike swarm was emplaced at 605.7 ± 1.8 Ma. We propose that the locally pervasive partial melting occurred due to high geothermal gradients and introduction of mafic melt in the lower crust. These processes significantly reduced the strength of the crust, eventually facilitating continental break-up. ©2019. American Geophysical Union. All Rights Reserved.