Timing of Breakup and Thermal Evolution of a Pre‐Caledonian Neoproterozoic Exhumed Magma‐Rich Rifted Margin

Abstract

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 breakup 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 approximately 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 breakup.

Evolution of a shear zone before, during and after melting

“…Atlantic rifting; Clerc et al 2015Clerc et al , 2018 as well as older, former Iapetus margins (e.g. Kjøll et al 2019). Seiland Igneous Province emplacement alongside the shear zones suggests that the Seiland Igneous Province was part of a magma-rich continental rift zone where the paragneiss formed ductile mid-crustal shear zones as illustrated in Figure 9.…”

Early­­­­–Middle Ordovician sedimentation and bimodal volcanism at the margin of Iapetus: the Trollhøtta–Kinna Basin of the central Norwegian Caledonides

“…Volcanic rocks and mafic dyke complexes indicate Late Neoproterozoic rifting of the ancient supercontinent, with subsequent break-up and separation of Baltica, Laurentia and Amazonia (which later became part of Gondwana) at around 620-550 Ma (e.g. Bingen et al 1998;Cawood et al 2001;Kjøll et al 2019;Tegner et al 2019), although the Late Neoproterozoic to Cambrian plate configuration prior to opening of Iapetus is still not fully resolved (e.g. Hartz & Torsvik 2002;Pisarevsky et al 2008;Levashova et al 2015;Torsvik & Cocks 2016;Nance & Murphy 2019).…”

Exhumation of the High‐Pressure Tsäkkok Lens, Swedish Caledonides: Insights From the Structural and White Mica 40Ar/39Ar Geochronological Record

“…Of these allochthons, UHP rocks are prominent in the SNC of the Middle Allochthon (e.g., Gee et al, 2013Gee et al, , 2020. The SNC comprises metasedimentary and metaigneous rocks that represent the extended passive margin of Baltica (e.g., Andréasson et al, 1992;Jakob et al, 2019;Kjøll et al, 2019). These rocks were subducted to mantle depths beneath a volcanic arc of Iapetus affinity during Caledonian orogenesis (e.g., Andréasson, 1987;Brueckner & van Roermund, 2004;Bukała et al, 2018;Gee, 1975;Janák et al, 2013;Klonowska et al, 2014Klonowska et al, , 2016Klonowska et al, , 2017Kullerud et al, 1990;Majka et al, 2014;Petrík et al, 2019;Santallier, 1988;Stephens & van Roermund, 1984).…”

“…Locally, some eclogites preserve pillow basalt textures (Kullerud et al, 1990). The mixture of continentally derived rocks, marbles, and the intrusions of the mafic bodies into the metasediments as well as submarine volcanism had led to the interpretation that the Tsäkkok Lens represents a hyper-extended portion of the Baltican passive margin, bordering the Iapetus Ocean (Kjøll et al, 2019;Kullerud et al, 1990).…”