Tonian and Silurian magmatism in Nordaustlandet: Svalbard’s place in the Caledonian orogen
William C. McClelland, Werner von Gosen, Karsten Piepjohn, "Tonian and Silurian magmatism in Nordaustlandet: Svalbard’s place in the Caledonian orogen", Circum-Arctic Structural Events: Tectonic Evolution of the Arctic Margins and Trans-Arctic Links with Adjacent Orogens, Karsten Piepjohn, Justin V. Strauss, Lutz Reinhardt, William C. McClelland
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The Nordaustlandet terrane of Svalbard plays a critical role in evaluating strike-slip displacements in the Caledonian orogen. Comparison of Silurian and Tonian magmatism in Nordaustlandet, East Greenland, and the Pearya terrane on Ellesmere Island provides a means to evaluate models of large-scale versus minimal displacements. Augen gneiss with an emplacement age of 972 ± 5 Ma demonstrates that the Tonian granite suite is coeval with the calc-alkaline Kap Hansteen volcanic rocks. Zircon from Silurian leucosomes, leucogranites, and granites is dominated by xenocrystic components, making it analytically difficult to isolate magmatic versus inherited age domains. Zircon systematics from a relatively undeformed Silurian granite (431 ± 5 Ma) resemble those of similar granites interpreted to be Tonian in age (e.g., Kontaktberget granite). Assuming less deformed granites of Nordaustlandet are Silurian, synemplacement or syntectonic deformation of Tonian augen gneiss and volcanic rocks is no longer required. Tonian magmatic rocks of Svalbard share a common origin with basement rocks of the Pearya terrane within a continental arc system, but are distinctly older than Tonian igneous and metamorphic rocks of East Greenland. Migmatite complexes and granite intrusions in Nordaustlandet, with ages ranging from 440 to 425 Ma, are coeval with granites in East Greenland that record the combined effects of subduction beneath Laurentia and mid-crustal melting. Migmatites in East Greenland are juxtaposed with low grade rocks by syn-contraction normal faults whereas migmatites show gradational contacts into lower grade rocks on Nordaustlandet. These differences in basement age and structural setting preclude proximity of the Nordaustlandet terrane with East Greenland during the 440–400 Ma continent-continent collision phase (Scandian) of the Caledonian orogen. Similarly, differences in depositional, magmatic, and metamorphic history between the Pearya terrane basement and Nordaustlandet terrane argue against simple offset of crustal fragments. The Pearya and Nordaustlandet terranes likely were not involved in the main phase of crustal thickening directly related to collision of Baltica and Laurentia, but rather resided on a convergent boundary north of the Scandian continent-continent collision zone, consistent with models of Gee and Teben’kov (2004) and Johansson et al. (2005).