Two garnet growth events in polymetamorphic rocks in southwest Spitsbergen, Norway: insight in the history of Neoproterozoic and early Paleozoic metamorphism in the High Arctic

Abstract

Geochronological studies in northern Wedel Jarlsberg Land, southwestern Svalbard (Norway), showed that the Tonian (c. 950 Ma) igneous rocks were subjected to metamorphism during the Torellian (c. 640 Ma) and early Caledonian (470–460 Ma) events. Predominant augen gneisses, derived from a Tonian protolith, are intercalated in that area, with schists comprising two distinct metamorphic mineral assemblages. The M1 (Torellian) assemblage containing garnet-I + quartz + plagioclase-I + biotite-I + muscovite-I was formed under amphibolite-facies conditions at c. 550–600 °C and 5–8 kbar (1 kbar = 100 MPa). The M2 (Caledonian) assemblage comprising garnet-II + quartz + plagioclase-II + biotite-II + muscovite-II + zoisite + chlorite crystallized at c. 500–550 °C and 9–12 kbar, corresponding to epidote–amphibolite facies conditions. The M2 mineral assemblage constitutes the pervasive Caledonian fabric of the schists that was subsequently reactivated in a left-lateral strike-slip shear regime. The subsequent c. 70° clockwise rotation of the original structure to its present position was caused by a large-scale passive rotation during the Paleogene Eurekan orogeny. The new pressure–temperature estimates suggest that metamorphic basement in the study area was consolidated during the Torellian middle-grade event and then overprinted by Caledonian moderate- to high-pressure subduction-related metamorphism. A following sinistral shear zone assembled the present structure of basement units. Our results pose a question about the possible extent of Torellian precursor to the Caledonian basement across the High Arctic and the scale of its subsequent involvement in early Caledonian subduction. In conjunction with previous studies, the results suggest a possible correlation between southwestern Spitsbergen and the Pearya Terrane in Ellesmere Island.

Magmatic and metamorphic events recorded within the Southwestern Basement Province of Svalbard

“…Retrograde blueschists from Nordenskiöld Land yielded P-T conditions of 14-18 kbar by 470-490 °C and confirmed a low 7-8 °C/km subduction gradient [25]. Majka et al [33] have reported on 9-12 kbar and c. 500-550 °C P-T conditions for the Caledonian metamorphic overprint of the Berzeliuseggene unit.…”

“…The retrograde blueschists can be traced south of Oscar II Land in Nordenskiöld Land near Marvågen and Orustosen as shown by Kośmińska et al [25]. Also, the polymetamorphic rocks of the Berzeliuseggene unit are overprinted by the HP metamorphic event of inferred Caledonian age ( [9,33] and references therein). The only reliable ages derived for the greenschist facies rocks come from the southern part of Wedel Jarlsberg Land and have been published by Manecki et al [34].…”

“…Ion microprobe zircon U-Pb dating yielded a 950 ± 5 Ma protolith crystallization age of the augen gneisses [32]. The Berzeliuseggene unit displays a tectonic contact with the Neoproterozoic Deilegga and Sofiebogen groups and other sediments of unknown, but probable Early Paleozoic age [9,32,33]. This contact has been interpreted as a rotated, large-scale sinistral shear zone, similar to the aforementioned Vimsodden-Kosibapasset zone [33].…”

“…In contrast, the Isbjørnhamna Group and the Berzeliuseggene unit have been studied in detail with regard to pressure-temperature (P-T) conditions using classical geothermobarometry and thermodynamic phase equilibria modeling. The maximum P-T conditions for the Isbjørnhamna Group were estimated at 11 kbar and 680 °C [30], whereas conditions for the Berzeliuseggene unit were constrained at 6 kbar and 600 °C [33].…”

“…However, the SBP also bears record of subduction-related Caledonian metamorphism of blueschist to eclogite grade (e.g. [25,33,36]). These high-pressure (HP) rocks in their type locality at Motalafjella are unconformably overlain by a Late Ordovician to upper Silurian sedimentary succession [2,43].…”