Late Neoproterozoic amphibolite-facies metamorphism of a pre-Caledonian basement block in southwest Wedel Jarlsberg Land, Spitsbergen: new evidence from U–Th–Pb dating of monazite

Abstract

Southwest Spitsbergen, Wedel Jarlsberg Land, consists of two Proterozoic crustal blocks with differing metamorphic histories. Both blocks experienced Caledonian greenschist-facies metamorphism, but only the southern block records an earlier pervasive M1 amphibolite-facies metamorphism and strong deformational fabric. In situ EMPA total-Pb monazite geochronology from both matrix and porphyroblast inclusion results indicate that the older M1 metamorphism occurred at 643 ± 9 Ma, consistent with published cooling ages of c. 620 Ma (hornblende) and 580 Ma (mica) obtained from these same rocks. This region thus contains a lithostratigraphic profile and metamorphic history which are unique within the Svalbard Archipelago. Documentation of a pervasive late Neoproterozoic Barrovian metamorphism is difficult to reconcile with a quiescent nontectonic regime typically inferred for this region, based on the occurrence of rift-drift sequences on the Baltic and Laurentian passive margins. Instead, our new metamorphic age implies an exotic origin of the pre-Devonian basement exposed in SW Spitsbergen and supports models of terrane assembly postulated for the Svalbard Archipelago.

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

“…Gavrilenko et al [13] reported a whole rock Rb-Sr age of c. 930 Ma for the Isbjørnhamna Group, based on a three-point isochron, whereas Balashov et al [3] dated zircon separated from the rhyolitic rocks of the Eimfjellet Group, using the traditional U-Pb whole grain technique, and obtained a lower intercept of concordia at 931 ± 54 Ma. These age estimates were discredited by Majka et al [29] and subsequent studies which highlighted methodological problems of the previous studies and presented new geochronological and geological evidence ruling out a possibility of Grenvillian metamorphism record in the Isbjørnhamna Group.…”

“…These authors suggested that the obtained ages reflected the latest Neoproterozoic re-heating and speculated that it might have been related to the opening of the early Iapetus Ocean. Majka et al [29] dated metamorphic monazite in the Isbjørnhamna Group and obtained an age of 643 ± 9 Ma (Fig. 2).…”

“…Both Stenian-Ectasian and Tonian igneous rocks (at least in the southern part of the SBP), as well as associated amphibolite facies metasedimentary successions, were affected by Late Neoproterozoic Torellian metamorphism (e.g. [29,32]). The Torellian Orogeny is also manifested in the form of a regional-scale angular unconformity [6,7], which divides substantially folded, but weakly metamorphosed, Early Neoproterozoic metasediments from overlying Late Neoproterozoic and less deformed metasediments.…”

Neoproterozoic metamorphic evolution of the Isbjørnhamna Group rocks from south-western Svalbard

“…Using previous geochronological data (Manecki et al 1998;Majka et al 2008) and thermometrical results presented in this paper, it is possible to reconstruct the cooling history of the Isbjørnhamna Group. Manecki et al (1998) Majka et al (2008) obtained a 643 My U-Th-total Pb age for monazites located both in the matrix of mica schists and enclosed in garnet porphyro- Neoproterozoic metamorphism in southern Svalbard J.…”

“…Manecki et al (1998) Majka et al (2008) obtained a 643 My U-Th-total Pb age for monazites located both in the matrix of mica schists and enclosed in garnet porphyro- Neoproterozoic metamorphism in southern Svalbard J. Majka et al blasts, constraining the growth of garnet in the Isbjørnhamna Group at 643 Mya.…”