Single-grain zircon dating of the metamorphic and granitic rocks from the Biscayarhalvøya-Holtedahlfonna zone, north-west Spitsbergen

The Biscayarhalvøya-Holtedahlfonna zone (BHZ) in north-western Spitsbergen is a north-south trending, narrow horst, with crystalline basement rocks exposed under a Devonian unconformity. Previous K-Ar, ⁴⁰Ar/³⁹Ar and Rb-Sr analyses have confirmed the occurrence of Caledonian thermal events, and Grenvillian ages have been obtained by conventional zircon U-Pb and single-zircon Pb evaporation methods. A total of 55 zircon grains from three samples (an augen metagranite, a micaceous schist and a granitic neosome of migmatite) have been analysed by the single-zircon Pb evaporation method. The grains with the age range of ca. 950-1100 My (million years) are the major component in all three samples, suggesting tectono-thermal activity in that period. The detrital versus resorption orgin of the rounded shapes of these grains from the granitic neosome is not clear yet. Therefore, the ages of the migmatization and of the sedimentary protoliths are not concluded. The youngest presumed detrital grain from the granitic neosome is 1060 My old. The metagranite, cutting the Richarddalen unit, yielded grains with an age of ca. 950 Mya. A granite dyke with an age range of 955-968 My cuts the Biscayarhuken unit in the northern Liefdefjorden area. These indicate the sedimentary protoliths of the Richarddalen and Biscayarhuken units are pre-Neoproterozoic. The youngest detrital zircon ages of ca. 940 My indicate Neoproterozoic sedimentary protoliths of the Solanderfjellet micaceous schists. A significant population of zircon grains with an age range of 1600-1900 My in all three samples suggests a wide exposure of these rocks in the source areas during Meso- and Neoproterozoic times. Several Archean ages have also been obtained. The results are generally conformable with those obtained from north-western Spitsbergen.     

Grenvillian and Caledonian evolution of eastern Svalbard – a tale of two orogenies

Svalbard is located in the north-west corner of the Barents Sea shelf and the Eurasian Plate, in a key area for interpreting Caledonian and older orogens in the Arctic region. Recent U–Pb dating in the Nordaustlandet Terrane of eastern Svalbard shows this terrane to consist of a Grenville-age basement, overlain by Neoproterozoic to early Palaeozoic platformal sediments, and intruded by Caledonian anatectic granites. Deformation, metamorphism and crustal anatectic magmatism occurred both during the Grenvillian (960–940 Ma) and Caledonian (450–410 Ma) orogenies. This evolution shows great similarities with that of eastern Greenland. In the classical model, eastern Svalbard is placed outboard of central east Greenland in pre-Caledonian time. Alternatively, it may have been located north-east of Greenland and transferred west and rotated anticlockwise during Caledonian continent–continent collision. In the Neoproterozoic, easternmost Svalbard may have been part of a wider area of Grenville-age crust, now fragmented and dispersed around the Arctic.     

Single-zircon Pb-evaporation and 40Ar/39Ar dating of the metamorphic and granitic rocks in north-west Spitsbergen

North-west Spitsbergen consists of a complex of Caledonian and Grenvillian crystalline rocks, situated at the north-west corner of the Barents Shelf. The aim of this study is to understand the extent of pre-Caledonian basement rocks and their protoliths. Micas and zircon grains from six rocks from north-west Spitsbergen have been dated by the ⁴⁰Ar/³⁹Ar and single-zircon Pb-evaporation methods. Two grey granites yielded Late Caledonian mica ⁴⁰Ar/³⁹Ar and zircon ages of ca. 420-430 My, with inherited zircon grains as old as 1725 My. Zircon grains from a gneissose granite xenolith in a grey granites gave crystallization ages of ca. 960 My; some grains from a migmatite neosome show similar ages. Zircon grains yielding Archean and late Palaeoproterozoic ages (1600-1800 My) are interpreted as xenocrysts of detrital origin. The youngest ages obtained from detrital zircon grains from a greenschist facies quartzite of the Signehamna unit are ca. 1800 My. Similar schists are included as xenoliths in the 960 My old gneissose granite; therefore, the sedimentary protoliths of the unit are Mesoproterozoic. The dating results suggest a significant tectonothermal event during Grenvillian time; subsequent Caledonian events had less extensive thermal effects. However, it is still a matter of debate whether Grenvillian or Caledonian metamorphism produced the majority of the migmatites. A large population of zircon grains with Late Palaeoproterozoic ages suggests a wide surface exposure of rocks of this age in the source area, with some Archean zircons.