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

Abstract

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.