Grenvillian single-grain zircon Pb age of a granitic rock from the southern island of Hesteskoholmen, Liefdefjorden, northwestern Spitsbergen, Svalbard

A NS trending, narrow zone of crystalline basement occurs from Biscayarhalv oya to Holtedahlfonna in northwestern Spitsbergen and is composed of variou s metasedimentary and igneous rocks, including granites. Previous isotopic age determinations on these rocks are by the K-Ar, Rb-Sr, Ar-40/Ar-39 and c onventional zircon U-Pb method and yielded the Caledonian and Grenvillian a ges. Thr single-grain zircon Pb evaporation method has recently been applie d to solve complex problems and this is the first report by the method. A granitic rock, syntectonically intruded into the phyllitic metasediments of the Biscayarhuken formation, which is the uppermost lithotectonic unit i n the metamorphic rocks of the zone, was dated on four zircon grains, yield ing a narrow age range from 955 +/- 4 to 968 +/- 9 Ma, 961 +/- 4 Ma in aver age. This age of ca. 960 Ma is considered to be the age of intrusion, based on the occurrence and zircon morphology, which is roughly simultaneous wit h the formation of the phyllitic cleavages of the surrounding metasediments . The data obtained imply that the Caledonian events did not reset the Pb i sotope system of zircon and major metamorphism occurred during the Grenvill ian time in the Biscayarhuken formation, accordingly, the protolith age of the metasediments is Mesoproterozoic.