ADDITIONAL RB-SR AND SINGLE-GRAIN ZIRCON DATINGS OF CALEDONIAN GRANITOID ROCKS FROM ALBERT-I LAND, NORTHWEST SPITSBERGEN

Previous K-Ar and Rb-Sr datings of the metamorphic and granitic rocks from the northwestern basement region of Spitsbergen mainly show the c ooling time of the rocks, except for a Rb-Sr isochron age of the Horne mantoppen granitoid. New samples were collected during several years o f geological mapping in the area and the Rb-Sr whole rock isochron and single-grain zircon evaporation methods were applied to the Hornemant oppen granitoids and the grey granites. A dioritic dyke was also dated by the latter method. The bulk rock chemistry study shows that most o f both granitic rocks are of the S-type and probably post orogenic, wi th distinctive incorporation of crustal materials. The isotopic data a lso support this interpretation. The results of the Rb-Sr isotope anal yses, 412 +/- 4.8 Ma and the zircon Pb evaporation age of 424 +/- 56 M a, confirm the previous age of the Hornemantoppen granitoid, 414 +/- 1 0 Ma. An older zircon age of 547 +/- 19 Ma is considered to be the min imum age of inherited zircon. Zircons from the grey granites suggest a n age of ca. 420 Ma with a large error. Field relations demonstrate th at the grey granites are older than the Hornemantoppen granitoids. A m inimum inherited zircon age, 952 +/- 20 Ma, has been obtained from the grey granites. Three multi-grain Pb ages, 423 +/- 22 Ma (2 grains), 4 61 +/- 42 Ma and 561 +/- 93 Ma (the last two 3 grains) were considered to be mixed ages. Although no definitive evidence for the presence of Grenvillian granites in this area has been obtained in the present st udy, preliminary results from the multi-grain zircon evaporation metho d, carried out in the Russian laboratory at Apatity, infer Paleo- and Mesoproterozoic protoliths for the metamorphic rocks of northwestern S pitsbergen.