GRENVILLIAN BASEMENT AND A MAJOR UNCONFORMITY WITHIN THE CALEDONIDES OF NORDAUSTLANDET, SVALBARD

Variously deformed granites and migmatites underlie the Neoproterozoic successions of Nordaustlandet, Svalbard. New U-Pb zircon investigatio ns of two types of intrusions from Lapponiahalvoya, northwestern Norda ustlandet, have yielded ages of 93 +/- 8 and Ma (Kontaktberget granite ) and 961 +/- 17 Ma (Lapponiafj ellet granite), confirming previous, l ess precise multi-grain, Pb-Pb evidence of Grenvillian igneous activit y. The Kontaktberget granite cuts metasediments (Brennevinsfjorden Gro up), that are tightly folded and are separated from overlying volcanic and volcaniclastic rocks (Kapp Hansteen Group) by a major discordance (the Botniahalvoya Unconformity). The latter has been mapped in north western Nordaustlandet and is inferred to exist in central areas betwe en Vestfonna and Austfonna. The Kapp Hansteen Group, with associated i ntrusive quartz porphyries, are probably a late orogenic expression of the Grenvillian igneous activity. The lack of igneous rocks in the ov erlying Neoproterozoic succession (Murchisonfjorden Supergroup) sugges ts that the base of this major unit is also a significant unconformity . This new evidence emphasizes the importance of the belt of Grenvilli an tectonothermal activity, referred to here as the Nordauslandet Orog eny, that reaches from the classical provinces of North America and so uthern Scandinavia, via the East Greenland Caledonides, into the high Arctic.