SHRIMP U-PB GEOCHRONOLOGY AND METAMORPHIC HISTORY OF THE SMALLEFJORD SEQUENCE, NE GREENLAND CALEDONIDES

SHRIMP U-Pb zircon ages have been obtained for a sample of metasedimen tary gneiss from the Smallefjord sequence in the NE Greenland Caledoni des. Pre-1000 Ma zircons are interpreted as being detrital in origin; the source region contained late Archaean rocks (or younger sediments or inheritance-rich granites derived from them) and several groups of Proterozoic rocks as young as c. 1035 Ma. Zircons with Pb-206/U-238 ag es between c. 980 Ma and 890 Ma are interpreted to have grown during a discrete high grade metamorphic event. Six analyses provided a mean P b-206/U-238 age of 955 +/- 13 Ma, which is interpreted as recording 'G renvillian' metamorphism of the Smallefjord sequence. This supports re gional correlation of the Smallefjord sequence with the Krummedal sequ ence of central E Greenland which has previously yielded evidence for tectonothermal activity at c. 1000 Ma, and provides a further indicati on of a northern arm of the Grenville-Sveconorwegian orogen in the Nor th Atlantic region. Further growth of metamorphic zircon within the Sm allefjord sequence occurred during the Caledonian orogeny at 445 +/- 1 0 Ma (Upper Ordovician). This is consistent with estimates of a Middle -Upper Ordovician age for Caledonian orogenesis in central E Greenland . It contrasts with previously published SHRIMP U-Pb zircon dating of Lower Devonian (404 +/- 6 Ma) metamorphism within the basement gneisse s of the Dove Bugt area north of the Smallefjord sequence outcrop. SHR IMP dating of zircons therefore indicates polyphase Palaeozoic metamor phism within the NE Greenland Caledonides.