SHRIMP U-Pb geochronology and metamorphic history of the Smallefjord sequence, NE Greenland Caledonides

Abstract

SHRIMP U-Pb zircon ages have been obtained for a sample of metasedimentary gneiss from the Smallefjord sequence in the NE Greenland Caledonides. 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 206 Pb/ 238 U ages between c. 980 Ma and 890 Ma are interpreted to have grown during a discrete high grade metamorphic event. Six analyses provided a mean 206 Pb/ 238 U age of 955 ± 13 Ma, which is interpreted as recording 'Grenvillian' metamorphism of the Smallefjord sequence. This supports regional correlation of the Smallefjord sequence with the Krummedal sequence of central E Greenland which has previously yielded evidence for tectonothermal activity at c. 1000 Ma, and provides a further indication of a northern arm of the Grenville-Sveconorwegian orogen in the North Atlantic region. Further growth of metamorphic zircon within the Smallefjord sequence occurred during the Caledonian orogeny at 445 ± 10 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 gneisses of the Dove Bugt area north of the Smallefjord sequence outcrop. SHRIMP dating of zircons therefore indicates polyphase Palaeozoic metamorphism within the NE Greenland Caledonides.