Abstract
Reconnaissance Pb-Pb step-leaching analyses have been carried out on garnet and kyanite from the Krummedal supracrustal sequence in East Greenland, yielding respectively Neoproterozoic and Caledonian ages. These data support previous analyses suggesting that the Krummedal supracrustal sequence, widespread in southern parts of the East Greenland Caledonides, was affected by both an early Neoproterozoic and a Caledonian thermal event. Titanite and apatite fractions from the underlying crystalline basement rocks were analysed in order to obtain metamorphic ages, as a contrast and supplement to the numerous existing protolith ages on orthogneisses. The titanite yielded a date of 486 ± 15 Ma which, if interpreted as a true age, is older than the usual range of Caledonian ages in East Greenland. The significance of this date is uncertain, but one possibility is that it reflects extension and subsidence taking place prior to Caledonian collision. The apatite, in contrast, yielded a very young Caledonian date of 392 ± 24 Ma that may reflect the cooling of the basement gneisses to < 500°C subsequent to collision.
Highlights
The thrust sheets incorporate Archaean and Palaeoproterozoic orthogneiss complexes overlain by a thick late Mesoproterozoic – early Neoproterozoic metasedimentary succession known as the Krummedal supracrustal sequence; the latter is structurally overlain by the Neoproterozoic Eleonore Bay Supergroup and Tillite
The late Mesoproterozoic – early Neoproterozoic Krummedal supracrustal sequence is widely distributed in the southern part of the East Greenland Caledonides between 70° and 74°N (Fig. 1; Higgins 1988)
The analyses reported in this paper are the first Pb step-leaching (PbSL) analyses reported on rocks from the Caledonian orogen of East Greenland
Summary
Reconnaissance Pb-Pb step-leaching analyses have been carried out on garnet and kyanite from the Krummedal supracrustal sequence in East Greenland, yielding respectively Neoproterozoic and Caledonian ages. The titanite yielded a date of 486 ± 15 Ma which, if interpreted as a true age, is older than the usual range of Caledonian ages in East Greenland The significance of this date is uncertain, but one possibility is that it reflects extension and subsidence taking place prior to Caledonian collision. Stepwise leaching of the mineral phases increases the data spread in uranogenic (207Pb/204Pb – 206Pb/204Pb) and thorogenic vs uranogenic (208Pb/204Pb – 206Pb/ 204Pb) diagrams, and as a consequence the precision of Pb/Pb isochrons is improved (Frei et al 1997) Another advantage of the method is that the corresponding uranogenic and thorogenic Pb ratios of the different leach solutions can be observed, and a signature of Pb-containing microscopic mineral inclusions revealed. The thrust sheets incorporate Archaean and Palaeoproterozoic orthogneiss complexes overlain by a thick late Mesoproterozoic – early Neoproterozoic metasedimentary succession known as the Krummedal supracrustal sequence; the latter is structurally overlain by the Neoproterozoic Eleonore Bay Supergroup and Tillite
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