U-Pb geochronology of late Neoproterozoic augen granites in the Moine Supergroup, NW Scotland: dating of rift-related, felsic magmatism during supercontinent break-up?

摘要

Within the Caledonides of Caithness, Scotland, the Neoproterozoic metasedimentary rocks of the Moine Supergroup are intruded by minor sheets of strongly deformed granite. U–Pb sensitive high-resolution ion microprobe zircon ages of 599 ± 9 Ma (Berriedale augen granite) and 588 ± 8 Ma (Braeval augen granite) are interpreted to date emplacement during the late Neoproterozoic. These augen granites are therefore unrelated to either Knoydartian (c. 870–790 Ma) or Caledonian (c. 470–420 Ma) orogenesis in the Scottish Highlands. Intrusion was, however, broadly contemporaneous with late Neoproterozoic extension of the Laurentian margin during continental break-up and the opening of the Iapetus Ocean. In the context of evidence for rift-related mafic magmatism within the Dalradian basin in Scotland at c. 600 Ma, as well as contemporaneous anorogenic magmatism along the margins of the developing Iapetus Ocean in the Appalachians, we propose that the protoliths of the augen granites were probably emplaced during continental break-up. The new data broaden the extent of this late Neoproterozoic magmatic event in the Scottish Highlands: other deformed granites that are at present undated but have been assumed to be of Caledonian age therefore require reinvestigation.