New isotopic age determinations from the Northern Highlands and their implications (online lecture)

Professor Matthew Thirlwall, Royal Holloway, University of London

Over the last 15 years we have carried out a large number of new age determinations on metamorphic rocks from the Moine and its basement inliers. Some of these are published and some not. They include Lu-Hf and Sm-Nd ages on garnets and Rb-Sr ages on white mica and biotite. Lu-Hf garnet ages are probably close to crystallization ages, while Sm-Nd garnet ages in most cases represent stages on a cooling trajectory. This can clearly be demonstrated on a few samples where core Sm-Nd ages are younger than rim Lu-Hf ages. White mica ages are in some cases substantially younger than Sm-Nd garnet, and in other cases agree well.

We see a long record of Proterozoic events, but there are few if any Archean ages on the basement inliers. There are almost no Silurian ages, neither from garnets, nor from white micas, implying that Scandian events did not result in substantial new garnet or mica growth. White mica ages largely cluster in the late Ordovician around 445Ma, and are clearly too old to reflect final collision.

Background reading: Bird, Anna; Cutts, Kathryn; Strachan, Rob; Thirlwall, Matthew F.; Hand, Martin. (2018): First evidence of Renlandian (c. 950–940 Ma) orogeny in mainland Scotland : Implications for the status of the Moine Supergroup and circum-North Atlantic correlations. Precambrian Research, 305, p. 283-294.

Matthew was an undergraduate in Oxford and did his PhD in Edinburgh, on the Late Caledonian volcanic rocks. Since 1984 he’s been employed by Royal Holloway University of London, and the last few years has enjoyed only 20% employment.

This lecture will be held as a Zoom meeting. Society members for whom we have email addresses will be sent an invitation a few days before the event. If you are a member but are not on our email list, or a non-member who would like to join the meeting, please email the society’s meetings secretary to request an invitation.