A POTENTIAL NON-LAURENTIAN PROVENANCE FOR THE SOUTHERN HIGHLAND GROUP OF THE DALRADIAN SUPERGROUP, SCOTLAND

The Ediacaran (to Early Ordovician?) Southern Highland Group (SHG)—the youngest member of the Dalradian Supergroup—crops out within the Grampian Terrane, Scotland. The SHG hosts the classic, c. 470 Ma Barrovian and Buchan metamorphic series that formed during the Grampian Orogeny.

The Portsoy–Duchray Hill Lineament (PDHL), a km-scale high-strain zone and lithotectonic mélange that crops out in the coastal region of Portsoy, Scotland defines the western limit of the SHG. The PDHL incorporates three distinct populations of igneous rocks: pre-Iapetan, rift-related bimodal units (c. 600 Ma); pre-Grampian, mafic and ultramafic units (c. 490 Ma), and syn-Grampian-metamorphic gabbros (c. 470 Ma). The two latest (Paleozoic) igneous populations within the PDHL are temporally and geochemically distinct, related to pre-Grampian arc magmatism and syn-Grampian, decompression-related magmatism, respectively.

Published detrital zircon spectra from the stratigraphically lowest levels of the SHG have a strong Grenville (1100–900 Ma) signature. However, detrital zircon spectra from the Boyndie Bay Group and Macduff Slates, at stratigraphically higher levels of the SHG, lack this Grenville signature; similarly, detrital white mica from the Boyndie Bay Group has a strong Pan-African (600–550 Ma) signature but no Grenville signature. Glacial dropstones occur within the Macduff Slates, and may represent a cryptic Early Ordovician glaciation or, if older, an example of Ediacaran glacial units that, outside of the SHG, are only known from Baltica and the various continental fragments that would form Gondwana (Hoffman & Li, 2006).

Garson & Plant (1973) proposed that ultramafic units along the PDHL may represent vestiges of the Iapetus Ocean or an associated seaway that was closed during the Grampian Orogeny. Detrital mineral age and glaciogenic observations within the SHG (which only occurs east of the PDHL) are consistent with an allochthonous origin for the sediments that the Barrovian/Buchan metamorphism affected. The SHG within the Barrovian and Buchan Blocks may represent an exotic, Gondwana-derived (Peri-Gondwanan) terrane that collided with the Laurentian margin sometime after 490 Ma and prior to 470 Ma.

References

Garson, Plant, 1973. Nature 242, 34.

Hoffman, Li, 2006. Palaeogeogr Palaeoclim Palaeoecol 277, 158.