The Central Highland Migmatite Complex (CHMC) forms the stratigraphically lowest part of
the central and eastern Grampian block. Baker (1986) reported garnet amphibolite pods near
Tomatin in the Dava Succession of the CHMC that display relict diopside-plagioclase
symplectites and plagioclase + hornblende collars around garnets, indicating the prior
existence of omphacite and pyrope-grossular-rich garnet. This observation has been largely
ignored, but has profound implications for Neoproterozoic geodynamics in Scotland, because
eclogite-gneiss assemblages often indicate transient subduction of continental lithosphere, and
they are commonly associated with exhumation structures and successor basins. Clearly, it is
important to establish the extent, timing and structural context of eclogite-facies
metamorphism in the Grampian block, so a renewed investigation of this early, high-pressure
episode is now under way. This contribution is based upon preliminary observations by the
author and previous studies on the general geological context by numerous other workers.
Diopside-plagioclase symplectites and plagioclase-hornblende collars around garnets have
been confirmed in the Dava Succession, and have now been found at three further localities
within the Glen Banchor Succession of the CHMC. Preliminary observations indicate that the
amphibolites shared the migmatisation of the gneisses, hence the eclogite-facies
metamorphism predates migmatisation (840Ma) and the later (~800Ma) D2 shear-zones that
separate the CHMC from the Grampian Group. Two other types of metabasite are found
associated with the CHMC; a suite of metagabbros in the shear zone complex between the
CHMC and the Grampian Group has been variably transformed to garnet amphibolite.
Intrusion post-dates migmatisation, predates D2 shearing and they lack petrographic evidence
for an eclogite-facies overprint. A further unit of striped garnet amphibolites lying above the
shear zone in inliers at Ord Ban and Kincraig exhibits plagioclase collars around garnets, but
has not yet yielded evidence for omphacite. Their stratigraphic position is uncertain and its
resolution is important for determination of the upper extent of high-pressure metamorphism
and thus the position of intra-Dalradian unconformities. The age of high-pressure
metamorphism is thus constrained to lie between 840Ma and either the youngest reliable
detrital zircon age in the Glen Banchor succession (900±17Ma) or possibly the depositional
age of the Ord Ban metasediments.
The lowest stratigraphic unit of the Grampian group, the Glenshirra subgroup, contains metarudites
(e.g. Garva Bridge Formation). Their relationship with the CHMC is obscure, but
recent work on their sedimentary facies suggests an alluvial fan environment related to
“extension”. In this sense they are possibly analogous to late-orogenic extensional molasse
basins related to exhumation of high-pressure rocks. In this case, the eclogite-facies episode
would have been restricted to the CHMC and would not have affected the overlying
Grampian Group or higher Dalradian units. It might also be speculated that the ~800Ma D2
shear zones accommodated some of the exhumation of the CHMC eclogites.
Original languageEnglish
StatePublished - Apr 2008
EventHighlands Workshop - Edinburgh, United Kingdom

Workshop

WorkshopHighlands Workshop
CountryUnited Kingdom
CityEdinburgh
Period24/04/0825/04/08

ID: 210136