Eclogite-facies metamorphism in the Central Highland Migmatite complex, Grampian Highlands

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 molasses 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.