Grampian migmatites in the Buchan Block, NE Scotland

Abstract

Rocks exposed along the Scottish coast between Fraserburgh and Inzie Head contain information critical to understanding the evolution of the Buchan Block, the type locality for low-P, high-T regional metamorphism, and its relationship with the rest of the Grampian terrane, one of the major tectonostratigraphic components of the Scottish Caledonides. The ~8 km long section traverses a regional network of shear zones and, at the highest grades around Inzie Head, passes into the core of the Buchan Anticline, a large-scale open fold that is commonly regarded as a late structure, post-dating metamorphism. The metasedimentary rocks increase in grade from upper amphibolite to granulite facies and preserve unequivocal evidence for partial melting. The diatexite migmatites around Inzie Head, along with other gneissose units within the Buchan Block, have been regarded as allochthonous Precambrian basement rocks that were thrust into their current position during the Grampian orogenesis. However, field observations show that the onset of in situ partial melting in metapelitic rocks, which was associated with the formation of garnet-bearing aplites and associated pegmatites, occurred around Fraserburgh, where shear fabrics are absent.Thus, the rocks preserve a continuous metamorphic field gradient that straddles the shear zone network. This observation supports an alternative interpretation that anatexis was the result of mid-Ordovician (Grampian) metamorphism, rather than an older tectonothermal event, and that the Inzie Head gneisses are autochthonous. Using an average mid-Dalradian pelite as a plausible representative protolith, phase equilibria modelling satisfactorily reproduces the observed appearance and disappearance of key minerals providing that peritectic garnet produced with the first formed melts (represented by the garnet-bearing aplites) depleted the source rocks in Mn. The modelled metamorphic field gradient records a temperature increase of at least 150 °C (from ~650 °C near Fraserburgh to in excess of 800 °C at Inzie Head) but is isobaric at pressures of 2.7–2.8 kbar, suggesting the Buchan Anticline developed synchronous with partial melting. The Buchan Anticline is likely an expression of crustal thinning and asthenospheric upwelling, which produced voluminous gabbroic intrusions that supplied the heat for Buchan metamorphism.