Evidence for a genetic granite-migmatite link of the Dalradian of NE Scotland

The Inzie Head gneisses of the NE Dalradian are syntectonic metapelitic migmatites containing numerous sill-like bodies of granite. Leucosomes preserve evidence for efficient deformation-controlled segregation and interconnection of melt, from intergranular pockets to sheets of leucogranite hundreds of metres thick. Although the physico-chemical system is complex, the geochemistry of in situ decimetre-scale leucosomes and discrete metre- to decametre-scale leucogranite sheets within the migmatites suggests derivation from a common metapelitic source. A comparison with spatially and temporally related Grampian Granite plutons and garnet-bearing aplites that occur at shallower crustal levels supports a genetic granite-migmatite link. The compositions of the granitic rocks are interpreted to be linked via fractional crystallization of a parental magma and escape of the fugitive melt from a cumulate residue. Leucogranite sheets within the migmatics have compositions that are similar to experimentally determined first-formed melts at the appropriate P-T conditions, and are interpreted to approximate parental liquids. Smaller-scale leucosomes contain the accumulated products of fractional crystallization and variable quantities of entrained solids, principally biotite. The Grampian Granites and aplites represent evolved liquids that segregated from source. The evolution of Grampian Granite melts involved varying degrees of K-feldspar-dominated fractional crystallization that is consistent with magma ascent; removal of a maximum of 15-20% cumulate material is implied. The composition of the aplites suggests a multistage evolution that involved plagioclase-dominated fractionation of a fugitive melt batch.