Partial melting in the Inzie Head gneisses: the role of water and a petrogenetic grid in KFMASH applicable to anatectic pelitic migmatites.

Abstract A sequence of prograde isograds is recognized within the Dalradian Inzie Head gneisses where pelitic compositions have undergone variable degrees of partial melting via incongruent melting reactions consuming biotite. Three leucosome types are identified. At the lowest grades, granitic leucosomes containing porphyroblasts of cordierite (CRD-melt) are abundant. At intermediate grades, CRD-melt mingles with garnetiferous leucosomes (GT-melt). At the highest grades, CRD-melt coexists with orthopyroxene-bearing leucosomes (OPX-melt), while garnet is conspicuously absent. The prograde metamorphic field gradient is constrained to pressures of 2–3 kbar below the CRD-melt isograd, and no greater than 4.5 kbar at the highest grade around Inzie Head. A petrogenetic grid, calculated using thermocalc, is presented for the K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) system for the phases orthopyroxene, garnet, cordierite, biotite, sillimanite, H2O and melt with quartz and K-feldspar in excess. For the implied field gradient, the reaction sequence predicted by the grid is consistent with the successive prograde development of each leucosome type. Compatibility diagrams suggest that, as anatexis proceeded, bulk compositions may have been displaced towards higher MgO content by the removal of (relatively) ferroan granitic leucosome. An isobaric (P = 4 kbar) T–aH2O diagram shows that premigmatization fluids must have been water-rich (aH2O > 0.85) and suggests that, following the formation of small volumes of CRD-melt, the system became fluid-absent and melting reactions buffered aH2O to lower values as temperatures rose. GT- and OPX-melt formed by fluid-absent melting reactions, but a maximum of 7–11% CRD-melt fraction can be generated under fluid-absent conditions, much less than the large volumes observed in the field. There is strong evidence that the CRD-melt leucosomes could not have been derived by buoyantly aided upwards migration from levels beneath the migmatites. Their formation therefore required a significant influx of H2O-rich fluid, but in a quantity insufficient to have exhausted the buffering capacity of the solid assemblage plus melt. Fluid : rock ratios cannot have exceeded 1 : 30. The fluid was channelled through a regionally extensive shear zone network following melt-induced failure. Such an influx of fluid at such depths has obvious consequences for localized crustal magma production and possibly for cordierite-bearing granitoids in general. [ABSTRACT FROM AUTHOR]