0000000001182866
AUTHOR
R.w. White
Field and petrographic evidence for partial melting of TTG gneisses from the central region of the mainland Lewisian complex, NW Scotland
The central region of the mainland Lewisian complex is dominated by granulite-facies tonalite–trondhjemite–granodiorite (TTG) gneisses that are highly depleted in some mobile trace elements (Cs, Rb, Th and U) relative to amphibolite-facies TTG gneisses elsewhere in the Lewisian complex and to the average composition of TTG gneisses worldwide. Over almost half a century of research there has been vigorous debate as to the origin of this depletion, in particular with respect to the role of partial melting and melt loss. Here we provide field and petrographic evidence that TTG gneisses across the central region partially melted during granulite-facies (Badcallian) metamorphism. Partial melting…
Phase equilibrium constraints on conditions of granulite-facies metamorphism at Scourie, NW Scotland
The metamorphic evolution of a metapyroxenite and metagabbro from Scourie, NW Scotland, is investigated using phase equilibria modelled in the NCFMASHTO (Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O) system. The calculated stability fields for the observed assemblages in each rock overlap and imply conditions of 8.5–11.5 kbar and 875–975 °C for the peak of granulite-facies (Badcallian) metamorphism. The lack of any evidence for the former presence of garnet in the metapyroxenite suggests that the rocks cannot have reached pressures much in excess of those recorded at the metamorphic peak. The growth of coronas of plagioclase, orthopyroxene and magnetite replacing garnet in the metagabbro is consi…