Field and petrographic evidence for partial melting of TTG gneiss within the central region of the mainland Lewisian complex, NW Scotland

Abstract

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 occurred largely by fluid-absent incongruent reactions consuming plagioclase, quartz, hornblende and biotite to produce melt and peritectic clino- and orthopyroxene. The preservation of dry, granulite-facies assemblages requires loss of melt, consistent with the presence of an interconnected network of leucosomes and larger felsic sheets that probably record segregation and transfer of melt to higher crustal levels. Regardless of whether or not partial melting and melt loss can explain fully the unusual geochemical signature of the central region TTG gneisses, these fundamental processes did occur.