Field and petrographic evidence for partial melting of TTG gneisses from 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. Supplementary material: Figure S1, showing additional field photographs of boulders from Poll Eòrna, is available at www.geolsoc.org.uk/SUP18580 .

New constraints on granulite facies metamorphism and melt production in the Lewisian Complex, northwest Scotland

“…Many of the metagabbroic layers contain leucocratic quartz‐ and plagioclase‐rich segregations (leucosomes), indicating partial melting (Johnson et al., ). Small‐scale leucosomes, interpreted to have formed by local in‐situ melting, occur in the melanocratic host and commonly contain large grains or accumulations of euhedral clinopyroxene (Figure c).…”

“…Although the Archean–Proterozoic Lewisian Complex of northwest Scotland is one of the most widely studied high‐grade terranes on Earth, key aspects of its tectonothermal evolution remain under debate (e.g. Johnson, Fischer, & White, ; Johnson et al., ; Park, ; Wheeler, Park, Rollinson, & Beach, , and references therein). In particular, uncertainty concerning the peak metamorphic P – T conditions for the c .…”

High-precision U–Pb dating of complex zircon from the Lewisian Gneiss Complex of Scotland using an incremental CA-ID-TIMS approach

“…It is composed of Archean gneisses which have been variably reworked during the Proterozoic and has proven to be a key region in the study of crustal growth and continental tectonics (e.g. Moorbath et al, 1969;Hamilton et al, 1979;Rollinson and Windley, 1980;Whitehouse and Moorbath, 1986;Heaman and Tarney, 1989;Coward, 1990;Whitehouse and Bridgwater, 2001;Rollinson, 2012;Johnson et al, 2013). The Lewisian gneisses, are predominantly tonalite-trondhjemite-granodiorite (TTG) gneisses but also feature gneisses of sedimentary origin, metamorphosed ultramafic rocks, mafic dykes and alkali granitoids, all of which are variably deformed and metamorphosed (Tarney and Weaver, 1987;Goodenough et al, 2013;MacDonald et al, 2013).…”

Anatexis of ultrahigh-pressure eclogite during exhumation in the North Qaidam ultrahigh-pressure terrane: Constraints from petrology, zircon U-Pb dating, and geochemistry

“…3F). These microstructures are interpreted as trapped melt pockets that crystallized within the residual melanosome, and they provide strong evidence for in situ partial melting (Sawyer, 2008;Johnson et al, 2013). …”

“…Numerous laboratory experiments and studies of natural samples have revealed many fundamental aspects concerning the P-T conditions and the processes of partial melting of crustal rocks (Holness and Clemens, 1999;Holness and Sawyer, 2008;Holness et al, 2011;Sawyer, 1999Sawyer, , 2008Sawyer, , 2010Zheng et al, 2011;Johnson et al, 2012Johnson et al, , 2013Chen et al, 2013). However, most high-grade metamorphic rocks generally experience protracted cooling and retrograde evolution, which lead to extensive deformation-driven recrystallization and complete solidifi cation of the melts.…”