Petrogenesis of rare-metal pegmatites in high-grade metamorphic terranes: a case study from the Lewisian Gneiss Complex of north-west Scotland

Abstract

a b s t r a c tMany rare metals used today are derived from granitic pegmatites, but debate continues about the origin of these rocks. It is clear that some pegmatites represent the most highly fractionated products of a parental granite body, whilst others have formed by anatexis of local crust. However, the importance of these two processes in the formation of rare-metal pegmatites is not always evident.The Lewisian Gneiss Complex of NW Scotland comprises Archaean meta-igneous gneisses which were highly reworked during accretional and collisional events in the Palaeoproterozoic (Laxfordian orogeny). Crustal thickening and subsequent decompression led to melting and the formation of abundant granitic and pegmatitic sheets in many parts of the Lewisian Gneiss Complex. This paper presents new petrological, geochemical and age data for those pegmatites and shows that, whilst the majority are barren biotite-magnetite granitic pegmatites, a few muscovite-garnet (rare-metal) pegmatites are present. These are mainly intruded into a belt of Palaeoproterozoic metasedimentary and meta-igneous rocks known as the Harris Granulite Belt.The rare-metal pegmatites are distinct in their mineralogy, containing garnet and muscovite, with local tourmaline and a range of accessory minerals including columbite and tantalite. In contrast, the biotitemagnetite pegmatites have biotite and magnetite as their main mafic components. The rare-metal pegmatites are also distinguished by their bulk-rock and mineral chemistry, including a more peraluminous character and enrichments in Rb, Li, Cs, Be, Nb and Ta. New U-Pb ages (c. 1690-1710 Ma) suggest that these rare-metal pegmatites are within the age range of nearby biotite-magnetite pegmatites, indicating that similar genetic processes could have been responsible for their formation.The peraluminous nature of the rare-metal pegmatites strongly points towards a metasedimentary source. Notably, within the Lewisian Gneiss Complex, such pegmatites are only found in areas where a metasedimentary source is available. The evidence thus points towards all the Laxfordian pegmatites being formed by a process of crustal anatexis, with the formation of rare-metal pegmatites being largely controlled by source composition rather than solely by genetic process. This is in keeping with previous studies that have also challenged the widely accepted model that all rare-metal pegmatites are formed by fractionation from a parental granite, and raises questions about the origin of other mineralised pegmatites worldwide.

Origin and petrogenetic implications of garnet from Rb‐rich pegmatites in North Qinling Orogen, China

“…Granitic pegmatites can form through fractionation of parental granites (e.g., Černý, ; Černý et al, ; Černý & Ercit, ; Jahns & Burnham, ) or direct anatexis of country rocks (Martin & De Vito, ; Nabelek et al 1992a, b; Shaw et al, ; Simmons et al, ; Simmons & Webber, ). Based on whole‐rock geochemistry, the Ziyugou pegmatites show both S‐type and A‐type affinities.…”

“…Granitic pegmatites can form through fractionation of parental granites (e.g., Černý, 1991;Černý et al, 2012;Černý & Ercit, 2005;Jahns & Burnham, 1969) or direct anatexis of country rocks (Martin & De Vito, 2005;Nabelek et al 1992a, b;Shaw et al, 2016;Simmons et al, 1996;Simmons & Webber, 2008 to those of typical A-type granitoids (e.g., Konopelko et al, 1997;J. Zhang et al, 2012).…”

“…Some scholars have proposed an igneous origin and considered that granitic pegmatites are derived from fractionation of parental granites (e.g., Černý, ; Černý & Ercit, ; Černý, London, & Novak, ; Jahns & Burnham, ). Others argue that direct anatexis of country rocks could lead to pegmatite formation (Martin & De Vito, ; Nabelek, Russ‐Nabelek, & Denison, ; Nabelek, Russ‐Nabelek, & Haeussler, ; Shaw et al, ; Simmons, Foord, & Falster, ; Simmons & Webber, ). Besides, hydrothermal processes also play an important role in mineralization and some unique textures within pegmatites (cf.…”

“…The origin of granitic pegmatites in orogenic belts has interested many geoscientists mainly because (a) granitic pegmatites are important sources of rare metals such as Li, Cs, Be, Nb, Ta, and REE (Černý & Ercit, ; London, ); (b) they can provide important information regarding the evolution of orogenic belts (e.g., Bongiolo, Renac, Piza, Schmitt, & Mexias, ; Henderson & Ihlen, ; Shaw et al, ). There are basically two genetic models for pegmatites in orogens.…”

Petrogenetic and Compositional Features of Rare Metal Pan-African Post-Collisional Pegmatites of Southwestern Nigeria; A Status Review

“…At present, there are two hypotheses that define the processes responsible for the formation of pegmatite melts: (1) extended fractional crystallization of a granitic magma at the scale of a pluton (Leake, 1990;London, 2005;Martin and De Vito, 2005), and (2) direct formation by low degree partial melting of crustal rocks in the middle to deep levels of Earth's continental crust influenced by the presence of fluids (Romer and Kroner, 2016;Shaw et al, 2016;Muller et al, 2017;Lv et al, 2018). Several examples of rare-metal pegmatites formed by fractionation of a granitic melt are typical in the Superior province of Ontario, Canada (Selway, 2005), Yichun, China (Belkasmi et al, 2000 and the Adola belt, southern Ethiopia (Mohammed, 2017).…”