Assessing the Validity of Negative High Field Strength-Element Anomalies as a Proxy for Archaean Subduction: Evidence from the Ben Strome Complex, NW Scotland

Abstract

The relative depletion of high field strength elements (HFSE), such as Nb, Ta and Ti, on normalised trace-element plots is a geochemical proxy routinely used to fingerprint magmatic processes linked to Phanerozoic subduction. This proxy has increasingly been applied to ultramafic-mafic units in Archaean cratons, but as these assemblages have commonly been affected by high-grade metamorphism and hydrothermal alteration/metasomatism, the likelihood of element mobility is high relative to Phanerozoic examples. To assess the validity of HFSE anomalies as a reliable proxy for Archaean subduction, we here investigate their origin in ultramafic rocks from the Ben Strome Complex, which is a 7 km2 ultramafic-mafic complex in the Lewisian Gneiss Complex of NW Scotland. Recently interpreted as a deformed layered intrusion, the Ben Strome Complex has been subject to multiple phases of high-grade metamorphism, including separate granulite- and amphibolite-facies deformation events. Additional to bulk-rock geochemistry, we present detailed petrography, and major- and trace-element mineral chemistry for 35 ultramafic samples, of which 15 display negative HFSE anomalies. Our data indicate that the magnitude of HFSE anomalies in the Ben Strome Complex are correlated with light rare earth-element (LREE) enrichment likely generated during interaction with H2O and CO2-rich hydrothermal fluids associated with amphibolitisation, rather than primary magmatic (subduction-related) processes. Consequently, we consider bulk-rock HFSE anomalies alone to be an unreliable proxy for Archaean subduction in Archaean terranes that have experienced multiple phases of high-grade metamorphism, with a comprehensive assessment of element mobility and petrography a minimum requirement prior to assigning geodynamic interpretations to bulk-rock geochemical data.

Origin(s) and geodynamic significance of Archean ultramafic–mafic bodies in the mainland Lewisian Gneiss Complex, North Atlantic Craton

“…7). Moreover, this trace element pattern, whereby apparent negative HFSE anomalies are associated with LREE enrichment, strongly resembles those described in selected samples from the Ben Strome Complex (Guice et al, 2018b), with localised LREE enrichment by metasomatism associated with H 2 O and CO 2rich fluids during amphibolitisation. Corresponding enrichment of Ba and Sr in the Ben Dreavie samples -typically fluid mobile elements -suggests that similar, secondary, processes may have also generated the anomalies in this instance, particularly given the proximal location of the Ben Strome Complex (Fig.…”

“…Strome and each complex studied here Abbreviations: LDM = Loch an Daimh Mor; LEC = Loch Eilean na Craoibhe Moire; NSB = North Scourie Bay; GnS = Geodh' nan Sgadan; chonN = chondrite normalised; pmN = primitive mantle normalised; PGE = platinum-group elements. * major, minor and trace elements are from Guice et al (2018b). PGE are from this study.…”

“…rocks. Moreover, as demonstrated by Guice et al (2018b), metasomatic effects in the LGC can be both cryptic and extremely localised. These authors show that the LREE -elements typically considered relatively immobile -were enriched in selected samples during interaction with CO 2 -and H 2 O-rich fluids, generating apparent negative high field strength element (HFSE) anomalies which, out of context, may have been interpreted as evidence for subduction-related magmatism.…”

“…10b). As outlined in the introduction to this discussion, the complex tectonothermal history experienced by the LGC has likely resulted in (often cryptic) element mobility that can only be constrained by detailed investigations of individual localities (Guice et al, 2018b;Rollinson and Gravestock, 2012). Consequently, these plots should be treated with caution.…”

“…temperatures, making them relatively resilient to the subsequent effects of high-temperature metamorphism and associated partial melting (Bowen, 1956). Therefore, if the effects of alteration and associated metasomatism, which are often cryptic, can be constrained (Guice, 2019;Guice et al, 2019Guice et al, , 2018b, ultramafic-mafic rocks have the potential to establish the physical manifestations of Archaean geodynamic processes. Specifically, confident identification of residual mantle rocks associated with age equivalent intrusions and submarine lavas (as part of an ophiolite that displays structural and lithological relationships comparable to Phanerozoic examples) would represent extremely strong evidence in favour of plate tectonics having operated (Bédard et al, 2013;Kamber, 2015;Rollinson, 2007).…”