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.
Highlights
Geosciences 2018, 8, x FOR PEER REVIEWGeochemical fingerprinting, which commonly involves plotting immobile element ratios on bivariate
Such an interpretation—whereby the light rare earth-element (LREE) enrichment of clinopyroxene and amphibole, and associated bulk-rock high field strength-elements (HFSE) anomalies shown by the group 2 and 3 ultramafic rocks result from fractional crystallisation of an initial magma inherently depleted in the HFSE—is contradicted by the evidence presented in this study
Based on these combined characteristics, the HFSE anomalies displayed by the groups 2 and 3 samples are most consistent with processes involving interaction with a CO2 and H2 O-rich hydrothermal fluid associated with amphibolitisation and the Laxfordian metamorphic event, which represents the second phase of amphibolite-facies metamorphism experienced by the Lewisian Gneiss Complex (LGC)
Summary
Geochemical fingerprinting, which commonly involves plotting immobile element ratios on bivariate. Geochemical fingerprinting, which commonly involves plotting immobile element ratios on proxy, which involves the relative depletion of high field strength-elements Ti) on normalised trace-element plots (Figure 1; referred to as the ‘HFSE anomaly’), is [1,2,3] One such proxy, which involves the relative depletion of high field strength-elements Despite being generated alternative suchmagmatism, as crustal contamination [5]HFSE or via interaction with the used by as evidence for mechanisms, subduction-related application of the anomaly requires caution, as itlithospheric can be generated by alternative mechanisms, such asauthors crustal contamination [5] or that sub-continental mantle
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