Guice, G. L., McDonald, I., Hughes, H. S.R., Schlatter, D. M., Goodenough, K. M., MacDonald, J. M. and Faithfull, J. W. (2018) Assessing the validity of negative high field strength-element anomalies as a proxy for Archaean subduction: evidence from the Ben Strome Complex, NW Scotland. Geosciences, 8(9), 338. (doi: 10.3390/geosciences8090338)
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Publisher's URL: http://www.mdpi.com/2076-3263/8/9/338
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.
| Item Type: | Articles |
|---|---|
| Additional Information: | G.L.G. would like to thank The Society of Economic Geologists (Graduate Fellowship Award) and Geological Society (Timothy Jefferson Fund) for generous bursaries that provide the funding for the fieldwork that underpins this study. |
| Keywords: | HFSE, rare earth-elements (REE), Archaean geodynamics, Lewisian Gneiss Complex, ultramafic-mafic complex, granulites, amphibolitisation, element mobility. |
| Status: | Published |
| Refereed: | Yes |
| Glasgow Author(s) Enlighten ID: | Faithfull, Dr John and MacDonald, Dr John |
| Authors: | Guice, G. L., McDonald, I., Hughes, H. S.R., Schlatter, D. M., Goodenough, K. M., MacDonald, J. M., and Faithfull, J. W. |
| Subjects: | Q Science > QE Geology |
| College/School: | University Services > Library and Collection Services > Museum and Art Gallery College of Science and Engineering > School of Geographical and Earth Sciences |
| Journal Name: | Geosciences |
| Publisher: | MDPI |
| ISSN: | 2076-3263 |
| ISSN (Online): | 2076-3263 |
| Published Online: | 08 September 2018 |
| Copyright Holders: | Copyright © 2018 The Authors |
| First Published: | First published in Geosciences 8(9): 338 |
| Publisher Policy: | Reproduced under a Creative Commons License |
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