Archean ultramafic-mafic complexes have been the focus of important and often contentious geological and geodynamic interpretations. However, their age relative to the other components of Archean cratons are often poorly-constrained, introducing significant ambiguity when interpreting their origin and geodynamic significance. The Lewisian Gneiss Complex (LGC) of the northwest Scottish mainland -a high-grade, tonalite-trondhjemite-granodiorite (TTG) terrane that forms part of the North Atlantic Craton (NAC) -contains a number of ultramafic-mafic complexes whose origin and geodynamic significance have remained enigmatic since they were first described. Previous studies have interpreted these complexes as representing a wide-range of geological environments, from oceanic crust, to the sagducted remnants of Archean greenstone belts. These interpretations, which are often critically dependent upon the ages of the complexes relative to the surrounding rocks, have disparate implications for Archean geodynamic regimes (in the NAC and globally). Most previous authors have inferred that the ultramafic-mafic complexes of the LGC pre-date the TTG magmas. This fundamental age relationship is re-evaluated in this investigation through re-mapping of the Geodh' nan Sgadan Complex (where tonalitic gneiss reportedly cross-cuts mafic rocks) and new mapping of the 7 km 2 Ben Strome Complex (the largest ultramafic-mafic complex in the LGC), alongside detailed petrography and spinel mineral chemistry. This new study reveals that, despite their close proximity in the LGC (12 km), the Ben Strome and Geodh' nan Sgadan Complexes are petrogenetically unrelated, indicating that the LGC (and thus NAC) records multiple temporally and/or petrogenetically distinct phases of ultramafic-mafic Archean magmatism that has been masked by subsequent high-grade metamorphism. Moreover, field observations and spinel mineral chemistry demonstrate that the Ben Strome Complex represents a layered intrusion that was emplaced into a TTG-dominated crust. Further to representing a significant re-evaluation of theLGC's magmatic evolution, these findings have important implications for the methodologies utilised in deciphering the origin of Archean ultramafic-mafic complexes globally, where material suitable for dating is often unavailable and field relationships are commonly ambiguous.
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.
Suprasubduction zone (SSZ) ophiolites of the northern Appalachians (eastern North America) have provided key constraints on the fundamental tectonic processes responsible for the evolution of the Appalachian orogen. The central and southern Appalachians, which extend from southern New York to Alabama (USA), also contain numerous ultra- mafic-mafic bodies that have been interpreted as ophiolite fragments; however, this interpretation is a matter of debate, with the origin(s) of such occurrences also attributed to layered intrusions. These disparate proposed origins, alongside the range of possible magmatic affinities, have varied potential implications for the magmatic and tectonic evolution of the central and southern Appalachian orogen and its relationship with the northern Appalachian orogen. We present the results of field observations, petrography, bulk-rock geochemistry, and spinel mineral chemistry for ultramafic portions of the Baltimore Mafic Complex, which refers to a series of ultramafic-mafic bodies that are discontinuously exposed in Maryland and southern Pennsylvania (USA). Our data indicate that the Baltimore Mafic Complex comprises SSZ ophiolite fragments. The Soldiers Delight Ultramafite displays geochemical characteristics—including highly depleted bulk-rock trace element patterns and high Cr# of spinel—characteristic of subduction-related mantle peridotites and serpentinites. The Hollofield Ultramafite likely represents the “layered ultramafics” that form the Moho. Interpretation of the Baltimore Mafic Complex as an Iapetus Ocean–derived SSZ ophiolite in the central Appalachian orogen raises the possibility that a broadly coeval suite of ophiolites is preserved along thousands of kilometers of orogenic strike.
The geodynamic regime(s) that predominated during the Archean remains controversial, with the plethora of competing models largely informed by the felsic lithologies. Ultramafic–mafic rocks displaying distinctive geochemical signatures are formed in a range of Phanerozoic geotectonic environments. These rocks have high melting points, making them potentially useful tools for investigating Archean geodynamic processes in highly metamorphosed regions. We present the results of field mapping, petrography, traditional bulk-rock geochemistry and platinum group element geochemistry for 12 ultramafic–mafic bodies in the Lewisian Gneiss Complex, which is a highly metamorphosed fragment of the North Atlantic Craton in NW Scotland. Our data indicate that most of these occurrences are layered intrusions emplaced into the tonalite–trondhjemite–granodiorite-dominated crust prior to polyphase metamorphism, representing a significant re-evaluation of the magmatic evolution of the Lewisian Gneiss Complex. Of the other occurrences, two remain ambiguous, but one (Loch an Daimh Mor) has some geochemical affinity with abyssal/orogenic peridotites and may represent a fragment of Archean mantle, although further investigation is required. The ultramafic–mafic bodies in the Lewisian Gneiss Complex thus represent more than one type of event/process. Compared with the tonalite–trondhjemite–granodiorite host rocks, these lithologies may preserve evidence of protolith origin(s), with potential to illuminate the tectonic setting(s) and geodynamic regimes of the early Earth. Supplementary material: Bulk rock geochemical data is available at: https://doi.org/10.6084/m9.figshare.c.4878588
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.