Introduction:
When, how, and why plate tectonics began on Earth remain among the most
important unresolved questions in plate tectonic theory (e.g., Bauer et
al., 2020; Beall et al., 2018; Brown and Johnson, 2018; Condie and
Puetz, 2019; Hansen, 2007; Harrison, 2009; Korenaga, 2011; Nutman et
al., 2020; Stern, 2008; Tang et al., 2020). Investigations of plate
tectonic initiation have significant implications for questions
associated with the evolution of early terrestrial planets, including
(1) whether early Earth experienced any pre-plate tectonic global
geodynamics/cooling after the magma ocean stage (e.g., Bédard, 2018;
Collins et al., 1998; Lenardic, 2018; Moore and Webb, 2013; O’Neill and
Debaille, 2014); and (2) why other terrestrial planets in the solar
system appear to lack plate tectonic records (e.g., Moore et al., 2017;
Stern et al., 2017; cf. Yin, 2012a; Yin, 2012b).
Many proposed signals for the initiation or early operation of plate
tectonics on Earth are controversial due to the issue of non-uniqueness.
For instance, the origin of Hadean zircons from the Jack Hills of
western Australia have been contrastingly interpreted as (1) detrital
crystals from felsic magmas generated by ~4.3 Ga plate
subduction (Harrison, 2009; Hopkins et al., 2008); (2) zircons
crystallized via impact heating and ejecta sheet burial (Marchi et al.,
2014) or (3) low pressure melting of Hadean mafic crustal materials
(Reimink et al., 2020). Similarly, researchers continue to debate
whether the presence of Archean low-Ti mafic lava (also termed as
boninite or boninitic basalts) must indicate subduction initiation as
early as ~3.7 Ga (cf. Pearce and Reagan, 2019; Polat and
Hofmann, 2003). Another example is how a ~3.2 Ga shift
in zircon Hf-isotope signatures has been variably interpreted to
indicate the onset of plate tectonics (Næraa et al., 2012) or enhanced
mantle melting during a proposed Earth’s thermal peak (Kirkland et al.,
2021). Due to these equivocal interpretations, the initiation of plate
tectonics has been suggested to be ≤3.2 Ga using geological records that
are generally considered unique to plate tectonics (e.g., paired
metamorphic belts, ultra-high pressure terranes, and passive margins)
(e.g., Brown and Johnson, 2018; Cawood et al., 2018; Stern, 2008; cf.
Bauer et al., 2020; Foley et al., 2014; Harrison, 2009; Korenaga, 2011;
Nutman et al., 2020). The ≤3.2 Ga onset of plate tectonics requires
early Earth tectonic evolution to be non-uniformitarian, involving some
form of single-plate stagnant-lid tectonics (e.g., Bédard, 2018; Collins
et al., 1998; Moore and Webb, 2013).
One proposed signal of early plate tectonics is the preservation of
phaneritic ultramafic rocks in Eo- and Paleoarchean terranes. However,
the issue of non-uniqueness also extends to their interpretations. In
the Eoarchean Isua supracrustal belt and adjacent meta-tonalite bodies
exposed in southwestern Greenland (Fig. 1a ), some dunites and
harzburgites have been interpreted to represent melt-depleted mantle
rocks that were emplaced on top of crustal rocks during the Eoarchean
plate tectonic subduction (e.g., Friend and Nutman, 2011; Nutman et al.,
2020; Van de Löcht et al., 2018), similar to how modern ophiolitic
ultramafic rocks are preserved in collisional massifs (e.g., Boudier et
al., 1988; Lundeen, 1978; Wal and Vissers, 1993). In contrast,
Szilas et al. (2015) argue that
dunites and harzburgites in the Isua supracrustal belt can be
interpreted as crustal cumulates based on their geochemical signatures.
Crustal cumulates are not exclusive to plate tectonics, and have been
used to explain the emplacement of other phaneritic ultramafic rocks in
Eo- and Paleoarchean terranes. Examples include phaneritic ultramafic
rocks in the Eoarchean Tussapp ultramafic complex of southwestern
Greenland (McIntyre et al., 2019), the Paleoarchean East Pilbara Terrane
of northwestern Australia (e.g., Smithies et al., 2007), and the
Paleoarchean Barberton Greenstone Belt of South Africa (e.g., Byerly et
al., 2019). Therefore, ultramafic rocks in the Isua supracrustal belt
potentially formed in a different tectonic setting compared to those of
other early Archean terranes. Because all early Archean terranes
preserve voluminous tonalite-trondhjemite-granodiorite (TTG) suites
surrounded by deformed, dominantly mafic supracrustal belts (e.g.,Fig. 1 ; also Condie, 2019), a different origin for the Isua
supracrustal belt may be an artifact of interpretive non-uniqueness. If
the phaneritic ultramafic rocks of the Isua supracrustal belt can be
similarly interpreted to have cumulate or volcanic origins (which is
questioned by many recent studies, see section 2 for a review), then
these rocks cannot be used as unequivocal indicators of plate tectonics.
This contribution explores the origins of Isua ultramafic rocks via
analysis of new and published geochemical and petrological findings,
including comparative analysis of the key Isua rocks and similar rocks
from settings considered representative of hot stagnant-lid tectonics
[In this study, we follow tectonic taxonomy from Lenardic (2018)].
The Paleoarchean geology of the
East Pilbara Terrane is widely accepted as representing hot stagnant-lid
tectonics (Hickman, 2021; Johnson et al., 2014; Smithies et al., 2007,
2021; Van Kranendonk et al., 2004, 2007); Pilbara ultramafic samples are
also investigated in this study (Fig. 1b ) as examples of
ultramafic rocks from non-plate tectonic regimes. We also compare the
petrology and geochemistry of Isua ultramafic rocks with compiled (1)
ultramafic cumulate rocks; (2) modelled ultramafic cumulate rocks; (3)
melt-depleted mantle rocks from plate tectonic settings; and (4)
modelled melt-depleted mantle rocks. We then examine whether the
generation of Isua and Pilbara ultramafic rocks is compatible with the
predictions of hot stagnant-lid tectonics. Our findings help to evaluate
whether plate tectonics is indeed required to explain the Eoarchean
assembly of the Isua supracrustal belt.