Figure 11. Evolutionary diagrams for Isua and Pilbara
ultramafic rocks. Ultramafic rocks from both terranes can be interpreted
via similar hot stagnant-lid tectonic models. Ultramafic rocks are
initially cumulates formed during cooling of magmas in hot stagnant-lid
settings that feature voluminous volcanism. During solidification of
these cumulates, REE-enriched but HSE-depleted melts interacted with
cumulus phases. These cumulates were then variably deformed and/or
metamorphosed during tectonic events that either represent (1)
shortening, corresponding to volcanic burial, plate-breaking or plate
tectonic subduction (panel a); or (2) intra-crustal diapirism
corresponding to gravitational instability (panel b). Later,
mostly static (talc/carbonate/serpentine) alterations further modified
the petrology and geochemistry.
ConclusionsSome ultramafic rocks preserved in or near the Isua supracrustal belt
have been interpreted as tectonically emplaced mantle peridotites that
require >3.7 Ga onset of plate tectonics (e.g., Nutman et
al., 2020; Van de Löcht et al., 2018). In contrast, this study shows
that cumulates and mantle rocks may have similar primary rock
textures, and whole-rock geochemistry and igneous mineral assemblages
generated by olivine-dominated rocks interacting with melts.
Differences between Isua and Pilbara ultramafic rocks may largely have
resulted from different alterations and/or deformation experienced by
these rocks, which are also consistent with crustal conditions
(Waterton et al., 2022; Mueller et al., pre-print). In contrast, other
characteristics of these rocks, such as certain types of spinel
geochemistry (e.g., Fe-Ti trends in Cr#-Mg# space, Barnes and
Roeder, 2001) as well as cumulate textures, appear to be unique to
cumulates. Furthermore, melts that have interacted with Isua and
Pilbara ultramafic rocks should be co-genetic melts generated in magma
chambers or deep mantle, which cannot be explained by sub-arc mantle
origins. Thus, we conclude that no features preserved in ultramafic
rocks of the Isua supracrustal belt and East Pilbara Terrane are
diagnostic of plate tectonic-related mantle slices, but instead are
compatible with crustal cumulates. Again, it is important to note that
these interpretations do not exclude plate tectonic origins for the
formation of the Isua supracrustal belt (e.g., Van Kranendonk, 2010;
Nutman et al., 2020), but they permit a hot stagnant-lid tectonic
origin for this terrane, consistent with previous studies for the belt
(Ramírez-Salazar et al., 2021; Webb et al., 2020; Zuo et al., 2021).
Therefore, because the East Pilbara Terrane can also be explained in
terms of a hot stagnant-lid setting (e.g., Collins et al., 1998; Van
Kranendonk et al., 2007), no tectonic shift between the Eoarchean and
Paleoarchean is required. Short episodes of local plate tectonic
processes during the Eo- and Paleoarchean might be possible, as
regional stagnant-lid processes may have coexisted with local plate
tectonic processes in early terrestrial planets (e.g., Van Kranendonk,
2010; Yin, 2012a; Yin, 2012b). Nonetheless, our findings show that a
≤3.2 Ga initiation of plate tectonics is viable.