5.5. A model for emplacement, metamorphism and alteration of Eo-
and Paleo-Archean phaneritic ultramafic rocks
Here we describe a common evolutionary pathway for ultramafic rocks of
early Earth terranes in the context of the hot stagnant-lid tectonic
regimes such as heat-pipe tectonics (Moore and Webb, 2013) and partial
convective overturn tectonics (Collins et al., 1998), which have been
proposed for such terranes. Ultramafic rocks of early Earth could have
initially crystallized from high-magnesium, fluid-rich magmas, either as
ultramafic volcanic flows [e.g., komatiites,
Byerly et al. (2019)],
intrusions, or crustal cumulates at the bases of lava flows or magma
chambers (Fig. 11 ). Some of these rocks could have experienced
interactions with co-genetic melts, such as HSE-depleted melts derived
from the deep mantle (Fig. 11 ). Later, these ultramafic rocks
could have been metamorphosed under crustal conditions (e.g.,
greenschist or amphibolite facies conditions) that may or may not have
been associated with significant deformation and mineral phase
transformation. In the case of the Isua supracrustal belt, amphibolite
facies metamorphism was accompanied by deformation during, at the end
of, or after heat-pipe cooling (e.g., Ramírez-Salazar et al., 2021; Webb
et al., 2020; Zuo et al., 2021). These P-T conditions are capable of
producing olivine + serpentine ± Ti-humite ± carbonate ± talc
metamorphic assemblages (Fig. 11a ). Primary igneous textures in
olivine-rich cumulates could have been preserved by concentrating most
of the strain into other phases (e.g., Yao et al., 2019; Zuo et al.,
2021). Alternatively, growth of metamorphic olivine from dehydration
breakdown of strongly oriented serpentine minerals could also produce a
B-type olivine CPO (e.g., Nagaya et al., 2014). In contrast, hot
stagnant-lid volcanism during the Paleoarchean time would have been less
rapid in terms of long-term deposition and burial rates versus the
Eoarchean Isua supracrustal belt, and thus would have led to a
relatively hot lithosphere for the East Pilbara Terrane (Moore and Webb,
2013; Webb et al., 2020), potentially permitting intra-crustal partial
convection via gravitational instability (Fig. 11b ; Collins et
al., 1998). The metamorphic conditions experienced by the exposed
Pilbara rocks may have been lower, and deformation may have been weaker
(e.g., Collins et al., 1998; Wiemer et al., 2018), especially in rocks
located far from the margins of the granitoid bodies (e.g., François et
al., 2014) such as the samples studied here (Fig. 1b ).
Consequently, Pilbara ultramafic samples only preserve evidence for
greenschist facies metamorphism without identifiable strain
(Fig. 3 ). Post-deformational alterations (such as talc,
carbonate, or serpentine alterations) might have further modified these
ultramafic rocks as well as nearby supracrustal rocks in the following
>3 billion years (Fig. 11 ).