The ca. 3.6-3.2 Ga East Pilbara Terrane (EPT) formed by a distinct vertically accretive geodynamic mode during predicted highest planetary heat flow, prior to the onset of Wilson-style plate tectonics at ca. 3.2 Ga. Three major greenstone volcanic cycles suggest that mafic proto-crust formed by repeated mantle overturns, which in turn gave rise to the formation of continental crustal material (Wiemer et al., 2018). However, although generally ascribed to the operation of mantle plumes, the greenstone sequences feature a broad variety of intercalated rock types and emplacement or depositional styles. Thus, source and genetic processes remain debated. Additionally, differences in the compositional range between successive mantle overturns have been attributed to secular changes in the mantle source (Smithies et al., 2007), but may equally reflect variation in lithospheric structure (e.g., Niu, 2021). We report on the discovery of cumulate-type anorthosite, clinopyroxenite, and olivine-chromite subvolcanic bodies emplaced during the oldest recognized mantle overturn (Warrawoona Group). The finding is exceptional in that these dense ultramafic-mafic rocks have poor preservation potential during dome-keel development that affected the EPT at the end of each mantle overturn. Petrological-geochemical data suggest that these cumulates represent remnants of the magma plumbing system and played a fundamental role in the generation of diverse greenstone compositional types from a common komatiitic parent. Integration of EPT-wide geochemical data and previous petrologic and isotopic constraints indicates that each of the three major mantle overturns was characterized by (i) a similar source, likely representing small-scale convective ambient upper mantle, and (ii) the re-occurrence of identified main greenstone compositional types, reaffirming the importance of the cumulates. Observed variations in the greenstone compositional range and concomitant decrease in added supracrustal thickness between successive mantle overturns imply a parallel evolution of the lithosphere. To this end we explore how the range of greenstone compositions, and their temporal variations inform on the lithospheric evolution of the EPT during its development into a mature long-term stable continental lithosphere. We propose that lithospheric thickening and the coeval emergence of continental crust led to the development of a mechanically strong proto-plate that ultimately promoted Wilson-style rifting.

Reference(s)

Niu, Y, 2021, Lithosphere thickness controls the extent of mantle melting, depth of melt extraction and basalt compositions in all tectonic settings on Earth – A review and new perspectives: Earth-Science Reviews, v. 217, 10361417. 

Smithies, RH, Champion, DC, Van Kranendonk, MJ, and Hickman, AH, 2007, Geochemistry of volcanic rocks of the northern Pilbara Craton, Western Australia: Geological Survey of Western Australia, Report 104, p. 1-47.

Wiemer, D, Schrank, CE, Murphy, DT, Wenham, L, and Allen, CM, 2018, Earth’s oldest stable crust in the Pilbara Craton formed by cyclic gravitational overturns: Nature Geoscience, v. 11, p. 357–361.