Geochemical variation of intraplate magmatism and its sources in the Kaapvaal craton (Southern Africa) from Paleoarchean to present: implications for the evolution of its mantle lithosphere.

The Kaapvaal craton, which hosts rocks as old as 3.6 billion years, began to stabilize around 3.2 billion years ago with the progressive development of its subcontinental lithospheric mantle (SCLM). This craton, which has undergone continuous and relatively well-preserved geological evolution through numerous episodes of erosion, sedimentation and magmatic additions, provides a unique setting for studying the evolution of geodynamics. This study focuses primarily on the interpretation of existing literature and some newly acquired data for HFSE, REE and Lu-Hf and Sm-Nd isotopes, due to their immobility during low temperature alteration processes.

The Palaeo-Mesoarchean transition marks significant changes in both geodynamic (from vertical to transitional tectonics) and magmatic processes. During this period, magmatism changed from mixed calc-alkaline and tholeiitic affinities derived from 'asthenospheric' sources to a pure tholeiitic series stemming from the (proto-)SCLM and/or asthenosphere. Almost all Meso- and Neoarchean igneous units show 'arc-like' geochemical signatures (e.g., negative Nb, Ta and Ti anomalies), mainly due to crustal assimilation. Only a few occurrences may reflect clear fluid metasomatism of the mantle sources. A major shift occurred in the early Paleoproterozoic, characterized by a protracted magmatic phase lasting over 500 Myr, with events showing significantly high Th/Nb ratios. These signatures are likely to be from refertilized sources, linked to a major metasomatic event and hence a significant but transient subduction (or comparable) process at the Archean-Proterozoic transition. After 1830 Ma, the frequency of magmatism decreases significantly, leaving only three major events (at ~1400, 1100 and 180 Ma), if we except kimberlites and lamproites. During this time, the Kaapvaal craton gradually evolved into a component of larger continental masses (i.e., proto-Kalahari then Kalahari cratons). However, these post-Paleoproterozoic events show much more diverse magmatic source signatures than earlier periods, including contributions from previously unobserved sources (e.g., OIB-like or enriched mantle OIB).

The data suggest that most (but not all) post-Paleoarchean magmatic events are associated with plume-related activity involving melting of the SCLM at shallow depths (i.e., spinel mantle), consistent with conclusions from previous studies that considered these events separately.  However, a relatively inert Late Mesoarchean SCLM alone would not be sufficient to generate such widespread and long-lasting magmatic activity, particularly if it exhibits enriched geochemical signatures. Overall, the geochemical data support an evolving SCLM that underwent periods of (local) melting, depletion and refertilization.