Igneous apatite geochemistry indicates early cratonization of continents

Abstract

Processes and mechanisms accounting for the stabilization of Archean (4.0–2.5 Ga) continental crust remain a matter of debate. Over the last decades, major efforts have been made to determine the chemical and isotopic composition of rocks belonging to the Tonalite-Trondhjemite-Granodiorite (TTG) suite, i.e. those forming the bulk of Archean continental crust, as well as late-Archean sanukitoids, the typical marker of cratonization. The extensive use of zircon elemental and isotopic signatures has indisputably been an unrivalled source of information; yet it has also biased interpretations through the prism of a single mineral, hence not reflecting the whole geological history of these magmas. To extend our understanding of early continent stabilization, a pivotal aspect of Earth’s evolution, a fresh perspective is necessary. Here, we present in-situ analyses of igneous apatite, Ca₅(PO₄)₃(OH,Cl,F), from Archean granitoids exposed in the eastern Kaapvaal craton for major/trace elements and U–Pb/Sr isotopes. The trace element signatures of these apatite crystals, with a clear enrichment in LREE and an elevated LREE/HREE, resemble that of apatite from sanukitoids and Phanerozoic I-type granites, a signature which can be blurred at the whole-rock scale. We interpret this signature as indicating that the studied granitoids are formed via interaction between (i) a TTG melt, formed via partial melting of a subducting oceanic crust and (ii) a mantle component, causing chemical depletion of the mantle domain involved and thus production of long-lived and stable lithospheric keels pivotal in the long-term preservation of Archean lithosphere at the Earth’s surface. Therefore, the identification of this signature—in igneous apatite from Paleo to Meso-Archean TTGs of the eastern Kaapvaal—indicates an early onset of cratonization in this region.