Geochronological and geochemical constraints on the formation and evolution of the mantle underneath the Kaapvaal craton: Lu-Hf and Sm-Nd systematics of subcalcic garnets from highly depleted peridotites

Subcalcic garnets carry the major inventory of most trace elements of their host harzburgites and are thus proxies of the bulk composition. We used single garnet grains from heavy mineral concentrates from the Kaapvaal craton (Roberts Victor and Lace mine) to determine the major and trace elements and the Sm-Nd and Lu-Hf isotope systematics of these highly depleted members of the peridotitic suite. The combination of the results with previous work from the Finsch mine (Lazarov et al., 2009a) allowed us to reconstruct the formation and evolution of the mantle underneath the Kaapvaal craton. Several lines of evidence from major and trace elements suggest that mantle melting was mainly at shallow pressures followed by subduction into the garnet stability field. A 3.22 Ga metasomatic event underneath the East block occurred in a previously depleted mantle (epsilon Hf = +16) which was sufficiently stabilized by that time to hold a crust with tonalite-trondhjemite-granodiorite (TTG's) and greenstone belts. Such high epsilon Hf values can be produced within a few hundred million years by 25% non-modal fractional melting in the spinel stability field. This is the first prove of a mantle underneath the East block with an age similar to a 3.65 Ga crustal age. Before the amalgamation around 2.88 Ga, oceanic lithosphere was created between the W- and E-block around 2.95 Ga (group RV1 samples from the Roberts Victor) and subducted underneath the W-block. Another mantle portion (group RV2 garnets) from Roberts Victor yielded a seeming age of 3.27 +/- 0.15 Ga with epsilon Hf = +17.6. It actually results from an enrichment process in a highly depleted mantle about 2.8-2.9 Ga ago. This may have been the depleted mantle wedge above the subduction and final collision between the West and the East block. The creation of a cratonic nucleus for the West-block is unknown until 3.2 Ga when the oldest mantle T-RD and oldest crustal zircon ages are reported. Such ages were not directly obtained from the study of the subcalcic garnets but the high positive epsilon Hf value (+25) of the 2.62 Ga enrichment age from Finsch and the existence of a highly depleted mantle wedge before 2.8-2.9 Ga at Roberts Victor suggests that the depletion occurred several hundred million years earlier. Subsequently, the 2.6-2.8 Ga old voluminous Ventersdoorp volcanics poured over a major part of the unified Kaapvaal craton, an event which also may have modified portions of the mantle underneath Finsch. The attachment of the Kheis-Magondi belt to the Kaapvaal craton from the West caused further modification around 1.90 Ga. The latest stages of craton-scale metasomatism recorded in the subcalcic garnets lie between 0.9 and 1.3 Ga as can be seen in the Sm-Nd isotope systematic. This age range coincides with the Namaqua-Natal belt orogeny. The present study and previous work on subcalcic garnets show that they can be excellent recorders of multiple mantle events which can be correlated with the tectonomagmatic evolution of the craton. (c) 2013 Elsevier Ltd. All rights reserved.