Two episodes of fluid migration in the Kaapvaal Craton lithospheric mantle associated with Cretaceous kimberlite activity: Evidence from a harzburgite containing a unique assemblage of metasomatic zirconium-phases

Abstract

In this study we combine textural evidence with mineral chemical, mineral structural and isotopic data in an attempt to reconstruct the history of metasomatic events recorded in a spinel-harzburgite xenolith which was sampled by a Cretaceous kimberlite of the central Kaapvaal Craton, South Africa. Two episodes of interaction between the host rock and silicate-rich high-density fluids can be distinguished which together gave rise to a uniquely complex assemblage of LILE–HFSE-rich phases including zircon together with both monoclinic (baddeleyite) and cubic (tazheranite) zirconium oxide, srilankite and a new Mn–Fe-rich member of the pyrochlore-group of phases. The pre-metasomatic assemblage is olivine+orthopyroxene+Cr-spinel. Subsequent modal metasomatism formed phlogopite+K-richterite+crichtonite-group (lindsleyite–mathiasite) phases+Nb–Cr-rich rutile+srilankite+zircon+Fe–Ni-sulfide. K-richterites are strongly zoned in Ca, Na, Fe and Cr with up to 2.3wt.% Cr2O3 which is the highest Cr content reported so far for K-richterite. SIMS U–Pb dating of the zircons yields ages in the range from 81±2 to 91±2 (2σ) Ma which are indistinguishable from emplacement ages of Cretaceous kimberlites in the Kimberley area. The age spread is interpreted as a result of minor re-setting of the U–Pb isotopic system. The coincidence between zircon and kimberlite eruption ages further supports a temporal and genetic link between Cretaceous kimberlite activity and hydrous potassic metasomatism in the central Kaapvaal Craton lithosphere and limits the residence time in the mantle of metasomatized peridotites to <~5–10Ma. Thermobarometry of the harzburgite yields 750–760°C at 3GPa with a redox state of +0.9 to +1.5 log units relative to FMQ. Infiltration of a hot and alkali-rich (kimberlitic?) high-density fluid with aSiO2 lower than that defined by olivine+orthopyroxene into the cool metasomatized peridotite led to partial breakdown of K-richterite, crichtonite-group phases, zircon and sulfides and formed Al–Ti–Ni-rich spinel+K–Cr priderite+Nb-rich titanite+baddeleyite+tazheranite+Mn–Fe-betafite. In addition, a second generation of Ni-poor and Mn-rich olivine formed along with Ti-rich K-richterite, phlogopite and clinopyroxene. Both tazheranite and Mn–Fe-betafite were unambiguously identified with electron microprobe analysis combined with selected area diffraction using FIB–TEM. Mn–Fe-betafite is new member of the pyrochlore group of phases and the tazheranite from this study represents the first occurrence reported from an upper mantle environment.