Garnet peridotite xenoliths and xenocrysts from the Monk Hill Kimberlite, South Australia: insights into the lithospheric mantle beneath the Adelaide Fold Belt

Abstract

A total of 73 peridotitic mantle xenoliths and a set of garnet and clinopyroxene xenocrysts from the recently discovered Jurassic Monk Hill kimberlite (UCO-H77A) in South Australia were used to constrain the thermal and compositional structure of the lithospheric mantle beneath the Adelaide Fold Belt, which is located at the southeastern margin of the Australian craton. The xenoliths contain mostly lherzolitic mineral assemblages (garnet + cpx + opx ± chromite), but lack preserved olivine as a result of alteration. Pressure and temperature estimates for the suite of xenoliths (73 samples) follow an array from ∼1•2 GPa and 650°C to ∼5•0 GPa and 1300°C, which reflects the conductive geothermal gradient for this region at the time of kimberlite emplacement (Jurassic, ∼189 Ma). Based on the projected intercept of the geotherm with the mantle adiabat, the maximum depth of the lithospheric mantle beneath the Monk Hill kimberlite is estimated to be around 160–180 km, with the base of the lithosphere lying marginally outside the diamond stability field. The results challenge previously proposed paleogeotherms for this region, which are either significantly hotter or significantly cooler. Sm–Nd isotope data for high-T garnet and clinopyroxene megacrysts define a robust isochron (189 ± 17 Ma), which reflects the Jurassic emplacement age of the Monk Hill kimberlite. This indicates that minerals from deeper parts of the lithosphere were in isotopic equilibrium and exhumed during the kimberlite eruption from temperatures above the Sm–Nd closure temperature for garnet and clinopyroxene. Within the suite of peridotite xenoliths from Monk Hill, abundant low-T (<1000°C) xenoliths can be distinguished from a less common high-T (>1000°C) population. The high-T xenolith population is characterized by titanium-enriched compositions, suggesting that the deeper parts of the lithosphere were affected by pervasive melt metasomatism. This interpretation is supported by the trace element compositions (rare earth elements, high field strength elements) of the garnet and clinopyroxene xenocrysts.