Mineral chemistry and U-Pb SHRIMP geochronology of xenoliths and xenocrysts from the Aries micaceous kimberlite: Constraints on the composition and age of the central Kimberley Craton, Western Australia

Abstract

The Neoproterozoic (~815 Ma) Aries micaceous kimberlite intrudes the central Kimberley Basin, northern Western Australia, and contains a suite of 27 serpentinised ultramafic xenoliths, including spinel-bearing, and rare, metasomatised, phlogopite-biotite and rutile-bearing types, along with minor granite xenoliths. This suite of xenoliths provides new data on the composition and age of the central Kimberley Craton. Proton probe trace-element analysis of pyrope and chromian spinel grains derived from heavy mineral concentrates from the kimberlite has been used to define a Proterozoic geotherm for the central Kimberley Craton, of ~35-40 mW/m2. Lherzolitic chromian pyrope, that is highly depleted in Zr and Y, and Cr-rich magnesiochromite xenocrysts (class 1), probably were derived from reduced, depleted garnet peridotite mantle at ~150 km depth. We interpret the highly depleted chromian pyrope xenocrysts to have formed by exsolution from high temperature orthopyroxene in former harzburgite. Sampling of shallower levels of the lithospheric mantle by magmas in the north, and north extension lobes of the Aries kimberlite entrained high-Fe chromite xenocrysts (class 2), and aluminous spinel-bearing xenoliths, where both spinel compositions are anomalously Fe-rich for spinels from mantle xenoliths. This Fe-enrichment may have resulted from Fe-Mg exchange with olivine during slow cooling of the peridotite host rocks. Episodes of Fe-Ti-rich metasomatism in the spinel-facies Kimberley mantle are the likely source of Ti-phlogopite-biotite + rutile and Ti-enriched aluminous spinel ± ilmenite associations in several ultramafic xenoliths. U-Pb SHRIMP 207Pb/206Pb zircon ages for one granite (1851 ± 10 Ma) and two serpentinised ultramafic xenoliths (1845 ± 30 Ma; 1861 ± 31 Ma) indicate that the granitic basement and lower crust beneath the central Kimberley Basin are Palaeoproterozoic in age.