Perovskite geochronology and petrogenesis of the Neoproterozoic Mad Gap Yards ultramafic lamprophyre dykes, East Kimberley region, Western Australia.

The Mad Gap Yards ultramafic lamprophyre (UML) dykes in the East Kimberley region of northern Western Australia form part of a widespread Neoproterozoic (~ 842–800 Ma) alkaline mafic–ultramafic magmatic province in the north, east and central regions of the Kimberley Craton of Western Australia. The NE-trending Mad Gap Yards dykes lie at the southeastern margin of the Kimberley Basin adjacent to the Greenvale Fault and intrude the Paleoproterozoic Elgee Siltstone. The dykes are classified as alnöite, and contain abundant macrocrystic olivine in a groundmass of phlogopite, perovskite, spinels, diopside, apatite, andradite–hydroandradite, serpentine, calcite, pseudomorphs after melilite and rare gittinsite. Mantle-derived olivine macrocrysts have compositions in the range Mg#_91–92, similar to moderately refractory peridotite from other parts of the Kimberley Craton, whereas magmatic olivine phenocrysts have Mg#_88–90. Olivine and chromian spinel were the earliest phenocrysts; they record equilibration temperatures of ~ 1030–920℃ under moderately reducing conditions with fO₂ values below the fayalite-magnetite-quartz (FMQ) oxygen buffer (Δ FMQ = mostly − 0.8 to − 1.7 log units). Magnetite rims and groundmass grains crystallised at ~ 850–740℃ under more oxidising conditions with Δ FMQ ~ + 0.6 to − 0.75 log units. Perovskite is well preserved in parts of the dykes and indicates crystallisation inside this fO₂ range. The perovskite yielded a SHRIMP ²⁰⁶Pb/²³⁸U age of 842 ± 8 Ma. The Mad Gap Yards dykes carry rare partially altered spinel-peridotite xenoliths containing olivine (Mg#_86.3–90), Cr-diopside, enstatite and Al-Cr spinel, and well as mantle xenocrysts of Cr-Al spinel and Cr-diopside. Bulk rock trace-element geochemistry, Cr-diopside thermobarometry and Sr–Nd-isotopic compositions of perovskite suggest that the UML magma was derived from partial melting of a garnet-bearing asthenospheric mantle source at ~ 200 km depth. Nd depleted-mantle model ages (T_DM) for perovskite range from 1106–865 Ma and broadly correlate with regional mantle metasomatism during the Yampi Orogeny (~ 1000–800 Ma). Rare evidence of mantle metasomatism by LILE and HFSE-enriched melts has been found in the form of priderite and loveringite replacing Mg-rich ilmenite in an olivine macrocryst. The timing of emplacement of the Mad Gap Yards UML dykes at ~ 842 Ma correlates with the early stages of the breakup of Rodinia. [ABSTRACT FROM AUTHOR]