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

Abstract

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˚C 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˚C 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

Mapping the Structure and Metasomatic Enrichment of the Lithospheric Mantle Beneath the Kimberley Craton, Western Australia

“…Kimberlite magmatism during the Neoproterozoic (∼800–850 Ma) was focused on the North Kimberley Province, and to a lesser extent, the East and Central Kimberley Provinces. This magmatism occurred during the early stages of the breakup of the Rodinia supercontinent (Downes et al., 2016, 2023; Jaques & Milligan, 2004), and coincided with several small‐ to medium‐scale volcanic events elsewhere on the Australian continent, including the Gairdner large igneous province (LIP) (827 Ma, Wingate et al., 1998), Mundine Well Dyke Swarm (755 Ma, Li et al., 2006; Wingate & Giddings, 2000) and Boucaut Magmatic Event (788 Ma; Armistead et al., 2021), as well as aillikite and related magmatism within the Gibson Desert (806 Ma, Webb Kimberlite Province) (Western Australia). Formation of these LIPs and related magmas has been attributed to a “superplume” event which led to continental rifting and the breakup of Rodinia (Li et al., 2006).…”

“…The Neoproterozoic (∼800–850 Ma) kimberlites and lamprophyres of the Kimberley Craton appear to have formed in response to early stage plume impingement on the metasomatized roots of the North Australian Craton during the incipient stages of the breakup of Rodinia. The (limited) isotopic data available suggest that the magmas were derived from the asthenosphere (Downes et al., 2023; Edwards et al., 1992; Fielding & Jaques, 1986; Graham et al., 1999) in contrast to the significant input of enriched CLM seen in the isotopic compositions of the Argyle and West Kimberley lamproites (e.g., Jaques et al., 2018 and refs therein). The high equilibration temperatures of the sub‐calcic Cr diopside xenocrysts and abundant picroilmenite in the Skerring kimberlite likely reflects a refertilization event involving plume‐derived magma at or close to the eruption age of Skerring (Taylor et al., 1999).…”

“…(1999), which included samples from Argyle, Bow Hill, Ellendale, Pteropus, Seppelt and Skerring and the Mad Gap Yards lamprophyre from Downes et al. (2023) (Table 1). Garnet inclusions in diamonds from Argyle and Ellendale were taken from Jaques et al.…”

“…Alkaline volcanic rocks across the craton host a variety of mantle cargo that includes xenocrysts of Cr diopside, pyrope and almandine-rich garnet, ilmenite, Cr and Al-rich spinel, and diamond. Although rare, mantle xenoliths include diamondiferous and non-diamondiferous coarse garnet-bearing lherzolite, harzburgite and wehrlite, and spinel peridotite (Downes et al, 2007(Downes et al, , 2023Graham et al, 1999;Jaques, 1986;Jaques et al, 1990;Luguet et al, 2009;Wyatt et al, 1998).…”

“…The Argyle lamproite, and by association, the Lissadell Road and Seagull's Dyke lamproites, are the oldest with Argyle having an age of 1,177 ± 47 Ma (Pidgeon et al., 1989). The Bow Hill and Mad Gap Yards ultramafic lamprophyres have ages of 815 ± 20 Ma (Pidgeon et al., 1989) and 842 ± 8 Ma (Downes et al., 2023) respectively (Table 1). The Argyle lamproite is a large multiphase pipe (47 ha) composed of diamondiferous quartzose volcaniclastic olivine lamproite (predominantly lapilli tuff) and intruded by sparse olivine lamproite dykes (Jaques, 1986; Rayner et al., 2018).…”