Late Archaean Ti-rich, Al-depleted komatiites and komaiitic volcaniclasticrocks from the Murchison Terrane in Western Australia

Greenstone belts in the northern Murchison Terrane of the Yilgarn Craton co ntain an extensive suite of 2.9-3.0 Ga, porphyritic komatiites and komatiit ic volcaniclastic rocks. These unusual Ti-rich Al-depleted komatiites have been sampled at Gabanintha and are characterised by higher incompatible-ele ment abundances than most suites of Barberton-type Al-depleted komatiites. They form a petrogenetically related group with similar Ti- and incompatibl e-element-rich, Al-depleted porphyritic komatiites and komatiitic volcanicl astic rocks from Karasjok in Norway, Dachine in French Guiana and Steep Roc k - Lumby Lake in Canada (here called Karasjok-type komatiites). Their Al-d epletion results from magma generation at depths of > 250 km in the presenc e of residual majorite-garnet. The porphyritic textures and abundance of am ygdales and volcaniclastic rocks typical of this type of komatiite are feat ures of hydrous ultramafic magmas. The incompatible-element-rich ultramafic rocks from Dachine contain diamonds that were most likely picked up as par ent magmas interacted with mantle lithosphere that had been hydrated and ch emically modified. Consequently the interaction of Karasjok-type komatiite magmas with thick, island arc or continental mantle lithosphere may have re sulted in their elevated water and incompatible-element contents. The occur rence of Karasjok-type komatiite lavas and volcaniclastic rocks in the nort hern Murchison Terrane suggests that during the Late Archaean that terrane had a hydrated, metasomatised or subduction-modified mantle lithosphere.