Late Archaean Ti–rich, Al–depleted komatiites and komatiitic volcaniclastic rocks from the Murchison Terrane in Western Australia

Abstract

Greenstone belts in the northern Murchison Terrane of the Yilgarn Craton contain an extensive suite of 2.9–3.0 Ga, porphyritic komatiites and komatiitic volcaniclastic rocks. These unusual Ti–rich Al–depleted komatiites have been sampled at Gabanintha and are characterised by higher incompatible‐element abundances than most suites of Barberton‐type Al–depleted komatiites. They form a petrogenetically related group with similar Ti– and incompatible‐element‐rich, Al–depleted porphyritic komatiites and komatiitic volcaniclastic rocks from Karasjok in Norway, Dachine in French Guiana and Steep Rock‐Lumby Lake in Canada (here called Karasjok‐type komatiites). Their Al–depletion results from magma generation at depths of >250 km in the presence of residual majorite‐garnet. The porphyritic textures and abundance of amygdales and volcaniclastic rocks typical of this type of komatiite are features of hydrous ultramafic magmas. The incompatible‐element‐rich ultramafic rocks from Dachine contain diamonds that were most likely picked up as parent magmas interacted with mantle lithosphere that had been hydrated and chemically modified. Consequently the interaction of Karasjok‐type komatiite magmas with thick, island arc or continental mantle lithosphere may have resulted in their elevated water and incompatible‐element contents. The occurrence of Karasjok‐type komatiite lavas and volcaniclastic rocks in the northern Murchison Terrane suggests that during the Late Archaean that terrane had a hydrated, metasomatised or subduction‐modified mantle lithosphere.