Geochemistry of komatiites from Forrestania, Southern Cross Province, Western Australia: Evidence for crustal contamination

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The 2.9 Ga-old Forrestania greenstone belt is intermediate in character between typical > 3.0 Ga and ~ 2.7 Ga belts in containing both Al-depleted and Al-undepleted komatiites. The extensive nickel exploration database on the belt provides a unique opportunity to examine geochemical variations in the context of volcanological evolution. Within each of the six komatiitic units in the greenstone belt there is a transition from komatiites (18–28 wt.% MgO in the liquid) to high- and low-magnesium komatiitic basalts (5–18 wt% MgO) with stratigraphic height, and much of the chemical variance within the komatiitic suite can be explained in terms of the addition or fractionation of olivine. There are, however, aspects of the incompatible-element signatures which must be attributed to other factors. The more primitive komatiites from the lower ultramafic units are Al-depleted (Al 2O 3/TiO 2 ~ 10.5–13.0) and depleted in both light and heavy REE ( La Sm n ~ 0.5, Gd Yb n ~ 1.5 ). Komatiites from the uppermost ultramafic units are variably less depleted in Al ( Al 2O 3 TiO 2 ~ 16.5–20.5 ). This is interpreted to reflect decreasing retention of garnet in the source region of the Forrestania komatiites over time, and possibly a decrease in the depth of melting. Within the Al-depleted suite there are also anomalous enrichments in SiO 2, FeO, TiO 2 and Zr, which correlate with regional variations in the composition of substrate metasedimentary rocks (banded iron formation versus felsic tuff). Coupled with field evidence for trough structures at the base of lava channel sequences, these chemical anomalies provide strong evidence for supracrustal thermal erosion by komatiite lavas. The Forrestania data indicate that 2.9 Ga-old komatiites were generated over a range of melting conditions spanning those typical of older and younger Archaean sequences, and that contamination by supracrustal thermal erosion can influence komatiite chemistry on a regional scale.