Iron-rich alkaline magmatism in the Archean Wawa greenstone belts (Ontario, Canada)
Kitayama et al
The finding of Fe-rich picritic basalts on Mars has stimulated interest in Fe-rich magmatism on the early Earth. Rare vestiges of Fe-rich magmatism have been identified in remnants of old crust, but many other lithologies reported as ‘enriched’ or ‘Al-depleted’ komatiites in Archean terranes may also be derived from Fe-rich primitive magmas. Although absent today and rare in the Phanerozoic, Fe-rich magmatism may thus have been widespread on the early Earth. Knowledge of the mineralogy and geological setting of Archean Fe-rich igneous rocks may reveal the nature of volcanic activity on the early Earth and possibly Mars.
We conducted a detailed petrological study of some of the best exposed of these terrestrial Fe-rich igneous rocks, which occur in the Pulpwood-Playter Harbours Sequence of the Heron Bay volcanics (∼2.6-2.7 Ga) in the Wawa oceanic plateau association of the Superior Province (Canada). Rocks from these Fe-rich igneous bodies range from serpentinized dunite and clinopyroxenite to differentiated gabbro. They followed the crystallization sequence: olivine ± chromite, clinopyroxene, Fe-Ti oxide and plagioclase. Whole-rock major and trace element analyses show that these Fe-rich bodies are mildly alkaline, and are enriched in incompatible trace elements. The parental magma that we calculated for the Fe-rich bodies is a strongly Fe-rich (∼19 wt.% FeOtot) alkaline magma, with at least ∼9 wt.% MgO. It represents the most Fe- and trace element-enriched end-member of the Wawa greenstone belts and may be the Archean equivalent of modern alkaline basalt.
Magmatic clinopyroxene compositions indicate that crystal fractionation occurred in a small closed-system under low-pressure conditions without significant recharge of magma. Our crystal accumulation models confirm that a closed body of this ferropicritic alkaline magma can undergo progressive crystal accumulation and differentiate to produce the Fe-rich rocks of Pulpwood-Playter Harbours. The observed range of trace elements is mainly controled by increasing proportions of inter-cumulus liquid retained in the rocks during their crystallization and the gabbros represent residual liquid portions of thick lava flows or sub-volcanic sills undergoing extensive closed-system differentiation (∼80% crystal fractionation).
Detailed mapping of the Fe-rich bodies revealed that they are underlain by tholeiitic lava and overlain by some volumetrically minor Fe-rich lava. These upper Fe-rich lavas are more enriched in Fe and incompatible trace elements compared to the lower tholeiites and correspond to mixtures between a tholeiitic magma and the Fe-rich alkaline magma parent to the cumulate rocks. The close spatial association between the tholeiites and alkaline ferropicritic bodies at Pulpwood-Playter Harbours suggests a mantle source in which local regions enriched in Fe- and trace elements melted to produce volumetrically minor, mildly alkaline ferropicritic magmas.