Archaean boninite-like rocks in an intracratonic setting

Abstract

Boninites and high-Mg norites show considerable compositional overlap. While the petrogenesis of both may involve large ion lithophile element (LILE) replenishment of a mantle source, boninites appear restricted to convergent margin settings, while many high-Mg norite suites intrude intracratonic rifts. Rocks with boninitic compositions are extremely rare in Archaean terranes, where they are also interpreted to have formed at convergent margins. High-Mg, LILE-enriched basaltic rocks form a ca. 2950 Ma sub-volcanic sill within the 3010–2935 Ma Mallina Basin, of the Pilbara Craton, northwest Australia. They are spatially associated with low-Ti tholeiitic intrusions and a high-Mg diorite (sanukitoid) suite. They have a strong boninitic affinity, with high SiO 2 (52–54 wt%), Mg # (62–67) and Al 2O 3/TiO 2 (62–73), low TiO 2 (0.24–0.28 wt%) and strong LILE enrichments. Primitive-mantle normalised La/Gd and Gd/Yb ratios of ∼4.5 and ∼0.6, respectively, produce prominent ‘U-shaped’ normalised trace element patterns. Despite such close compositional affinity to boninite, these rocks probably formed more than 40 Myr after regional magmatism and 60 Myr after the last regional event linked to subduction. Geological evidence suggests that the Mallina Basin formed in an intracontinental setting, not at a convergent margin, and so high-Mg norite provides a more realistic analogy for the Mallina boninite-like rocks in terms of tectonic setting. In old terranes, the tectonic setting of rocks with boninite-like compositions must be interpreted with caution. Trace element modelling shows that contamination by felsic crust is a plausible explanation for LILE enrichments in the Mallina boninite-like rocks, but the scarcity of suitable contaminants makes this explanation unlikely. Petrogenesis of the boninite-like rocks was closely associated with that of the high-Mg diorite (sanukitoid) suite. LILE enrichments in high-Mg diorite require that the mantle beneath the entire Mallina Basin was variably metasomatically enriched by an earlier subduction event at, or before, ca. 3015 Ma. The refractory source for the boninite-like rocks may have also been enriched during this event.