ARCHEAN BONINITES OF THE MURCHISON DOMAIN: A NEW COMPONENT IN YILGARN CRATON GEODYNAMIC MODELS

Following the recognition that Archean siliceous high-magnesian basalts of the Yilgarn Craton were komatiitic magmas that had undergone crustal assimilation, the idea of Archean boninites fell out of general favour. Studies of boninitic rocks in the Abitibi belt, initially by Kerrich et al. (EPSL, 1998), firmly established, however, that the igneous suite was present in Archean terranes. The recent identification of low-metamorphic grade boninites in the Murchison Domain, which preserve distinctive boninites textures as well as chemical compositions, has brought the Yilgarn Craton boninite story full-circle. A survey of the comparatively sparse geochemical data for volcanic rocks across the Murchison indicates that mafic-intermediate rocks (SiO₂ ~ 48 – 55 wt%; MgO ~ 14 – 6 wt %) with low TiO₂ (~ 0.3 – 0.7 wt.%), high Al₂O₃/TiO₂ (30 – 60) and low Zr/Y (1.4 – 3.0) are widespread. Where REE data are available, the rocks are commonly characterized by depletion of the MREE versus the LREE and HREE.

The apparently widespread occurrence of boninites across the Murchison implies that a significant re-evaluation of regional geodynamic models is required, consistent with recent studies indicating a complex pre-amalgamation history for the craton. The Murchison boninitic rocks display an association with komatiites as now documented for some, but not all, occurrences in other Archean cratons. The strong prior depletion of their mantle sources also makes the boninites particularly sensitive and useful monitors of mantle fluxes. Accordingly, the Murchison boninites will play a key role in resolving the contentious issue of how mantle plumes and plate tectonics may have co-existed at 3.0 – 2.7 Ga. We will report the first results from a new study in the Youanmi Terrane of the Murchison Domain to establish the full extent of the boninitic rocks and their tectono-stratigraphic context.