Petrography and geochemistry of an unusual Fe-rich basaltic komatiite from Boston Township, northeastern Ontario

The Boston Creek Flow is a thick, Archean layered basaltic komatiite lava flow located 16 km south of Kirkland Lake, Ontario, within the Abitibi greenstone belt. It is superficially similar to the layered flows of Munro Township in that it consists of a basal pyroxenite overlain sequentially by peridotite, pyroxenite, gabbro, and pyroxene spinifex-textured rock and exhibits the typical cumulate komatiite crystallization sequence: olivine–clinopyroxene–plagioclase. The spinifex-textured unit is, however, unusually thick (33 m), and in comparison with other Archean lavas of similar magnesium content, the flow is Fe and Ti enriched and Al and Y depleted. CaO/Al2O3 and Al2O3/TiO2 ratios of 2–3 and 5, respectively, also distinguish it from other Archean magnesium-rich volcanic rocks.The preservation of typical tholeiitic chemistry in lavas underlying the Boston Creek Flow precludes metamorphism and metasomatism as explanations of the flow's enigmatic geochemistry. This is supported further by relict igneous textures exhibited by Ti-bearing oxides in the pyroxenite, gabbro, and spinifex-textured units of the flow. The presence of trellis-textured ilmenite lamellae in titanomagnetite, the textural evidence for symplectic intergrowth of titanomagnetite and clinopyroxene during crystallization, and the failure of assimilation models to explain the coupling of Fe and Ti enrichment with Al depletion suggest rather that the unusual geochemistry of the flow is a primary igneous feature. The Boston Creek Flow is more Al depleted and Fe and Ti enriched than Barberton and Minnesota Al-depleted komatiites (ADK) of the same magnesium number. It is therefore an Fe-rich, layered ADK.