Differentiation processes in an unusual iron-rich alumina-poor Archean ultramafic/mafic igneous body, Ontario

Abstract

Detailed field and petrographic observation and geochemical study of the Boston Creek ferropicrite (BCF), a layered Fe-rich Al-poor Archean ultramafic igneous body, has yielded important insight into the controls on differentiation processes in ferropicrite liquids. Presence of flow top breccia, spinifex, vesicles (amygdules), and altered glass together with chert on its top, and the fact that it is in direct contact with pillowed and massive basalt flows, strongly suggest an effusive origin. Anomalous thickness (30 m) of the spinifex-textured layer compared to thick and differentiated komatiitic basalt and typical tholeiitic basalt flows of similar magnesium contents results from the great abundance of Ca, Fe and Mg relative to Al and high contents of volatiles in the parental liquids and supercooling-induced rapid crystallization, which together increased the interval of clinopyroxene crystallization. Relatively anomalous lithological (clinopyroxenite to diorite), textural (pegmatitiic to fine-grained), and geochemical (Mg-numbers between 45 and <20) complexity in the gabbroic layer results from the high volatile content of the parental liquids and the crystallization of Fe-Ti oxide prior to plagioclase, and consequent displacement, mixing, and entrapment of low-density, volatile-enriched residual liquids. Variations in the development of pegmatite, liquid segregation pockets, and PGE mineralization in the gabbroic layer along strike reflect variations in the extent of within-flow displacement, migration, and mixing of residual liquids.