HARRISITIC TEXTURES IN THE CENTER-HILL COMPLEX, MUNRO TOWNSHIP, ONTARIO - PRODUCT OF DIFFUSION-LIMITED GROWTH

A spectacular occurrence of harrisitic textures has been observed near the upper margin of the Centre Hill complex, a cyclically layered maf ic intrusion emplaced within komatiitic and tholeiitic volcanic rocks of the Archean Abitibi Subprovince in the Canadian Shield. The harrisi tic textures, interpreted as pseudomorphs after fayalite, reach up to 50 cm in length and consistently extend away from the upper sill margi n. They poikilitically enclose crystals of the surrounding matrix, ind icating that they originally formed within a crystal-liquid mush. The harrisitic fayalite crystals are interpreted to have grown in situ und er far from equilibrium conditions, where growth was limited by the su pply of nutrients to the crystal boundaries. Mineral growth conditions were simulated using a diffusion limited aggregation (DLA) algorithm to produce branching clusters qualitatively similar to the observed te xtures. Based on this model, it is proposed that the original fayalite crystals formed in the late stages of differentiation cycles. where d iffusivity was low due to increasing viscosity and Fe-Si enrichment of the interstitial melt. Mineral growth was initiated by an imposed the rmal gradient established through the injection of primitive magma int o the chamber.