Evidence for Differentiation by Crystal Fractionation in Theo's Flow, Canada

Theo's Flow, a 120-m thick differentiated lava flow in Munro Twp, Ontario, is an unusual magma body which calls for an unusual formation mechanism. New systematic sampling of the flow for geochemistry and petrography offers additional clues to its post-emplacement differentiation. Evidence for differentiation comes from both petrography and geochemistry. Theo's has four lithologic units: a basal peridotite (3-9 m), a thick pyroxenite (50 m) and gabbro (40 m), and a capping (8-12 m) hyaloclastite. Contacts between lithologic layers are gradual, mostly marked by changes in modal mineralogy, although there is also a distinct change in plagioclase morphology concurrent with its modal increase. From pyroxenite to gabbro, plagioclase shifts from fine, interstitial sprays to large laths in a subophitic intergrowth with pyroxene. Whole rock and pyroxene compositions display typical fractionation trends (e.g. Fe/Mg, Ti, Zr, Nb increase upsection). Furthermore, a weighted sum of 22 whole rock compositions from the internal layers match the hyaloclastite composition well for most non-mobile elements. All these factors suggest formation of the three internal layers by fractional crystallization and evolution of a magma whose composition is represented by the hyaloclastite. Any proposed differentiation process must address some specific observations. Cluster and CSD analysis reveal that pyroxene grains grew in clusters under steady-state conditions of nucleation and growth. Pyroxene grain size is nearly uniform throughout the pyroxenite, suggesting thermal conditions were maintained over a long crystallization interval. Progression of a typical solidification front would produce a fine-grained roof crust beneath the quenched top, but there is no evidence of such a layer in Theo's. Unlike magmas for which solidification fronts are invoked, Theo's parent magma was highly mafic and Al-poor, yielding a low viscosity magma (4 Pa-s) that crystallized only pyroxene over a long temperature interval. We will consider two formation models in which differentiation was driven by crystallization at flow top and bottom, but accumulation only occurred at the base. One model addresses the possible role of convection.