Mafic Mineralogy of Ferroaugite Syenite from the Coldwell Alkaline Complex, Ontario, Canada

A 1500 m thick sheet-like body of ferroaugite syenite is divisible mineralogically into an upper and lower series of syenites. The lower syenites are characterized by well developed igneous layering defined by mafic cumulus minerals. The syenites are saturated to oversaturated and contain as cumulus phases alkali feldspar, olivine (Fa83–93), ferroaugite (Di50Hd45Ac5−Di5Hd90Ac5) and ilmeno-magnetite. Amphiboles which crystallized from the intercumulus liquid range in composition from ferrohastingsitic hornblende to ferroedenitic hornblende to ferroactinolitic edenite. The upper series are coarse grained cumulates with poorly defined layering and abundant patch pegmatites. Cumulus phases are alkali feldspar, olivine (Fa93), and acmitic-hedenbergite (Di5Hd50Ac5−Ac50Hd50). Intercumulus liquids are peralkaline and crystallized to aenigmatite and amphiboles which range in composition from ferrorichteritic katophorite to ferrorichterite, Patch pegmatites are peralkaline rocks composed of ferrorichterite, ferroactinolite, alkali feldspar, aenigmatite, quartz and zircon. Extreme differentiation of ferroaugite syenite magma generates residua which are ironrich, oversaturated and peralkaline. Initial and final temperatures of crystallization are estimated from mineral stability data to be 800–900 °C to 500–550 °C respectively. Thermodynamic data and mineral compositions indicate that during crystallization the oxygen fugacity of the magma decreased from approximately 10−15 to 10−23–10−24 bars. Ferroaugite syenite pyroxene compositional trends are similar to those of undersaturated peralkaline syenites (llimaussaq) and demonstrate that acmite enrichment trends are independent of silica activity and take place under decreasing oxygen fugacities.