Mineralogy of an Appinitic Hornblende Gabbro and Its Significance for the Evolution of Rising Calc-Alkaline Magmas

Abstract

The magmatic and sub-solidus evolution of calcic amphiboles and Fe–Ti oxides was investigated in the Neoproterozoic Frog Lake pluton, Nova Scotia, Canada, in order to understand the relationship between the history of hydrous magma and the resulting mineralogy. The pluton occurs as sheet-like bodies of hornblende gabbro and hornblendite, with lesser tonalite dykes and granite bodies, interlayed with screens of medium-grade metamorphic country rock. Small, diffuse clots of felsic minerals are present in the gabbro. The subsolidus growth of actinolite occurs in early clinopyroxenes and amphiboles. Ilmenite is the dominant Fe–Ti oxide, as interstitial magmatic crystals. The increase of Mn towards the margin of the ilmenite crystals indicates a gradual increase in oxygen fugacity with time, leading to the precipitation of titanite and ferrohypersthene. The replacement of titanite by ilmenite and ilmenite lamellae in the amphiboles suggests subsequent reducing conditions during the sub-solidus crystallisation. The gabbros in the coeval, but apparently shallower, Jeffers Brook granodiorite laccolith have dominant magnetite and Mg-rich subsolidus amphiboles, which are indicative of high oxygen fugacity. The differences between the two plutons suggest that there was a greater flux of hydrothermal water through the sheet-like architecture of the Frog Lake pluton.