The petrology of the Tilting Harbour igneous complex, Fogo Island, Newfoundland

Abstract

The Tilting Harbour complex, Fogo Island, Newfoundland, displays a mineralogical range from olivine-bearing pyroxenites to diorites. The engulfing granite is not genetically related to the complex. On field, petrographic and chemical evidence the complex can be divided into two distinct series. The older magma is of quartz tholeiite composition with an Mg/(Mg + Fe) ratio consistent with derivation directly from the mantle by melting under fairly low pressure (10 kbar (10 × 105 kPa)) under hydrous conditions. The second magma is of olivine tholeiite composition and produced magnetite-rich gabbros. It is not a primary melt from the mantle. Subsequent differentiation produced a range of liquids from olivine-normative to silica-rich, which is best explained by hornblende fractionation. Both magma series gave rise to websterite cumulates. However, the removal of pyroxene did not produce a significant change in residual liquid composition. The Ni content of the quartz-tholeiite magma is 75 ppm and suggests a partition coefficient of about 40 between mantle olivine and the liquid. The pyroxenes that crystallized from both liquids contained five times the Ni content of the magma. The partition coefficients for Ni and Cr between hornblende and liquid, determined from the chemical variation of the magnetite-rich gabbro suite, are both approximately 2.The contamination of the granite by melting of microdiorite was selective, only the fusible constituents being assimilated while refractory components were not incorporated.