MINERALOGY AND PETROLOGY OF ORBICULAR IJOLITE FROM THE PRAIRIE LAKE CARBONATITE COMPLEX, MARATHON, ONTARIO

Orbicular ijolite is hosted by contemporaneous ijolite rocks at the 1.1 Ga Prairie Lake Carbonatite Complex (Marathon, Ontario), and is the only known occurrence of this textural type in a rock of this composition. Orbicular textures represent rare crystallochemical reactions, which occur during the later stages of emplacement(s) and crystallization of the magmas. The Prairie Lake ijolite orbicules occur along densely packed bands in equigranular nepheline-rich ijolite and range from 3 to 6 cm in diameter. Some orbicules are oblate and exhibit deformation and show magmatic sedimentation textures within the host ijolite. From a macroscopic perspective, the orbicules show great variability in the cores, with some having an equigranular texture similar to the host matrix, whereas others exhibit quench textures. Radial modal zoning is common closer to the cores with tangential concentric zoning (up to 40 discrete zones) of distinct mineral banding towards the rims of the orbicules. The mineralogy of the orbicules includes: nepheline, diopside, calcite and apatite, andradite-melanite garnet, biotite, titaniferous magnetite, perovskite, secondary calcite, and cancrinite. The mineralogy of the host ijolite is similar- with less alteration and no secondary calcite present. The only difference between the orbicules and the host ijolite is their unique textures. Quenched textures are identified in the orbicules, notably in the core with the presence of both fine- and coarse-grained quench textures. The orbicules are interpreted to represent interaction of a partially crystallized quenched ijolitic melt, which was in contact with a second pulse of consanguineous ijolite magma. Immersion in the latter resulted in annealing of orbicules, in which previously formed quenched ijolite was recrystallized producing the monominerallic concentric layers sequentially from the margins towards the center of the orbicule.