GLIMMERITE XENOLITHS IN EARLY PROTEROZOIC ULTRAPOTASSIC ROCKS FROM THE CHURCHILL PROVINCE

Phlogopite-rich xenoliths (less-than-or-equal-to 3 cm in diameter) occ ur in ultrapotassic feeder dykes and flows of the Christopher Island F ormation (ca. 1.84 Ga) throughout a 600 X 300 km area in the central C hurchill Province in the District of Keewatin. The glimmerite xenolith s are subspherical to prolate, with smooth and well-rounded surfaces. Most have a concentric rind of distorted and recrystalized phlogopite that is interpreted to result from collisions with the conduit walls d uring magma ascent. Mineral inclusions in the rinds are typically flat tened and boudinaged. The core of the xenoliths is dominated by coarse -grained phlogopite and carbonate minerals with igneous textures. Mino r phases in the core and rind are calcite, dolomite, strontianite, chr omite, chromian diopside, Al-poor amphibole, titanite, zircon, apatite , rutile, sulfides, epidote, barite, goethite, quartz, and fluorite. P hlogopite from xenoliths [average Cr2O3 1.2 wt%, TiO2 0.7 wt%, atomic Mg/(Mg+Fe(T)) 0.92] is more primitive than that occurring as phenocrys ts in the host magma [average Cr2O3 0.1 wt%, TiO2 2.5 wt%, Mg/(Mg+Fe(T )) 0.82]. The xenoliths resemble MARID (Mica - Amphibole - Rutile - Il menite - Diopside) xenoliths found in kimberlites, but are relatively depleted in Ti-bearing phases. The glimmerite xenoliths are interprete d to be fragments of mantle pegmatites and wall-rock cumulates, formed by solidification of primitive ultrapotassic melts with phlogopite +/ - magnesiochromite +/- chromian diopside on the liquidus, and carbonat e minerals +/- a silica polymorph crystallizing near the solidus.