Bruce Craig Jago thesis abstract

The Ham diatreme and dyke are post-late Silurian intrusions located in north-central Somerset Island and are the most northerly known kimberlites in theSomerset Islandkimberlite province.  The Ham diatreme, which consists of three petrographically distinct varieties of kimberlite, formed as a series of fluidized intrusions at the intersection of several regional fracture sets.  Type 1A kimberlite is petrographically similar to the Ham dyke (a single intrusion located 1.5 km to the east) and forms the flanks of the Ham diatreme. This dark, massive rock contains phenocrysts and xenocrysts of garnet,olivine, chrome-diopside, phlogopite, spinel and carbonate in aserpentine-carbonate groundmass containing carbonate and serpentine emulsion textures.  Type 1B kimberlite, which occupies the central portion of the Ham diatreme, is a highly altered, light green, seperentine-carbonate-rich rock formed by the prograde serpentinization and carbonatization of Type 1A kimberlite. This alteration occurred during the degassing of structurally lower portions of the Ham diatreme.  Type 2 kimberliteis a carbonate-rich mineralogical equivalent of Type 1A kimberlite and formed as a late stage dyke within the Ham diatreme.

Pre-fluidization phenocrysts include Mg-rich olivine (Fo_89-93), low Cr, (<3.5 wt.% Cr₂O₃), high Ti (>0.3 wt. % TiO₂) pyrope-garnet, Al-rich, Ti-poor (<2.00 wt % TiO₂) aluminous-magnesium chromite (Cr/Cr + Al = 0.18 - 0.85) and Ti-rich phlogopite(1.0 - 4.6 wt. % TiO₂). Post-fluidization microphenocrysts include Mg-rich olivine (Fo_89-93),Ti-rich phlogopite (2.5 - 4.0 wt. % TiO₂) and spinel which evolved from Ti-bearing (2.00 wt. % TiO₂), titan-magnesium-aluminous-chromiteto Fe³⁺- and Ti-rich (max. 17.0 wt. % TiO₂) magnesium-ulv_spinel - ulv_spinel-magnetite.  Atoll spinels, formed prior to the complete crystallization of the kimberlite groundmass are present in the Ham dyke but extensive resorption of magnesium-ulv_spinel-ulv_spinel-magnetite and titan-magnesium-aluminous-chromite in the Ham diatreme has precluded their persistence.

Xenocrysts formed by the disaggregation of garnet and spinel lherzolites include Cr-rich (3.5 - 10.0 wt. % Cr₂O₃), Ti-poor (<0.30 wt. % TiO₂) pyrope-garnet, Mg-rich olivine and chrome-diopside.  Pressure temperature estimates from garnet lherzolite xenoliths range from 36 to 37 kb and 1031 to 1146EC corresponding to a depth of origin of 110 to120 km.

Multiple discriminant analysis demonstrates that cluster analysis can only distinguish between garnets of grossly different chemistry and paragenesis and that major and minor element variation diagrams are required to separate statistically,chemically similar garnets within a paragenesis.

Geophysical studies may be used to delineate kimberlite subcrop patterns and structural elements which may have controlled the intrusion of the kimberlites.

A copy of the thesis can be downloaded here