Controls on the Emplacement Style of Coherent Kimberlites in the Lac de Gras Field, Canada

Abstract In the Lac de Gras (LDG) kimberlite field, Northwest Territories, Canada, coherent kimberlites (CKs) occur as tabular dykes, pipe-shaped diatremes, and irregular bodies without well-defined geometries. Combining morphology of CK with occurrence fragmented olivine microcrysts allows distinction four types at LDG: (1) dykes no broken olivine; (2) but probable sheet geometry (3) pipe-filling (pfCK) abundant (4) pfCK olivine. These features suggest an intrusive origin for type 1 and, probably, 2 CK; a high-energy extrusive emplacement 3 low-energy or 4. Here, we compare petrographic whole-rock, spinel compositional data pfCK, units to understand factors controlling their variable styles. Extrusive contain more groundmass phlogopite monticellite, lower carbonate/silicate mineral abundance ratios significantly dolomite pleonaste-spinel abundances compared CK. This indicates greater CO2 loss higher H2O/CO2 in melt phase during emplacement. Lower incompatible element concentrations CKs different chromite Ti# rim Mg# indicate derivation from distinct primitive compositions. The feature ɛNdi marginally ɛHfi compositions than CK, pointing sources. findings strongly imply that primary were largely responsible differences styles LDG. Low-energy pfCKs have similar Mg#, hence, samples. Their may depend on local factors, such changing stress regimes, slightly volatile concentrations. Both might reflect waning stages volcanic sequences resulting eruption segregated magma column started pipe excavation explosive gas-rich (volcaniclastic kimberlite), followed by less energetic melt-rich (pfCK). hypothesis underscores dyke vs pipe-forming (and filling) hence fundamental control explosivity kimberlites.