Mineralogy, geochemistry, petrogenesis and structural relationships of the Aillik Bay alkaline intrusive suite, Labrador, Canada

Foley, Stephen Francis (1982) Mineralogy, geochemistry, petrogenesis and structural relationships of the Aillik Bay alkaline intrusive suite, Labrador, Canada. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Aillik Bay alkaline intrusive suite comprises dykes of alkaline lamprophyre (sannaite and olivine sannaite), kimberlite and carbonatite. Structural and mineralogical criteria indicate that the dykes are related to an intrusive centre of nephelinite-carbonatite type situated beneath the Labrador Sea to the northeast of the study area. -- Dykes were emplaced in three structural episodes; two concentric sets are separated in time by a dominant radial set. Sannaites make up the first set and the bulk of the second whereas kimberlites and carbonatites exclusively occupy the third dyke set. Formation of segmented dykes is attributed to flow instabilities enhanced by a volatile-rich fluid moving ahead of the magma. This fluid was presumably exsolved from the magma as a result of pressure reduction during emplacement, and also assisted in the formation of parallel fracture zones adjacent to kimberlites and carbonatites. -- Sannaites are characterised by leucocratic ocelli which are frequently zoned: a central zone dominated by carbonate and analcite gives way to an outer zone of Fe-mica, pyroxene, nepheline, K-feldspar and analcite. The outer zones were formed by segregation of late-stage melt. One sample bears globules which are clearly the result of liquid immiscibility. Immiscibilty and segregation are accompanied by concentration of incompatible elements. Groundmass mineralogy shows chemical evolution similar to nepheline syenites. -- Minerals in kimberlite delineate a more complex history, beginning at depth in a low oxygen fugacity environment. Kimberlites lack high pressure equilibrated diamond 'marker' minerals, and thus diamond potential is low. Carbonatites typically exhibit relict kimberlitic textures. Sannaite and olivine sannaite were derived by flow differentiation from a parental magma, the composition of which is defined. All rock types were derived by partial melting of an incompatible element enriched mantle source. Structural inheritance permitted successive emplacement of rocks representing progressively smaller and deeper derived melt fractions via the intrusive centre.

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