David G. Laderoute MSc thesis abstract

Thesis Title: 
The Petrography, Geochemistry and Petrogenesis of Alkaline Dyke Rocks from the Coldwell Alkaline Complex, Northwestern Ontario
David G.

Dyke rocks occurring in the Coldwell alkaline intrusive complex, N.W. Ontario, were studied by various petrographic and geochemical means.  The resulting observations indicate that 5 types of mafic alkaline lamprophyres, distinguished on the basis of chemistry and mineralogy according to the scheme of rock (1977), and 1 type of felsic tinguaite occur in the complex (in approximate order of abundance):

    a)     ocellar camptonites
    b)   analcite tinguaites
    c)    sannaites
    d)   quartz-bearing camptonites
    e)   amphibole camptonites
    f)     monchiquites

Lamprophyres are characterized mineralogically by phencrysts of clinopyroxene and brown amphibole, and remnant phencrysts of olivine (all commonly zoned from Mg-rich cores of Fe-rich rims) in a groundmass of Fe-Na-clinopyroxene, brown amphibole, biotite, plagioclase and alkali feldspar (except in the case of the monchiquites, which contain an isotropic glassy material in place of feldspar), nepheline, calcite and minor opaque and accessory phases.  All phases are commonly altered to a variety of secondary products.  Quartz camptonites are distinct in containing incompletely resorbed quartz phenocrysts.  All lamprophyres contain varying abundances of ocelli, which are globular segregations of carbonate minerals and silicate phases such as scapolite, chlorite, epidote, zeolites and fluorite.  Tinguaites contain phenocrysts of alkali feldspar, nepheline, analcite and minor riebeckite in a groundmass of the former three phases plus aegerine and minor accessory phases.

Application of multivariate statistical routines to major and minor element data from these rocks suggests that the quartz camptonites are distinct from the other lamprophyres on the basis of various chemical parameters other than silica.  These routines are incapable of distinguishing the other lamprophyres on the basis of chemistry.  Variation diagrams constructed from the major element data suggest that all of the dyke rocks except the quartz camptonites form a magmatic series.  Studies of trace element abundances in these rocks suggest that they are related as a magmatic series by a fractional crystallization scheme involving those phases occurring as phenocrysts, crystallizing under varying physio-chemical conditions, with the parental liquid being a LREE- and volatile component-enriched alkali olivine basalt related to the terminal stages of the Centre 2 magmatism in the complex.  The quartz camptonites are derived separately from a similar liquid which was contaminated with silica-rich material, or from a thoeliitic parent related to the Centre 1 activity in the complex.  Ocelli result from immiscibility of a carbonate or silicate liquid coexisting with the magma from which the dykes formed.  Studies of material from dyke margins and corresponding centres suggest that minor flow differentiation has occurred.  Following their emplacement, all dykes were subjected to varying degrees of alteration resulting from late-stage magmatic and post-magmatic processes.

A copy of the thesis can be downloaded here