petrology of syenites from centre iii of the coldwell alkaline complex, northwest ontario
abstract
the coldwell alkaline complex, situated on the north shore of lake
superior, is of neohelikian age. it is intruded into archean supracrustal
rocks of the superior province, near marathon, ontario. the complex has a
diameter of 25 km and consists of three major magmatic centres. each
centre represents the focus of a cauldron subsidence event. from earliest
to latest they are: centre i-saturated alkaline rocks with peralkaline
oversaturated residua; centre il-miaskitic undersaturated alkaline rocks;
and centre ill-alkaline rocks with oversaturated residua. this thesis
documents the rocks of centre iii.
centre iii magmatism of the coldwell alkaline complex is represented
by alkali feldspar syenites and alkali feldspar quartz syenites. in order
of intrusion from earliest to youngest these are (1) synneuritic
magnesio-hornblende syenite, (2) perthitic ferro-edenite syenite, (3)
contaminated ferro-edenite syenite and (4) quartz syenite. amphiboles are
the major mafic phase in all syenite types. their evolutionary
compositional trend ranges from magnesian hastingsite in the magnesiohornblende
syenite to riebeckite in the quartz syenite. pyroxenes are
ti-al rich in the earliest syenites, but evolve to na-fe rich members in
the youngest. the contaminated syenites result from the assimilation of
xenoliths of oligoclase basalts which are interpreted to represent
contemporaneous lavas. varying degrees of contamination lead to an
apparent plethora of syenite types.
the centre iii syenites are miaskitic and metaluminous with normative
quartz content increasing from the earliest to the latest varieties. all
of the syenites are enriched in u, th, ree, and zr, reflecting the
presence of zircon, chevkinite, ree-carbonates, nb-rutile and aeschylite.
centre iii syenites have petrological affinities with a-type granites and
are interpreted to have been formed by partial melting of lower crustal
material.
collections
- retrospective theses [1604]