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.