Petrographic and chemical study of a diabase dike in Gatineau Park, Quebec.

Swarms, of diabase dikes have been reported in all Provinces of the Precambrian Shield. One of these is the Buckingham swarm which extends into the region of the Gatineau Park. A fresh exposure of one of these dikes was sampled to study ids petrographic and chemical characteristics and any variations which might reflect the nature of its formation. The physical and chemical characteristics of the dike compare closely to those of Hawaiian tholeiitic basalts, and the sub-ophitic texture of this dike resembles that of typical diabase dikes. The dike appears to be a member of a swarm of diabase dikes of probable Ordovician age, and are located in the Grenville province of the Canadian Shield. Some variations in composition exist across the dike. (a) Mafic minerals decrease in quantity inward while plagioclase increases inward. (b) Grain size graphs plot in the shape of a bell curve, rather than parabolic. (c) The characteristic sub-ophitic texture is replaced by porphyritic texture in the contact samples. (d) An content of plagioclase decreases inward. (e) Silica content of bulk rocks decreases outward. (f) Rock compositions appear to have erratic variation in the zones near the contacts. The observed variation in texture and chemical composition within the study dike may be attributed to a combination of factors including; contamination, secondary alteration, and some form of differentiation