A lithogeochemical study of the St. Lawrence granite, Newfoundland

The St. Lawrence map area is underlain mainly by a sequence of Proterozoic flows, pyroclastics and volcanogenic sediments which are unconformably overlain by a Cambrian sequence of sandstone, siltstone, greywacke, shale and locally nodular limestone and conglomerate. These formations were subsequently intruded (in chronological order) by gabbro, the St. Lawrence fluorite-bearing alaskite granite, and later acidic and basic dykes. The alaskite granite has been dated isotopically by rubidium-strontium at 330 ± 10 million years. -- The St. Lawrence granite is a pink to red granitic rock characterised by a scarcity of ferromagnesian minerals. Petrographic studies show it to be composed essentially of quartz, orthoclase and albite with minor amounts of riebeckite, aegirine, biotite, fluorite, magnetite and hematite. Perthitic and granophyric textures are common. -- Two hundred and fifty rock samples were collected in the map area and 140 granitic rocks were analysed for major elements and 220 for trace elements, showing that it is distinctly high in SiO₂ (average 76.7 per cent), Zr (average 516 ppm), Rb (average 295 ppm) and F (average 1308 ppm), and low in A1₂O₃ (average 10.9 per cent), Ca0 (average 0.36 per cent), Sr (less than 10 ppm) and Ba (average 70 ppm). Mineralogically, it contains relatively sodic plagioclase (average An₁₀) and silica- undersaturated mafic minerals such as riebeckite and aegirine and this is reflected in normative acmite. These chemical and mineralogical features classify the St. Lawrence granite as peralkaline. -- Normative Q-Ab-Or plot of the analyses fall in a field centred on the ternary minimum at 0.5 kb, suggesting a temperature of crystallisation of the granite at about 780°C. It has been considered to be a relatively dry hot melt because of its low PH₂0 minimum composition (i.e. a hypersolvus granite) by the presence of tuffisites and the lack of pegmatites, all also suggesting a shallow depth of intrusion. The magma was probably formed by limited fractional melting at the base of the continental crust or by fractionation of a rather uniform and relatively calcalkaline acid parent. -- The St. Lawrence granite is mineralogically and chemically very similar to the riebeckite-aegirine-bearing granites of Kudaru and Liruei of Nigeria. Although the Traytown granite of Newfoundland is riebeckite-bearing, there is a distinct difference in its trace elements compared to the St. Lawrence granite