Geochronology of the Big Spruce Lake alkaline intrusion
A zircon age of has been determined for the silica-saturated syenites of the Big Spruce Lake alkaline complex. Pb–Pb whole-rock ages of and 2155 ± 16 Ma were obtained for the silica-undersaturated rocks of the complex and for the complex as a whole, respectively. The Sm–Nd whole-rock data yield an age of 2183 ± 75 Ma for the complex as a whole. By weighting the ages by the inverse square of the standard deviations, an average age for the complex of 2174 ± 20 Ma is determined. However, it is felt the zircon age is the most reliable for the time of crystallization of the complex. Minor contamination by country rock (Sm–Nd model ages of 2467–2595 Ma) and metasomatism contribute to scattered Sm–Nd data points and large errors in the ages obtained for the five individual units. The Pb–Pb data enable a limiting estimate of about 50 Ma for the maximum time span for the intrusion of centres 1–4.