U–Pb zircon ages from the eastern Savant Lake – Crow Lake metavolcanic–metasedimentary belt, northwest Ontario
Five rocks from three areas in the eastern part of the Savant Lake - Crow Lake metavolcanic-metasedimentary belt have been dated by U-Pb analysis of zircons. The Sioux Lookout area is composed of a sequence of generally fault-bounded metavolcanic and metasedirnentary belts. The age of a felsic tuff in the Central volcanic belt is 2732.72;:; Ma. This is possibly the youngest sequence in the area. The Savant Lake volcanic belt comprises the Jutten volcanic unit at its base separated by an angular unconforrnity from the Handy Lake volcanic sequence. The age of a dacitic tuff near the base of the Handy Lake volcanic sequence is 2745.22 i:: Ma. The South Sturgeon Lake volcanic belt is composed of four volcanic cycles. The age of a felsic tuff from the top of the uppermost cycle is 2717.92::: Ma. The felsic Beidelman Bay pluton and the mafic Pike Lake pluton, which both intruded the lower art of the sequence, give ages of 2733.82;:; and 2732.721.; ~a, respectively. This establishes a minimum time span of 15.921): Ma for deposition of the sequence. This interval, along with the 27 Ma span between the ages of the Handy Lake sample and the volcanic sample from the South Sturgeon Lake area, suggests that the sequences were laid down by a series of discrete volcanic episodes. For samples subjected to multistage lead loss, the reliability of ages has been greatly enhanced by air abrasion of zircons. Several cases of suspected biasing of analyses by the air abrasion process have been encountered. It appears that this biasing problem can be overcome by selection and abrasion of low uranium grains that are free of imperfections.