Argon-40/argon-39 dating of the Kapuskasing diabase dyke swarm and lamprophyre dykes in the Kapuskasing Uplift of the Archaean Superior Province, Canada

The Kapuskasing dykes are a swarm of Proterozoic, NE-trending diabase dykes that are largely restricted to the southern portion of the Kapuskasing Uplift (KU), in the southern Superior Province of the Canadian Shield. To determine the age of these dykes, {dollar}sp{lcub}40{rcub}{dollar}Ar/{dollar}sp{lcub}39{rcub}{dollar}Ar step-heating analyses have been conducted on biotite separates from country-rock immediately adjacent to four Kapuskasing dykes. In addition, at one of these dykes, a detailed step-heating study was conducted on nine pairs of biotite and hornblende separates sampled from country-rock, at distances form the dyke from 0 to 99 meters away. In this detailed study, each biotite from within 9m of the dyke contact yielded an integrated date of less than 2100 Ma, and an age spectrum with significant plateau-segments, whereas those from farther away yielded older dates, reaching ca. 2400 Ma. Hornblende separates farthest away from the dyke contact had age-spectra dominated by ca. 2520 Ma plateaus, while hornblendes closer to the dyke had age-spectra which were more disturbed, and were consistent with partial argon loss related to dyke emplacement. Biotites adjacent to three other Kapuskasing dykes yielded integrated dates of less that 2100 Ma. The best interpretation of the data is that the dykes were intruded at 2038 {dollar}pm{dollar} 6 Ma.; Biotite from country-rock adjacent to the contact of a unique, vertically dipping, NE-trending diabase dyke near the ILFZ was dated at 1500 {dollar}pm{dollar} 7 Ma, and was taken as the time of dyke emplacement.; Step-heating analyses of biotite separates from three different N-E trending lamprophyre dykes in the southern KU were also conducted. Each biotite had an age-spectrum indicating minor excess argon contamination, but all had plateau-segments with dates in excellent agreement at ca. 1150 Ma. The best estimate for the age of these lamprophyre dykes is given by the average date of the two plateau-segments from the most detailed age-spectrum, at 1144 {dollar}pm{dollar} 7 Ma. (Abstract shortened by UMI.)...