Mineral age patterns in ca. 3700 my old rocks from West Greenland

Abstract

K Ar, 40Ar 39Ar and Rb Sr dates are reported for minerals from the ca. 3700 my-old Amîtsoq and Isua gneisses of the Godthaabsfjord area of West Greenland. K Ar dates on biotites and hornblendes range from about 1900 to 3500 my, with hornblendes having a much narrower range (ca. 2250–2750 my) than biotites. One biotite from Isua gives an impossibly high K Ar date of 4940 my. 40Ar 39Ar mineral dates are in close agreement with conventional K Ar dates over the entire range of apparent age values. The presence of minor amounts of excess argon is observed in the hornblendes, but radiogenic and excess argon in the biotites are completely homogenised and cannot be differentiated. Rb-Sr measurements on biotites are closely concordant and show that all biotites were completely open to diffusion of radiogenic 87Sr at about 1600–1700 my. This is the first proof of a regional thermal event at this time in the Archaean of West Greenland, although similar dates are well known from the Proterozoic belts to the north and south. The evidence suggests that those K Ar biotite dates greater than about 2700–2800 my result from excess radiogenic argon incorporated during a thermal event of about this age or, more probably, during the 1600–1700 my Sr isotope homogenisation event. Scatter of mineral dates below about 2700 my could also be due, at least in part, to overprinting by the 1600–1700 my event. K Ar mineral dates and an Rb-Sr mineral isochron from a pegmatite associated with the last major rock-forming event in the Godthaabsfjord area, namely the Qoˆrqut granite, indicate an age of emplacement of 2580 ± 30 my.