Mineral isotopic age relationships in the polymetamorphic Amîtsoq gneisses, Godthaab district, West Greenland
U-Th-Pb, Pb-Pb, Rb-Sr and K-Ar radiometric relationships in the minerals from six selected Amîtsoq gneiss samples reveal a complicated history of variable mineral response to polymetamorphism. K-Ar dates on biotite range from 2170 to 3220 m.y. (excess argon present), on hornblende from 2340 to 2510, and on a single muscovite at 1670 m.y. Rb-Sr whole rock results give an apparent isochron of at least 4065 m.y., but this result is likely fortuitous from a small sample selection since Pb-Pb whole rock analyses give ~ 3600 m.y. and the zircons in these rocks yield a concordia-discordia intersection at 3600 m.y. Rb-Sr mineral analyses generally give a confusing and variable pattern of isotopic relationships; but hornblende, K-feldspar, apatite, allanite and sphene appear to have last responded to metamorphism at 2200-2600 m.y. Rb-Sr in biotite, epidote and, in part, plagioclase have been affected by an event at ~ 1550 m.y. U-Th-Pb data from sphene, apatite and allanite give almost concordant dates at 2500-2600 m.y. soul 207Pb /204Pb vs soul 206Pb /204Pb plots yield two separate lines for apatite (slope age 2435 m.y.) and for sphene + allanite (slope age 2530 m.y.), indicating apatite to have a different (less-radiogenic) 'initial' Pb than that for sphene and allanite. A similar pattern is found for the soul 208Pb /204Pb vs soul 207Pb /204Pb plot for sphene and apatite. The Pb-isotopic composition of the feldspars is very homogeneous and the least-radiogenic of all components, pointing towards a homogeneous parent material for the now lithologically diverse Amîtsoq gneisses. Using a0 = 9.307, b0 = 10.294, C0 = 29.476, t0 = 4.56 b.y., ω = 6.9 and soul 232Th /204Pb = 27.1 ; the feldspars give a model Pb age of 3500-3600 m.y. by either U-derived or Th-derived Pb. The segregation of the present Amîtsoq gneisses from the homogeneous parent material was apparently accompanied by a U and Th loss with preservation or enrichment of Pb at ~ 3600 m.y. ago. No consistent treatment of the present U-Th-Pb data will produce viable data indicating an age > 3600 m.y. for the parent materials of the Amîtsoq gneiss. Petrographie observations generally concur with radiometric results and permit the postulation of the reaction: Hbl + K-feld→ biotite + epidote + sodic plag, to account for some of the effects of the latest metamorphism. The total internal radiometric evidence indicates three major metamorphic events affected the Amîtsoq gneisses close to 3600, 2500 and 1550 m.y.