The U-Th-Pb systematics of zircons from the type Nûk gneisses, Godthåbsfjord, West Greenland

Details The U-Th-Pb systematics of zircons from the type Nûk gneisses, Godthåbsfjord, West Greenland The U-Th-Pb systematics of zircons from the type Nûk gneisses, Godthåbsfjord, West Greenland

Jul 1981

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981gecoa..45.1099b&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 45, Issue 7, pp.1099-1109

Physics

6

Scientific paper

U-Pb analyses were made on zircons separated from different lithologies within the Nûk gneisses taken from Godthåb (Nuuk) townsite and from the south-west shore of Bjørneøen. Size fractions of the zircons from each rock yield linear discordia which give apparent crystallisation ages for the parents of the Nûk gneisses of 2890-3065 Myr. This spread of ages is not a result of multiple intrusion, but more likely results from different degrees of `diffusion' and metamorphic episodic lead loss by the zircons. Since U-Pb and Th-Pb discordance are similar, loss of radiogenic lead (and not U or Th-loss/gain) by radiation-damage diffusion and through metamorphism is the main factor causing discordance of the U-Pb dates. The results show that the discordia line given by the analyses of fractions of a single zircon population may not give a true crystallisation age when the rock has been strongly metamorphosed. This interpretation is supported by Rb-Sr whole rock analyses of the same rocks, in which the Nûk townsite rocks scatter about a reference isochron of 2840 Myr, while the Bjørneøen samples give an isochron (MSWD = 0.55) of 3076 ± 27 Myr and the initial ratio at 0.70205 ± 0.00009. The zircon results do not rule out the possibility that more than one metamorphic episode affected the original Nûk gneiss parents prior to 2520 Myr (Qôrqut granite intrusion). A date of 2660 Myr was obtained from an aplite intruded during a period of plutonic activity that included the formation of a linear belt of intense ductile deformation that extends through south-western Bjørneøen and Godthåb town.

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