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A super 40 Ar/ super 39 Ar and U/ Pb isotopic study of the Ilimaussaq Complex, South Greenland; implications for the super 40 K decay constant and for the duration of magmatic activity in a peralkaline complex

, : A super 40 Ar/ super 39 Ar and U/ Pb isotopic study of the Ilimaussaq Complex, South Greenland; implications for the super 40 K decay constant and for the duration of magmatic activity in a peralkaline complex. Chemical Geology 227(3-4): 258-273

Magmatism in the Gardar Province, South Greenland, is related to two main rifting events at 1280 Ma and 1180 to 1140 Ma. Little is known about the duration of the magmatic activity in a specific complex. The Ilimaussaq intrusion belongs to the second period of rifting and comprises an extraordinary diversity of granitic and syenitic rock types, which intruded and fractionated in three successive magmatic events. As the intrusion contains some of the most evolved, incompatible element-rich rocks on Earth, it was chosen for a detailed geochronological study to quantify the duration of melt production, intrusion, fractionation and cooling. Amphiboles, which are abundant in all rock types, including pegmatites and late magmatic veins, were dated using the (super 40) Ar/ (super 39) Ar technique. Since the solidus temperature of the most evolved melts is below the closure temperature of amphibole, and no later heating event occurred, the (super 40) Ar/ (super 39) Ar ages reflect the magmatic crystallisation and can be used to determine the duration of igneous differentiation. The (super 40) Ar/ (super 39) Ar plateau ages range between 1142.6+ or -2.2 Ma and 1152.3+ or -3.7 Ma using the Steiger and Jager [Steiger, R. H. and Jager, E., (1977). Subcommision on Geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359-362.] (super 40) K decay constant. These ages are younger than an U-Pb age of 1160+ or -5 Ma on baddeleyite from the first magma batch. Our results indicate that the (super 40) K decay constant of Steiger and Jager may be too high; a lambda similar to that proposed by Kwon et al. [Kwon, J., Min, K., Bickel, P. and Renne, P. R., (2002). Statistical methods for jointly estimating decay constant of (super 40) K and age of a dating standard. Math. Geol. 34, 457-474.] is required to make the (super 40) Ar/ (super 39) Ar match the U-Pb age.


DOI: 10.1016/j.chemgeo.2005.10.004

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