Physics
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agusm.v41a..03h&link_type=abstract
American Geophysical Union, Spring Meeting 2004, abstract #V41A-03
Physics
1035 Geochronology, 3640 Igneous Petrology, 9604 Cenozoic
Scientific paper
A cooling curve for the Skaergaard intrusion was first established by Norton & Taylor [1] using oxygen isotopes and transport theory. Subsequently, Hirschmann et al. [2] located a cooling curve in absolute time using 40Ar/39Ar ages on biotite and hornblende. Here, we present new, high-precision U-Pb results for zircon recovered from the Sandwich Horizon on Basistoppen. Our (concordant) age of 55.59±0.13 Ma (2σ ) is fully consistent with these earlier results, thereby confirming the validity of both the cooling model, temperatures deduced from mineral equilibria, and the standardization used in the Ar-Ar system. It is, however, at variance with a recent WR-apatite Lu-Hf age for the intrusion [3]. Furthermore, knowledge of the thickness of lava overburden at the time of intrusion also constrains the pressure in the cooling intrusion, and this is in turn consistent with existing estimates of 60±15 MPa. Andreasen et al. [4] recently demonstrated that lavas with compositions corresponding closely to deduced Skaergaard magmas are found relatively high up in the adjacent lava plateau. The new zircon age thereby allows us to precisely date this level; a result in general agreement with the 40Ar/39Ar stratigraphy of Storey et al. [5]. Moreover, the Skaergaard age reported here represents the first precise and accurate zircon age for Tertiary (Palaeogene) magmatism in Greenland, and serves as a useful reference point for assessing Ar-Ar ages determined on numerous lavas, sills, dykes and gabbros elsewhere in the North Atlantic Igneous Province. The 55.59 Ma U-Pb age reaffirms earlier conclusions that the Skaergaard intrusion was emplaced contemporaneously with continental flood basalts erupted during late Paleocene-early Eocene time, in the middle stages of Palaeogene NAIP magmatism. The timing was synchronous with incipient rifting of Greenland and northern Europe as reflected in the formation of new ocean crust (Chron 24r), but distinctly postdates basic central intrusive complexes (e.g. Rum, Cuillins, Mull) of the Hebridean province, Scotland [6]. [1] Norton, D., & Taylor, H.P. Jr. (1979) J. Petrol, 20, 421 [2] Hirschmann, M.M. et al. (1997) Earth Planet. Sci. Lett. 146, 645 [3] Barfod, G.H. et al. (2003) Chem. Geol. 200, 241 [4] Andreasen, R., Peate, D.W. & Brooks, C.K. (in press) J. Petrol. [5] Storey, M. et al. (1996) Eos, Trans. AGU, 77 (46), F839 [6] Hamilton, M.A. et al. (1998) Nature, 394, 260.
Brooks Kent C.
Hamilton Melinda A.
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