Nd, Sr and Pb isotope geochemistry of the Lower Lavas, E Greenland Tertiary Igneous Province

Abstract

Summary New Sr, Nd and Pb isotopic and trace-element data are presented for the Lower Lavas and some Plateau Basalts. Ranges of isotopic compositions are large among the lower and middle Vandfaldsdalen Formation lavas ( 87 Sr/ 86 Sr=0.7033–0.7073; 143 Nd/ 144 Nd=0.5124–0.5130; 206 Pb/ 204 Pb=15.1–17.5; 207 Pb/ 204 Pb=14.7–15.6; 208 Pb/ 204 Pb=35.0–39.1) and show a decrease in range of values with stratigraphic height. The youngest flows among the Lower Lavas have compositions similar to the Plateau Basalts of the late Skrænterne and Igtertiva Formations as well as of Nansen Fjord and Wiedemann Fjord, all of which are near to Icelandic composition ( 87 Sr/ 86 Sr=0.7031–0.7036; 143 Nd/ 144 Nd=0.5129–0.5131; 206 Pb/ 204 Pb=17.1–18.4; 207 Pb/ 204 Pb=15.3–15.4; 208 Pb/ 204 Pb=37.6–38.0). The lavas constitute two groups, one of which (group 2) is restricted to the lower parts of the succession. The source for this group can be modelled by mixing between an enriched old lithospheric mantle and an ocean island basalt (OIB)-type mantle with a relatively small component of depleted asthenosphere. The other group (group 1) is the most common and can be modelled by mixing between a typical Icelandic source component and a second, less enriched, lithospheric mantle component. Contamination from continental crust was of minor overall importance. The geochemical evolution in basic E Greenland Tertiary lavas is believed to reflect the interaction between the asthenospheric mantle, 3 Ga old continental lithospheric mantle and the mantle jet which is presently positioned under Iceland. With time the most enriched lithospheric source quickly decayed while the less enriched lithospheric source was important during most of Vandfaldsdalen Formation times. The OIB-type source prevailed throughout lava eruption with the asthenospheric contribution larger in the Miki Formation and the Plateau Basalts.