NEODYMIUM ISOTOPIC ANALYSIS OF CRETACEOUS ROCKS FROM SVALBARD, NORWAY

Cretaceous magmatism and volcanism forms a large footprint in the High Arctic area, and has been associated with a Large Igneous Province and opening of the Amerasian and possibly the Eurasian oceanic basins. Nd-Sm isotopic analysis was conducted on three diabases, 6 Cretaceous sandstones, and one Cretaceous iron ooid horizon from Svalbard, Norway, which was on the margin of this igneous activity. Initial ratios for the diabases are similar to those published from Franz Josef Land volcanics, which were also part of this igneous event. Calculated model ages are ca. 490 Ma. Isotopically, the Cretaceous sandstones form a distinct group with lower (older) values than the diabases, and than published values for underlying Triassic strata. The iron ooid sample plots distinctly apart from both groups, with higher Sm/Nd and an intermediate initial isotopic signature. Initial ratios in the sandstone tend to increase with stratigraphic ascent.

The isotopic data for the diabases suggest either contamination of Cretaceous mantle melts or derivation from a primitive mantle. The lack of a Cretaceous isotopic signature in the sandstones, despite containing 10-20% volcanic detritus, can be attributed to the fact that most of this debris is in the form of plagioclase, which tends to carry very low amounts of Nd. Abundant basement debris in these sublitharenites to lithic arenites dominates the isotopic signature. The change in initial ratios from the Triassic to the Cretaceous sandstones is attributed to changes in paleogeography associated with LIP development, so that older Mesoproterozoic basement rocks in NE Svalbard served as a source. This is consistent with Cretaceous paleocurrent data and environmental reconstructions. The iron ooid sample result is enigmatic, but may suggest some connection with Cretaceous magmatism.