The Skaergaard Layered Series, Part VII: Sr and Nd Isotopes

Abstract

The initial isotopic ratios of strontium and neodymium in the Skaergaard Layered Series vary both vertically and laterally, on every scale from the intrusion as a whole down to coexisting minerals in a single rock. The magma that filled the Skaergaard chamber was contaminated to various degrees with the metamorphic rocks through which it rose and was never completely homogenized after being intruded. The contamination was most pronounced in contact zones and aureoles around rare xenoliths. The greater concentrations of lithophile trace elements in the Upper Border Series was previously attributed to assimilation of buoyant fragments of gneiss that collected under the roof, but most of the rocks of the Upper Border Series are isotopically indistinguishable from those of the Layered Series. It is doubtful, therefore, that this part of the intrusion assimilated much more of the metamorphic basement than did the rest of the magma. Similarly, the marked increase in the concentrations of excluded elements in the upper part of the Layered Series is not matched by a change in the isotopic character of the rocks and cannot be attributed to a later influx of new magma. Analyses of minerals separated from rocks with exceptionally mafic or felsic modal compositions revealed marked inhomogeneities in the isotopic compositions of their constituent minerals. For example, coexisting plagioclase and pyroxene from closely associated anorthosites and pyroxenites have very different initial isotopic ratios of both strontium and neodymium. The same is true of mafic and felsic layers in modally graded gabbros. These differences are unrelated to the low-temperature alteration shown by oxygen isotopes. They must have been introduced when the original gabbro was largely crystallized and underwent local metasomatic replacement by nearly mono-mineralic mafic and felsic assemblages.