The Chilled Marginal Gabbro and Other Contact Rocks of the Skaergaard Intrusion

Abstract

Rocks in the outer selvage of the Skaergaard intrusion have a range of textures and compositions, and among these are materials representing quenched Skaergaard magma. Pristine chilled marginal gabbro (CMG), however, is not ubiquitous at the intrusive contact, because many of the “contact” rocks have been hydrothermally or metasomatically altered, contaminated with gneiss or olivine xenocrysts, while others contain accumulated minerals. Material representing quenched magma appears to be restricted to contact rocks that are texturally and mineralogically similar to diabase, and free of accumulated minerals. Where it exists, the CMG is found within one to three meters of the exposed intrusive contact except at the roof of the intrusion where its thickness is greater. CMG was distinguished from the diverse group of contact rocks by petrographic and geochemical screening of over 80 specimens. Samples of CMG from the eastern and western margins and from the roof of the intrusion have relatively uniform composition similar to that of ferrobasalt, and are noticeably richer in iron (mg-number=0·51-0·54), TiO2 K2O, and P2O5 than other unmodified contact rocks. CMG's also have trace element compositions distinct from most other rocks in the outer Marginal Border Series (MBS). They have incompatible element contents up to 3–6 times greater than in LZa-type cumulates, negligible Eu anomalies, and Ni and Cr contents and Ni/Cr ratios that are among the lowest of rocks in the outer MBS. The results of melting experiments corroborate selection of this material as CMG. The composition of glasses obtained from partial melting experiments of LZa-type cumulates are essentially identical to those of the CMG. The 1-atm. liquidus phase relations for one of the CMG samples (KT-39) is largely consistent with the sequence and composition of cumulus minerals observed with distance inward through the MBS and upward through the Layered Series. Solidification of magma at the outer margin of the intrusion is interpreted to have involved locally efficient quench crystallization followed by initial primocryst growth in an undercooled transition zone a short distance inward that finally extended into regions of near equilibrium crystallization. The similarity in composition between samples of chilled marginal gabbro from the exposed roof and sides of the intrusion, and those of reconstituted trapped liquid from early cumulates in the outer MBS suggests that a single magma, similar in composition to ferrobasalt, was parental to the Skaergaard intrusion. This interpretation corroborates geophysical evidence of a significantly smaller mass for the intrusion than that estimated by Wager, and provides a basis for revision of models of its chemical evolution. Samples chosen by Wager as chilled marginal gabbro belong spatially, texturally, and compositionally to the group of LZa-type cumulates in the MBS, and should no longer be regarded as chilled marginal gabbro.