The earliest history of the skaergaard magma chamber: A textural and geochemical study of the Cambridge drill core

Abstract

The Cambridge Drill Core provides a continuous sample of the lower part of the floor cumulates of the Skaergaard Intrusion, including ~150M of stratigraphy from the unexposed Hidden Zone. Bulkrock geochemistry together with olivine mineral compositions and augite-plagioclase-plagioclase dihedral angles from the drill core are interpreted as a record of the early history of the Skaergaard magma chamber. A detailed geochemical and microstructural study of the mode and morphology of augite reveals no unambiguous markers that can be used to pinpoint the first appearance of cumulus augite in the stratigraphy sampled by the drill core. The early history of the Skaergaard magma chamber involved the arrival of multiple small batches of magma, each with a variable load of olivine and plagioclase phenocrysts. The region of the core between -120 and -108.8m records at least three separate, but closely spaced, influxes of new magma, with more magma added at -85 and -65 m. The last influx of magma is recorded by the cumulates just below the lowest exposed horizons of the Layered Series, involving a large volume of magma that inflated the chamber to its final size. The Skaergaard magma chamber therefore formed by the progressive inflation of what was likely to have originated as a sill intruded at the discontinuity between the Precambrian gneisses and the overlying plateau lavas. The 'Skaergaard parental magma' should therefore be viewed as the integrated and mixed composition of the various magma influxes that filled the chamber.