Petrology of the Upper Border Series of the Skaergaard Intrusion
Abstract
The Upper Border Series crystallized downwards from the roof of the Skaergaard magma chamber. It met with the Layered Series that crystallized upwards from the floor in the Sandwich Horizon that contains the last and most evolved rocks of the intrusion. Previous investigations of the Upper Border Series (Naslund, 1984) have shown that the compositional trends of plagioclase, olivine and pyroxene largely mirror those of the Layered Series. At the same time it was argued that the crystallization sequence in Upper Border Series differed from the Layered Series in that apatite precipitated before magnetite that, in turn, appeared before Ca-rich pyroxene. From the existing data the magma from which the Upper Border Series crystallized was inferred to be enriched in SiO2, K2O, P2O5 and H2O relative to the magma in the lower parts of the intrusion. This has lead to the conception that the Upper Border Series crystallized from a chemically different magma. Here we present new petrography, mineralogy and bulk compositions for samples collected in three profiles through the Upper Border Series (Kilen, Hammerpas and Brødretoppen transects). Although euhedral apatite is present throughout most of the Upper Border Series, we interpret a marked increase in modal apatite late in the crystallization sequence as marking its first appearance on the liquidus at the crystallization front. The plagioclase An% at this level in the Upper Border Series is ∼40 and is identical with plagioclase An% at the level of apatite-in in the Layered Series. Similarly, we find that the plagioclase An% at the onset of FeTi-oxide and sulphide precipitation in the Upper Border Series (52 and 47, respectively) and Layered Series are alike. Finally, we interpret abundant augite in Upper Border Series rocks before magnetite-in as a cumulus phase. We therefore conclude that the crystallization sequences of the two series are identical. The new bulk rock data reveal that the Upper Border Series and the Layered Series are similar in the early stages of differentiation until after FeTi-oxides appear as primocrysts. In the more evolved rocks the Upper Border Series is gradually enriched in SiO2, K2O and incompatible trace elements relative to the Layered Series as shown by Naslund (1984). This is expressed by abundant interstitial granophyre pockets between the cumulus crystals in the Upper Border Series. However, we find that it is possible to explain the Upper Border Series as a mixture of cumulus minerals and reasonable liquid estimates. Thus the Upper Border Series appears to trap a higher fraction of residual liquid than is the case in the Layered Series. We conclude that the Upper Border Series crystallized from the same liquid as the Layered Series and mainly differs in a high amount of trapped liquid. This implies that the Skaergaard chamber had only one convecting magma body. References: Naslund, H.R.; Petrology of the Upper Border Series of the Skaergaard Intrusion, Journal of Petrology, Vol. 25, Part 1, pp 185-212, 1984
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V21A1958S
- Keywords:
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- 1036 GEOCHEMISTRY / Magma chamber processes