The Skaergaard Layered Series: I. Structure and Average Compositions

• Published in: Journal of petrology 1989-04, Vol.30 (2), p.363-397
• Main Author: McBIRNEY, A. R.
• Format: Article
• Language: English
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/30.2.363

Re-examination of the Skaergaard Layered Series in the light of more extensive field work and sampling shows that the lithologic zones vary laterally as well as vertically, in both their bulk chemical composition and their mineralogical assemblages. The margins of the zones differ from both the central part of the Layered Series and Marginal Border Series in being richer in FeO*, TiO2, K2, P2O2, and most excluded elements. Mafic minerals tend to be more abundant and more iron-rich, plagioclase is more albitic and more strongly zoned, and apatite and biotite are more abundant near the margins. When the average compositions of successive zones are compared, the abundances of most excluded components are seen to decline upward as far as Middle Zone then reverse their trends and increase through Upper Zone. P2O5 and K2O are negatively correlated in Upper Zones B and C, owing, perhaps, to separation of immiscible felsic liquids from the iron-rich magma. No evidence has been found for introduction of a new batch of less differentiated magma. Layered rocks have an average composition that is more mafic than that of homogenous rocks at the same level. Blocks that fell from the roof have the opposite relation; they are greatly enriched in felsic components compared to the original compositions of the Upper Border Series from which they came. Although some of the compositional variations may be consistent with differing degrees of fractionation of trapped liquids, no consistent relation has been found between the degree of fractionation and rates of crystal accumulation or cooling at the walls. Contamination with the metamorphic wall rocks, either by assimilation or by hydrothermal fluids, seems to have had only local effects and cannot account for the large-scale variations. At least some of the compositional differences must have resulted from late-stage processes that redistributed certain components after the intrusion reached advanced stages of solidification.