The geochemistry of part of the Fiskenaesset Anorthosite complex, West Greenland

• Main Author: Hutt, David K.
• Format: Text
• Language: unknown
• Published: Imperial College London 1974
• Subjects: Greenland
• Online Access: https://dx.doi.org/10.25560/9201; http://spiral.imperial.ac.uk/handle/10044/1/9201

The region of Qaqat Akulerit lies 60 km. to the east of Fiskenaesset, West Greenland. The region contains several outcrops of the Fiskenaesset anorthosite complex, an association of Archaean metamorphosed and deformed stratiform igneous rocks. Parts of this complex retain their original stratigraphy, and many original chemical features have been preserved,. Detailed field mapping of the region shows that two distinct types of anorthosite can be recognised* The first is very similar to the horizons of the anorthosite complex at Fiskenaesset, and is believed to be a direct continuation of those horizons. The second type differs structurally and lithologically from the first type; it forms linear horizons which have a wide range in lithology, whereas the first type occurs as massive bodies which have restricted lithological variations. 130 whole rock and 75 mineral analyses demonstrate that there are marked geochemical differences between the two types of anorthosite, and show that primary differentiation trends are preserved despite the fact that many of the rocks have been strongly deformed and migmatised. The first anorthosite type has a differentiation trend almost identical to that of the anorthosite horizons at Fiskenaesset, while the second type has an unusual trend characterised by a slight iron depletion upwards. This contrasts strongly with the differentiation of well-known stratiform intrusions which normally show fairly strong iron enrichment upwards. Detailed examination of the whole-rock and mineral chemistry shows that within the first type fractionation was controlled by plagioclase accompanied by clinopyroxene and amphibole. Crystallisation processes were similar in the leucocratic parts of the second type, but within a rhythmically layered mafic unit olivine, orthopyroxene and magnetite were the first phases to precipitate, A high water pressure or P are envisaged for the formation of 2 both types of anorthosite, with the exception of the rhythmically layered unit. This is thought to have crystallised under locally anhydrous conditions, which may have become hydrated soon after crystallisation. The chemical and mineralogical differences between the two types are ascribed to differences in original bulk chemical composition. Both types are believed to have been derived from different parts of a common ancestral magma, and both were intruded under essentially similar conditions.