Compositional Variation in the Pyroxenes and Amphiboles of the Kangerdlugssuaq Intrusion, East Greenland: Further Evidence for the Crustal Contamination of Syenite Magma

C. K. Brooks and R. C. O. Gill*
Institut for Petrologi, University of Copenhagen, Østervoldgade 10, DK-1350, Copenhagen
Department of Geology and Mineralogy, University of Oxford, Parks Road, Oxford OX3 7AW
*Present address: Department of Geology, Chelsea College, University of London, 552 Kings Road, London SW10 0UA.

Abstract: Pyroxenes and amphiboles from the under-saturated to over-saturated syenites of the Kangerdlugssuaq intrusion have been examined to see what light they throw on the two contrasting petrogenetic models for the intrusion. Pyroxene crystals are strongly zoned outwards from augitic to more acmitic compositions, with the most calcic cores present in the foyaite, contrary to the expected pattern if the foyaite is the most evolved rock type as previously postulated. Amphiboles, which are absent in the foyaites, show an exceptionally wide compositional field varying from actinolite through richterite and katophorite to arfvedsonite. Many are manganoan and potassian varieties. However, there is no consistent variation throughout the intrusion as previous work has suggested. These results are not favourable to the idea of a crystal fractionation model for the intrusion and we suggest that the foyaite is closest to the original magma which has hybridized with the enclosing gneisses and basalts to produce the over-saturated rocks. Such a model is consistent with the existing isotopic data.

Mineralogical Magazine; 1982 v. 45; no. 337; p. 1-9; DOI: 10.1180/minmag.1982.045.337.01
© 1982, The Mineralogical Society
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