Complex Sequential Pyroxene Growth in Tholeiitic Hypabyssal Rocks from Southern West Greenland

R. P. Hall, D. J. Hughes and C. R. L. Friend
Department of Geology, Portsmouth Polytechnic, Burnaby Road, Portsmouth, PO1 3QL
Department of Geology and Physical Sciences, Oxford Polytechnic, Headington, Oxford OX3 0BP

Abstract: Chemically complex pyroxenes which occur in early Proterozoic tholeiitic dolerite dykes in southern West Greenland have been investigated using back-scattered electron (BSE) imagery, X-ray mapping and electron microprobe analysis. A wide variety of compositions occur within individual pyroxene grains in these rocks. They can be explained by simultaneous nucleation of different pyroxenes, the evolution of domains around these nucleii as a response to differential chemical gradients and the sequential precipitation of different pyroxenes at progressively lower temperatures. As an example, the individual grains in one dyke sample contain domains of bronzite, hypersthene, magnesium pigeonite, augite, and subcalcic augite. Olivine in this sample varies in composition from Fo70 to Fo33, although individual grains are only weakly zoned. The wide variation in pyroxene and olivine compositions suggests ranges of crystallization temperatures from c.1250° to as low as 825°C. Such compositionally variable pyroxenes are possibly characteristic of hypabyssal tholeiitic rocks.

Keywords: pyroxenes • hypabyssal • back-scattered electron imagery • West Greenland

Mineralogical Magazine; September 1986 v. 50; no. 357; p. 491-502; DOI: 10.1180/minmag.1986.050.357.12
© 1986, The Mineralogical Society
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