Complex sequential pyroxene growth in tholeiitic hypabyssal rocks from southern West Greenland

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 Fo70to Fo33, although individual grains are only weakly zoned. The wide variation in pyroxene and olivine compositions suggests ranges of crystallization temperatures fromc.1250° to as low as 825°C. Such compositionally variable pyroxenes are possibly characteristic of hypabyssal tholeiitic rocks.

Mineral chemistry of volcanic sequences from Hole 917A, southeast Greenland Margin

“…The pyroxene crystallization trends in the tholeiitic series are generally complex because they depend upon numerous factors such as the chemistry of the lavas, the cooling conditions, the crystallization of plagioclase, and further re-equilibration with the liquid (Hall et al, 1986). The cooling rate seems to be the main factor that influences the compositional trends of the pyroxenes in Hole 917A.…”

Complex Pyroxene Assemblages of Proterozoic Dolerites, S.E. Greenland

“…We have so far examined the mineralogy of four representative samples of the dolerite dykes, as part of a major petrological study. There is at present some doubt about how these dykes correlate with the basic dykes of W. Greenland where three principal swarms have been recognised: (i) BN norites, (ii) MD dolerites, and (iii) Kangamiut dolerites and their metamorphosed equivalents (Zeck and Kalsbeek, 1981;Hall and Hughes, 1987). Eastwards, across the north Atlantic, a similar wide range of (meta-)norites, dolerites and gabbros in N.W.…”

“…LeBas, 1962;Nisbet and Pearce, 1977). The compositional variations occur both between different pyroxene grains and as extreme irregular zoning of individual grains, providing clear evidence of the complex sequential precipitation of different pyroxene phases (Hall et al, 1986).…”

“…Preliminary results of the petrological study of the S.E. Greenland dykes show that the compositions of the norites are distinctive and very similar to those of the BN and Scourie norite dykes (Weaver and Tarney, 1981;Hall and Hughes, 1987;authors' unpublished data), while the mineralogy and chemistry of the dolerite dykes closely resemble those features of the MD dolerites.…”

“…Greenland have been described in a previous report (Hall et al, 1986). Although the successive generations of these dykes are progressively Ferich, their pyroxenes do not show a simple sequential Fe-enrichment trend, such as that defined by successively differentiated units of layered intrusions and typified by the Skaergaard pyroxene evolution trend (Wager and Brown, 1968;Nwe, 1976).…”