Rit (Vísindafélag Íslendinga) - 01.06.1967, Síða 49
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ranging from 500 to 1800, perhaps a further indication of
the continental affinities of Icelandic magmas.
Granitic xenoliths have been discovered in Icelandic ba-
salt lavas in the last few years, notably in Surtsey (Sigurds-
son, 1966) and in cinder cones of western Iceland. By con-
trast, ultrabasic (eclogitic or peridotitic) xenoliths have not
been found although these rocks are often cited as the source
material of basalt magmas. A preliminary petrological study
of the granitic xenoliths has not so far revealed characters,
such as low temperature feldspar optics, which are normally
found in the large granite masses of continents, but it must
be remembered that the xenoliths have suffered many vicis-
situdes during transport in the basalt magma. Walker (1965)
claims on unspecified petrological grounds, that there is rea-
son for suggesting the presence of sial under Iceland, which
by remelting has contributed to the acid rocks seen at the
surface. It is hard to see how sial under Iceland can be recon-
ciled with Walker’s hypothesis on the formation of the Ice-
landic basalt pile hy crustal drift, as put forward in the
same paper.
By way of recapitulation we may state that the presence
of sial would account for the gravity anomaly, the great vo-
lume of acid magma erupted at the surface and the granitic
xenoliths, hut evidence from petrological and isotopic investi-
gations indicates fractionation from a basaltic parent magma
as a more likely source of the acid rocks in Iceland.
REFERENCES
Remmelen, R. W. van and M. G. Rutten, 1955: Tablemountains of north-
ern Iceland. E. J. Brill, Leiden, 217 pp.
Brown, G. M., 1963: Melting relations of Tertiary granitic rocks in Skye
and Rhum. Miner. Mag. 33, 533-562.
Bödvarsson, G. and G. P. L. Walker, 1964: Crustal drift in Iceland. Geo-
phys. J. Roy. astr. Soc. 8, 285-300.