Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 104

100
PÁLL imsland
is probably the reason for the vesiculation of the glass. This idea of a partly
fused already existing trachyte was the conclusion reached by Kirk (1953)
regarding the origin of these xenoliths. Acid xenoliths ranging in texture
from flinty pyrometamorphic and slightly fused granite or granophyre to
light, highly vesicular (frothy or pumiceous) glass lumps, found in numer-
ous localities within the volcanic zones of Iceland, have been described by
several authors (see Sigurdsson, 1968). Those found in the Grímsnes lavas
are mostly of the higly vesicular type, similar in appearance to Jan 5 and
142. Jakobsson (1966) believed them to be remelted intrusive acid rocks.
The vesicular glassy xenoliths ofSurtsey are explained by Sigurdsson (1968)
as the result of cotectic or near cotectic fusion of granitic rocks.
Jan 149, rhyolitic in composition, is quite different from the other
xenoliths and the rocks of the suite. Petrographically it is composed of about
equal amounts of quartz grains, and clear glass. It is higher in Si, Y and Zr
than all the other rocks and extremely low in Sr. The glass is feldspar like in
composition (4, Table 14) but contains some 13 per cent co in the norm.
Small iron-titanium oxide crystals occur in the rock. Adjacent to these the
glass is brown and darkens towards the mineral grains and becomes
enriched in Ti and Fe (5 & 6, Table 14). This shows that these minerals
were melting when the liquid became quenched. The composition of the
glass indicates that other melted minerals were feldspars, but the source of
the excess A1 is unknown. The great amount of quartz in the rock shows that
this is not a partly remelted trachyte. The quartz and the texture indicate
that the original rock might have been a metamorphosed sedimentary rock.
While the glassy xenoliths occur in the basalts of the rock suite, the
syenitic coarse-grained xenoliths, showing no or very small signs of fusion,
occur in the trachytes. The differences in chemical composition between the
syenitic xenoliths, the trachytic glassy xenoliths and the trachytes of the rock
suite are insignificant in all analysed elements. The syenites differ from the
others in being coarse-grained; otherwise the minerals are the same as is
their chemistry. The syenites might be formed by crystallization of trachytic
magma. When crystallized they are occationally picked up by, or otherwise
come into contact with, the hotter basic magmas. Then these syenites (and
trachytes) become partly or completely remelted, while they escape
unmelted or nearly so through the cooler trachytic magmas, which may also
pick them up.
b. Glasses of the coarse-grained xenoliths
Some of the gabbros contain a thin film of opaque glass between mineral
grains and minerals and small vesicles. The glass in Jan 148 has been
analysed and shows extreme heterogeneity (Table 15). An analysed point
(2) close to a plagioclase crystal is high in A1203 (28.93 per cent) and CaO
(13.84 per cent) but low in MgO (0.6 per cent), FeO1 (1.7 per cent) and
Ti02 (0.55 per cent). A point close to a clinopyroxene crystal (6) is relatively