Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 64
60 PÁLLIMSLAND
titanium oxides with scattered phenocrysts of feldspar and biotite.
The unmelted and unaltered state of the trachyte chips is rather surprising
and indicates that these were incorporated into the hot ankaramitic basalt
magma a very short time before its solidification, so that practically no
reaction time was given. If remelting and mixing of the trachyte had taken
place, this rock would not have been distinguishable from other low-Mg
basalts.
This unique rock in the Jan Mayen rock collection shows that the produc-
tion of low-Mg basalts by mixing of trachyte and high-Mg basalt magma
must be considered as a possible factor in the evolution of the Jan Mayen
rock suite.
The alternate case, where basaltic pieces or blocks are mixed with
trachyte, is apparently much more common in the Jan Mayen rocks. In
several trachyte occurrences, basaltic xenoliths are found in considerable
amounts. The contact of basalt and trachyte, seen in thin-section, shows no
signs of remelting, as is to be expected.
X. Hyaloclastites
These rocks belong to the rock suite but have not been generally analysed,
neither for whole rock compositions nor mineral or glass compositions.
Occasional solid cosanguineous bombs which occur in the tuífs of the
hyaloclastites have been analysed. These analyses are included with those of
the proper rock group. It is obvious from the chemical composition of these
bombs and from microscopic observations that the magmas of the hyaloc-
lastites do not differ from those of the lavas, as is to be expected from the
field occurrence of these rocks.
The diíferent lithological characters are the result of changed external
conditions at the eruption site or time. The hyaloclastites are basic with one
exception, the trachytic pumice breccia of Borga (Imsland, 1978a). The
mineralogy indicates that the rest is mostly of the pl-ol-cpx porphyritic type
basalts, but the other basic rock groups may also be represented in the
hyaloclastites.
The mineralogical descriptions given under individual groups above hold
for the hyaloclastites as well. The only difference worth mentioning is that in
the hyaloclastites the crystals may be fragmented. The degree of fragmenta-
tion of crystals and glass is diíferent from sample to sample as well as from
one place to another in the same hyaloclastite occurrence.
The glass of the hyaloclastites is mostly translucent, faintly brownish in
colour. Occasionally dark brown to black opaque glass particles occur,
intermingled with the others. The glass particles range from minute shards
to fragments more than 1 cm in diameter and are always more or less
vesicular.
Palagonitization has taken place to some degree, but mostly the glass is