Jökull - 01.12.1989, Page 88

sheet and the low grade of welding suggest a high
eruption column before it collapsed and the ignim-
brite sheet was formed (Cas and Wright, 1987). The
volume of the Herfell ignimbrite amounts to at least
5 km3.
RESULTS OF GEOCHEMICAL AND
MICROSCOPIC ANALYSES
GEOCHEMICALINVESTIGATIONS
In Iceland rocks of tholeiitic, transitional and
alkalic character have been found. Rocks of the
Tertiary lava pile, however, seem to belong solely to
the tholeiitic suite, no alkalic rocks have been
discovered so far. Dearnley (1954) described alkalic
rocks like crinanite, an olivine and analcite-bearing
ultrabasic rock, and trachyte in Loðmundarfjörður.
A major goal of the remapping was to take samples
of the rocks and to determine their major element
content. The rock samples were taken in a recon-
naissance fashion rather than for a thorough petro-
logical investigation, and the results should be con-
sidered accordingly (Tab. I). Rocks with such a low
silica content as crinanite have never been found in
Iceland, and it is possible that Dearnley did not
recognize the secondary character of analcite in the
basaltic rocks, thus taking the vesicle-filling analcite
as a primary mineral of the rock. Some of the basal-
tic rocks have indeed relatively high alkali contents
(samples 6 and 7), but crinanite and trachyte have
not been found, even though the rock samples were
taken as close as possible to the sample points
described by Dearnley (1954). Because of their sub-
microscopical fine structure rhyolite and dacite lava
flows (samples 10 to 13) can only be distinguished
with the help of geochemical analyses and calcula-
tion of the CIPW norm (Tab. II). Incomplete melting
of quartz or a high water content may be responsible
for the unusual low totals of the rhyolites.
MICROSCOPIC ANALYSES OF THE ROCKS
The rhyolites and dacites show vitrophyric or
microlitic textures. Macrophenocrysts are pla-
gioclase (Anl0-15), sanidine, quartz and opaque
oxides. Numerous fractures in the minerals indicate
a high viscosity of the lavas. Phenocrysts occur in
all three zones of the lava flows and are evenly dis-
tributed among them. The structure of the matrix
ranges from glassy to microcrystalline. Therefore an
exact microscopic description of the minerals is not
possible. The glassy matrix has frequently been
altered into microcrystalline fan-like or mosaic-like
aggregates of those minerals that would have been
crystallized out of a slowly cooling magma. Spheru-
lites consist of radiating aggregates of alkali
feldspar and quartz (Fig. 13). They are the most fre-
quent devitrification products.
A further structural feature of the matrix is the
flow foliation, which is also detectable in the thin
sections. The layers represent different degrees of
crystallinity, and layering also results from vesicles
arranged parallel to the flow. In thin sections the
"frozen" movements of a cooling and laminarly
flowing melt are visible (Fig. 7). The melt flowed
around phenocrysts and thus complicated folds and
whirls were frequently created.
INTERPRETATION AND CONCLUSION
In the remapped area in Loðmundarfjörður pla-
teau basalts bank against the silicic lava flows of the
Kækjuskörð rhyolitic volcano. No updoming or
flexuring of the basaltic rocks in contact to the rhy-
olite is visible. The formation of the rhyolite as
subaerial lava flows is indicated by ubiquitous auto-
brecciation and ramp structures in the rhyolitic
rocks. This is interpreted, in contrast to Dearnley
(1954), as a small volcano built mainly of Tertiary
rhyolite flows. The volcano was subsequently
covered by plateau basalts and the Herfell ignim-
brite.
Silicic rocks in Iceland are mostly confined to
central volcanoes, because only here the high output
triggers an anatectic melting of crustal material
(Oskarsson and others, 1982). The Kækjuskörð rhy-
olitic volcano represents an unusual volcanic form
and differs from other outcrops of silicic rocks. It
formed a volcanic edifice without the typical
features of a central volcano, e.g. dyke swarm, cal-
dera formation or another kind of downsagging area,
but has a much larger volume than eruption products
from parasitic vents, e.g. Breiðdalur (Walker, 1963).
86 JÖKULL, No. 39, 1989