Jökull - 01.12.1979, Page 70

rocks are commonly found to be broken chips of
solid rock which have reacted with the magma and
can therefore be suggested to be xenoliths.
Gabbroic nodules found in the first mugearitic
extrusives of the 1973 Eldfell (Heimaey) eruption
in the Vestmannaeyjar volcanic system contain
hypersthene and are therefore xenolithic in the
alkalic magma, although they had reacted with the
magma to form hornblende and kaersutite.
No peridotitic or eclogitic nodules have been
found with certainty in Icelandic rocks.
Volume of extruded rocks
In Table 2 are shown estimates of the extruded
volumes of various rock types (groups) in the three
rock series of the eastern volcanic zone (Fig. 7)
along with estimates for all the volcanic zones. It is
seen that the production of the various rock types
within the three series is very different.
Compared with the five transitional alkalic and
the one alkalic system, the two tholeiitic systems
have by far the highest productivity of basalts.
Similarly the bulk of the evolved rocks are
produced in transitional alkalic systems. It appears
that the ratio of evolved rocks to basalts is 1 — 2
order of magnitude higher in the alkalic systems
than in tholeiitic areas. It may be unwise to
compare these systems, since they may be in a
different stage of maturity and since their tectonic
environment is different and the ratio of extruded
magma to underground-stored magma can not be
determined. However, these differences are
probably of some genetic significance. For
example, partial melting models suggest, that given
a constant composition of the mantle source region,
olivine tholeiites correspond to a factor of five to
eight times more melting than alkali olivine
basalts, and indeed field data suggest that the
tholeiitic systems produce about seven times more
basalts than the alkalic systems per unit area.
As five of the six active transitional alkalic
systems are found in the eastern zone, the
percentage of evolved rocks is lower when all the
volcanic zones are included, or about 8 per cent,
including the basaltic andesites. These are lower
values than usually calculated in Iceland.
Estimates from the Tertiary formations indicate
14—40 per cent of evolved rocks, but, these values
may however by misleading, since much of the field
work has centered on the old central volcanoes,
where the bulk of the evolved rocks is exposed.
PALAGONITIZATION AND
ZEOLITE FACIES METAMORPHISM
Metamorphic rocks do not outcrop in Iceland,
and extensive geophysical studies, as well as Sr and
Pb isotope investigations strongly suggest that sial
material does not exist under Iceland. Formerly,
the common occurrence of acid xenoliths in the
basic extrusives was thought to indicate the
existence of such a sial layer, but more detailed
Fig. 10. The rate of pala-
gonitization and con-
solidation in Surtsey
tephra as a function of
temperature and time.
Based on surface obser-
vations and 11 localities
between 1968—1976.
From Jakobsson 1978.
68 JÖKULL 29. ÁR