Jökull - 01.01.2005, Page 42

Árni Hjartarson
Figure 6. Alkali-silica diagram of geochemical data of rock samples from the stratigraphic units of the Tinná
Group (Hjartarson et al. 1998, Hjartarson 2003). – Efnagreiningar á bergsýnum úr Tinnáreldstöðinni.
Intrusions
Intrusions and dykes related to the Tinná Central Vol-
cano are not conspicuous. The most common dykes
in the area are basaltic ones, ranging between 1 and
5 m in thickness, but they can reach up to 40 m
in thickness. The average thickness is around 4 m.
These dykes penetrate the strata pile below the vol-
cano and many of them cut the formations of the vol-
cano itself, though none are seen to be connected to
a distinct volcanic formation. Acid dykes are seen
in Vesturdalur only near the river Fossá. As a gen-
eral rule the dyke intensity becomes higher with in-
creasing depth; they are rare at the highest exposed
levels but can occupy 10-20% of the rock volume in
dyke swarms at low levels. In the Skagafjörður Val-
leys the dyke intensity is much lower, i.e. about 7% at
the maximum. Dyke swarms, which are often associ-
ated with magma chambers and central volcanoes in
Iceland (Guðmundsson 2000), have not been detected
in the Skagafjörður Valleys.
Intrusions other than dykes are small and rare.
A few cone sheets from the last episode of the vol-
cano have been found near river Fossá in Vesturdalur,
as has already been mentioned. The mutual reason
for the lack of intrusions and dyke swarms is that
the volcano is so moderately eroded, and its roots
are located deep below the valleys. This is also in-
dicated by a low degree of alteration and zeolitisa-
tion. The Tinná Group is mostly situated inside the
uppermost alteration zones i.e. the zeolite-free zone
and the chabazite-thomsonite zone, showing that the
group was never buried below a thick pile of more
recent lavas (Walker 1960). No signs of geothermal
fields or high-temperature areas have been found in
the vicinity of the volcano. Such areas are associated
with most central volcanoes in Iceland (Sæmundsson
1979).
Geochemistry
About 30 analyses of rock samples from all the main
stratigraphic units of the Tinná Central Volcano have
been carried out (Hjartarson et al. 1998, Hjartarson
2003). Figure 6 shows an alkali-silica diagram of the
samples according to international standards (IUGS)
(Le Maitre et al. 1989, Figure B.13). The conclusion
of the analysis is that basic, andesitic and acidic rocks
from the volcano follow a tholeiite trend. The trend,
appearing here, is close to the one that has been found
for the Þingmúli Central Volcano in East Iceland and
is often used for comparison (Carmichael 1964, Hjart-
arson et al. 1998, Hards et al. 2000). The main rhy-
olite formation of the Tinná Central Volcano is very
high in silica, or up to 75% SiO2. The geochemical
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