Jökull - 01.12.1982, Page 85

also the most common clay filling in cracks and
fractures above 1500 m depth. Mixed-layer
smectite/chlorite and swelling chlorites are
found commonly beneath 1500 m depth and also
tn a few samples at upper levels.
Chlorite occurs sporadically below 1000 m
depth.
Illite and mixed-layer illite/smectite occur in
some sedimentary interlayers.
Celadonite is common in thesediments. Italso
occurs in vesicles in the tholeiite lavas.
Pyrite is common in fracture fillings and is also
dispersed in the groundmass ofthe basalts. It is
especially common in the neighbourhood of in-
trusions.
Epidote is only recorded in two of the drill-
holes, below 1880 m and 2200 m depth respecti-
vely. A continuous occurrence of epidote is only
obtained in the deepest drillhole at depths below
2400 m. Prehnite is recorded in a few samples
near the bottom of the deepest drillhole.
Amphibole has also been observed in a few
samples at the bottom of the exposed section.
CONCLUSIONS
By the study of drill cuttings from the deep drill-
holes on Laugaland one has gained knowledge
about the extension of the 1500 m thick lava pile
exposed in Eyjafjördur, the succession ofalteration
zones and dyke distribution by depth.
The extension ofthe lava pile consistsofan upper
formation with common olivine-tholeiite basalt
lavas and porphyritic lavas. Below 600 m tholeiitic
lavas are dominant.
The laumontite alteration zone succeeds the mes-
olite-scolecite alteration zone at 260-700 m below
sea-level. On the average the top of this alteration
zone is found at 2 km depth below the original
surface of the lava pile. The top of the epidote
alteration zone is about 4 km below the original
surface. This gives a fossil geothermal gradient of
about 70°C/km, using 260°C as the temperature of
formation for epidote (Kristmannsdóttir 1979). The
same fossil gradient is obtained by plotting the
depth to the top of the laumontite zone against the
estimated minimum temperature of formation (see
Kristmannsdóttir 1979). The regional geothermal
gradient in Eyjafjördur is only a few degrees lower
than the fossil one. The crust in Laugaland is re-
presentative for a normal part of the crust far from
central volcanoes. Fossil geothermal gradients of
60-70°C/km have been estimated in similar crustal
environments elsewhere in Iceland. Thermalgradi-
ents measured on the flanks of the zones of active
volcanism and rifting where alteration is going on
are 30-50°C higher than those fossil gradients. The
fossil gradients may not necessarily represent the
maximum gradients in the evolution of the crust.
Also the diíference in lithology between the Quat-
ernary and Tertiary formation could have an in-
fluence on the thermal conditions and convection of
water in the crust.
The dyke intensity shows a general increase by
depth in the crust at Laugaland.
REFERENCES
Björnsson, A. and K. Sœmundsson 1975: (Geothermal
activity in the vicinity of Akureyri). Jarðhiti í
nágrenni Akureyrar. OSJHD 7557, des. 1975. A
mimeographed report ( in Icelandic) from the
National Energy Authority.
Bjömsson, A., K. Samundsson, S. Einarsson, E. Thor-
arinsson, S. Arnórsson, H. Kristmannsdóttir, A. Guð-
mundsson, B. Steingrímsson and P. Thorsteinsson
1978: A mimeographed report on the status of
research in the Eyjaíjörður region, OS-7827, 139
pp.
Kristmannsdóttir, H. 1979: Alterationof basaltic rocks
by hydrothermal activity at 100-300°C. Proceed-
ings of the Sixth International Clay Conference,
Oxford 1978: 359-367.
Walker, G. P. L. 1960: Zeolite zones and dike distri-
bution in relation to the structure of the basalts
ofeastern Iceland.J. Geol. 68:515-528.
Manuscript accepted 2 July 1982.
ÁGRIP
BERGGERÐIR OG UMMYNDUN
í BORHOLUSNIÐUM FRÁ EYJAFIRÐI
Hrefna Kristmannsdóttir, Orkustofnun
Laugaland í Ongulsstaðahreppi er helsta
vinnslusvæði Hitaveitu Akureyrar. Dýpsta borhol-
an þar er 2820 m á dýpt og með rofna staflanum í
fjöllunum umhverfis fæst því um 4 km þykkt snið
gegnum basaltstaflann í Eyjafirði. Er það dýpsta
JÖKULL 32. ÁR 81