Jökull - 01.12.1982, Page 83

Fig. 2. Stratigraphic section
through drillholes in the
Laugaland area. For location
see Figure 1. Also shown is a
section through a drillhole
(TN-1) in the Ytri-Tjarnir
area about 1 km north of the
Laugaland area. The location
of main alteration zones is
indicated in the sections.
Mynd 2. Jarðlagasnið frá
norðaustri til suðvesturs gegnum
borholur á Laugalandi og eina
borholu á Ytri-Tjörnum, um 1 km
norðan Laugalands. Afstaða bor-
holanna á Laugalandi er sýnd á
mynd 1. Helstu ummyndunarbelti
eru einnig sýnd á sniðinu.
the drillholes vary from coarse grained dolerites to
very finegrained basalts. The margins are fractured
and often with clay fillings. From such fractured
clay-filled dyke margins many serious drillhole coll-
apses have happened. The dyke margins are comm-
only oxidized and highly zeolitized.
alteration of the rocks
In general the lavas are not very intensely alter-
ed. Of the olivine in olivine— tholeiite basalts only
pseudomorphoses are left. Reaction rims are comm-
only developed around pyroxene crystals and a to-
tal replacement of pyroxene occurs in the most alt-
cred lavas. The plagioclase is more resistant. In
plagioclase phenocrysts calcite filled cracks are
coramon and albitization is obserx ed in a few sam-
ples at great depths.
Fine grained groundmass in thebasalts is replac-
ed mainly by clay minerals. Pores and cracks are
filled by zeolites, calcite and clay minerals. The
zeolites crystallize in vesicles and fractures in the
lavas. The greatest amount of zeolites is encounter-
ed from the scoriaceous top or bottom of the lavas.
Due to mainly the impervious nature of the rocks
the fossil altcration pattern is very slightly affected
by the present geothermal activity. The porosity of
the basalts is assumed to be about4% (measurcd in
one core). Thus the main fiow of water must be
assigned to dyke margins and faulted blocks.
In a few samples we have observed disturbances
in the fossil alteration zones which can surely be
related to the present activity.
As mentioned previously the top of the mesolite
— scolecite zone is at about 300 m elevation in the
mountains of Eyjafjördur. The maximum elevation
of the drillholes is 60 m and they all start in the
mesolite — scolecite zone:
JÖKULL 32. ÁR 79