Jökull - 01.12.1980, Blaðsíða 37

Fig. 1. Distribution of dykes along the north shore of Reydarfjördur, eastern Iceland.
Numbers refer to the sampled dykes of Table I and Piper et al. (1977). The inset diagram
shows the position of this dyke swarm in relation to the generalised distribution of dykes in
eastern Iceland after Walker (1959).
Mynd 1. Gangar norðan við Reyðarfjörð. Tölur eiga við Töflu 1. Litla kortið sýnir dreifingu ganga á
stœrra svœði þar eystra.
must be hidden downdip, and it is also ob-
served (Fig. 1) that the dykes do not occur in
discrete zones of one polarity but are
throughly mixed; this must reflect the broad
belt of dyke intrusion prevailing at any one
time.
PETROLOGIC CHARACTERISTICS
OF OPAQUE PHASES
The effects of deuteric oxidation (800—
500°C) on the magnetic constituents of basal-
tic rocks are defined by experimental petro-
logy (Lindsley 1962), field measurements (Sato
and Wright 1966) and petrologic inves-
tigations (Wilson and Haggerty 1966, Ade-Hall
et al. 1968, 1971). They are frequently over-
printed by a later response due to hydrother-
mal alteration (500—150°C) a ubiquitous
effect in the Reydarfjördur area where the
lava pile has been buried and reheated, and
the groundwater mobilised (Walker 1960) with
concomitant deposition of zeolites in vesicles
and low temperature alteration of susceptible
mineral phases; at present levels of exposure
this has taken place over the probable tem-
perature range of about 100—300°C. The
effects of hydrothermal alteration on opaque
phases are quite distinct from those of deuteric
oxidation when the extent of the latter is
limited, but there is little to distinguish these
effects at high deuteric oxidation states; in
these cases a resolution of the two types of
alteration may be difficult or impossible
(Ade-Hall et al. 1971).
The six classes of deuteric oxidation used in
this study are:
Class T. Unaltered and unoxidised tit-
anomagnetite. This is susceptible to
hydrothermal alteration when temperatures
reach 100—150°C (Ade-Hall et al. 1968),
which in eastern Iceland implies burial to
depths in excess of 900 meters, when patches
of ferrirutile granules appear. At temperatures
in excess of 300 °C white flecks of tit-
anohematite may appear.
Class 2: A small number of exsolution
JÖKULL 30. ÁR 35