Jökull - 01.12.1980, Page 36

Opaque Petrology, magnetic Polarity and
thermomagnetic Properties in the Reydarfjördur Dyke
Swarm, Eastern Iceland
C. F. BIRD* and
J. D. A. PIPER,
Department of Geophysics, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, England.
ABSTRACT
This paper summarises the petrographic charac-
teristics of opaque phases in 78 dykes of the Reydar-
fjördur swarm (ca. 12 m.y. old) in eastern Iceland,
and relates them to magnetic polarity and Curie
points. Titanomagnetites belong to deuteric oxidation
classes I to III, and sulphide content is inversely
related to oxidation class. Reversed dykes tend to be
more deuterically-oxidised than normal dykes, a cor-
relation noted in some other basaltic rocks. Magh-
emitisation is related to low temperature hydrothermal
alteration and this latter effect suppresses double
Curie poinls which are only found in dykes exhibiting
low deuteric oxidalion.
INTRODUCTION
It is now nearly two decades since petrologic
diíferences were noted between rocks of nor-
mal and reversed magnetisations (see sum-
mary by Wilson and Haggerty 1966). A quan-
titative analytical approach to this problem
had to await the detailed study of the mag-
netic remanence carriers in igneous rocks, and
in particular the fine subdivision of deuteric
oxidation states in basaltic titanomagnetites
by Haggerty (Watkins and Haggerly 1965).
Subsequent analyses of, for example, Colum-
bia Plateau lavas (Wilson and Watkins 1967)
and Scottish Tertiary dykes (Ade-Hall and
Wilson 1969) showed that reversed rock units
averaged typically higher oxidation states
than normal ones. Since the primary nature of
the reversed magnetic field is no longer in
doubt, this remains a petrologic enigma sugg-
esting that core processes (controlling primary
field polarity) in some way affect the chemis-
try of magmas fractionated from the upper
mantle.
Icelandic volcanics, although extensively
sampled for palaeomagnetic research, have
not yet been widely studied in the context of
magnetic-petrologic correlations. Watkins and
Haggerty (1968) did not find any significant
correlations in 7 dykes from eastern Iceland
but it is probable that their sample was not
large enough to show any correlations. Of the
present sample of 100 dykes from the northern
shore of Reydarfjördur (Fig. 1) 36 were nor-
mal, 43 reversed and 21 intermediate after
alternating field demagnetisation, (sites are
defined as normal or reversed if the virtual
geomagnetic pole lies within 40° of the mean
geomagnetic field axis); magnetic-petrologic
correlations are only made here using dykes
with defined normal or reversed polarity. The
proportion of normal and reversed dykes is
comparable to the contemporaneous part of
the Eastern Iceland lava succession sampled
by Dagley et al. (1967) and the total number of
dykes is somewhat smaller than the number of
lava units in the equivalent succession (Piper et
al. 1977). It is inferred that the exposed dyke
swarm is largely capable of having fed the
observed lava pile although some dyke feeders
* Present address: Esso Resources Canada,
Exploration Dept., 500 6th Av. SW, Calgary,
Alberta, Canada.
34 JÖKULL 30. ÁR