Jökull - 01.01.2013, Blaðsíða 13

The Stardalur magnetic anomaly, SW-Iceland
(Sigurgeirsson, 1970b,c, 1979) which in turn aided
in identifying hidden remains of other such centers
(Kristjánsson et al., 1977; Kristjánsson, 1987). In
the case of the Stardalur anomaly, much progress was
made in 1968–1973 by locating its main peak, drilling
to obtain samples from what appears to be the main
source body of this peak, and studying their proper-
ties. However, the momentum in mapping of both the
volcanic centers in Iceland and the lava pile outside
the currently active volcanic zones has diminished
somewhat since 1980. Therefore, our understanding
of the geophysical anomalies accompanying the cen-
ters is still very uncertain. Thus, the Reykjavík (Jóns-
son and Kristjánsson, 2002) and Stardalur anomalies
are almost the only ones where detailed magnetic sur-
veys have been carried out in the last three decades.
Indeed the general problem of the origin of anomaly
lineations in Iceland has not been solved: for instance,
it is not known whether the positive anomalies which
coincide with fissure swarms on the Reykjanes penin-
sula (Kristjánsson, 1970, 1972) are due to primary or
secondary remanence or even to induced magnetiza-
tion, in buried extrusives and/or intrusives.
As stated above, our general conclusion is that
the Stardalur magnetic anomaly peak is due a se-
quence of lava flows which were erupted in rapid suc-
cession within a caldera. These along with much
of the other caldera-filling material were emplaced
in a short period (subchron) of normal geomagnetic
polarity within a long reverse-polarity chron, pre-
sumably the Matuyama. The lavas had high con-
centrations of unusually pure magnetite which ap-
pears in part to be produced from titanomagnetite by
solvus exsolution, in part to consist of a fine-grained
type derived from olivine by high-temperature oxy-
exsolution. The magnetic phases do not give rise to
single-domain magnetic properties in the bulk rock, as
would be observed in for instance the shape of strong-
field hysteresis curves (Day plots) or high median de-
structive fields of the natural remanence. However,
the properties of the core samples are also not entirely
characteristic for multi-domain magnetite, cf. the to-
tal absence of viscous magnetization.
It is clear that unusual coinciding factors caused
the lavas at Stardalur to have a mean remanent magne-
tization intensity of around 15 times that for Tertiary
lavas in Iceland. Some authors (e.g. Helgason et al.,
1990; Vahle et al., 2007) consider this strong rema-
nence to have been caused by secondary hydrothermal
alteration, but the question of what processes could
have brought such alteration about in Stardalur and
not in general elsewhere, remains open. Judging from
the stability and uniformity of the remanence direc-
tions in the drill core, it seems more plausible that the
remanence is largely primary, dating from a period of
rapid filling of the caldera by extrusives. However,
in that case it needs to be explained how the lavas
retained their strong remanence intensity through the
hydrothermal alteration which they have clearly suf-
fered.
The presence of a relatively intense ambient ge-
omagnetic field would have aided in the acquisition
of the strong remanent magnetization, regardless of
whether this remanence was of primary or secondary
origin. Unfortunately, currently available methods are
unlikely to be successful in determining the paleo-
field intensity. So far, not much has been found out
about the origin and the shaping of the source struc-
ture, or why its lavas originally acquired their peculiar
chemical and petrographic properties.
It is clear that much additional geophysical and
geological work needs to be done at the central-
volcano complexes of Iceland. This need is exem-
plified by the magnetic anomaly at Hvanneyri (Sig-
urgeirsson, 1979; Figure 5a,b) whose main peak has
interesting similarities with the Stardalur anomaly.
The source of the Hvanneyri anomaly may lie in for-
mations related to the Hafnarfjall volcano and its as-
sociated gravity anomaly; alternatively, it seems pos-
sible that these belong to a separate volcanic center
extending across the innermost part of Borgarfjörður.
The relation of these formations to the major uncon-
formity in the Borgarfjörður area (see Jóhannesson
and Sæmundsson, 2009) is also uncertain.
Acknowledgements
Þorbjörg Ágústsdóttir carried out the gravity mea-
surements at Stardalur. Finnur Pálsson processed the
magnetic field data and drafted Figure 3, Rósa Ólafs-
dóttir drafted the other figures. The author acknowl-
edges constructive reviews of the original manuscript
JÖKULL No. 63, 2013 13