Jökull - 01.01.2017, Blaðsíða 24

Holocene lavas in Bárðardalur, NE-Iceland
ratios of a sample erupted through a young basaltic
crust, free from the influence of seawater-alteration,
can be expected to be similar to the ratios of its man-
tle source (e.g., Sigmarsson et al., 1992; Furman et
al., 1995; Sigmarsson and Halldórsson, 2015). Fur-
thermore, as at many oceanic islands, it is well estab-
lished that the Icelandic mantle displays notable varia-
tions in radiogenic isotopes which are almost entirely
related to primary variability of the underlying man-
tle (e.g., Stracke et al., 2003; Thirlwall et al., 2004).
Therefore, when such tracers are coupled to geochem-
ically well-characterized eruptive units, they probably
represent the most powerful tool available to assign
volcanic units to their mantle source, and therefore a
likely volcano of origin.
Here, we present a geochemical study of early-
Holocene large lava flows in the Bárðardalur region.
The primary aim is to determine the eruptive site of
these lavas and elucidate their petrological context
within the NRZ. In order to do this, we compare the
chemical and isotopic characteristics of the lavas from
Bárðardalur to previously published data from Bárð-
arbunga and other volcanic systems of the NRZ. Ad-
ditionally, we report chemical and isotopic data for
several eruptive units from the region north of Vatna-
jökull and from the Bárðarbunga central volcano. The
notion that Bárðarbunga has produced large-volume
lava flows that travelled vast distances, not only to-
wards the south coast of Iceland but also towards
the north (Figure 1), needs further testing, not least
because of its significance for improving our under-
standing of the magmatic history of the NRZ.
GEOLOGICAL SETTING AND
SAMPLING
Northern segment of the Bárðarbunga fissure swarm
The eruptive record of the Bárðarbunga-Veiðivötn
system indicates that similar to the southern part, the
northern part of the fissure swarm has also produced
several lava flows of significant volume during the
Holocene time period (e.g., Hartley and Thordarson,
2013; Sigmarsson and Halldórsson, 2015). Holocene
lava fields in the Dyngjuháls region cover vast areas
north of Vatnajökull, reaching from Gæsavötn in the
west to Urðarháls in the east (Figure 2) (Sigurgeirsson
et al., 2015). The Dyngjuháls lavas are partly cov-
ered by younger lavas of the nearby shield volcano
Trölladyngja (<4500 years BP; Sigurgeirsson et al.,
2015). Owing to the ruggedness of the region, it has
not been studied to the same extent as the Veiðivötn
region, despite a large number of Holocene eruptions
there. The nearby volcanic unit Urðarháls, located
to the east of Dyngjuháls (Figure 2), shows evidence
of glacier erosion (Sigbjarnarson, 1988), suggesting
a pre-Holocene age. The largest and most promi-
nent crater row in the region is Gígöldur, situated to
the northeast of Urðarháls (Figure 2). The Gígöldur
crater field represents a large eruptive unit, made up
by tens of fissures, craters and crater rows that are
connected in a complex way (Sigbjarnarson, 1988).
The craters show signs of explosive eruptions towards
the end of the last glacial period, suggesting high
rates of volcanic activity around 10 kyr BP. More-
over, these craters are the likely source of numerous
prehistoric lava flows that disappear under younger
lava units (e.g., Trölladyngja) (Sigbjarnarson, 1988;
Hansen and Grönvold, 2000). The Krepputunga lava,
which covers a substantial area to the E and NE of
the crater row, has been suggested to originate from
Gígöldur (Sigbjarnarson, 1988; Vilmundardóttir and
Kaldal, 1991; Hjartarson and Kaldal, 2004; Hjartar-
son, 2004). Hrímalda, situated to the west of Gígöldur
(Figure 2), also formed from many eruptions occur-
ring towards the end of the last ice age (Sigbjarnarson,
1988).
The Holocene lavas of the Bárðardalur region
A prominent prehistoric lava field of the NRZ occurs
in the narrow Bárðardalur valley (Figure 1), where
Holocene lava units can be traced northwards, almost
reaching the north coast of Iceland. Earlier studies
of these lava units in Bárðardalur are generally de-
scriptive geological and petrographic studies (Tómas-
son, 1967; Sigbjarnarson, 1988; Vilmundardóttir and
Kaldal, 1991; Hjartarson and Kaldal, 2004; Hjart-
arson, 2004). Based on field observations and pet-
rographic characteristics, Sigbjarnarson (1988) sug-
gested the Gígöldur crater row as the eruptive cen-
tre of the majority of plagioclase-rich lavas found in
Bárðardalur valley. More recent studies conducted
JÖKULL No. 67, 2017 19