Jökull - 01.01.2017, Blaðsíða 39

Svavarsdóttir et al.
Bárðardalur – Southern segment of the Bárðarbunga
fissure swarm (the Veiðivötn region)
In Figures 8e and f, whole rock major and trace el-
ement analyses of samples from the Veiðivötn re-
gion (Jakobsson, 1979; Manning and Thirlwall, 2014)
are plotted together with whole rock and groundmass
analyses of the lavas of Bárðardalur valley. Due to the
plagioclase-rich character of lavas in this region, these
whole rock analyses are likely skewed towards lower
K2O and MgO, relative to the compositional field de-
fined by Bárðarbunga tephra. These whole rock anal-
yses are therefore less likely to define a representa-
tive liquid line of descent for Bárðarbunga magmas.
Although the samples from Bárðardalur valley are
generally more primitive than the Veiðivötn products,
they follow a nearly identical evolutionary trend, both
with respect to major and trace elements. Also seen
by comparison with Bárðarbunga tephra, this suggests
that Bár-1 and Kinnarhraun samples represent the
primitive endmember of the Bárðarbunga products.
This endmember is chemically similar to glass com-
positions of the Kistufell table-mountain (9.27–12.56
wt% MgO / 0.05-0.09 wt% K2O; Breddam, 2002),
the most primitive magma identified in this region,
which has ratios of radiogenic isotopes (87Sr/86Sr
= 0.70304–0.70308; 143Nd/144Nd= 0.51306-0.51310;
208Pb/204Pb = 37.935-37.964) that are similar to the
lavas found in Bárðardalur valley.
The Þjórsárhraun lava, being the largest lava flow
from the Veiðivötn region, shares many characteris-
tics with the Kinnarhraun and Bárðardalshraun lavas.
These are all large lava flows that flowed from the Ice-
landic highlands towards the lowlands during an ac-
tive period following deglaciation in the region (Vil-
mundardóttir, 1977; Hjartarson, 2011). Their petro-
logical characteristics are also nearly identical, all be-
ing highly porphyritic lavas with variable amounts of
plagioclase macrocrysts, reaching up to ∼30 vol%
in some samples (Halldórsson et al., 2008). This
raises the question whether these large lava flows
were emplaced during the same eruption. To inves-
tigate this idea, groundmass and whole rock analyses
from the lavas of the Bárðardalur region were plot-
ted with groundmass analyses from Þjórsárhraun lava
from Halldórsson et al. (2008) (Figure 8e and f). The
Þjórsárhraun lava groundmass is more evolved, as it
displays generally higher K2O, Ba, Y and lower MgO
contents relative to groundmass of the Kinnarhraun
and Bárðardalshraun lavas. Significantly, the lack of
intermediate chemical composition between Þjórsár-
hraun lava, on one hand, and the Kinnarhraun and
Bárðardalshraun lavas, on the other, supports the no-
tion that these lavas were emplaced in separate erup-
tive events. However, these lavas fall on the same
geochemical trend with respect to major and trace ele-
ments, possibly implying that they originate from the
same volcanic system.
Bárðardalur – Bárðarbunga central volcano and
northern segment of the Bárðarbunga fissure swarm
Hansen and Grönvold (2000) published major and
trace element analyses of glass samples from
Hrímalda and Gígöldur, which confirm their some-
what primitive nature (Hrímalda: 7.9–8.8 wt% MgO;
Gígöldur crater row: 7.7–9.3 wt% MgO). Thus, their
analyses suggest that Hrímalda and Gígöldur resem-
ble the lavas in Bárðardalur valley somewhat more
than Bárðarbunga products from the Veiðivötn region.
In the following text, new analyses of Bárðarbunga
basement rocks, Dyngjuháls, Urðarháls, Hrímalda,
and Gígöldur will be systematically evaluated and
compared to the lava flows of Bárðardalur valley.
The analyses of the basement rocks of the Bárðar-
bunga central volcano are much more scattered than
analyses of the lavas of Bárðardalur (Figures 9a and
b). Significantly, two Bárðarbunga samples (SAL–
307 and SAL–312) plot well outside the representa-
tive chemical range of Bárðarbunga basalts, with high
TiO2, high Zr and Y but low MgO contents, signifying
highly evolved basalts, commonly observed within
mature caldera systems in Iceland (e.g., Nicholson et
al., 1991; Hartley, 2012). Two other Bárðarbunga
samples plot also outside the chemical trend of the
Bárðarbunga basement rocks (SAL–308, SAL–314)
with low TiO2 and MgO contents relative to other
samples. These are samples with large amounts of
plagioclase macrocrysts, similar to what has been de-
scribed for Kinnarhraun and Bárðardalshraun lavas
(Figure 4b). If these four anomalous samples are ex-
cluded, the chemical composition of Bárðarbunga and
the lava flows of Bárðardalur are clearly within the
34 JÖKULL No. 67, 2017