Jökull - 01.01.2017, Blaðsíða 36

Holocene lavas in Bárðardalur, NE-Iceland
DISCUSSION
Division of the Bárðardalshraun lava flow field
No consensus has so far been reached concerning
a definitive number of Holocene lava flows present
in Bárðardalur valley. Inspection of samples from
Bárðardalur reveals a clear distinction between the
aphyric Útbruni lava, on one hand, and ultraphyric
Kinnarhraun and Bárðardalshraun lavas, on the other.
However, the Kinnarhraun and Bárðardalshraun lavas
share many of the same petrological characteristics,
making a distinction between these units difficult.
Field evidence implies different eruption ages for
these lava units, as the Útbruni lava is found interbed-
ded between them (e.g., Hjartarson, 2004). Elec-
tron microprobe analyses of the mineral phases from
Kinnarhraun and Bárðardalshraun lavas demonstrate
that four samples collected from Bárðardalshraun lava
have groundmass plagioclase and olivine microcrysts
with distinctively lower An and Fo contents, respec-
tively, relative to other samples collected from the
Kinnarhraun and Bárðardalshraun lavas. Petrographic
observations and calculated KD values of Na-Ca and
Fe-Mg exchange for plagioclase and olivine micro-
crysts, respectively, establish that they were in equi-
librium with the host melt. Therefore, olivine and
plagioclase in these four samples from Bárðardals-
hraun lava must have crystallized from more evolved
melt than in the other samples (see details in Svav-
arsdóttir, 2017). This justifies the division of the
Bárðardalshraun lava into two separate groups: Bár-1
(B15–04, B15–06, B15–07, B15–08, B15–09), which
is compositionally very similar to the Kinnarhraun
samples, and Bár-2 (B15–02, B15–13, B15–14, B15–
15), which is relatively more evolved. Major, mi-
nor and trace element analyses of groundmass sam-
ples from Kinnarhraun and Bárðardalshraun lavas re-
inforces the division of Bárðardalshraun lava into two
sample groups (Figures 4b and d). It is notewor-
thy that samples from the two Bárðardalshraun lava
groups are found in different parts of Bárðardalur val-
ley. In this regard, we point out that a lava front
is present within the Bárðardalshraun lava west of
Svartárvatn lake. The Bár-2 samples are all situated
to the south of this lava front, and therefore the di-
vision of the Bárðardalshraun lava into two separate
flows, Bár-1 and Bár-2, is indicated in Figure 7. Al-
though the groundmass of Bárðardalshraun lava is bi-
modal in terms of major and trace elements, the Bár-1
and Bár-2 flow units, along with Kinnarhraun lava,
share many common isotopic characteristics, suggest-
ing origin from a single volcanic system.
Source provenance of the Bárðardalur lavas:
Chemical constrains
On the basis of major, minor and trace element com-
positions, the source provenance of the lavas from the
Bárðardalur region will be evaluated in the following
sections. We choose to focus this discussion on five
representative major, minor and trace elements (MgO,
K2O, Ba, Zr and Y) that all display variations which
lie significantly outside the error associated with their
analyses. Due to its proximity to Bárðardalur, we will
start by evaluating the connection to the Askja vol-
canic system. We will then compare the lava units in
Bárðardalur valley systematically to some previously
studied eruptive units and tephra layers from the Bárð-
arbunga volcanic system. Finally, we will explore the
relation of the lava units in Bárðardalur to the base-
ment rocks of Bárðarbunga and eruptive units north
of Vatnajökull (Dyngjuháls, Urðarháls, Gígöldur, and
Hrímalda).
Bárðardalur – Askja
Large fissure-fed lavas and lava shields have erupted
at various locations along the fissure swarm with the
Askja volcanic system in postglacial times (e.g. Hart-
ley, 2012). Kuritani et al. (2011) reported that the vol-
canic products of Askja contain generally less than 3
vol% of macrocrysts of plagioclase and olivine, which
is quite different from the lavas in Bárðardalur and
makes them less susceptible to biases due to crys-
tal accumulation. In Figures 8a and b, whole rock
and groundmass analyses from Bárðardalur valley are
plotted along with whole rock analyses of basaltic
volcanic products associated with the Askja volcanic
system (Hartley, 2012). We note that most basalts
from Bárðardalur have lower K, and Ba/Y than Askja
basalts, although those from Útbruni overlap with the
Askja basalts. Askja is therefore regarded as an un-
likely source for the Bárðardalur and Kinnarhraun
lavas.
JÖKULL No. 67, 2017 31