Jökull - 01.12.1986, Blaðsíða 34

Fig. 2. Alkali : silica diagram indi-
cating the chemical trend of the
Hengill tholeiitic series. Numbers
1 —20 represent sample numbers (see
Fig. 3.). The square indicates the cal-
culated high-MgO liquid composit-
ion. The encircled areas are taken
from Jóhannesson (1975) and Jak-
obsson (per. comm. 1982), and the
dividing lines are from Jakobsson
(1979)
Fig. 3. Pyroxene quadrilateral
(Poldervaart & Hess 1951) showing
the composition of clinopyroxenes
from the Hengill volcanic system.
The line is the pyroxene trend for the
Thingmuli suite, a central volcano in
eastern Iceland (Carmichael 1967).
Legend, H1 : Haedir olivine tholeiite,
H2 : Jórutindur tholeiite, H6 : Nesja-
hraun tholeiite, H11 : Maelifell pic-
rite basalt, H13 : Sleggja icelandite,
H15 : Hengill tholeiite.
Petrographically, rocks in the area define a tholei-
itic suite ranging in composition from picritic basalt
to rhyolite (Hardardóttir 1983). The picrites represent
one end member of the discontinuous tholeiitic suite
that forms the Hengill central volcano, shown on an
alkali — silica diagram in Fig. 2. In terms of evolution
the picrites are followed by basalts ranging from fairly
primitve olivine tholeiite to mildly quartz normative
ones. The more primitive tholeiites are characteristi-
cally coarse-grained and composed of glomerocrysts
and phenocrysts (10 vol. %) of olivine (Core:
Fo85-86.5’ rim: Fo7i-85.5)’ plagioclase (core: An85_?8,
rim An68_88 6) and sparse clinopyroxene (Fig. 3) while
the more evolved ones have a core composition in the
range Fo77_80. Phenocrysts of plagioclase are found in
all the lavas. All the grains analysed are bytownite
An70_90 in composition, except for a few rim and one
core composition, which are labradorite (An57_69 5)-
Phenocrysts of clinopyroxene are found in most of the
eruptive units (Fig. 3). These rocks make up most of
the volume of the volcano. At one locality, Sleggja in
southwest of Hengill, icelandites and rhyolites are
found and an additional icelandite at Stapafell, just
south east of Maelifell. These are all very fine grained
and in most cases contain very sparce micropheno-
crysts or glomerocrysts of plagioclase, pyroxene and
opaque minerals.
Saemundsson (pers. comm. 1985) subdivided the
area into two main volcanic zones, each with different
structural settings. These are the Hengill mountains
32