Jökull - 01.12.1986, Blaðsíða 35

and the eastern mountain complex which includes
Hrómundartindur, Hagavíkurfjöll and Katlatjarnir.
The Hengill area thus probably contains two active
volcanic subsystems, that of Hengill and another
centered on Hrómundartindur to the east (Saemunds-
son, pers. comm. 1986).
Numerous porphyritic high-MgO picritic basalts
have been discovered in Iceland and the possibility
that such lavas may represent high-MgO Iiquids is of
crucial importance to an understanding of basalt
petrogenesis.
In this paper the main emphasis is on the Maelifell
picritic basalt which belongs to the eastern Hrómund-
artindur volcanic system. By combining data from
microprobe analysis with detailed petrography, I have
attempted (1) to test whether or not the olivine pheno-
crysts were in equilibrium with the groundmass liquid
and (2) to determine the origin of the picritic basalt.
the picrite basalt
The Maelifell picrite basalt is from the Weichselian
period (Saemundsson 1967). The sample studied was
taken from pillows west of Ölfusvatnsá and corre-
sponds to material extruded late in the eruption.
Maelifell is composed mostly of pillow Iava, but
hyaloclastites are found in the northern part of the
mountain. The pillow lava has been divided into
older and younger units by Saemundsson (1967). The
younger unit was not considered here owing to the
extensive alteration (Saemundsson, pers. comm.
1981) it has suffered.
Chemical composition. Glass in the picrite basalt
occurs both residually in the matrix, in the pillow
nnds (quenched glass), and as inclusions in pheno-
crysts. Table 1 shows the quenched glass composition
°f pillow rinds, and the whole-rock composition. The
Quenched glass contains much Iower Mg than the
whole rock; with consequently higher percentages of
other major elements.
Petrography. The samples of the older pillow lavas
contain olivine phenocrysts (38.6 vol.%), with micro-
phenocrysts of Cr-spinel and macrophenocrysts of
clinopyroxene (8.9%) in a subophitic to spherulitic
groundmass composed of: pyroxene (29%), plagioclase
(16.5%), olivine (0.9%), opaque minerals (6.1%; in-
cluding the Cr-spinel microphenocrysts) and glass.
Rare plagioclase phenocrysts (Ango) have been found
(Saemundsson 1967) and are interpreted to be xeno-
crysts (Grönvold & Óskarsson, pers. comm. 1983).
The samples are fresh except for a few olivine grains,
which are oxidized along fractures. The interstitial
Table 1. Composition of quenched glass* (micro-
probe analyses) in pillow rinds, whole-rock compo-
sition (XRF analyses) of the Maelifell picrite basalt
(taken from Hardardóttir 1983) and the calculated
high-MgO liquid composition.
sample glass whole- liquid
rock composition
Si02 47.05 45.11 46.8
tío2 1.04 0.59 0.8
ai2o3 14.83 8.67 11.7
Cr203 0.07 0.23
MgO 8.78** 25.87 18.0
FeO 9.44 8.79 8.1
Fe203 not anal. 1.29 1.1
MnO 0.17 0.17 0.17
CaO 14.20 8.68 11.6
Na20 1.65 0.84 1.14
K20 0.03 0.02 0.03
p2o5 0.04 0.02 0.03
Total 97.30 100.28 99.47
Mg/(Mg+- Fe2+) 0.62 0.84
* average of six analysis
** values range from 8.60 to 8.97
glass is devitrified, as is in part the quenched glass
found in pillow rinds.
Two types of olivine phenocrysts are observed:
those that exhibit reverse chemical zoning (group I)
and those that are normally zoned or unzoned (group
II). Both types are subhedral in shape and reach up to
1 cm across. Group-I olivine is, on average, 2.2 mm
across in thin section. The larger grains are character-
ized by large (>500 pm) crystallized silicate-melt in-
clusions up to 1.8 mm across. Numerous small inclu-
sions of glass (<50 pm) are also commonly included in
the group-I olivine. Euhedral zoned plagioclase con-
taining devitrified, irregularly shaped inclusions of
glass is occasionally included in the reversely zoned
olivine phenocrysts. Primary Cr-spinel has not been
found enclosed in the olivine belonging to group I
with two exceptions where they were located at the
rim. In two cases, bright green anhedral clinopyroxene
was found moulded on (or surrounding) olivine of
group I.
Group-II olivine averages 2.5 mm across in thin
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