Jökull - 01.12.1983, Qupperneq 69

TABLE 3. Description of Analysed Rock Samples.
TAFLA 3. Lýsing á sýnum.
S4 ROCK NAME: Palagonite Basalt Breccia
LOCALITY: Unit C — from just below of the top ofjökulhaus South.
PETROGRAPHY: A buíf coloured, crudely bedded, volcanic breccia consisting of vesicular volcanic
fragments and matrix, showing varying degrees of alteration of the black volcanic glass matrix to the buíf
coloured hydrated palagonite form. Both fragments and matrix appear to have been derived from the same
magma type, hence are products of the same eruption. They contain fairly abundant small plagioclase
phenocrysts together with scarce olivine and augites.
S5 ROCK NAME: Basalt — Transitional Alkalic Type
LOCALITY: The east bank of the Skógaíjallsgil river just below the mouth of the gorge — thought to belong to
the top of Unit B, but a slight measure of uncertainty as mayjust be a giant fallen block from Unit C in the
Skógarfjall cliff.
PETROGRAPHY: It contains abundant larger labradorite plagioclase phenocyrsts (about 16 vol. %) together
with scarce augite phenocrysts (2%) but lacks olivine. The matrix is characterised by an abundance ofsmall
plagioclase laths together with small pyroxene and opaque oxide (titanomagnetite) grains.
S7 ROCK NAME: Basalt — Transitional Alkalic Type
LOCALITY: Unit C — from the “valley fill” sequence on the south face of Skógafjall.
PETROGRAPHY: Very similar to S5 with abundant large labradorite plagioclase phenocrysts (14 vol.%)
and scarce large augites (2%). Again no olivine apparent.
S8 ROCK NAME: Hawaiite — Alkali Olivine Basalt Series.
LOCALITY: From just east of Skáli mountain hut on the E-W volcanic ridge in the col area of Fimmvördu-
háls between Eyjafjallajökull and Mýrdalsjökull.
PETROGRAPHY: Scarce large phenocrysts (about 5 vol.% in total) of plagioclase, olivine and augite in a
uniformly fairly coarse-grained matrix of the same three minerals together with titano-magnetite grains and
some interstitial dark brown, semi-opaque, glass. This lava thus appears to have undergone two periods of
crystal growth prior to its final eruption and quenching. Consequently the large phenocrysts of plagioclase and
augite are chemically zoned, in particular the plagioclases which have distinct outer rims of more sodic
composition (andesine) around central labradorite cores.
S12 ROCK NAME: Ankaramite — Picritic Olivine Tholeiite.
LOCALITY: Unit A — at the southern base of Jökulhaus North by the glacier edge.
PETROGRAPHY: A very striking coarsely porphyritic lava with abundant large olivine and augite pheno-
crysts (about 20 vol.% of each) and zeolite lined vesicles in a fine grained matrix rich in mafic minerals.
S20 ROCK NAME: Basalt — Olivine Tholeiite.
LOCALITY: Unit B — large lobe in palagonite breccia sequence towards the top of Jökulhaus North.
PETROGRAPHY: A porphyritic lava with large phenocrysts of olivine (6 vol.%), augite (11%) and
labradorite plagioclase (6%) in a medium grained fairly plagioclase rich matrix. A specimen of this lava type
from lower on Jökulhaus North was found containing gabbroic xenoliths.
S21 ROCK NAME: Basalt — Transitional Alkalic Type.
LOCALITY: Unit B — interpreted as being younger than S20 and one ofa series ofmassive flow units capping
Unit B on the top of Jökulhaus North.
PETROGRAPHY: An essentially aphyric lava of uniform medium -fine grained size with abundant plagio-
clase, augite and titano-magnetite grains and scarce olivine.
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