Acta naturalia Islandica - 01.07.1964, Side 25

THE ANKARAMITES OP HVAMMSMÚLI, EYJAPJÖLL, SOUTHERN ICELAND 23
olivine phenocrysts show dendritic magnetite exsolutions, and may contain
minute needles of spinel.
Pl. Vla shows a large phenocryst of olivine, sitting in the groundmass
of feldspar laths, but containing granular groundmass in an embayment. This
phenomenon seems to be a further evidence of the close genetic relationship
between the two textural types in the quarry. The phenocryst was carried
about by the moving magma. The groundmass in the embayment shows that
the phenocryst came at one stage into touch with the granular mass. Protected
by the phenocryst, the granular substance remained in its hole in spite of the
change of enviroment.
The olivines are slightly altercd to iddingsite. The occasional plagioclase
phenocrysts tup to 2.5 mm long) contain small magnetite crystals in the core.
Rock III. The highly porphyritic rock in the quarry.
Phenocrysts occupy over 60% of the volume. The rock is the coarsest
grained type at Hvammsmúli; the groundmass is dominated by plagioclase laths
ca. 0.4—0.5 mm long. The texture is ophitic, since the optically continuous
pyroxene areas are large enough to enclose entire plagioclase laths (F. Walker
1957).
The olivine phenocrysts are rounded in outline and fresh, the pyroxenes
irregular. Reaction rims are not obvious, except by the ophitic intergrowth
of the margins with feldspar laths of the groundmass. Some of the olivines
possess a curious conchoidal fracture round the rim but are unfractured in
t.he centre. The fracture may define a compositional break. A few of the oli-
vines show dendritic magnetite exsolutions. The phenocrysts are similar in
size as those of Type II but much more numerous.
The ore occurs in largish poikilitic areas defined by and including feld-
spars. The rock was intruded into the other rock types of the quarry (See later).
Rock IV. The granular rock above the quarry.
Phenocrysts occupy less than 30% of the volume. The rock is very similar
to Type I: the groundmass is granular, relatively coarse at the base, and grows
finer upwards. The phenocrysts are rather well formed and carry a reaction
rim. A sharp, almost vertical, junction is seen between the light coloured Type
IV and the dark quarry rock. Type IV is thought to be later. Next to the con-
tact the (otherwise) light coloured Type IV has a narrow zone, 10—15 cm
wide, which is dark, due to the alteration of olivine and ore. Neither rock is
chilled at the margin. Rock IV has average density of 3.1 as opposed to all the
others, which have a specific gravity about 3.04,