Jökull - 01.12.1982, Page 75

In the tholeiites of the Pleistocene centres the
olivine may be totally or partly altered to iddingsite
or other secondary minerals, and interstitial glass is
almost invariably altered to green or brovvn-colour-
ed fine-grained mineral aggregates. Vesicles are
sometimes filled with calcite (± aragonite) and/or
various zeolite minerals (see Prestvik 1979 for de-
tails).
Tholeiitic icelandites and icelandites
fhese rock types occur both as lava flows and as
hyaloclastite units. The analysed material does not
mclude many icelandites, a rock type that is difficult
to identify in the field, especially in the hyaloclastite
sequences, where they tend to resemble both
aphanatic tholeiites and tholeiitic icelandites. In
the field icelandites were mapped together with
these more basic rocks and not as a separate type
(Prestvik 1979). It is possible, therefore, that the
icelandites may be underrepresented among the
analysed material.
Porphyritic types occur among these rocks
( Table 1), but the aphyric (or nearly aphyric) ones
are by far the most abundant. The groundmass
texture of the tholeiitic icelandites varies from inter-
granular or subophitic to pilotaxitic and hyalopi-
litic. The icelandites which commonly display flow-
banding, at least on a microscopic scale, usually
have hyalopilitic matrices.
In the tholeiitic icelandites zoned plagioclase
phenocrysts commonly have sodic bytownidc cores
lAn70-75)> and rims ranging from AnH to An()..
Microphenocrysts cluster around An.,,54. Plagio-
clase microphenocrysts in basic icelandite (SiO.,
~56%) are sodic labradorite (An_5_), whereas
andesine (An3_ 37) microphenocrysts occur in the
more evolved icelandite (SiO,2 ~58%) of the 1727
Oræfajökull eruption.
Small olivine phenocrysts of tholeiidc icelandites
are relatively rich in the forsterite component (Fo(.?
78), which in the icelandite it is down to Fo47 4g.
Olivine has not been found in all the icelandite
samples. Clinopyroxene usually occurs as micro-
phenocrysts (0.1-0.3 mm) in both tholeiitic iceland-
ites and icelandites. Occasionally bigger pheno-
crysts (up to 1.2 mm) are found. The composidon
varies from Wo^.jEn^Fs^ for phenocrysts to VVo+4
En3_Fs2| in microphenocrysts of tholeiitic icelandit-
es and from Wo^Ertj.jFs^ to VVo.j()Enj.,Fs3(| in ice-
landite. Microphenocryst and matrix oxides are tit-
anomagnetites in both tholeiidc icelandites (TiO.,
= 24-44%) and in icelandite (TiO., = 28%).
Dacites and rhyoliles
I n the field these rocks occur as small domes, lava
flows, hyaloclastites, airfall ash layers, and dikes.
Laminar llow structures are usually observed. Some
of these rocks are obsidians or pitchstones, but more
usually the rocks are crystalline or partly devitrifi-
ed. Most rocks are slightly feldspar porphyritic and
occasionally phenocrysts of olivine and pyroxene
are found as well, especially in thedacites (Table 1).
Oxide minerals are commonly observed as micro-
phenocrysts in both dacites and rhyolites. In a
couple ofsamples tiny needles ofmatrix amphibole
and secondary quartz have been observed. Quartz
is a common groundmass phase in the crystalline
rhyolite, but it has never been found as a pheno-
cryst.
The composition of feldspar phenocrysts ranges
from oligoclase/andesine (An28_38) in dacites to
oligoclase (An1[j24) and anorthoclase (Or13Ab65
An2,,) in rhyolites. MatrLx feldspar in rhyolite is
anorthoclase (Or17Ab7jAn10). Phenocrysts of oliv-
ine (0.1-1.0 mm ) range from Fo^19 in dacite down
to Fo3 in rhyolite. Clinopyroxene phenocrysts (0.5-
1.5 mm) in dacites range from ferroaugites (VVo+3
En,9Fs28 to Wo45En17Fs38) to almost hedenbergite
(Wo+9En85Fs42 5 to VVo^ErtjFs^g) in rhyolite.
Orthopyroxene (Wo3En.(Fs43) has been found in
spherulite-like aggregates together with oligoclase
crystals in a glassy matrix of one rhyolite sample.
The oxides are titanomagnetites (TiO., = 22-24%)
in dacites, but range towards magnetite (TiO,, =
7-12%) in rhyolites. In some of the rhyolites tiny
zircon crystals occur.
Intrusive rocks
In the eroded section of the Breidamerkuríjall
center both gabbro and granophyre are exposed.
Gabbro is exposed only at a few localities along
Breidárlón whereas a larger intrusion at Bæjarsker
is a granophyre.
The gabbro is medium grained (0.5-5 mm) with a
hypidiomorphic to allotriomorphic - granular text-
ure. The principal minerals are plagioclase (An_g0)
and clinopyroxene accompanied by abundant oxide
ores. Subordinate amounts of altered olivine, green
and brown amphibole and some small grains of
biotite are also present. The chemical analyses of
JÖKULL 32. ÁR 71