Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 56

52
PÁLL IMSLAND
from the amphibole described as xenocryst in the pl-ol-cpx porphyritic
basalts.
VIII. Trachytes
Whole rock analyses: Jan 35, 36, 38, 77, 79, 81, 83, 88, 90, 92, 168,
169, 170, 171 & 172
Mineral analyses from: Jan 35, 36, 77, 81, 88, 168 & 172
The rocks of this group are high-silica intermediate rocks, SiO2=60—66
wt. per cent and MgO < 1.25 wt. per cent. The total alkali content ranges
from 10.23 to 13.25 wt. per cent, the Na^O/K^O ratio ranging from 0.84—
1.50, but mostly above 1.
The rocks are moderately porphyritic. As in the preceding group the main
phenocryst phase is feldspar. Biotite is the second most important pheno-
cryst phase and occurs in most of the samples. Other phenocryst phases that
usually occur are iron-titanium oxides and clinopyroxene. Apatite, olivine,
amphibole and zircon occasionally occur as microphenocrysts. Megacrysts
of amphibole surrounded by an extensive mineral corona also occasionally
occur.
In this group, as in all the feldspar phyric groups, the feldspars are the
most varying minerals in composition and appearance. Inter- and intra-
sample variation is greater than in the previous group, but the characteris-
tics are similar. The feldspar phenocrysts range in size from micropheno-
crysts and up to 4 mm. They range from perfectly euhedral single crystals
(Fig. 34) to panidiomorphic aggregates (Fig. 35) or to extensively internally
resorbed remnants, enclosed in a marginal envelope (Fig. 36). In composi-
tion they range from plagioclase of more than An8o composition to andesine-
oligoclase and through anorthoclase to K feldspar of Or50 composition. The
groundmass feldspar is mostly orthoclase, but anorthoclase also occurs
among the groundmass minerals. The feldspar reladons of the trachytes are
more thoroughly described by Imsland (1980).
The clinopyroxene forms euhedral microphenocrysts of Fe enriched salite
to Ca rich, Fe poor ferroaugite. The ferroaugite has a green colour (Fig. 37),
which decreases to almost colourless with decreasing Fe content.
The iron-titanium oxides of this group are usually much more altered
than those of the basic rocks. They occur mostly as groundmass grains and
microphenocrysts of titanomagnetite compositions. Some samples contain
sparse anhedral phenocrysts. They show either exsolved feebly magnetic,
greyish lamellae (Fig. 38) or distinct patches of a different grey colour (Fig.
39). These oxidized microphenocrysts contain members of the pseudo-
brookite series, inter alia. Four samples contain unexsolved micropheno-
crysts, which vary in overall composition from Usp63_47. The micropheno-
crysts ofjan 172 contain small blebs of a yellow sulfide. In addition to the
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