Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 197
mineral chemistry and relationships
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small phenocrysts, primarily occurring as single crystals (in contrast to
glomerocrysts). Mostly these show normal zoning of small but evenly
changing composition from the centre towards the margins. Occasionally a
relatively thin marginal zone of more drastic chemical change occurs
enclosing these. A variety occurs where the normal zoning is not as even as
regards chemical changes, but shows slight oscillations, and in such cases
the crystals also tend to be slightly bigger. In the few rocks of this group
which contain plagioclase glomerocrysts the individual crystals in the
glomerocrysts are microphenocrysts of the same characteristics as the single
crystal types. Occasional relatively big phenocrysts occur in this group of
rocks. These are always internally resorbed with a continuous marginal
zone enclosing the resorbed core. The resorption cavities are mostly filled
with glass but occasionally they are filled with crystalline groundmass,
especially the bigger ones. As regards plagioclase the pl-ol-cpx porphyritic
basalt group is more complex. The rocks may be divided into two main
groups; one of plagioclase glomerocrysts and another of single crystal
phenocrysts. The individual crystals of the glomerocrysts have the same
textural features as in the previous group and need no further description.
The single crystal phenocrysts have cores offour textural types: 1) unzoned,
2) oscillatory zoned, 3) patchy zoned, and 4) internal resorption. These
cores are enclosed in marginal zones of relatively contrasting compositions.
In most cases the marginal zones are poorer in the An component than the
cores, but a few samples have An richer margins. Occasionally there is a
relatively well defined zone (of varying thickness from crystal to crystal) of
small resorption cavities between core and margin. These resemble the
resorption cavities of the cores and they all contain glass or groundmass as
do the less common cases in the pl-ol porphyritic rock group. Of the four
textural types of the cores the unzoned and the oscillatory zoned types are
probably equally common, while the patchy zoned one is scarce. The
corroded cores are common and were originally probably of both the
unzoned and the zoned types.
K enrichment of the feldspars begins in the intermediate rocks. These
rocks contain either single crystals of plagioclase and alkali feldspar com-
positions, occurring as separate grains, or they contain grains zoned from
plagioclase to alkali feldspar or in other cases they may contain panidio-
morphic aggregates of alkali feldspar phenocrysts only.
II. The composition of the feldspars
The overall range of the feldspar composition is from plagioclase of An90
composition through the plagioclase series towards ~An25 composition, and
from plagioclases of these low An contents through anorthoclases to alkali
feldspars of Or60 composition. The basic rocks contain only plagioclase,
showing a range of Ang0_33. The intermediate rocks, on the other hand,