Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 316

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PÁI.I. IMSI.AND
case regarding Mg in ilmenites. A continous increase of the amount of
geikielite in solid solution with ilmenite is found to take place from ocean
bottom tholeiites through oceanic island tholeiites, transitional basalts and
alkaline basalts to kimberlites, the typical host of magnesian ilmenite.
5) The feldspars show a compositional range from calcic plagioclase
(An90) through sodic plagioclases (down to An2s) and anorthoclases to alkali
feldspars of Or60 composition. The alkali feldspars occur in the highly
evolved rocks only, while the plagioclases are present in all rock types,
except the wehrlite. In the ankaramites the plagioclase is however an
extremely scarce exception as a phenocryst phase. The feldspars, especially
the plagioclases, show extreme variable textural features and a complex
pattern of zoning and resorption. If taken as a single group there are
therefore diííiculties when it comes to evolution and genesis. The complexity
of these features is solved within the framework formed by the explanations
oífered by the literature on feldspar zoning and structure as well as by
crystallization experiments. In the basalts the cores of the plagioclase
phenocrysts seem to start to crystallize at nearly 1200°C and at depths
corresponding to 10 kb pressure. En route to the surface, or during a magma
reservoir delay, these may become partly resorbed as the result of magma
mixing and/or changes in physical conditions. They are finally enclosed in
an envelope of a new feldspar, where extremely late crystallization takes
place below 1040°C during surface cooling. In the case of the evolved rocks,
the more evolved trachytes seem to have crystallized the alkali feldspar
before plagioclase. 'I’his takes place at shallow depths and somewhat above
1000°C. The less evolved trachytes and the tristanites, on the other hand,
crystallize plagioclase before alkali feldspar and at higher temperatures,
probably over 1100°C. The groundmass of these rocks crystallizes at
~850°C. In these feldspars Mg and Fe are present as trace elements and are
concentrated in late crystallized minerals relative to early ones. This holds
true for both plagioclase and alkali feldspar. Sr is found to reach a maximum
in the plagiocases of intermediate An content. Ba is high in the alkali
feldspars but low in the plagioclases. It shows the tendency to concetrate in
the late crystallized plagioclases and in the early formed alkali feldspars.
Sparse data suggest that Ti is generally present as a trace element in the
plagioclases and that it has the tendency to concentrate in the late formed
crystals relative to the early ones.
6) Apatite occurs as a phenocryst phase in close association with iron-
titanium oxides in basalts to trachytes. It is most conspicious in the low-Mg
basalts and basaltic tristanites. These apatites may show a slight brownish
colouring, caused by minute opaque oxide needles. The C1 content of the
apatites is relatively constant and low, while the F- and OH contents vary.
The apatites of the hydrous mineral xenolith have a higher C1 content than
those of other rocks. This apatite is further shown to be sector zoned as
regards the volatile components. Si, Sr, Mg, Mn, and Fe are generally