Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 215

mineral chemistry and relationships
211
extrusion, i.e. the groundmass crystallization, while the inner zoning then
reflects crystallization during ascent where the compositional change
reflects the efíects of evenly decreasing temperature and pressure as well as
changing liquid composition. Occasionally the plagioclase micropheno-
crysts reveal slight oscillations in the zoning. These phenocrysts tend to be
slightly bigger than the others. These may have started to crystallize prior to
the others; perhaps slightly before the magma ascent started, but later than
the relatively scarce big phenocrysts which occur in these rocks and always
have internally resorbed cores.
In the more evolved rocks, the tristanites and trachytes, both plagioclase
and alkali feldspar occur. As a general rule, both the feldspars are normally
zoned from centre to margin. Resorption is not uncommon and occurs
primarily in the plagioclases. The plagioclases are most frequently found as
cores of phenocrysts, typically enclosed by relatively albitic or by alkali
feldspar margins and have compositons ranging from very calcic (An82)
towards anorthoclase. These cores may be oscillatory zoned, evenly
normally zoned or unzoned. In some cases they have been resorbed, in other
cases not. The resorption cavities either contain glasses and groundmass
minerals or have been filled by more Ab rich plagioclase or alkali feldspar by
crystallization corresponding to the marginal one (patchy zoning) in com-
position. The alkali feldspars occur as phenocrysts, outer zones on plagio-
clase and as groundmass grains. Their zoning pattern is from Or rich
towards more albitic. Anorthoclase cores may though be marginally zoned
by more Or rich feldspar. These Or rich marginal zones frequently contain a
thin zone of very small mineral inclusions, probably indicating a period of
relatively rapid growth. The feldspars of these evolved rocks indicate a
rather simple crystallization history for some samples but more complex for
others. Jan 175, e.g., which contains microphenocrysts with euhedral to
subhedral cores of andesine to anorthoclase (An32_22) enclosed in a
relatively thick marginal zone of alkali feldspar (Or5i_35) and has an alkali
feldspar as a dominant groundmass phase, probably reflects a rather simple
crystallization. Samples as Jan 36, on the other hand, containing pheno-
crysts of varying compositions, ranging from anorthoclase (Or34) to plagio-
clase (up to An82) and showing resorption of some grains (while not of
others) and in addition having an andesine groundmass feldspar, must
surely reflect a more complex crystallization history. In these more complex
cases, some of the plagioclases may be looked upon as xenocrysts and the
rocks as having crystallized from magmas having some past history of
mixing or complex modification.
In summary, the zoning features of the plagioclases of the basalts reveal a
complex crystallization history. The earliest plagioclases indicate a varying
magma viscosity during initial plagioclase crystallization, resulting in
unzoned to oscillatory zoned cores of phenocrysts. These, in most cases,
later became resorbed and marginally zoned by more albitic plagioclase