Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 314

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PÁIX IMSLAND
5) Phenocrysts are present in the rocks throughout the entire wide
compositional range of the rock suite. In some rock groups they are
extremely prominent, as for instance in the ankaramites, where they may
constitute over 50 per cent of the rock volume. In other groups they may be
present as scarce microphenocrysts only, as for instance in the basaltic
tristanites. In spite of a generally porhyritic rock suite, such practically
aphyric rocks are present as a minority in most compositonal types of the
rock suite. In the ankaramites and basalts the groundmass of the rocks is
generally microgranular to intergranular in texture. Ophitic textures do not
occur. In the more evolved rocks, the groundmass is generally of trachytic
texture.
6) A variety of coarse-grained xenoliths occur in the rock suite. These are
composed of the various minerals that occur as phenocrysts throughout the
lavas. There is thus a close relationship between the genesis of the xenoliths
and the lavas that contain them. The wehrlites occur in the ankaramites
only and the phenocrysts of the ankaramites are partly the same as the
wehrlite minerals. Syenites occur in the trachytes and tristanites and are
composed of the trachyte phenocrysts. Gabbros occur in the basalts and so
on. These xenoliths may be in various stages of fusion, if present in lavas of
higher magma temperatures than the solidus temperature of the xenolith
rock. Small amounts of compositionally heterogeneous glasses may thus be
present in the gabbros. The hydrous mineral xenolith contains glass closely
similar to the basaltic tristanites in composition. The syenitic xenoliths are
found partially to nearly totally fused, and thus composed of a clear glass of
trachytic composition. An example of a partially fused rock giving a
rhyolitic glass and unfused quartz remnants is known, but as an exception.
B. MINERALOGY
1) The olivines range from F092-51 in composition, from kink-banded
anhedral to unstressed and euhedral in appearance, and from wehrlite
xenoliths and ankaramite lavas to trachytes in occurence. When compared
to the clinopyroxene, the olivine, in contrast to the cases of most other rock
suites, has a relatively small effect on the rock suite evolution. In the late
olivines, Mn is highly enriched. Ca is generally present and in relatively
high concentrations. Ni is primarily found in the early olivines.
2) The clinopyroxenes range from chromian diopside through titan-sal-
ites to ferroaugite in compostion, and from anhedral to euhedral crystals of
complex zoning (including sector- and patchy zoning) in apperance. Clino-
pyroxene occurs as a major phase in all rock and xenolith types, except in
the nearly aphyric low-Mg basalts and the hydrous mineral xenolith. It is
the mineral of greatest influence in the evolution of the mafic and basic
magmas. The early ones are Cr rich but Ti- and A1 poor diopsides, which
become Cr poor and Ti- and A1 rich salites as the evolution goes on.