Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 45

PETROGRAPHY
41
The titan-salite has a similar composition to the ankaramitic one. One
sample, Jan 66, contains scarce, sector zoned, green coloured, iron enriched
salite (salite-ferrosalite) as well as the titan-salite.
The composition of the olivine phenocrysts of the Mg richer rocks of the
group is about Fo90, but the microphenocrysts range in composition from
Fo90_71. The olivine phenocrysts of the Mg poorer rocks on the other hand
range from Fo82-79-
The chrome-spinel phenocrysts and inclusions are the same as those in
the ankaramites.
The groundmass is microgranular to intergranular as in the ankaramites
and consists of the same minerals; labradorite (An68_6i), titan-salite and
oxides. Occasional samples contain small phlogopite flakes.
III. Pl-ol-cpx porphyritic basalts
Whole rock analyses: Jan 6, 7, 8, 9, 11, 14, 15, 18, 19, 20, 27, 29, 40, 50,
51, 52, 55, 60, 62, 68, 69, 73, 80, 84, 85, 87 & 173
Mineral analyses from: Jan 15, 20, 27, 52, 69, 80 & 87
As a group these rocks comprise variable basalts which have the common
feature of containing plagioclase, olivine and clinopyroxene phenocrysts.
About one third of the analysed rocks of the rock suite fall in this group,
which is the most common rock group in the field as well.
The MgO concentration range is from 8.5 to 4 wt. per cent. The group
may be divided into two subgroups; one where plagioclase phenocrysts
dominate over the mafic phenocrysts (Fig. 23) and the other vice versa, the
plagioclase dominating one being much more common. The total amount of
phenocrysts diífers within the group. Some rocks are only scarcely porphyri-
tic, while others are highly porphyritic (Fig. 24). This holds for both
subgroups. Some samples contain both microphenocrysts and phenocrysts
which range up to more than 1 cm in diameter.
The clinopyroxene phenocrysts are the same as those described for the
ankaramites. The titan-salite is much more abundant and the chromian
diopside occurs only as cores surrounded by the titan-salite.
Fhe olivine phenocrysts still appear as both the unstressed and the kink-
banded anhedral types, though the latter type is much scarcer in the rocks of
this group than the preceding ones. The olivine composition ranges mostly
from ho90_72, the compositional range of individual samples being much
narrower. The most extreme range is found in Jan 27 (Fo89_75). Others are
relatively homogeneous as e.g. Jan 80 (Fo90_ss) or Jan 20 (Fo79_76). Occa-
sionally resorbed crystals of widely different composition occur. In Jan 15
which contains olivine phenocrysts of the composition Fo74_72, scattered
resorbed Fo65 crystals may also be found.
Fhe plagioclase phenocrysts are the most varying phase of this group.
1 hey range in size from microphenocrysts up to at least 5 mm in diameter