Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 50

46
PÁLL IMSLAND
titan-salite. This amphibole is brown in colour and strongly pleochroic. It is
kaersutitic in composition.
The groundmass of these rocks is microgranular to intergranular, occa-
sionally intersertal. In the intersertal samples the glass is usually dark
coloured and at least one sample may be described as microporphyritic
tachylite Qan 18). The groundmass minerals are plagioclase laths which
vary in composition from sample to sample from An78_65, as well as titan-
salite and oxides. The oxides are mostly titanomagnetite. Ilmenite occurs in
many samples in small amounts, both as separate grains and as lamellae in
titanomagnetite. Chromc-spinel occurs occasionally as a groundmass mine-
ral. Olivine is usually not a groundmass mineral or occurs as such only in
small amounts. Flakes of phlogopite frequently occur in the groundmass of
these rocks. When the rocks are vesicular, these flakes extend from the walls
of vesicles into the cavities or straight across them. The ílakes may occa-
sionally reach the size of a few mm.
IV. Pl-ol porphyritic basalts
Whole rock analyses: Jan 24, 42, 47, 48, 49, 53, 54, 56, 58, 59,
61, 63, 64 & 91
Mineral analyses from: Jan 48, 59, 61 & 63
This group is in most respects similar to the preceding one except that
clinopyroxene phenocrysts no longer appear and phenocrysts are generally
more scarce. MgO varies from an upper limit of 6 wt. per cent to as low as 4
wt. per cent, which is the same as the lower limit of MgO in the preceding
group. In most of the samples olivine phenocrysts are scarce relative to
plagioclase phenocrysts and microphenocrysts always dominate the pheno-
cryst content. Oxides occur as microphenocrysts of iron-titanium oxides
only. Chrome-spinel is absent in this group.
The olivine phenocrysts are mostly euhedral and may have a semi-
skeletal shape. The Mg rich kink-banded olivine still appears but is
extremely scarce. The microphenocrysts range in composition from Fo78_72
except in Jan 63, which is a coarser grained rock which has solidified in a
vent pipe. Here the olivine composition is Fo6o_5g.
The plagioclase phenocrysts are mostly euhedral, normally zoned micro-
phenocrysts. Some samples contain scattered, slightly bigger phenocrysts,
which usually show internal resorption as described from the plagioclase in
the previous group. The plagioclase compositions vary from sample to
sample, but are mostly between An75_56.
The iron-titanium oxides range from groundmass size and up to micro-
phenocrysts. These are mostly titanomagnetite, but ilmenite occurs both as
separate grains and as composite type intergrowth (Haggerty, 1976). The
titanomagnetite microphenocrysts range in composition from Usp72_56 and
the ilmenite from Hm8_3. The groundmass grains have similar compositions