Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 60

56 PÁLLIMSLAND
titanomagnetite the groundmass of Jan 81 and 35 contains small laths of
ilmenite.
The biotite is the second most important phenocryst phase of this group
(after the feldspars) and may occur in considerable amounts. The composi-
tion is the same as in the tristanites, but the amount is greater in the
trachytes, where it occurs in almost all samples. It ranges from euhedral
crystals (Fig. 40) through embayed examples to anhedral remnants. They
may be unaltered, but most often they show the beginning of oxide
exsolution at the crystal edges (Fig. 41). Occasionally they are totally
altered to a fine-grained aggregate of oxides and silicates (Fig. 42).
Apatite occurs occasionally, mostly associated with the oxide grains as in
the previous groups described.
Small scattered microphenocrysts of kaersutitic amphibole occur in some
of the samples (Fig. 43). Large crystals (up to 4 mm) of this same mineral
occur in Jan 36 (Fig. 44), where it is surrounded by an extensive corona of
silicates and oxides, very similar in appearance to the xenocrysts occurring
in the pl-ol-cpx porphyritic basalts. The corona minerals are: biotite,
titanomagnetite, apatite, salite and kaersutite. Irregular patches of basaltic
hornblende occur in the groundmass of the samples and may enclose
clinopyroxene microphenocrysts (Fig. 45).
Olivine is rather common as very small microphenocrysts in Jan 172. It
occurs as euhedral crystals of Fo74. Euhedral to subhedral, scattered,
slightly altered Fo53_5i microphenocrysts occur in Jan 168.
Euhedral small microphenocrysts of zircon are seen in Jan 77.
The groundmass of these rocks is usually trachytic, composed of feldspar
laths and small grains of oxides and clinopyroxene. The groundmass of Jan
77 is not fully crystallized and is composed of semi-oriented feldspar laths
and clear glass.
IX. Mixed rocks
Whole rock and mineral analyses: Jan 174
Jan 174 is an unusual rock in the rock suite ofjan Mayen, a basalt rock
crowded with trachyte chips. The trachyte only shows extremely small signs
of remelting and makes up about 10 per cent of the rocks.
The whole rock chemistry is the same as that of the MgO poorer pl-ol-cpx
porphyritic basalts and the pl-ol porphyritic basalts. MgO is 5.4 wt. per
cent.
The mineralogy of the basalt component is the same as in the ankaramitic
basalts, i.e. olivine, kink-banded (Fo8a) and unstressed (Fo75) and titan-sal-
ite phenocrysts with exceptional plagioclase microphenocrysts of labradorite
(An71_68), in an intersertal groundmass.
The trachyte component is mostly a trachytic groundmass composed of
anorthoclase to alkali feldspar (Or34Ab55Ann — Or^gAb^oAnJ and iron-