Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 36

32
PÁLL IMSLAND
undersaturated. This applies to the basic rocks and some oí'the intermediate
rocks. In spite of up to 8.26 per cent ne in the CIPW norm of the basic rocks
nepheline has not been positively identified in thin-sections (small patches
of low-refractory minerals occurring in some of these rocks may contain
nepheline or analcime). The majority ofthe trachytes is silica oversaturated,
containing up to 6.82 per cent qz in the norm. Quartz is however not seen in
thin-section. The saturation conditions of the rocks are shown graphically in
Fig. 9, where the range of normative ol, ne, and qz are drawn for each rock
group.
Trachyte
Tristanite
Basaltic tristanite
Aphyric basalt
Pl-ol porph. basalt
Pl-ol-cpx porph. basalt
Ankaramitic basalt
Ankaramite
qz 10
Trachyte
Tristanite
Basaitic tristanite
Aphyric basalt
Pl-ol porph basalt
Pl-ol-cpx porph. basalt
Ankaramitic basalt
Ankaramite
I----------1-----------1-----------1----------1-----------1-----------1----------1----------1
qz 10 5 0 5 10 15 20 25 30 ol
7. in CIPW norm
-1-----------1------------1-----------1------------1-----------1------------1-----------1
5 0 5 10 15 20 25 30 ne
-------------1------ 7. in CIPW norm
Fig. 9. Range of the normative components qz, ne, and ol in individual rock groups
of the Jan Mayen rock suite.
I. Ankaramites
Whole rock analyses: Jan 10, 12, 13, 17, 21, 23, 28, 30, 65, 166 & 167.
Mineral analyses from: Jan 10, 12, 30, 65 & 166.
The ankaramites are the most mafic rocks of the Jan Mayen rock suite.
They are rich in Mg, containing from 10 to at least 18.5 wt. per cent MgO
and they are highly porphyritic (Fig. 10), occasionally composed ofup to 65
per cent phenocrysts.
In thin-section the phenocrysts range in size from microphenocrysts and
up to more than 1 cm in diameter. In the field they are occasionally seen to
reach a few cm. The phenocrysts are mostly clinopyroxene and olivine.
Chrome-spinel occurs both as phenocrysts in the groundmass and as
inclusions in the olivine and clinopyroxene phenocrysts. Plagioclase pheno-
crysts are rare and small compared to the others.
The clinopyroxene phenocrysts range from diopside to salite in composi-
tion. The diopside is usually rich in chromium, containing up to 1.3 wt. per
cent Cr203. With decreasing Cr the diopsides show an enrichment of both
Ti and A1 and the salites are rich in both these elements, containing up to