Rit (Vísindafélag Íslendinga) - 01.06.1984, Side 32

6. PETROGRAPHY
A. INTRODUCTION
In this chapter a general description of the rocks and their minerals is
given. In the rock suite section some general comments on the rock suite are
given and then each rock group is described in following subsections. Whole
rock and mineral chemistry is dealt with in subsequent sections. The general
mineral compositions and range of the most influencing major elements are
given here for orientation. In the subsections on individual rock groups the
description is arranged so that the samples are listed first, where whole rock
chemical analyses and mineral analyses have been made. This is followed by
a short note on the characteristic chemistry of the groups and some general
comments on the phenocrysts of the group. The main text is a description of
the mineral phases. Finally the groundmass textures are briefly described.
In the subsections on the xenoliths the description is similarily arranged.
The subsection on the hyaloclastites briefly describes those rocks, which
belong to the rock suite, but have not been analysed. The mineral composi-
tions given in the text are calculated from microprobe analyses, where minor
elements are excluded.
In hand specimen the rocks do not differ from other related volcanic
rocks. A description of their occurrence in the field has been given pre-
viously by the author (Imsland, 1978a), and a somewhat more detailed
mineralogical description appears in Imsland (1980).
B. THE ROCK SUITE
The rock suite is highly differentiated, having a Differentiation Index
ranging from 11 to 91. This differentiated character is clearly revealed by
the lithology of the rocks of the suite, which range from highly porphyritic
ankaramites through various types of phyric and aphyric basalts and low-
silica intermediate rocks to porphyritic trachytes. The total amount of
phenocrysts of some samples may reach up to about 65 per cent of the rock.
The amount of phenocrysts of the analysed samples is estimated, the result
being shown in the histogram in Fig. 5. About 50 per cent of the analysed
samples represent rocks where the phenocryst content is from 5 to 65 per
cent of the rock and almost 90 per cent of the rocks contain > 1 per cent of
phenocrysts. The range of the total phenocryst content of individual rock