Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 100
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PÁLL IMSLAND
This indicates that plagioclase plays no role in the ankaramites. Plagioclase
phenocrysts only occur in these rocks as extremely scarce small phenocrysts.
As mentioned in chapter 6, the amount of the titan-salite in the ankaramites
decreases with increasing maficity, while the chromian diopside amount
increases. This is confirmed by the Cr increase and Ti decrease with
increasing Mg in the ankaramites.
The increasing amount of K, Na and Rb in the trachytes with decreasing
Mg is reflected by an increasing content of alkali feldspar phenocrysts.
Biotite accounts for the excess increase of K over that of Na. The greatly
increasing Zr content of the trachytes is reflected in the occurrence of
occasional small euhedral zircon grains.
The P maximum in the Mg poor basalts and Mg rich intermediate rocks
is in harmony with the rather frequent occurrence of apatite phenocrysts in
these rocks. These apatites are usually associated with iron-titanium oxides
and both iron and Ti show a maximum in these rocks as well.
C. XENOLITH COMPOSITIONS
The whole rock compositions of the xenoliths are given in Table 13.
Published chemical analyses of oceanic island xenoliths are rather few but
plentiful descriptions indicate that xenoliths generally occur on the islands
and show a wide variety in chemical range and mineral compositions. In the
literature however the major emphasis is on the coarse-grained xenoliths.
Daly (1965) describes granitic, syenitic, dioritic, gabbroic and peridotitic
xenoliths from Ascension. Le Maitre (1965) describes gabbroic and perido-
titic xenoliths from Gough Island and Bennell (1969) describes gabbroic to
peridotitic xenoliths from the Azores and so on. The coarse-grained xeno-
liths of Jan Mayen are of a similar compositional range as those found on the
other islands. In addition to the coarse-grained xenoliths, which may
contain minor amounts of glass, silicic xenoliths, containing up to 50 or
almost 100 per cent clear acid glass, occur on Jan Mayen.
I. Coarse-grained xenolith compositions
The coarse-grained xenoliths are wehrlites, gabbros, hydrous mineral
gabbro-like rocks, and syenites.
The wehrlite (151, Table 13) is composed of about equal amounts of
olivine and clinopyroxene with minor chrome-spinel and a trace of phlogo-
pite. Accordingly it is a highly magnesian rock (32.5 per cent MgO) rich in
Cr and Ni and poor in the LIL elements. It contains 47.15 per cent Si02, 8.3
per cent Fe203‘, 9.85 per cent CaO and only 1.62 per cent A1203. The
composition is thus not unusual for an ultramafic xenolith except that the
Ca amount is high relative to most ultramafic xenoliths. This is in agree-