Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 102

98 PÁLLIMSLAND
ment with the fact that the rock contains clinopyroxene in great amounts,
but no orthopyroxene.
Generally the compositions of the gabbro xenoliths (146 & 148, Table 13)
are similar to those of the basalts of the rock suite. The gabbros are slightly
lower in Si than the basalts. The Ca content is slightly higher and the total
iron content is similar to the highest ones found in the basalts. The gabbros
are lower in K than any rocks of the suite and resemble tholeiites in their
LIL element concentrations. They are however slightly ne-normative.
The hydrous mineral xenolith (329, Table 13) is a ne-normative basic
rock, similar to the gabbros in chemical composition but quite different in
mineralogy, containing three hydrous minerals (amphibole, biotite and
apatite) in great amounts. It is rather low in Si02 (43.75 per cent) but
relative to the gabbros it is high in Sr and the LIL elements, especially P2O5
(2.12 per cent) and low in Ca, Ni and Cr.
The syenites (324, Table 13) are allotriomorphic rocks composed of
feldspars and some accessory minerals. They are high-silica intermediate
rocks of identical chemistry to the silica rich trachytes of the rock suite.
The assemblage of coarse-grained xenoliths on Jan Mayen thus shows a
wide scatter of chemical and mineralogical compositions.
II. Xenolith glass compositions
The compositions of the glasses occurring in the xenoliths are given here,
while some other features closely related to the glasses are also described.
a. The glassy xenoliths
The composition of the glassy xenoliths is trachytic in the case of Jan 5
and 142, but rhyolitic in the case of Jan 149 (Table 13). The trachytic
examples are very similar to the trachytes of the rock suite in composition
regarding most elements.
Jan 5, which is composed of clear glass with minor amounts of feldspar
microlites, may be slightly contaminated by the basaltic eruption products.
This is indicated by the slightly higher V, Cr, Co, Ni and Ti contents, than
are found in the trachytes.
Jan 142 is composed of about equal amounts of feldspars and clear glass.
Glass compositions are given in Table 14. The glass is lower in Si than both
the feldspar and the whole rock. It is higher in both Na and K than the
whole rock, lower in Na than the oligoclase and lower in K than the alkali
feldspar. The overall composition of the glass resembles feldspars, contamin-
ated by some mafic component. This xenolith might thus be a partly
remelted trachyte, where the mafic components and some feldspars are
melted while the opaque oxides and some feldspars are still unmelted at the
time of quenching. The melted mafic components were probably mostly
biotite, which is common in the trachytes. The escape of its volatile content