Rit (Vísindafélag Íslendinga) - 01.06.1984, Side 9

ABSTRACT
Jan Mayen is a small volcanic island. The rocks form a practically
continuous rock suite with a “Daly gap” regarding number of eruptions, but
probably not regarding volume relations. The rock suite is potassic alkaline
and ranges from ankaramites through various basalt types and low-silica
intermediate rocks to trachytes. It contains various types of comagmatic
coarse-grained xenoliths, ranging from wehrlites to syenites in mineralogy.
The rock suite is generally porphyritic, but aphyric rocks of various com-
positional types are present. The most porphyritic rocks are the ankara-
mites, which in extreme cases may be over 50 per cent phenocrysts.
The phenocrysts are, in the ankaramite case, an early chrome-spinel,
chromian diopside and a kink-banded olivine (F090). These minerals crystal-
lize out of the most primitive mafic magmas of the magma system at
pressures exceeding 18 kb and at temperatures between 1400 and 1325°C.
The ankaramites further contain an unstressed, somewhat less magnesian
olivine and a titan-salite pyroxene. These minerals crystallize in the con-
tinuation of the wehrlite crystallization, during the early rise of the wehrlite
containing mafic magmas, primarily though before the lower stability level
of plagioclase is reached and above 1200°C. During these early crystalliza-
tion events, the magmas may be extensively fractionated. This wehrlite and
ankaramite fractionation produces Mg rich basaltic magmas.
These basic magmas continue to crystallize titan-salite and olivine, but
these are soon accompanied by plagioclase and some titanomagnetite at
~1200°C and ~10 kb. Within this category the bulk of the rock suite is
contained. This crystallization may be fractionating and thus produce more
evolved basalts, although this does not occur without modification.
Following the basalts in the rock suite are the low-silica intermediate
rocks, aphyric basalts and basalticjristanites. These are generally pheno-
cryst poor rocks. The crystallization of hydrous minerals starts and becomes
important in these rocks. Minor apatite may have crystallized out at more
primitive stages. Other minerals of major importance in these rocks are the
feldspars, which here for the first time become alkaline. This magma is
produced in the crust by remelting of amphibolites, and is not related to the
more primitive magmas of the suite through crystal fractionation.
The most evolved rocks of the suite are the tristanites and trachytes,
which are highly alkaline silica rich intermediate rocks, which contain
feldspars and biotite as the principal phenocryst phases. Crystal fractiona-
tion may be operating in this rock group, but it is not evolved from the