ABSTRACT
The island of Jan Mayen is the northernmost active volcano on the
Mid-Atlantic
Ridge. The rocks of Jan Mayen belong to the potassic series of the
alkaline
rocks and appear to belong to the straddle type association. The
ankaramites
and alkali olivine basalts are characterized by the presence of
large xenocrysts
of rimmed chromium diopside, titaniferous salite, olivine (Fo83 to
Fo88),
magnetite and sometimes plagioclase (bytownite rimmed by
labradorite).
Phenocrysts of olivine (Fo74) and plagioclase (andesine) are
present in
several rocks. These and phenocrysts lie in a matrix composed of`
titaniferous
salite, olivine (Fo58), plagioclase (andesine), magnetite, biotite
and
sometimes ilmenite. Latite andesites contain large crystals of
plagioclase
of an andesine-oligoclase composition rimmed by oligoclase or
alkali feldspar,
magnetite, biotite, hornblende and sometimes highly altered
clinopyroxene.
The trachytes are characterized by phenocrysts of aegirine-augite,
anorthoclase, magnetite, biotite and sometimes andesine rimmed by
an alkali
feldspar. The phenccrysts of the latite andesites and trachyte lie
in a
groundmass consisting of andesine, alkali feldspar, magnetite,
hematite
and biotite.
Trace element whole rock geochemistry indicates that the rocks of
Jan
Mayen were probably derived from the mantle from slightly
different source
regions based on different Rb/Sr ratios for the mafic rocks of
Nord Jan
and Sor Jan. The rocks formed by the partial accumulation of
titaniferous
salite, plagioclase, olivine and magnetite from an alkali basalt
magma
followed by the intrusion and capture of xenoliths and xenocrysts
of ultramafic
or mafic rocks, which may have been in the process of
accumulation. This
accumulation process is supported by the results of least squares
magma
mixing models and Rayleigh fractionation curves.
White, C.A., 1979. Petrology and mineral chemistry of some
Jan
Mayen volcanics. Unpublished MSc. thesis, State University
of New
York at Albany.
197pp., +x.
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