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
Mayen volcanics. Unpublished MSc. thesis, State University
York at Albany.
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