Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 19

15
PREVIOUS STUDIES OFJAN MAYEN ROCKS
(1965), gives an analysis ofjan Mayen trachyte, a separate analysis of its
groundmass and partial analyses of its sanidine and coexisting plagioclase
phenocrysts. Berlin & Henderson (1969) added Sr and Ba to these analyses.
Carmichael’s (op.cit.) main conclusion in this connection is that “if two-
feldspar salic, particularly oversaturated, liquids are accounted derivatives
by fractional crystallization of basaltic magma, then the parental basalt
must itselí be potassic. The problem of the origin of the two-feldspar
potassic salic liquids therefore becomes a problem of the origin of the
potassic nature of the parental basalts”.
Rocks are described in an article on the geology of the north eastern
corner of Jan Mayen by Roberts & Hawkins (1965) and another on the
petrology of this area by Hawkins & Roberts (1972). Ten basalt and 3
ankaramite analyses are given and 10 olivine and 16 clinopyroxene analyses.
In the former article the authors do not deal much with the petrology but
conclude that the rocks belong to the alkali basalt series and are undersatu-
rated, highly potassic rocks similar to certain other island rocks (Gough,
Iristan da Cunha, and Azores). They further conclude that many of the
rocks are accumulative, that ankaramites are unusually abundant and that
the partent is a trachybasalt. Regarding the phenocrysts they conclude that
the diopsidic augite (chromian diopside) and olivine (often showing glide
lamellae) are xenocrysts implying the existence of a magma reservoir. In the
latter article on the other hand they conclude that the olivine xenocrysts/
phenocrysts are unrelated to the composition of the rocks they occur in; the
Mg/Fe ratios and the Ni concentrations are more similar to those of
pendotite and dunite olivines than to basaltic ones. The rocks, they say,
have undergone only moderate differentiation at near surface pressures and
the variation in rock types cannot be adequately explained by crystal
difl'erentiation under such conditions. The chromian diopsides and the Mg
nch olivine (xenocrysts) have crystallized before the titan-salites and the
Mg poorer olivine (phenocrysts) and at higher pressures. Spinel, garnet and
orthopyroxene were not found in the rocks. The first differentiation product
of the magma at high pressure could have been Mg rich olivine and
chromian diopside giving wehrlite. The wehrlite fractionation would give a
residual liquid, enriched in K relative to Na and rich in A1 and Ti, thus
being able to produce the primitive Jan Mayen magma. The formation of
the primitive melt could have taken place in the following way: Upper
mantle temperature rose and was high enough for a sufíicient time for the
wehrlite conditions to be attained. Still rising temperature would result in
fractional melting and formation of a basaltic liquid. At lower temperature
again this liquid would originally precipitate wehrlite, if held at 25 to 30 kb
pressure, producing the primitive, K, A1 and Ti enriched, Jan Mayen
magma. The xenocrysts are then wehrlite cumulates, accidentially brought
to the surface. The scarcity of trachyandesites on Nord-Jan leads these
authors to the conclusion that a long-lived magma chamber under Nord-Jan