Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 319

SUMMARY AND CONCLUSIONS
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minerals in each case, is needed. Nevertheless, the result is not in good
agreement with the analysed rocks, especially regarding the alkali elements
and Mn.
5) A test on this least-squares calculation model, using the trace ele-
ments, shows that the model may be imitating the natural process quite
nicely in the case of early fractionation and probably the basalt evolution as
well. In the case of the evolved rocks, on the other hand, a crystal
fractionation is apparently not responsible for evolving these rocks from the
more primitive ones. Most likely, though, the process was operating along
with other processes.
D. CRUSTAL EVOLUTION AND PETROGENESIS
1) The evolution of the North-Atlantic area is complex relative to most
other oceanic areas. The reason for the active volcanism in Jan Mayen and
its alkaline nature are hidden in this complexity.
2) During the continental break-up, at the time of the opening of the
North Atlantic area, and also during the repetition of that process 30 m.y.
later, at the previously formed continental margin, the oceanic crustal
segment now found under and immediately east ofjan Mayen, was formed.
This oceanic crust inherited a highly fractionated nature during these
processes. This crust is now driven by the oceanic spreading process along
the Jan Mayen fracture zone into the high thermal gradient extending
towards the south from the end of the Mohns spreading ridge axis.
3) On entering the high temperature zone, this crust responds by partial
melting. The result is a ne-normative, basic, alkaline magma of relatively
low Mg content, when the amphibolites of the crust (exemplified by the
hydrous mineral xenolith) are remelted, and a more trachytic magma, in the
case of the minimum melting of the hydrated basalts of this crust, which is
alkali enriched since the time of continental break-up. These two magmas
may be erupted in their pure state, they may be modified by crystal
fractionation, and they may mix. These magmas or their products mix with
the primitive mantle-derived magma and its derivative magmas and thus
cause the alkalinity of the rock suite as a whole. By the derivative magmas is
here meant the crystal fractionated derivatives of the most primitive
magmas.
4) The alkalinity of the rock suite is thus a crustal-induced phenomenon,
rooted in the early origin of the crust (through which the rock suite is
erupted and within which it is partly produced) and its later entrance into a
thermal anomaly. The non-alkaline mantle magmas are not found on the
surface in an unmodified state. Relative to spreading centre volcanism, this
mantle magma is small in amount, while the crustal-derived magmas are
relatively voluminous.