Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 303

PETROGENETIC RELATIONSHIPS
299
result of a fractionation of chromian diopside and forsterite rich olivine.
Chrome-spinel would be stable here as well, but its role in the fractionation
is minor. The highly magnesian basalts produced by this wehrlite fractiona-
tion may be further fractionated at lower pressure and temperature, by a
titan-salite and slightly less magnesian olivine, and assisted by titanomagne-
tite and plagioclase at the later stages, to produce the more common basalts
of moderate Mg content and relatively high K content, without much
modification of this process by others. This is apparently as far as the crystal
fractionation process has gone, in evolving the rock suite of Jan Mayen,
without serious modifications. Trace element chemistry of the rocks of the
rock suite, evolved beyond this stage, excludes the possibility that these
rocks are a direct crystal fractionation continuation of the less evolved rocks.
Crystal fractionation may have taken place, and apparently did, in these
highly evolved rocks, but they are not produced from the basalts by that
process. The analyses of the data are not sufficiently detailed to allow
quantitative modelling of the evolutionary process of the evolved rocks, but
in the next chapter their mode of origin and evolution is explained in a
qualitative way.
D. THE EVOLUTION OF THE NORTH ATLANTIC
AREA - ITS EFFECT ON THE JAN MAYEN
MAGMA SYSTEM AND THE COMPOSITIONS
OF ITS PRODUCTS
Detailed geophysical studies have been undertaken in the North Atlantic
area. Compared to other oceanic areas its evolution is quite complex.
Nevertheless, there is agreement between numerous authors who have
written on the subject on most major points in this evolution. A simplified
picture will be drawn up here mostly following Talwani & Eldholm (1977)
and Talwani (1978). A short summary chapter on this matter is given by the
author (Imsland, 1978a) in describing the geology ofjan Mayen. The main
features of interest in this discussion are drawn on a map in Fig. 132. The
summary presented here serves as the basis for putting the Jan Mayen
magma system into its spatial and temporal position with respect to the
evolution of the crust of the area, within the framework of global tectonics,
and the influence this evolution might have on the products, age, and
behaviour of this magma system.
Since the opening of the North Atlantic, about 60 m.y. ago, the Mohns
ridge has been an active spreading ridge. It is thus a stable element in the
global tectonics of this area, compared to the ridge segments south of it. At
the position just north ofjan Mayen it was offset by the eastern segment
(now extinct) of the Jan Mayen fracture zone, towards the Aegir ridge. This
Aegir ridge is now extinct but at this time it was the spreading axis of the