Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 305

PKTROGENETIC RKEATIONSHIPS
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been active since. Most authors take the so called Iceland plateau axis as an
active ridge for a short time, from 24 to 18 m.y. ago. This axis would then be
responsible for the segmentation of the Greenland coast. Vogt et al. (1980)
claim this axis to be nonexistent as a spreading axis and ascribe all the
activity, since the Aegir ridge extinction, to the presently active Kolbeinsey
ridge (Iceland-Jan Mayen ridge by some authors), which then ruptured
Greenland some 24—30 m.y. ago.
Jan Mayen lies over 200 km east of the spreading axis of the Kolbeinsey
ridge, which is responsible for its eastward drift. Just east ofjan Mayen the
bottom slopes down to the deep Norway basin, which has a thin “normal”
oceanic crust formed by the Aegir ridge activity or perhaps partly by a small
isolated ridge segment of the same age and activity (Grönlie et al., 1979) as
the Aegir ridge. Jan Mayen sits on the northern end ofthejan Mayen ridge,
just south of the Mohns ridge. Between the Mohns ridge and Jan Mayen is
the Jan Mayen fracture zone, which there is 2000 m deep and 20 km wide
(at the bottom) according to Johnson & Campsie (1976). According to
Sörnes & Navrestad (1976) the crust is about 18 km thick underjan Mayen.
Evans & Sacks (1979) claim the crust of the Iceland plateau just west and
southwest ofjan Mayen to be of identical thickness, but this part has not
been claimed to contain any continental remnants. This crustal thickness
does have a stronger resemblance to the Icelandic crust (Pálmason, 1971)
than to the typical oceanic crust. According to Grönlie et al. (1979) the
continental section in the Jan Mayen ridge apparently does not extend
through the length of the entire ridge. It extends as far north as 70.5° and
does thus not underlie the Jan Mayen island itself. According to these
authors this “microcontinent” is about 75 km wide. Judged from the more
detailed profiles of Gairaud et al. (1978) the continental rock sections in this
“microcontinent” may be considerabiy narrower. The ridge is further shown
by these authors to be considerably faulted, most probably internally offset
and rifted as well (between 30 and 24 m.y. ago), all ofwhich bears witness to
a complex history. The ridge is covered by thin flat-lying sediments and
sedimentary rocks of both post- and pre-rifting age, i.e. 24 m.y. (Gairaud et
al., 1979; Johnsson, 1975). D.S.D.P. drilling sites 346, 347 and 349 are all on
top of the Jan Mayen ridge. None of these holes reached the depth of the
supposed continental rocks. The presence of these is thus in fact unproved
(Talwani, 1978). The complex evolution of the area has a number of
consequences when it comes to magma genesis and magma evolution, as
will be shortly discussed in this chapter.
The Mohns ridge volcanism is poorly known, as are its products. Taking
the Mohns ridge as a “normal” oceanic spreading ridge, a view for which
there are at present no objections and which the 60 m.y. of continuous
production clearly indicates, its products would be of the “normal” oceanic
tholeiite type, and the ridge would give rise to a “normal” oceanic crust.
There is thus a clear compositional as well as structural contrast between