Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 307

PETROGENETIC RELATIONSHIPS
303
crust and it has a complex history. Indications that some continental
segment is retained in the plate at its northern extremity (i.e. in the Jan
Mayen area) are few. Apparently no recognizable continental rock xenoliths
have been brought to the surface by the Jan Mayen eruptions. The Sr
isotopes and the Rb-Sr chemistry of the Jan Mayen rocks reported by
Lussiaa-Berdon-Polve & Vidal (1973) and O’Nions & Pankhurst (1974) fa.ll
clearly within the relatively narrow compositional field characterizing
oceanic basaltic rocks when compared to the compilation of Powell & Bell
(1974). This narrow field, though, within the wider field, characterizes
continental basaltic rocks and is thus not conclusive. On the basis of
geophysical data, Grönlie et al. (1979) believe the continental segment in
the Jan Mayen ridge to end somewhere about 50 km south ofjan Mayen
island. All this points to the absence of a continental rock segment under Jan
Mayen.
How this material will be affected and how it will respond, when entering
the thermal anomaly, depends on the temperature and the temperature
pattern in the thermal anomaly, the constitution of the crust entering it, and
the time the material spends within it.
Let us consider the material first. Whether a pure continental segment lies
in the crust below Jan Mayen or not, this crust is the former break-up zone
of a continent and is thus in many respects not typical for the ocean bottom.
The sharp escarpment east and southeast of Jan Mayen, where the shallow
ocean bottom between Iceland and the Jan Mayen fracture zone slopes
down to the deeper Norway basin, was the continental margin of Greenland
between 60 and 30 m.y. ago, formed during the initial break-up of the North
Atlantic area. Just inside this margin, a new break-up occurred, when the
Aegir ridge became extinct and the spreading axis moved west. These two
break-up episodes must have affected its surroundings to some extent. The
first step in continental break-up is a thermal updoming of the area (Gass,
1972) and magma injection which is followed by the gradual thinning of the
continent and its final separation. This is a process ofsome millions ofyears
duration. Metamorphic reactions are thus given time to take place and the
continentai crust may thus be expected to be able to readjust to the rising
temperature. Similarly there is time for fractionation of the new material in
a favourable environment. During the formation of the East African rift
system the compositional trends of the highly fractionated volcanic rock
suites produced change from strongly alkaline through transitional to
tholeiitic. This compositional pattern, of decreasing alkalinity, coincides
with the thinning of the continent and its final disappearance (Mohr, 1972;
King & Chapman, 1972; Barberi et al., 1970 and 1975), where the crust
becomes a “typical” ocean bottom. During the break-up of the North
Atlantic continent discussed here, a similar evolution most likely took place.
Nielsen (1975) equates the present spreading process in the Afar region with
the flexuring and dike injection process of Eastern Greenland during the