Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 308

304
PÁLL IMSLAND
Tertiary era. The basalt province of Eastern Greenland displays a variety of
rock compositions (Noe-Nygaard, 1976; Deer, 1976; Brooks et al., 1979). To
the author’s knowledge, no investigations have been made into the effect of
the continental break-up process on the older continental rocks. Neither
theoretically nor practically therefore are the older continental rocks of
Eastern Greenland of assistance in this respect. On a regional scale the
Eastern Greenland basalt province has been unsystematically studied, but
rocks of both alkaline and tholeiitic trends occur, both primitive and
evolved. The Jan Mayen crust was, prior to the present Jan Mayen
volcanism, a part of this formation. It is thus not a typical tholeiitic ocean
bottom originally, but rather the break-up zone of a continent, composed of
the variety of basalt types characterizing such zones, and more or less
fractionated derivative rocks. It apparently does not contain any segments
or blocks of recognizeable continental crust. Quantification of this modifica-
tion of a “typical” oceanic crust is practically impossible, but qualitatively it
would be characterized by a slight enrichment of the felsic magmatic
components, but primarily by the incompatible elements.
On the basis of Pálmason’s (1973) kinematic model of crustal accretion at
oceanic ridges and available data on temperature pattern in the oceanic
crust, Óskarsson et al. (1979) have shown that the prograde metamorphism
of the oceanic crust all takes place within the rift zones. Here the basalts of
the crust are extensively hydrothermally altered and the rocks converted to
zeolite, chlorite-epidote, greenschist, and low-grade amphibolite facies
mineralogy (as the depth increases), and at depths, where the anatexis sets
in, to high-grade amphibolite and granulite facies mineralogy. This would
be the stratified mineralogy of the fractionated oceanic crust entering the
thermal anomaly at Jan Mayen.
The temperature pattern in this thermal anomaly is a relatively shallow
maximum or bulge of the isotherms close to the fracture zone and its fading
out in the east, south, and west directions. The drifting plate thus bypasses a
temperature maximum and its northern edge goes through the highest
temperature. The present position of Jan Mayen island (the maximum
volcanic activity) in relation to the temperature maximum is not obvious,
because of the poorly known structure of the Mohns ridge. Apparently, Jan
Mayen is now somewhat west of the shallow top of the temperature
maximum. The size (the steepness of the isotherm pattern) of the anomaly is
also unknown, as is its exact temperature. According to the model of
Oxburgh & Turcotte (1968) the temperature is however high enough,
several tens of kilometers (probably over 100) away from the maximum
bulge, to allow partial melting of hydrated basalts in the oceanic ridges. Jan
Mayen island extends over 50 km south-west from the fracture zone and
thus gives a minimum extent of the thermal anomaly in that direction.
The time that the material spends within the thermal anomaly depends
on the size of the anomaly and the drift rate of the oceanic plate. The rate of