Jökull - 01.12.1979, Síða 74

isotopic analyses, as well as geophysical evidence
and geotectonic ideas.
There is no doubt that Iceland is a pronounced
anomaly with regard to the Mid-Atlantic Ridge
south and north of Iceland. Extensive dredging on
the ridge both north and south of Iceland has
shown that MORB tholeiites are dominant on the
crest of the ridge. As noted above, there is a clear
gradient (Fig. 9) across Iceland with regard to
many elements. No less important is the fact that
the variety of rock types is much higher in Iceland.
The volume of basalt types, which are rare on the
abyssal ridge sections, as transitional and alkalic
types, is also much higher. No intermediate or acid
rocks are known with certanity from the submarine
parts of the ridge in the North Atlantic, whereas in
Iceland they may consititue some 8—13% of the
volume in the upper part of the crust. It is still a
matter of dispute whether these relations are
indicative or not of the existence of a mantle plume
beneath Iceland. It is, however, safe to conclude,
that Iceland is a plume or hot spot in the sense that
it is one of the major volcanic centers on earth, with
a very high discharge rate of volcanic rocks.
In terms of sea-floor spreading the Greenland-
Faeroes Ridge can be considered as a trace of the
Iceland hot spot since the opening of the North-
Atlantic. Petrological research in Iceiand, the
Faeroe Islands and in the Hebridean province
indicates that the bulk of the magma erupted by
the hot spot has been similar throughout the period
of development of this part of the North-Atlantic.
No final conclusions on the petrogenesis of the
rocks of Iceland can be presented here. Many
problems of the petrology of Iceland are being
actively considered at present. Discussion centers
perhaps on three main problems. 1. The origin of
the primitive members of the three rock series, and
the possible relation between them. 2. The origin of
the acid rocks and, 3. the nature of the suggested
mantle plume beneath Iceland and its influence on
the origin of the igneous rocks of Iceland. Some of
these problems are indeed not new in petrology and
are relevant outside Iceland.
SELECTED REFERENCES
Bailey, D. K. and R. Macdonald, 1970: Petro-
chemical variations among midly peralkaline
(comendite) obsidians from the oceans and
continents. Countr. Min. Petrol 28: 340—351.
Bailey, J. C. and A. Noe-Nygaard, 1976: Chemistry
of Miocene plume tholeiites from Northwest
Iceland. Lithos9: 185—201.
Blake, D. H., R. W. D. Elwell, I. L. Gibson, R. R.
Skelhorn and G. P. L. Walker, 1965: Some
relationships resulting from the intimate
association of acid and basic magmas. Q. J.
Geol. Soc. Lond. 121: 31 — 49.
Brooks, C. R. and S. P. Jakobsson, 1974: Petro-
chemistry of the volcanic rocks of the North
Atlantic ridge system. In Kristjánsson, L. (Ed.):
Geodynamics of Iceland and the North Atlantic
area: 139—154, Dordrecth, Reidel Publ. Comp.
Carmichael, I. S. E., 1964: The petrology of
Thingmúli, a Tertiary volcano in eastern Ice-
land. J. Petrol. 5: 435—460.
Carmichael, I. S. E., 1967: The mineralogy of
Thingmúli, A Tertiary volcano in eastern Ice-
land. Am. Miner. 52: 1815—1841.
Fridleifsson, I. B., 1973: Petrology and structure of
the Esja Quaternary volcanic region, southwest
Iceland. Unpubl. Ph. D. thesis, Univ. of Oxford.
Grönvold, K., 1972: Structural and petrochemical
studies in the Kerlingarfjöll region, central Ice-
land. Unpubl. Ph.D. thesis, Univ. of Oxford.
Grönvold, K., and H. Mákipáá, 1978: Chemical
composition of Krafla lavas 1975—1977.
Nordic Volc. Inst. 78 16: 49 pp.
Hart, S. R., J.-G. Schilling and J. L. Powell, 1973:
Basalts from Iceland and along the Reykjanes
Ridge: Sr-isotope geochemistry. Nature 246:
104—107.
Imsland, P., 1978: The petrology of Iceland. Some
general remarks. Nordic Volc. Inst. 78 08: 26
pp.
Jakobsson, S. P., 1972: Chemistry and distribution
pattern of Recent basaltic rocks in Iceland.
Lithos 5: 365—386.
Jakobsson, S. P., 1978: Environmental factors
controlling the palagonitization of the Surtsey
tephra, Iceland. Bull. Geol. Soc., Denmark 27,
Spec. Iss.: 91 —105.
Jakobsson, S. P., 1979: Petrology of Recent ba.salts
of the eastern volcanic zone, Iceland. Acta Nat.
Islandica 26: (in press).
Jakobsson, S. P., A. K. Pedersen, J. G. Ronsbo and
L. M. Larsen, 1973: Petrology of
mugearite-hawaiite: Early extrusives in the
1973 Heimaey eruption, Iceland. Lithos 6:
203—214.
Jakobsson, S. P., J. Jónsson and F. Shido, 1978:
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