Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 310

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PÁLL IMSLAND
main result of this subsidence will be to intensify the magma generation and
thus to add life to the volcanic system.
The Sudurland volcanic zone in Iceland (Imsland, 1978b) is a non-rifting
volcanic area, erupting mildly alkaline rocks of heterogeneous compositional
trends (Imsland, 1978b; Óskarsson et al., 1979; Jakobsson, 1979). It is
situated south of the volcanic rift zone, which erupts tholeiitic rocks and
crosses Iceland from the northern coast towards the Reykjanes ridge. Its
relation to the highly productive rifting zone is the same as Jan Mayen’s
towards the Mohns ridge, close to a fracture zone.
Óskarsson et al. (1979) have explained this mildly alkaline volcanism of
the Sudurland volcanic zone as the result of the high thermal gradient
extending south from the rift zone into the oceanic tholeiitic crust of the
Sudurland non-rifting volcanic zone. This oceanic tholeiitic crust is drifting
towards the east along the Sudurland fracture zone, from its place of
generation and fractionation in the Reykjanes volcanic rift zone around 75
km to the west. The heterogeneous mildly alkaline magma trends are the
results of various mixing proportions of three end member magmas avail-
able in the magma system. These three are: 1) a primitive tholeiitic mantle
magma of the rift zones, injected into the older crust, 2) a ne-normative
basalt magma produced by remelting of the high-grade amphibolized
oceanic crust, and 3) a silicic (rhyolitic to rhyodacitic) magma produced by
minimum melting of the hydrated basalts. Here two crustal generated
magma types are at hand and one primitive mantle derived magma. The
crustal generated magmas are the remelting products of an unusually thick
oceanic ridge crust and the mantle magma is the bulk product of a “typical”
oceanic ridge mantle.
An identical model may be used to account for the Jan Mayen case, in a
qualitative version. The geological features are much the same, as described
above. The fundamental difíerence is that the Sudurland crust is of a purer
oceanic tholeiite origin, only slightly fractionated previous to rejuvenation of
volcanism, while the Jan Mayen crust is generated in a continental break-up
zone, later to be remobilized by another break-up event and most likely
highly fractionated during these events. This fractionation would be pri-
marily characterized by alkali- and incompatible element enrichment. This
difference is in agreement with the results as regards rock compositions. In
the Jan Mayen case the crustal derived magmas thus should show increased
alkalinity, as is exemplifled by the nonporphyritic rocks. The mantle derived
magma is ofa deep-rooted origin (70—80 km, see Fig. 126) and ofankarami-
tic composition. It comprises a relatively small part of the total magma. As
shown in the above chapter, this mantle derived magma apparently goes
through some crystal fractionation modification. In relatively primitive
compositions this magma bears the imprint of some alkali contamination.
Of the other end member magmas (the crustal derived ones), the most silicic