Jökull - 01.12.1979, Page 73

Fig. 13. A section through the Reykjadalur central volcano, western Iceland, showing the meta-
morphic zones around it. Large dots, chabazite-thomsonite zone; medium dots, analcime zone; small
dots, mesolite-scolecite zone; horizontal lines, laumontite zone; crosses, epidote zone. From
Jóhannesson 1975.
probably represents a true regional metamorphic
facies.
Superimposed on these incipient regional burial
metamorphism zones are local hydrothermal
aureoles associated with central volcanoes. The
inner aureoles may bear chlorite, epidote, calcite,
quartz, laumontite, garnet and occasionally
pyroxenes and amphiboles. Laumontite and calcite
may be prominent in the outer zones. Fig. 13 shows
one example of such a thermal dome, in this case
the metamorphic zones around the Tertiary
Reykjadalur central volcano, western Iceland.
Here it is found that the high-grade mineralization
is closely related to the intrusive activity in the
volcano. True greenschist facies has only been
found outcropping at one locality, the Geitafell
central volcano in SE-Iceland.
ICELAND AS PART OF THE MID-ATLANTIC
RIDGE
Iceland is situated in the North Atlantic where
two large physiographic structures meet, the
Mid-Atlantic Ridge and the Grennland-Faeroes
Ridge (Fig. 14). Because of the elevated
topography, high heat flow and discharge of
volcanic rocks, Iceland has been defined as an
anomaly on the spreading Mid-Atlantic Ridge.
Iceland and similar regional highs on the mid-
ocean ridges have been proposed as surface
manifestations of mantle upwellings and called
“hot spots” or “hot mantle plumes” where partial
melts from the mantle are discharged through a
pipe. The plume models are based on extensive
geochemical work, mainly trace element and Sr-
Fig. 14. Map showing the
Tertiary to Recent vol-
canic areas of the North
Atlantic and the main
physiographic structures.
From Brooks and Jakobs-
son 1974.
JÖKULL 29. ÁR 71