Jökull


Jökull - 01.12.1979, Page 73

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
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