Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 168

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tive content of olivine varying 0.3-18%. The eastem volcanic
zone is, on the other hand, divided into three regions. At the
southemmost end there are alkali-olivine basalts, then transitional
alkali basalts and tholeiites in the north-east with normative com-
positions ranging from olivine 10% to quartz 5%.
As expected, all the lavas in the Krísuvík-area are tholeiitic, the
composition being either quartz or olivine normative. The average
composition is just about saturated with normative quartz.
Oxidation has considerable influence on the normative composi-
tion, so that oxidized samples tend to be quartz normative. Chayes
(2) suggested that a sample is oxidized if the Fe203/Fe0 ratio is
higher than 0.6. Irvine and Baragar (3) on the other hand suggested
that samples are oxidized if the percentage Fe203 is higher than
percentage Ti02 plus 1.5. Using these criteria 8 analyses needed
correction and the ferric/ferrous ratio was adjusted according to
the FMQ-buffer (4). However, even after this correction 13 ana-
lyses are still quartz normative. This is in contradiction to Jakobs-
son, (1) who maintains that only olivine normative tholeiites are
found in the Reykjanes-Langjökull volcanic zone. But comparing
the analyses with those used by Jakobsson suggests that the norma-
time classification may not be the most suitable method of dist-
inguishing between the different basalt types in Iceland.
The slight variation in the main components observed can be
explained by differentiation of plagioclase, ohvine and pyroxene
(see fig. 2), and microscopic investigation shows that plagioclase
and olivine phenocrysts are present in most samples although oh-
vine phenocrysts are much less abundant.
Fig. 3 shows the analysis from the Krísuvík-area compared to
some other Icelandic basalts. The chemical composition of the fis-
sure lavas is to some extent similar to the beginning to the Askja
differentiation (5). The Krísuvík-area includes a fairly active high
temperature area and it is cut by a fault swarm. These two fea-
tures as well as the intense volcanic activity suggest that the
Krísuvík-area is a part of an active central volcano (6), although
intermediate and acid rocks are absent. Perhaps the affinity of the
Krísuvík-area with Askja is caused by similar development of the
two central volcanoes which affects their chemical characteristics.