Jökull - 01.12.1986, Page 33

The Petrology of the Maelifell
Picrite Basalt, Southem Iceland
VIGDÍS HARDARDÓTTIR
National Energy Authority, Geothermal Division
Grensásvegur 9, 108 Reykjavík, Iceland
abstract
The Maelifell picrite basalt has primitive character-
istics and contains phenocrysts of Cr-spinel, Cr-Al-
endiopside, and two populations of olivine, one re-
versely zoned (noted for the first time in picrite basalt)
and the other more Mg-rich and normally zoned. The
presence of these two types of olivine indicates that
niixing has occurred in the picrite. Results of calcu-
lations give MgO contents of approximately 13 and 18
wt.% and liquidus temperatures of 1264°C and 1334°C,
respectively, for the two end-members involved in mix-
lng, inferred to have occurred at a temperature above
1223°C.
INTRODUCTION
Many volcanic systems in Iceland are recognized,
however, realtively few have been studied in detail
with regard to petrology and mineralogy (Carmichael
1964, 1967, Grönvold 1972, Franzson 1978, Jakobs-
son 1979). The Hengill area in the Western Volcanic
Zone (Fig. 1) has been the subject of numerous investi-
gations such as geophysical studies (Einarsson 1951,
1962, Foulger & Einarsson 1980, Björnsson & Hersir
1981, Foulger 1984, Hersir et al. 1986), stratigraphi-
cal (Saemundsson 1962, 1965, 1967), petrological
(Hardardóttir 1983, Risku-Norja 1985) but mainly
focussed on hydrothermal phenomena (Tómasson &
Saemundsson 1967, Tómasson & Grönvold 1973,
Torfason et al. 1983, Björnsson et al. 1986). The area
is almost entirely built of volcanic rocks of Weich-
selian and Postglacial age (Saemundsson 1967).
Picrite is known in two localities associated with the
Hengill volcanic swarm: Maelifell, and Midfell, which
was not included in this project.
Fig. 1. The volcanic zones of Ice-
land divided into the 29 volcanic sys-
tems (shaded). The position of the
differentiated central complexes
(Pleistocen-Postglacial) which are
thought to be connected with the sys-
tems are shown by cross-hatching.
The square shows the location of the
Hengill volcanic system. Taken from
Jakobsson (1979).
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