Náttúrufræðingurinn - 1984, Síða 24

SUMMARY
Icelandic rock types II
Olivine tholeiite
by
Sveinn P. Jakobsson
lceland Museum of Natural History
P.O. Box 5320
125 Reykjavík
The article describes the rock type oli-
vine tholeiite, with the Recent Þráinsskjöl-
dur lava shield (Fig. 1) in SW-Iceland as an
example. The lava contains about 1 per-
cent by volume of olivine phenocrysts (Fo
80-85) with small inclusions of chrome
spinel (Fig. 2). The groundmass consists of
labradorite, clinopyroxene, olivine, and
interstitial titanomagnetite and ilmenite.
A chemical analysis of the Þráinsskjöldur
lava (Table I) indicates a MgO content of
10.06 percent which is within the typical
range for Icelandic olivine tholeiite. Most
olivine tholeiites form lava shields with
pahoehoe surfaces.
Olivine tholeiite being a part of the volu-
minous tholeiitic rock series in Iceland, is
one of the most common rock types. It is
estimated that some 190 km3, or 40 per-
cent of the total production of volcanic
rocks during Recent time in Iceland, is
olivine tholeiite. Considering older forma-
tions also, probably some 30 percent of the
exposed formations since Miocene is oli-
vine tholeiite. The production of tholeiitic
rocks in Iceland has probably always been
confined to an axial rift zone, as during
Recent times (Fig.3).
18