Jökull - 01.12.1982, Page 73

Petrography, chemical Characteristics and
Nomenclature of Öræfajökull Rocks
T. PRESTVIK
Geological Institute, University of Trondheim,
Hogskoleringen 6, N-7034 Trondheim NTH, Norway
ABSTRACT
The petrographicJeahires of Pleistocene to Recent volcan-
ic rocks ojthe Orœfi district in southeast Iceland are describ-
ed. The chemical characteristics oj the rocks are discussed,
and it is concluded that they constitute a tholeiitic series. The
Jollowing classification based on Differentiation Index
(D.I.) isproposedfor this series: D.I.< 35: tholeiite, D.I.
— 35-50: tholeiitic icelandite, D.I. =50 - 70: icelandite,
D.I. = 70-82.5: dacite, and D.I. > 82.5: rhyolite.
INTRODUCTION
Oræfajökull is an active central volcano situated
some 50 km to the southeast of the eastern zone of
rifting and volcanism in Iceland (Fig. 1). Thorarins-
son (1958) discussed the history of Öræfajökull and
the geological features in some detail, and conclud-
ed that the activity ofÖræfajökull has been minor in
post-glacial dme. Recently Prestoik (1979) present-
ed a geological map of Öræfajökull and described
the field relations. He concluded that the Öræfi
district comprises remnants of two older (Pleistoc-
ene) volcanic centers, one in the area of Skaftafells-
jökull to the west and the other in Breiðamerkurfjall
to the east. The presently active Öræfajökull central
volcano is situated between and partly upon the
eroded sections of these two older centers. Prestvik
(1979) concluded that the rocks ofall these volcanic
structures are more or less similar to each other and
constitute a subalkaline rock series with a typical
tholeiitic trend.
1 he rock suite of'Öræfajökull also contains some
hybrid intermediate and silicic volcanic rocks with
calc-alkaline affmity. An account of petrologic fea-
tures of these rather uncommon Icelandic rocks was
given by Prestvik (1980) who concluded that this
calc-alkaline suite originated by mixing of rhyolitic
magmas with a ratherprimitive basaltic component
which may be either solid rock fragments and/or a
basaltic liquid.
The present account gives a rather detailed
petrographic description of the main rock types and
a discussion of the chemical characteristics, classif-
ication, and nomenclature of the common Öræfa-
jökull rocks. The names of rock types used in the
petrographic description are those proposed in the
discussion of nomenclature below.
PETROGRAPHY
Tholeiites.
These rocks occur as lava flows, hyaloclastite
units, and dikes. Both porphyritic and aphanatic
types are common, even though the slighdy por-
Fig. 1. The volcanic zones of Iceland with
affinity of rocks indicated. Redrawn after
Jakobsson (1972) and Imsland (1978).
Xlynd I. Eldvirk svteði á Islandi.
JÖKULL 32. ÁR 69