Jökull - 01.12.1989, Page 77

Tertiary Silicic Rocks in the Area
of the Kækjuskörð Rhyolitic Volcano, Eastem Iceland
LÚÐVÍK E. GÚSTAFSSON
Commercial College oflceland
Ofanleiti 1, IS-109, Reykjavík, Iceland
BERNHARD LAPP
and
LUTZ THOMAS
Institutfur Geologie
Freie Universitat Berlin
Altensteinstr. 34, D-1000 Berlin 33, F.R.G.
MANUELLAPP
Institutfur Geologie und Dynamik der Lithospháre
Georg August Universitát Göttingen
Goldschmidtstr. 3, D-3400 Göttingen, F.R.G.
abstract
In the summer of 1987 geological mapping and
structural investigations were carried out in the
Loðmundarfjörður area in Eastern Iceland. A small
volcano, the Kœkjuskörð rhyolitic volcano, was
niapped. It is composed mainly of silicic rocks and
could have formed as an independent eruption unit,
hut its origin as a parasitic volcano oflarger central
volcanoes not far away cannot be excluded. In the
area from Borgarfjörður Eystri in the north to Seyð-
lsfjörður in the south there were probably four cen-
tral volcanoes active in Tertiary times: the Dyrfjöll
central volcano, the Breiðavík central volcano, the
Uerfell central volcano and a fourth central volcano
now nearly completely eroded and concealed below
sea level, the Seyðisfjörður central volcano.
Rhyolites and dacites of the Kœkjuskörð rhyolitic
volcano are described and analysed, and geochemi -
cal analyses of basaltic rocks of the adjacent lava
pile are also presented along with the CIPW norms
of the rocks. The silicic rocks are interpreted, in
contrast to former views, as lavas flows. Five
different silicic lava flows could be identified.
In the area of the Kœkjuskörð volcano at least
three different ignimbrite formations occur. Two of
them are early products ofthe Kœkjuskörð volcano.
The third, a partly welded ignimbrite sheet, was
erupted from the Herfell central volcano. Relatively
good exposures in the mountains to the north and
west of Loðmundarfjörður enabled an isopach map
ofthis ignimbrite sheet to be constructed.
INTRODUCTION
The northemmost part of eastem Iceland around
the fjords of Borgarfjörður Eystri and Loðmundar-
fjörður is one of the geologically least known parts
of Iceland, even though it contains the second larg-
est outcrop of silicic rocks in the country and thus
JÖKULL, No. 39, 1989 75