Jökull - 01.12.2006, Side 59

Reviewed research article
Pleistocene rhyolitic volcanism at Torfajökull, Iceland: eruption
ages, glaciovolcanism, and geochemical evolution
David W. McGarvie1, Raymond Burgess2,
Andrew G. Tindle1, Hugh Tuffen3 and John A. Stevenson3
1Department of Earth Sciences, The Open University, UK
2School of Earth, Atmospheric and Environmental Sciences, University of Manchester, UK
3Environmental Science Department, Lancaster University, UK
Abstract — The Torfajökull central volcano lies in Iceland’s southern flank zone (a non-rifting zone) and last
erupted in the 15th century. Peralkaline rhyolites from its pre-Holocene formations have been dated by the
Ar-Ar method. Ages from 67±9 ka to 384±20 ka indicate Pleistocene eruptions, with the oldest age (384 ka)
also being from the most evolved rhyolite (a pantellerite). The oldest age indicates that a mature and evolved
magmatic-volcanic system was well established by the mid-Pleistocene and that the central volcano has been
active for at least 400 ka. Good correlation is found between the Ar-Ar ages of sustained rhyolite eruptions into
ice sheets (i.e. rhyolite tuya formation) and oxygen isotope stages dominated by cold conditions. This is the first
stage of developing a new proxy that uses rhyolitic glaciovolcanic edifices to provide estimates of past Icelandic
ice sheet thicknesses. The geochemistry of the dated samples corroborates earlier work showing a simple but
enigmatic trend of steadily-decreasing alkalinity towards the present (i.e. older rocks are more evolved). The
new ages reveal a hitherto-unrecognised drop in rhyolite alkalinity after 83 ka, which may be linked to the
evacuation of c. 16 km3 of rhyolite during a subglacial eruption into the last (Weichselian) ice sheet, for which
two new and overlapping Ar-Ar ages of 67±9 ka and 72±7 ka have been obtained. This rhyolite eruption, which
is the largest known from Torfajökull, heralded a major change in the magma system as all subsequent eruptions
are of small volume (<0.3 km3), dominated by subalkaline compositions, and characterised by interactions with
mafic magmas. This change may be linked to lower rhyolite magma replenishment rates and/or to the increasing
influence of rift zone volcano-tectonics.
INTRODUCTION
Central volcanoes of the Icelandic flank zones
Icelandic volcanic zones are classified into rift zones
and flank zones (Sæmundsson, 1974, 1979). The rift
zones represent the approximate trace of the Mid-
Atlantic Ridge through Iceland, and consequently
they are characterised by the volcano-tectonics of ac-
tive spreading (Figure 1). In the rift zones volcanism
is presently focused within a number of discrete vol-
canic systems, each of which has two components: a
central volcano and fissure swarm(s). At mature and
well-developed rift zone central volcanoes that are
well exposed (e.g. Askja, Krafla) modest volumes of
magma (a few km3) are stored in the mid- to up-
per crust, small but frequent (tholeiitic) basalt erup-
tions occur (usually much less than 1 km3), geother-
mal activity is usually evident, and silicic magmas
(chiefly rhyolites) have been erupted (Sæmundsson,
1974, 1979; Sigurdsson and Sparks, 1981; Jonas-
son, 1994). The associated fissure swarm(s), up to
120 km in length, appear to erupt only (tholeiitic)
basalt, with volumes varying from <1 km3 up to
20 km3 (Sæmundsson, 1979). In the flank zones lit-
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