Jökull - 01.12.2006, Síða 65

Pleistocene rhyolitic volcanism at Torfajökull, Iceland
SAMPLE DESCRIPTIONS
Ring fracture rhyolites
On the basis of corroborating evidence from morpho-
stratigraphy, chemistry, mineralogy, and eruptive lo-
cation, McGarvie (1985) hypothesised that the ring
fracture rhyolites were the products of one large
glaciovolcanic eruption. A tentative volume estimate
of 26.4±7.6 km3 was given with the caveat that high
uncertainties existed due to both incomplete map-
ping and incomplete geochemical characterisation of
the eruptive units. Following publication of the first
detailed geological map of Torfajökull (Sæmunds-
son and Friðleifsson, 2001) and mapping undertaken
during this study, the original estimate has been re-
evaluated and a new minimum estimate of 16 km3
is considered to be more realistic. This has been
derived from revising the volume estimates of the
glaciovolcanic edifices now considered to belong to
the ring fracture rhyolites: Rauðfossafjöll (5.5 km3),
Illihnúkur (2.2 km3), Kirkjufell (2.5 km3), Laufafell
(3.3 km3), and the scattered outcrops around the
southern and eastern margins of the Torfajökull ice
cap (2.5 km3). See Figure 2 for localities.
Samples for Ar-Ar dating were selected from two
widely separated rhyolite tuyas: Illihnúkur (sample
TJ97-18) in the east and SW Rauðfossafjöll (sample
TJ97-29) in the west (Figure 2). Illihnúkur forms a
slightly arcuate ridge (oriented roughly N-S) rising to
1131 m from a base level at c. 670 m. It has an es-
timated volume of 2.2 km3. Phreatomagmatic tephra
exposed within gullies cut into its western flank is
overlain by subaerial lava flows, and strongly suggests
that Illihnúkur is a rhyolite tuya (c.f. Tuffen et al.,
2002). Sample TJ97-18 is a porphyritic obsidian with
3% phenocrysts collected from a lava flow near the
summit. Phenocrysts are dominated by euhedral and
unzoned feldspars (anorthoclase) up to 2 mm in size
but mostly around 0.5–1 mm, with minor amounts
of ferroaugite, fayalitic olivine (Fo8), and titanomag-
netite that rarely exceed 0.5 mm in size, set in a flow-
banded and microlite-rich glass (A.G. Tindle, unpub-
lished data). This sample is a mildly-peralkaline rhyo-
lite (agpaitic index of 1.0) and is a comendite accord-
ing to the classification scheme of Macdonald (1974).
The base-summit height difference of 460 m suggests
that the ice sheet was at least this thick at the time of
eruption. This sample has an Ar-Ar age of 72±7 ka.
The Rauðfossafjöll area is a site of multiple and
seemingly-contemporaneous rhyolite glaciovolcanic
eruptions (McGarvie, 1984; Tuffen, 2001; Tuffen et
al., 2002), with an estimated volume of 5.5 km3 (Mc-
Garvie, 1985). It comprises four well-developed tuyas
that are aligned in a NE-SW orientation reflecting
eruption along linear fissures, together with associ-
ated peaks, ridges, and various marginal glaciovol-
canic units. Sample TJ97-29 was collected from the
summit of the most south-westerly tuya, which rises
to 1174 m from a base level of 800 m. The sum-
mit region of SW Rauðfossafjöll is flat-topped and
comprises sub-horizontal lava flows with occasional
pumiceous carapaces; features interpreted by Tuffen
et al. (2002) on the nearby SE Rauðfossafjöll tuya
as indicating eruption of the uppermost part of the
tuya into a subaerial environment. Sample TJ97-29 is
a porphyritic obsidian with 2% phenocrysts collected
from a lava flow on the southern side of the sum-
mit plateau. Phenocrysts are dominated by euhedral-
subhedral and unzoned feldspars (anorthoclase) up
to 1 mm in size but mostly around 0.5, with mi-
nor amounts of ferroaugite-ferrohedenbergite and il-
menite that are mostly less than 0.3 mm in size, set
in a flow-banded glass containing sparse microlites
(A. G. Tindle, unpublished data). This sample is a
mildly-peralkaline rhyolite (agpaitic index of 1.0) and
is a comendite according to the classification scheme
of Macdonald (1974). The base-summit height differ-
ence of 290 m suggests that the ice sheet was at least
this thick at the time of eruption. This sample has an
Ar-Ar age of 67±9 ka.
Older rhyolite tuya
One sample (TJ97-22) was collected from Gvendar-
hyrna (Figure 2), an isolated and prominent edifice
that may be an eroded tuya, as indicated by its strong
vertical development (that suggests confinement by
ice), remnants of a substantial (c. 70 m thick) lava
cap, and rare exposures of phreatomagmatic tephra
lying stratigraphically beneath the capping lava (c.f.
Tuffen et al., 2002). Gvendarhyrna forms a distinc-
tive and short NNE-SSW oriented ridge with a vol-
ume of c. 0.2 km3 that has a summit elevation of
JÖKULL No. 56 63