Jökull


Jökull - 01.12.2006, Page 62

Jökull - 01.12.2006, Page 62
David W. McGarvie et al. One solution to this stratigraphic problem is to de- termine the absolute ages of representative volcanic units. However this poses two problems: firstly that a dominance of subglacial eruptives may require a large number of samples to be dated before even a rudimentary stratigraphy can be established, and sec- ondly that at the present time the dating of such young rocks is technically difficult, and even the most well- developed method (Ar-Ar dating) is limited because of the high uncertainties generated when analysing young rocks with low potassium concentrations (see Hawkesworth et al., 2004, and references therein). Rocks high in potassium are preferred because they give both lower uncertainties and more meaningful ages, but Iceland’s most K-rich volcanic rocks are evolved rocks (mostly rhyolites but including tra- chytes and mugearites) that are restricted to the flank zones (Sæmundsson, 1974, 1979). With these con- straints in mind, a realistic initial approach to unravel- ling the morphological and geochemical evolution of a flank zone central volcano is to date a small number of carefully-chosen samples that form a skeletal but robust stratigraphy constrained by absolute age deter- minations. The Torfajökull central volcano The Torfajökull central volcano is Iceland’s largest rhyolite complex (Walker, 1966) and lies in the south- ern Iceland flank zone (Figure 1). Its volcanic prod- ucts comprise c. 250 km3 of subglacial and subaerial rhyolite (with minor amounts of basalt-intermediate compositions) which cover an area of c. 450 km2 (Sæmundsson, 1972; McGarvie, 1984). The central volcano is essentially a dissected rhyolite plateau within which a suspected caldera measuring 18 km by 13 km has been mapped by Sæmundsson and Friðleifsson (2001) (Figure 2). Incomplete and poor exposure of this elliptical structure does not allow for the amount of downsagging or the nature of any syn-caldera fill to be ascertained. Vigorous and per- sistent geothermal activity is concentrated within the suspected caldera, and this has led to extensive hy- drothermal alteration of the rocks within and adjacent to the structure (Friðleifsson and Sæmundsson, 2001). Shallow seismicity focussed in central areas suggests that shallow magma intrusion may be ongoing (Soos- alu and Einarsson, 2004; Soosalu et al., 2006). Torfajökull is still active, and its last eruption in 1477 AD produced two small rhyolite lava flows on its northern flanks (Sæmundsson, 1972, 1988; Larsen, 1984). Volumetrically, Holocene rhyolites represent only c. 1% of the exposed volcanic pile (about 2 km3), and yet (compared to the Pleistocene rocks) they have been exceptionally well studied (e.g. Blake, 1984; Larsen, 1984; Mörk, 1984; McGarvie, 1984; Mac- donald et al., 1990; Gunnarsson et al.; 1998; Stecher et al., 1999), due in large part to their good exposure, accessibility, and the existence of a relative stratigra- phy (Sæmundsson, 1972, 1988; McGarvie, 1985) un- derpinned by some tephrochronology (Larsen, 1984). A comparable understanding of the pre-Holocene geochemical and morphological evolution of Torfa- jökull has been hampered by the lack of a sufficient number of geochemical analyses from samples of un- equivocal stratigraphic position within the volcanic pile. Apart from information available in unpublished theses (McGarvie, 1985; Ívarsson, 1992), geochemi- cal analyses of Pleistocene rhyolites have been pub- lished only in McGarvie (1984) and McGarvie et al. (1990). These studies have emphasised an impor- tant distinction between Torfajökull’s Holocene and Pleistocene activity, with Holocene volcanism dom- inated by less-evolved (i.e. subalkaline) and small- volume (<0.3 km3) rhyolite eruptions accompanied by modest-to-extensive interactions with mafic mag- mas, whereas Pleistocene rhyolites are dominated by more evolved (peralkaline) compositions involving less extensive (or no) interactions with mafic magmas (McGarvie et al., 1990). Additionally, available ev- idence suggests that individual Pleistocene eruptions were generally larger than the Holocene rhyolite erup- tions (McGarvie, 1985; Ívarsson, 1992; Sæmundsson and Friðleifsson, 2001), with the most extreme exam- ple being the group of subglacial rhyolite edifices that surround the main rhyolite plateau (termed the “ring fracture rhyolites”) that McGarvie (1984) hypothe- sised were produced during one large eruptive event that took place during the last (Weichselian) glacial period (see Figure 2 for locations). 60 JÖKULL No. 56
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