Rit (Vísindafélag Íslendinga) - 01.06.1967, Page 43

39
and in the Setberg central volcano. Both these areas are
largely composed of Tertiary basalts with minor intrusives,
but are characterized by a high degree of alteration. The
secondary minerals present are epidote, albite, gamet and
quartz, i.e. an assemblage indicating something akin to the
green-schist facies. Einarsson’s suggestion that seismic layers
represent degree of alteration is supported here as far as re-
gards layer 3 but minor intrusions are probably also im-
portant.
The Icelandic acid rocks and the question of sial.
Acid rocks make up approximately 10—12% of the Ter-
tiary plateu in eastem Iceland (Walker 1959) and on Snæ-
fellsnes. Large tracts of the country are considered poor in
acid rocks, notably the Northwest Peninsula, where the geo-
logy is but little known. Nevertheless, rhyolite, andesite,
granophyre and especially acid tuffs must be considered an
important contribution to the Icelandic volcanic pile. Acid
rocks are not evenly distributed throughout Iceland, but oc-
cur most abundantly in the central volcanoes, of which at
least 42 have been found, ranging in age from Tertiary on-
wards, and of which at least six, Snæfellsjökull, Öræfajökull,
Askja, Torfajökull, Hekla and the Mývatn area, have shown
activity in Recent times (Fig. 2).
Clues to the origin of the Icelandic acid rocks have been
sought in mineralogical, chemical and isotopic criteria. Dif-
ferentiation of basic magma to give an acid residuum is a well
known and authenticated process and 2—5% of acid magma
is considered a likely yield of such a process, although a
theoretical maximum is of the order of 7—12% (Carmichael
1964). Such thorough extraction of the acidic fraction would
though undoubtedly show up in the basalt lavas. From the
point of view of volume alone it seems unlikely that the Ice-
landic acid rocks are solely the product of differentiation
from basalt magma. Petrological studies have not yet pre-