Jökull - 01.12.1982, Page 78

Fig. 3. Alkali-silica diagram of Öræfajökull rocks (black dots). The lines (1) and (2) represent divisions
between subalkaline and alkaline volcanic rocks as proposed by Miyashiro (1978) and Irvine and Baragar
(1971) respectively. rocks of the transitional Bouvetoya suite (Imsland et al. 1977) straddle line (2) at SÍO2
^ 60% and continue into the more silicic field as shown by trend line (3).
Mynd 3. Vensl alkali- og kísilinnihalds í bergsýnum frá Oræfaj'ókli. Línur (1) eða (2) marka skil milli mismunandi
bergflokka.
by partial melting of crustal rocks (Prestvik in prep;
Oskarsson et al. 1982).
It appears that the Öræfajökull series is a repres-
entative of the transition where tholeiitic and
Coombs trend rocks grade into each other (Miyas-
hiro 1978). The evolved basic and intermediate
rocks are, however, much more similar to evolved
tholeiites than to hawaiites and mugearites (Mac-
Donald 1960) of the Coombs trend alkalic series. In
spite of its slightly transitional character the Öræfa-
jökull series is therefore treated as a tholeiitic series
in the following section.
NOMENCLATURE
The above classification of rock series has drastic
consequences as to establish rock names of the int-
ermediate and silicic members of the Öræfajökull
rock series. If the series had been classified as trans-
itional alkalic, the evolved rocks would be hawaiite,
mugearite, benmoreite, oversaturated trachyte and
comendite (Miyashiro 1978), or basaltic andesite,
andesite, trachyte (?), and comenditic rhyolite as
proposed by Jakobsson (1979). On the other hand,
classified as members of a tholeiitic series the evolv-
ed rocks of Öræfajökull are basaltic icelandites, ice-
landites, dacites and rhyolites (Jakobsson 1979), tho-
leiitic andesite, icelandite, dacite and rhyolite (Irv-
ine and Baragar 1971), or tholeiitic icelandite, ice-
landite, dacite and rhyolite (Imsland 1978).
The term icelandite was introduced by Carmichael
(1964) for intermediate (andesitic) rocks ofthe thol-
eiitic Thingmuli suite in order to avoid confusion
with the common andesite of orogenic volcanism,
and has since been used widely for intermediate
rocks of tholeiitic series. For example, Stewart and
Thornton (1975) used the term icelandite for „ocean-
ic andesite”. The Iogical consequence of the adopt-
ion of the term icelandite is to use the composite
term tholeiitic (or basaltic) icelandite for rocks of
the „basaltic andesite” range. A tholeiitic series will
thus include the following main rock types: Thol-
eiite (eventually oceanite and olivine tholeiite in the
basic end), tholeiitic icelandite, icelandite, dacite
and rhyolite.
Since a rock series of tholeiitic affinity is supposed
to derive towards increasing content of the com-
ponents of petrogeny’s residual system, it should be
appropriate to choose an index that describes this
feature as differentation proceeds. The Diíferenti-
74 JÖKULL 32. ÁR