Jökull - 01.12.1982, Page 79

ation Index (D.I. = q+or+ab+ne; Thomton and
Tuttle 1960) is such an index and vvill be used in the
following.
If we examine the terms proposed above for
members of a tholeiitic series, the ranges for SiO„
D.I., or other indices for each rock type have not
been precisely defined.
In the Galapagos series the term Jerrobasalt was
used for rocks with D.I. from 35 to 50 (McBimey and
Williams 1969).Stewart and Thornton (1975) discussed
the nomenclature of intermediate rocks in oceanic
regions and characterized the term ferrobasalt as
‘awkward”. These authors proposed that the term
icelandite should cover the D.I. rangeof35-62.5. To
the present author this seems to be an unreasonable
division because it excludes most icelandite í'rom
the type locality (D.I. = 61.0-72.1) from the range
(Carmichael 1964). Furthermore, a group ranging in
D.I. from 35 to 62.5 (or more) would be very in-
homogeneous comprising rocks in the SiO.,-range
48-50% up to 60-65%. Based on the analysed rocks
from the Oræfi district and the discussion above I
propose that the term tholeiitic icelandite should be
used for rocks of a tholeiitic series ranging in D.I.
from 35 to 50. This is the same range that has been
used for “ferrobasalts”, and it corresponds rather
well with the Irvine and Baragar system used by
Imsland (1978) forvarious Icelandic rock series.
Carmichael (1964) proposed the term icelandite
for an intermediate composition, i.e. for rocks that
previously would have been termed andesite. Ac-
cording to Taylor (1969) the typical SiO.,-range for
andesite is 56-62%. I propose a lower D.I. limit of
50 for icelandite, corresponding approximately to
SiQ2 = 56%. However, tosetan upper D.I.limitfor
icelandite is not easy. Carmichael (1964) used ice-
landite as a term for rocks with D.I. up to 72.1 (SiO,,
~ 65%) leaving little or no room for the term dacite.
I o be consistent, the usage of this latter term too,
should be avoided in a tholeiitic series because dac-
ites are widely used for rocks oforogenic or calcalka-
line volcanic suites. However, the term dacite has
been widely used in Iceland and it is also maintain-
ed by Irvine and Baragar (1971). Furthermore, for a
few rocks of the Oræfi district is seems natural to use
the term dacite because in both textural and com-
positional respects they are clearly intermediate
between most icelandites and rhyolites. If D.I. = 70
and 82.5 are taken as limits between icelandites/
dacites and dacites/rhyolites respectively, we will
have borderlines that are rather consistent with the
usage of these terms previously.
In conclusion, the following classification (based
on Differentation Index, D.I.) is proposed for the
Oræfi series:
Tholeiite D.I. < 35
Tholeiitic icelandite D.I. = 35-50
Icelandite D.I. = 50-70
Dacite D.I. = 70-82.5
Rhyolite D.I. > 82.5
ACKNOVVLEDGEMENTS
The Jield work in the Öraji district was carried out in
1974/75 when the author held a oneyear Jellowship at the
Nordic Volcanological Institute in Reykjavik. The project
benejittedgreally Jrom the kind assistancegiven to the author
by the instilule’s stajj members. Finally, Páll Imsland is
thanked Jor his critical comments on the Jirst manuscript
drajt oj this paper.
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