Rit (Vísindafélag Íslendinga) - 01.06.1967, Page 46

42
bath and Walker (1965) and also by Heier et al. (1966),
in all some 29 samples, of which 11 are acid. The ratio
Sr87/Sr86 is governed largely by breakdown of Rb87 to Sr87
with time, giving higher Sr87 in older rocks. Moorbath and
Bell (1965) have demonstrated a striking difference in the Sr-
ratio of the Tertiary acid and basic rocks of north-west Scot-
land (Fig. 3), where the higher Sr-ratio of the acid rocks sug-
gests a source region of older crustal rocks, probably fusion
of pre-Cambrian sediments to give rise to a rhyolitic magma,
as indicated by Brown (1963) from a totally different ap-
proach. The few samples already analyzed from Iceland show
a rather constant Sr-ratio (Fig. 3) and no splitting of the
basic and acid samples into two groups is seen as in north-
west Scotland.
Moorbath (1965) has indicated, that the available Sr-iso-
tope data may be interpreted in two ways: The acid and
basic magma are closely related and the former has arisen
by differentiation from the basic magma. Or altematively,
if the acid rocks are derived by assimilation, the sialic source
must be less than 100 million years old, or possess an ab-
normal Sr-ratio.
On this evidence alone, differentiation from basic magma
must be considered a more attractive alternative for the ori-
gin of the acid rocks in Iceland. Nevertheless, the formation
of a young sialic crust in the area cannot be excluded, e.g.
by some process such as upwards migration of an acidic inter-
granular liquid derived from the upper mantle.
Contrary to prior claims that the K/Rb ratio remains con-
stant during differentiation and is similar in many igneous
rocks, it has been shown (e.g. Taubeneck 1965) that the ratio
may be a good index of differentiation and vary as a func-
tion of geological processes. An average K/Rb ratio of 240
is commonly accepted for continental rocks (Lessing et al.
1963). Study of the K/Rb ratio of Icelandic rocks may there-
fore have a bearing on the question of origin of the Icelandic
acid igneous rocks. Figure 4 is a plot of K against Rb for data