Jökull - 01.12.1979, Page 62

landite, icelandite (andesite), dacite and rhyolite.
The equivalent plutonic rocks (gabbro, etc.) have
also been observed. The major element
composition along with information on phenocryst
phases of four typical extrusive rock types of the
tholeiitic series is shown in Table 1. In this series,
close parallels to the tholeiitic magma types of the
classical Hebridean province can be demonstrated.
The transitional series is a hypersthene-
normative alkalic series. The basic rocks are usually
characterized by a high content of Fe and Ti, and
low Al. The basalts of this series will plot above the
division line in Fig. 2, but evolved compositions on
the other hand plot below the division line. This
series is made up of the following main rock types:
ankaramite, transitional basalt, basaltic andesite
(hawaiite), andesite (mugearite), trachyte (?) and
comenditic rhyolite. Equivalent plutonic rocks to
some of these compositions have been observed.
The majorelement composition of four typical rock
types of the transitional alkalic series is shown in
Table 1.
The alkalic series is of mild character, the rocks
are nepheline normative and plot above the di-
vision line in Fig. 2. The alkalic series is made up of
ankaramite (?), alkali olivine basalt, hawaiite
(trachybasalt), mugearite (trachyandesite), benmo-
reite, trachyte (?) and alkalic rhyolite. Equivalent
plutonic rocks have not been found with certainty.
The major element composition of three typical
members of this series is shown in Table 1. The
alkalic series of Vestmannaeyjar (Fig. 7) is nearly
identical to the Plateau Magma Type of the
Hebridean volcanic province.
Several typical differentiation trends within
these three rock series is shown with regard to their
alkali:silica content in Fig. 2. The plots of Post-
glacial basalts of the rock series is shown in Fig. 8.
Compositional variation within each rock series
is perceptible, i.e. the exact trend of differentiation
of a volcanic system may vary from that of others
belonging to the same rock series. However, each
rock series has characteristics which are retained,
e.g. the trends in the alkalksilica diagram of Fig. 2.
The occurrence and distribution of the rock series
will be discussed in the following sections.
TERTIARY TO PLEISTOCENE VOLCANIC
ROCKS
Tertiary
The petrology of the Tertiary rocks of Iceland
(about 16—3.1 m.y. ago) appears to be fairly
homogenous from a genetical point of view.
Volcanism in this period was probably confined to
an axial rift zone, similar to the present axial rift
zone, and only rocks belonging to the tholeiitic
series have been observed within the exposed
Tertiary volcanic systems of which there are
approximately 44. From a petrological point of
view large areas are still unknown, but the central
complexes of several individual volcanic systems
have been studied in detail, especially the Thing-
múli and Álftafjördurcomplexes in eastern Iceland
and the Setberg I, Reykjadalur and Hafnarfjall
complexes in western Iceland. These central
volcanoes generally consist of basic, intermediate
and acid lavas, pyroclastic rocks and a minor
amount of sediments. Intrusive activity is confined
to feeder dykes (dolerite), cone sheets (dolerite and
acid rocks) and irregular intrusions (mainly gabbro
and granophyre), a laccolith being recorded in one
case.
These studies indicate the existence of a
complete olivine tholeiite — tholeiite — basaltic
icelandite — icelandite — (dacite) — rhyolite series
of rocks. Picrite basalts are infrequent. At this point
it is appropriate to note that the field terms
frequently used in Iceland of olivine basalt,
tholeiite and plagioclase porphyritic basalt are not
genetical and apply to the macroscopical ap-
pearance, although these field types prove in most
cases to be tholeiitic.
The petrochemical trends of these Tertiary
central volcanoes are very similar, apart from
minor differences, e.g. parellel shifts in trend lines
as in the alkali: silica diagram of Fig. 2.
The mineralogy of the Tertiary volcanoes is
relatively simple. In Thingmúli, where the rock
suite has been suggested to be derived by a process
of fractional crystallization from a basaltic parent,
olivine is a ubiquitous phenocryst phase (Fo 85—
65) in the basic rocks, and is also found as a
groundmass constituent of the olivine tholeiite.
The olivine is absent in the tholeiites and the
basaltic icelandite, but reappears in the icelandites
and the acid rocks (as Fo 45), a recurrence
analogous to that of the olivine in the Skaergaard
intrusion. Plagioclase is the only feldspar, and the
groundmass constituent becomes progressivly more
sodic going from the basalts towards the acid end of
the series. In the basic rocks augite is rare as a
phenocryst phase; in the groundmass augite and
pigeonite coexist and vary in composition in a
60 JÖKULL 29. ÁR