Jökull - 01.12.1979, Page 63

Countryrock bosolts
k\N Epidote oureole
EZ3 Bytownite gabbro
FFpTl Igneous layered
! J '•H bytownite gabbro
fcv.ý'lyl Gabbro
E23 Gronophyre-potch gabbro I
[/. '//\ Quartz dolerite
I I 1 11 Centrol Epigranites
nn Tronsitional Epigranites
[___] Fa - Hd Epigranite
tnnr.1 Net veined complex
f‘AAA'1 Agglomerate
Foult
Fig. 3. Geological map of
the Vesturhorn intrusion,
southeast Iceland. After
Roobol 1974.
similar way to those of the Skaergaard intrusion. In
the icelandites (cf. Table 1, no. 3) both augite and
orthopyroxene have been found as phenocrysts
while in the acid rocks ferroaugite is the phenocryst
phase. Of iron-titanium oxides, both magnetite
and ilmenite are present. Magnetite plays a varied
role in the order of crystallization; in the
intermediate stages magnetite is found as a
phenocryst phase and is considered to play a vital
role in the course of fractionation at this stage by
controlling the Ti and Fe-content of the liquids.
Several large intrusions, which are apparently
not connected with central volcanoes, have been
mapped in southeastern Iceland, the most notable
being the Vesturhorn and Austurhorn intrusions
from Upper Miocene. In Vesturhorn (Fig. 3) there
are indications of more than 72 separate intrusive
bodies with rock compositions ranging from gabbro
through diorite to granite and granophyre. The
intermediate members of the series are thought to
be formed by mixing of the end members.
There is striking evidence from these intrusions
that acid and basic magmas have existed side by
side. Pillow-like masses of basic rock are found in
and chilled against granophyre, while in other
localities net-veined complexes are found, where
the basic rock is veined by acid rocks, see Fig. 4.
From a higher level in the crust, a number of
examples are known, from Tertiary as well as from
younger formations, of composite intrusions and
lavas. It has been suggested that in some cases the
mobility or even the uprise of the acid magma is
due to the transfer of heat from the coexisting basic
magma.
Layered igneous rocks are known from a few
localities in Iceland. The Thorgeirsfell gabbro
(Snaefellsnes) and gabbros of the Austurhorn
intrusion exhibit a faint layered structure. In the
Vesturhorn intrusion (Fig. 3) a block of a bytownite
gabbro with size-graded layering has been
described. In a Tertiary dolerite sill in Hrappsey,
western Iceland, large inclusions (up to 30.000 m2)
of bytownite anorthosite showing faint layering are
present, these anorthosites are thought to be
cumulates förmed at shallow depth from an olivine
tholeiite magma.
| [
Plio-Pleistocene and Uþper Pleistocene
Volcanism probably continued without
JÖKULL 29. ÁR 61