Acta naturalia Islandica - 01.07.1964, Page 29

THE ANKARAMITES OF HVAMMSMÚLI, EYJAFJÖLL, SOUTHERN ICELAND 27
face, the bands or „veins“ occur. In 3-D their symmetry seems to be mono-
clinic, because, seen from SE they are straight and parallel to the „fish“,
but on a perpendicular plane they 'wind about in an irregular fashion, (Fig. 6).
A unique object in these rocks, aligned parallel to the bands and the
„fish“, is a complete basaltic column With 3 segments (Pl. Vlla). A thin section
of its contact with the host rock shows that a certain amount of fusion has
taken place, because the contact is by no means clear. In the field, however, the
even surface of the column is very distinct from the uneven surface of the por-
phyritic host. The lower end of the column sticks into the dark area, and the
dark zone swings round it. Probably fusion of the basalt provided some iron
ore for the darkening of the rock.
These structures must be explained as mesoscopic flow structures. The
non-porphyritic bands are considered to be contemporaneous structures formed
in the magma in situ by the removal of phenocrysts by some agency, whereas,
the „fish“ and the column are inclusions in sensu stricto.
The magma liquid carried the phenocrysts and inclusions in suspension.
Its flow was laminar, as reflected by the parallelism of the elements.
The inclusions, ,,fish“ and column, occupy the lower levels of the rock
face, and the bands the upper. The shape and orientation of the „fish“, which
all lie with their narrow end pointing in the same direction, seems to indicate
a flow from right to left, i. e. the narrow end facing the flow direction. In-
clusions of this shape (but different composition) are quite common outside
the quarry. The main rock masses seem to represent gently dipping sheets of
rock, most likely sills. One would hardly expect such uniform laminar flow
in a lava.
The non-porphyritic ,,veins“ possess very delicate banding or lamination
(ca. 10 per cm) running parallel to their edges. In thin section the minunte
feldspar laths show very marked orientation parallel to the lamination. The
groundmass is granular. The laminae, which do not show well in thin section,
appear to be due to a slight concentration of magnetite grains in the darker
bands as compared with the lighter ones.
The „veins“ are composed of the groundmass material of the rock on
each side, the only difference between the two being the absence of phenocrysts
in the bands.
It seems to be difficult to reconcile the structure of the ,,veins“ and the
flow direction obtained for the inclusions (Fig. 6). A possible explanation is
that the „veins“ were originally more or less planar sheets caused by the
lamipar flow. While still in the plastic state a new phase of intrusion took place
disturbing the earlier mass locally, pushing it about and producing the folding.