Jökull - 01.12.1979, Page 69

in the Vestmannaeyjar system. The short
description on the mineralogy of the Setberg alkalic
suite applies also to the Vestmannaeyjar rocks. In
addition, basanitic segregation veins occur in the
basalts carrying nepheline, analcime, aegirine,
amphiboles and aenigmatite.
Fig. 8 shows the alkalhsilica plot of analyzed
Postglacial basalts from the western Reykjanes
Peninsula (Fig. 6) and the eastern volcanic zone
(F'g- 7).
There appears to be a relation between the
distribution of the petrological zones as defined
from the study of Postglacial rocks and the crustal
structure of Iceland. The depth to the inferred
upper mantle (layer 4) boundary is generally 8—9
km below the tholeiitic zones. Moving along the
alkalic flank zones away from the tholeiitic zones
(Fig. 1), the depth increases and reaches about 14
km depth in the distal ends, concordant with an
overall increase in the alkalinity of the alkali
basalts produced at the surface. The simplest
interpretation of these relations is, that the alkali
basalts are generated at greater depths and
consequently higher pressures than the tholeiites,
and may therefore represent different degrees of
melting of similar mantle material.
An important feature of the Postglacial basalts is
the compositional variation within the tholeiites of
the axial zone which reaches maximum in central
Iceland. Moving from typical mid-ocean ridge
basalt (MORB) compositions about 400 km south
of Reykjanes, the chemical transition is gradual
towards middle Iceland and the maximum or
minimum value encountered in the basalts in each
part of the rift zone rises or falls at the same time as
the scatter of values increases. In North Iceland,
however, an abrupt transition across the Tjörnes
Fracture Zone is indicated. Fig. 9 for example
shows the variation of KaO in the axial rift
tholeiites with distance along the ridge axis in the
Iceland area. The geochemical gradient across Ice-
land has been much discussed. Several
investigators favour a multiple source hypothesis in
a rising hot mantle plume to explain these rela-
tions.
Fig. 9 moreover indicates that maximum total
discharge rate in this region of the Mid-Atlantic
Ridge is reached just south of Central Iceland, with
an output per unit length of ridge at least 4—5
times higher than just south or north of Iceland.
Gabbroic nodules
Friable and porous gabbroic nodules are
common in the basic rocks, preferably in tholeiitic
lavas and tephras. The gabbroic nodules are
generally less than 6—8 cm in diameter, the grain
size being usually between 1 and 5 mm. Those
found in basalts only rarely show any reaction
relation with the host rock. Plagioclase is
commonly the dominating phase, in association
with clinopyroxene, olivine, orthopyroxene and
magnetite. Amphiboles and apatite are only rarely
observed. Some of the nodules exhibit igneous
layering and heteradcumulate and adcumulate
textures taken to be indicative of an accumulative
origin. There are strong indications that these
nodules are formed freely floating. On the basis of
general similarities of the host rocks, the nodules in
the basalts can be suggested to be autoliths formed
at shallow depth. Gabbroic nodules in andesitic
TABLE 2. Estimates on volume (km3) and relative abundance (%) of extruded rocks during Postglacial
time in the eastern volcanic zone, and all the active volcanic zones.
EASTERN VOLCANIC ZONE
POSTGLACIAL
ACTIVE ZONES
Rock type Tholeiitic Tran- sitional Alkalic Total km3 % Total km3 %
Basalt 107 65 3.7 : 176 87 390 92
Basaltic andesite* .... 0? 14.0 0.2 : 14.2 7 17 4
AndesiteT ? 3.7 0 : 3.7 2 5 1
Dacite-rhyolite 0 8.2 0 : 8.2 4 11 3
107 91 3.9 202 423
*Hawaiite-mugearite in the alkalic series
+ Benmoreite in the alkalic series
JÖKULL 29. ÁR 67