Jökull - 01.12.1979, Page 64

HBasic magma
Basic pillows
Mobile acid magma
Viscous acid magma
Fig. 4. Diagrammatic re-
presentation of the possible
relationship between, and
origin of, a net-veined
complex of Austurhorn —
type and a composite dyke
feeding a composite lava
flow. From Blake et al.
1965.
interuption from the Tertiary throughout the
Plio-Pleistocene (3.1—0.7 m.y.) into the Upper
Pleistocene. Flowever, the onset of glaciations
resulted in a drastical change of environmental
conditions. Mountains of hyaloclastites
(pyroclastic breccias and tuffs) and pillow lavas
piled up subglacially. During interglacial periods,
extensive lava flows were formed, as in the Tertiary
era. Many large olivine tholeiite lava shields are
known, particularly from the later interglacial
periods, as for example Lyngdalsheidi and Skálpa-
nes in southwestern Iceland and Vadalda and
Grjótháls in central northern Iceland. As the result
of volcanic activity under ice-cover the central
volcanoes are usually higher than those from the
Tertiary era. The best known of volcanoes dating
from this period are Setberg II and Kjalarnes-
Stardalur of Plio-Pleistocene age and Kerlingarfjöll
and Tindfjallajökull from the Upper Pleistocene.
As is the case with the Tertiary rock formations,
large areas of Plio-Pleistocene and Upper Pleisto-
cene age have not yet been studied petrologically,
although the main features are known. In the early
Plio-Pleistocene only tholeiitic rocks were
developed as in the Tertiary. The Kjalarnes-Star-
dalur central volcanoes are for example built up of
a tholeiidc suite nearly identical to the Thingmúli
suite previously described.
Later flank volcanic zones became active in
addition to the axial zones. The temporarily active
Skagi zone in western North Iceland at first pro-
duced tholeiitic rocks, but later rocks approaching
the transitional series in composition were extruded.
In the Snaefellsnes flank zone compositions may
have changed gradually on a regional basis through
time. In the Setberg area (Fig. 5) for example, a
tholeiitic suite was produced from the Setberg I
central volcano up to about 2.5 m.y. ago and later a
transitional suite (low in Fe and Ti) was erupted
from the Setberg II volcano from about 2.5 up to 0.7
m.y. ago. Finally, alkalic volcanism developed in the
area from approx. 0.7 m.y. ago up to present.
Similarily, a flank zone began to develop in
South Iceland south of river Tungná, perhaps some
2 m.y. ago. Rocks belonging to the transitional
series developed in volcanic systems in this flank
zone from that time and up to the present. In the
Upper Pleistocene and Holocene rocks belonging
to the alkalic series were produced in the Vest-
mannaeyjar volcanic system, the latest display of
volcanic activity occurring on Heimaey in 1973.
In the Setberg alkalic suite, ankaramite is
described as the most primitive member. This rock
carries abundant phenocrysts of diopsidic augite
and olivine (from Fo 89), along with plagioclase
(from An 92) and dark-brown spinel. Olivine and
plagioclase occur throughout the alkali basalt —
hawaiite — mugearite — benmoreite suite as
phenocrysts, the olivine having a compostional
range of Fo 85 in the basic rocks to Fo 55 in the acid
end and the plagioclase being An 81 to An 30 for
the same rocks. Augite is a very scarce phenocryst
phase, magnetite occurs in hawaiite, mugearites
and benmoreites and apatite as phenocryst in
62 JÖKULL 29. ÁR