Provenance and tectonic implications of Palaeoproterozoic
(c. 1740 Ma) quartz porphyry clasts in the basal Old Red
Sandstone (Lilljeborgfjellet Conglomerate Formation)
of northwestern Svalbard's Caledonides
Authors: HELLMAN F. J. , GEE D. G. , GJELSVIK T. , TEBENKOV A. M. 
Published: July 2002

The Lilljeborgfjellet Conglomerate Formation composes the lower
part of the alluvial
Siktefjellet Group of northwestern Spitsbergen's Old Red Sandstone
succession. Siktefjellet strata are
of late Silurian or early Devonian age, but lack precise age-diagnostic
fossils. They are unconformably
overlain by conglomerates and sandstones of the Red Bay Group, which contain
a well established fish
fauna of Lochkovian age. The Lilljeborgfjellet Conglomerate rests with
a major unconformity on
high-grade (with eclogites) schists and gneisses, with associated corona
gabbros and granitic gneisses.
Previous isotope-age studies have shown that these igneous rocks yield
U/Pb ages of c. 950 Ma, and
that the eclogite facies metamorphism may be of Caledonian or late Neoproterozoic
age. The high
P/high T rocks are intercalated with and overlain by schists
affected only by Caledonian amphibolite
facies metamorphism, recorded by 40Ar/39Ar and
Rb/Sr cooling ages of 400–430 Ma.

In the Lochkovian Red Bay Group of the Raudfjorden Graben, two horizons
of tuffites occur,
interbedded with sandstones. New studies of eight zircons from these volcanic
rocks have provided
single-zircon lead-evaporation ages of c. 950 and c.
1350 Ma; one yielded 440 Ma. All these zircons
are probably derived from the underlying basement rocks, the ages being
significantly older than the
Devonian host strata (c. 410 Ma).

The clasts in the Lilljeborgfjellet Conglomerate are generally angular
to subrounded and derived
locally from the underlying high-grade metamorphic complex. A subordinate
(usually less than 1%,
but up to about 10%) component of the clasts is a quartz porphyry that
is not known in the exposed
bedrock anywhere in northwestern Spitsbergen. The quartz porphyries are
better rounded than the
other clasts; however, the maximum diameter reaches 1.5 metres, indicating
that transport distances
are unlikely to have exceeded a few kilometres. Three quartz porphyry boulders
have been dated by the
single-zircon lead-evaporation method and shown to be of Palaeoproterozoic
age, yielding ages of
1735±4, 1736±5 and 1739±5 Ma that have not previously
been detected in the northwestern part of
Svalbard's Caledonides.

The quartz porphyry clasts show no evidence of the widespread high-grade
tectonothermal activity
of Mesoproterozoic and early Palaeozoic age that influenced northwestern
Spitsbergen. It is therefore
concluded that the most probable source of these clasts lies to the east
in the unexposed basement
beneath the Old Red Sandstones of the Andrèeland–Dicksonland Graben.
The Lilljeborgfjellet quartz
porphyry clasts are closely similar in age to the granitic rocks of Ny
Friesland. Whereas the latter
were subject to Caledonian high amphibolite facies metamorphism, the quartz
porphyry clasts have
only been affected by a low greenschist facies overprint. Nevertheless,
the similarity in age
suggests an affinity to Ny Friesland and it is proposed here that the Breibogen–Bockfjorden
Fault
defines the most important boundary between Svalbard's Caledonian
terranes.