NJG68-3-03
1988
Alternative to the Finnmarkian-Scandian interpretation on Magerøya, northern Norway
68
3
171-185

The mafic and ultramafic rocks of the Seiland lgneous Complex on Sørøya and Honningsvåg Igneous
Complex on Magerøya are much alike but have been thought to be of different ages, related to
Finnmarkian and Scandian orogenesis, respectively. Hand specimens from two different Honningsvåg
gabbros, however, each yielded three-point (cpx, plag, whole rock) internat isochrons, with Sm-Nd dates
of 508 ± 18 Ma and 475 ± 22 Ma. (Rb-Sr dates from the same mineral separates were 558 ± 91 Ma and
468 ± 18 Ma, respectively.) These new dates suggest that the Honningsvåg gabbros overlap in age with
the youngest mafic rocks of the Seiland complex, and are too old to have intruded the fossiliferous
Silurian rocks of the Magerøy sequence. By starting from these pre-Silurian dates and an unpublished
geologic map from 1958 by J. J. C. Geul, we believe we can show that the gabbros and the surrounding
contact-metamorphic rocks form a tectonic block, the Honningsvåg unit, which is separated by faults
from the regional-metamorphic rocks of the Magerøy sequence.
Way-up criteria in the Magerøy sequence of central Magerøya indicate that the sequence there is not
overturned and that the fossiliferous rocks and their correlatives lie in synforms at the top of the sequence,
passing downward through pelitic rocks into greywacke turbidites, which we equate with the Hellefjord
Schist Group on the islands farther west and on Porsangerhalvøya. The basal contact of the supposed
'Magerøy Nappe' over the top of the Kalak Nappe Complex in western Magerøya closely resembles
and is probably equivalent to the basal contact of the Hellefjord schist as exposed on Havøya and
Porsangerhalvøya within the nappe complex. If so, the Magerøy sequence is an integral part of the Kalak
Nappe Complex, and its Silurian fossils would show that the stacking of the Kalak Nappes was Scandian,
not Finnmarkian.
The Skarsvåg Nappe of northeastern Magerøya includes not only high-grade regional metamorphic
rocks but in our view also the underlying Skarsvåg and Opnan granites, which are equivalent and probably
a single continuous granitic sheet. The Skarsvåg Nappe clearly overlies the Magerøy sequence, whereas
the turbidites of the Honningsvåg unit may have been faulted upward from near the base of the Magerøy
sequence.

0029-196X