Abstract:
U-Pb ID-TIMS ages are reported for several magmatic and anatectic phases
from the Kalak Nappe Complex (KNC), northern Norwegian Scandinavian
Caledonides. Euhedral prismatic zircons from a suite of plutons intruding the
metaturbidite sequences of the Hellefjord Group, Sørøy-Seiland Nappe (upper
KNC) yielded intrusion ages of 440.9 ± 1.5 Ma for a syenogranite, and 435.9 ±
1.6 Ma and 436.7 ±0.8 Ma, respectively, for a granite and gabbro. The period of
magmatic activity was followed closely by a phase of deformation associated
with upper amphibolite-granulite facies metamorphism at c. 430 Ma. This
caused local anatexis in the granite and growth of magmatic zircon (429.5 ± 1.4
Ma), as well as the common growth of metamorphic titanite (431.4 ± 1 Ma and
427.8 ±2.7 Ma). These late Ordovician-early Silurian ages link this
tectonometamorphic activity to the Scandian phase of the Scandinavian
Caledonian orogeny. Evidence for Scandian tectonometamorphic activity is
also recorded at c. 425 Ma in the Klubben Group of the Olderfjord Nappe (lower
KNC). This is documented by the intrusion of a granodiorite pegmatite (425.9
±0.7 Ma) and anatexis of the Klubben Group (425.5 ± 1.3 Ma). In addition, the
Klubben Group records a c. 980 Ma Grenvillian age for anatectic veins
crosscutting an earlier fabric (980.9 ± 2.6 Ma, syenogranitic leucosome),
linking the initial evolution of the Klubben Group to the amalgamation of
Rodinia.
It is suggested that a previously undetected structural break exists in the
Sørøy Succession of the KNC at base of the Hellefjord Group. This is supported
by c. 441-436 Ma Scandian ages obtained for bimodal magmatic activity in the
Hellefjord, which does not relate to the intrusion of the SIP further down in the
Sørøy Succession at c. 570-560 and c. 530-520 Ma. In addition, Scandian
magmatic ages have not been detected in the Sørøy Succession underlying the
Hellefjord Group. However, these ages are coeval with bimodal plutons in the
overlying Magerøy Nappe (c. 440-436 Ma), which is composed largely of
metaturbidite sequences. The Hellefjord Group is suggested to represent an
extension of the Magerøy Nappe, which evolved in a ridge-trench intersection.
A tentative window of c. 430-425 Ma is suggested for the thrusting of the
Magerøy Nappe over the Kalak Nappe Complex, in light of the formation of
deformational fabrics in the Hellefjord Group at c. 430 Ma and the localised c.
425 Ma anatexis recorded in the Klubben Group.
