NEW U-PB GEOCHONOLOGY FOR THE HELGELAND NAPPE COMPLEX (HNC), NORWEGIAN CALEDONIDES: CONSTRAINTS AND CONUNDRUMS

The HNC consists of at least four nappes separated by east-dipping faults and shear zones. Structurally upward, they are the Sauren-Torghatten (S-T), Lower, Middle, and Upper Nappes. All contain metapelitic and quartzofeldspathic gneiss, marble, and calc-silicate rocks. Metamorphic grade is variable in the S-T Nappe (upper greenschist to upper amphibolite, with sparse migmatite). Migmatites characterize the Lower and Upper Nappes, but the Middle Nappe only reached amphibolite grade.

New U-Pb zircon and sphene ages (LA-ICPMS and SHRIMP) indicate that regional crustal melting occurred in the S-T nappe at about 480 Ma (migmatite leucosome at 483.8 ± 2.7 Ma, cordierite two-mica granite at 472.5 ± 4.7 Ma). Crust-derived granites in the Lower Nappe are 482 ± 4.2 to 470 ± 4 (Yoshinobu et al., 2002, Geology) and migmatites in the Upper Nappe yielded an igneous age of 479.2 ± 3.0 Ma. Metamorphic sphene from the Middle Nappe gave a lower intercept age of 475 ± 3 Ma. Evidently, high-grade metamorphism was coeval in all four nappes, but discontinuities in metamorphic grade across nappe boundaries show that metamorphism occurred prior to nappe emplacement. Moreover, detrital zircons from a pelitic schist in the S-T Nappe indicate a depositional age younger than 482 ± 4.8 Ma. These zircons are unusual in their high P2O5 contents, which is also characteristic of igneous zircons from ~coeval S-T migmatites and peraluminous plutons. The similarity in igneous and detrital zircon ages suggests active volcanism and/or rapid exhumation, deposition, and burial just prior to regional metamorphism.

In addition to inherited and detrital zircons of Proterozoic age, several S-T and Upper Nappe samples contain zircons with 490–525 Ma ages. This range overlaps the ages of ophiolite fragments in the HNC, but high P2O5 contents in these zircons suggest metapelitic rather than plagiogranitic source rocks. At present, no crustal melting or granite-forming event of this age is known from the HNC. The HNC is thought to be an orphan of Taconian orogenesis, with affinities to East Greenland rather than the Baltica. However, such ages are also absent from East Greenland, so that this 490 to 525 Ma zircon population is enigmatic.