REVISITING PRESSURE-TEMPERATURE-TIME PATHS FOR THE GEOTECTONIC HISTORY, EMPLACEMENT AND METAMORPHISM OF THE SEILAND IGNEOUS PROVINCE, NORWAY

The 575-565 Ma Seiland Igneous Province, Norway, is a mafic to ultramafic intrusive complex with carbonatite and related alkaline rocks suggesting extensional tectonics were active during emplacement. However, some studies promote models based on contractional tectonics. The models invoke three metamorphic events constrained using textures in metapelitic and metapsammitic rocks. The events were linked via a continuous anti-clockwise P-T-t path that included a HT/LP event (M1), followed by local heating (M2), then an isothermal high pressure event caused by Caledonian nappe accretion (M3) (Elvevold et al 1994). Thermodynamic modeling of metamorphic assemblages, combined with new geochronology, is used to test whether the protracted, complex tectonic history is viable, or whether other P-T-t paths are more realistic.

The assemblage Opx + Grt + Qz + Pl + Kfs + Spn + Ilm ± Crd defines M1 and is described in silica under-saturated anhydrous migmatized xenoliths. Metamorphic temperatures, in the presence of a pure CO₂ fluid, as low as 920°C are calculated, rising to 960°C in the presence of H₂O. The assemblage is independent of pressure, which was calculated to be <6.5 kbars.

The second assemblage (M2) is Grt + Kfs + Qz + Pl + Bt + Ilm in the presence of either Sil or Opx. Compositionally, the reaction from assemblage M1 to M2 will occur upon the addition of H₂O. The exclusivity of Sil versus Opx places metamorphic conditions between 730 and 770°C, but at pressures >7 kbars. Meanwhile, M3 is defined as a lower temperature, higher pressure assemblage of Grt ± Ky + Qz + Pl ± Kfs + Bt + Rt that requires calculated conditions of <700°C and pressures of >8 kbars.

Petrographic evidence favors emplacement of the Seiland Igneous Province during extensional tectonics, which cannot be reconciled by increasing pressures in samples collected at structurally similar depths. Furthermore, the ~130 m.y. gap between igneous emplacement and increasing pressures attributed to Caledonian orogenesis suggest the following conclusions: the time-span between tectonic events necessitates two separate metamorphic events, not a continuous P-T-t loop; and, M2, rather than being related to cooling after M1, is more likely related to Caledonian orogenesis and Barrovian prograde metamorphism.