The Lyngen Magmatic Complex ophiolite: preservation of pre- and syn-collisional structures and implications for the nappe thrusting sequence in the Northern Norwegian Caledonides

Abstract

The Lyngen Magmatic Complex (Northern Norway) marks a major discontinuity in the metamorphic gradient of the North Norwegian Caledonides nappe sequence. The Lyngen Magmatic Complex preserves pre- and syn-collisional structures and parageneses of the Caledonian orogeny. This study differentiates the two deformation events and offers a tectono-metamorphic model for the region, using a combination of field-based data, geochemical analysis, radiochronometric dating, and thermodynamic modelling. It is demonstrated that the precollisional event (D1) developed in a dextral strike-slip transpressional regime, generating Nsingle bondS-trending subvertical shear zones with subhorizontal shear displacement and steeply inclined isoclinal folding. The D1 metamorphic fabrics developed at 486 ± 9 Ma on a retrograde temperature-path, from amphibolite (680single bond800 °C, 0.5single bond0.9 GPa) to greenschist facies (300single bond450 °C, 0.3single bond0.75 GPa) conditions. The subsequent syn-collisional event (D2) produced a subhorizontal foliation, top-to-SE-directed thrust deformation at the base of the Lyngen Magmatic Complex. D2 rock fabrics represent typical nappe stacking structures during the Scandian collisional stage (∼430 Ma). The vertical D1 structures are overprinted by D2 fabrics in the thrust contact region only. Mineral assemblages crystallizing during D2 indicate a transition from lower amphibolite to greenschist conditions. Thermobarometry suggests significant re-heating after D1, with D2 maximum conditions ∼650 °C and 1.1 GPa at the base of the Lyngen Magmatic Complex and ∼558single bond610 °C and 0.8single bond1.3 GPa for the underlying graphite-bearing metasediments. Argon dating and the temperature difference between the Lyngen Magmatic Complex and the underlying units testify to out-of-sequence thrusting at 426 ± 7.5 Ma at the base of the Lyngen Magmatic Complex. The present results constrain the localization of the oceanic units of the North Norwegian Caledonides outboard and to the North of their present position. Consequently, stacking of uppermost units (likely of Laurentian origin) on top of the Lyngen Magmatic Complex occurred before their final emplacement onto the Reisa Nappe Complex (probably mostly Baltica-derived).