MULTIPLE PRE-SCANDIAN TECTONOTHERMAL EVENTS PRESERVED IN THE CENTRAL NORWEGIAN CALEDONIDES

Evidence of pre-Scandian age deformation and magmatism (~480 Ma to ~447 Ma) is well preserved in the Helgeland Nappe Complex in the Norwegian Caledonides. The current orogenic model suggests crustal thickening and magmatism began at ~497 Ma and was followed by a single rapid exhumation event at ~448 Ma. However, new P-T data from the 480 Ma Vega intrusion and associated host rocks support the possibility of an additional exhumation event at ~480 Ma.

The Vega intrusion is an enclave-bearing biotite muscovite granodiorite ± cordierite ± garnet ± sillimanite. Cordierite-bearing lithologies predominate in the southern and westernmost exposures. Peak amphibolite grade assemblages preserved in diatexites along and near the northeastern contact of the intrusion include kyanite + staurolite + garnet + biotite, garnet + biotite (retrogressed to chlorite), and kyanite + garnet + staurolite + sillimanite(?).

P-T estimates (GASP) indicate peak metamorphic pressures within or greater than the 7 to 8 kbar range. GASP equilibria intersect the kyanite-sillimanite equilibrium boundary at ~710° C and ~870° C. Temperature estimated from garnet-biotite exchange equilibrium in diatexite is ~600° C. P-T estimates, petrographic observations, and average Fe/(Fe+Mg) in garnet are consistent with diatexites originating by staurolite and/or muscovite dehydration melting (e.g. Spear et al., 1999; Pattison, 2001). Additionally, na-in-cordierite thermometry suggests cordierite phenocrysts equilibrated at ~750° C and cordierite-quartz nodules at ~660° C.

The stability of magmatic cordierite, implying lower pressures, to the south and west and higher pressures from diatexites to the east and northeast may be reconciled by two tectonic models, both of which include kyanite-stable regional metamorphism and melting followed by magma collection to make the pluton. In model 1 the pluton intrudes to depths partially within the cordierite stability field and is rotated during later deformation. In model 2 the pluton is emplaced at moderate to high pressures and exhumed into the cordierite stability field while in the magmatic state. Only model 2 necessarily supports a ~480 Ma exhumation event.