METAMORPHIC EVOLUTION OF THE TøNSVIKA ECLOGITE, TROMSø NAPPE - EVIDENCE FOR A NEW UHPM PROVINCE IN THE SCANDINAVIAN CALEDONIDES

Abstract

Eclogites within the Tromsø Nappe of the Uppermost Allochthon in the Scandinavian Caledonides record a complex P-T history, involving a prograde subduction-related stage. Information on the early prograde stage of the metamorphic evolution is provided by a relict amphibolite-facies inclusion suite within garnet. The inclusion assemblage comprises hornblende, oligoclase, albite, paragonite, biotite, and quartz. Included hornblende is chemically distinct from the secondary hornblende appearing in the matrix. Thermobarometric estimates on included hornblende and oligoclase yield pressures of ca. 1.3 GPa at 600-700°C. Maximum pressure conditions recorded are 3.36 GPa at 735°C, based on coexisting garnet, omphacite, and phengite, well within the coesite stability field. Post-eclogite decompression resulted in omphacite decomposition to three successive diopside + plagioclase symplectite stages: S1, S2, and S3. The width of the lamellar spacing (L) and the composition of the clinopyroxene vary as follows: Ls1 > Ls3 > Ls2 and XJds1 > XJds2 > XJds3 indicating that the three symplectites were formed at different P-T conditions. Thermobarometric estimates give 840°C at 1.41 GPa for S1, 700°C at 1.12 GPa for S2, and 740°C at 0.98 GPa for S3. Recrystallization under amphibolite-facies conditions occurred at 650-700°C, 0.9-1.0 GPa. The geothermobarometric results, together with previously published age dating of the rocks, indicates that initial uplift rates were very high, estimated at 3.6 cm/yr. Copyright © 2006 by V.H. Winston & Son, Inc. All rights reserved.