P-T CONDITIONS AND MICROSTRUCTURAL ANALYSIS OF UHP METAMORPHIC ROCKS DURING EXHUMATION, FJØRTOFT, WESTERN GNEISS REGION, NORWAY

Microdiamond-bearing garnet-kyanite gneiss produced during UHP metamorphism associated with the Scandian Orogeny (425-395 Ma) underwent extensive re-equilibration during exhumation. The retrograde assemblage present in the matrix of this gneiss includes garnet, kyanite, K-feldspar, plagioclase, biotite, quartz, rutile, and monazite. Quantitative analyses of these minerals were determined by the SEM and the Electron Microprobe (EMP). Garnet shows compositional zoning with the Fe/(Fe+Mg) ratio increasing from core to rim of the grain, which is interpreted to result from retrograde re-equilibration. The Ca zoning profile shows an increase from core to rim, and may drop off at rim; this zoning is in part interpreted to the original prograde growth zoning. Selected mineral analyses used with the TWEEQU program, yielded pressures of 12 to 14 Kbar and temperatures of 700 to 800 C. These estimates are interpreted to be synchronous with the growth of monazite in the matrix that occurred at 395 Ma. Combining these results with previously determined U-Th-Pb ages from monazite and metamorphic conditions indicate an average exhumation rate of 6.8 mm/yr. Microstructural analysis was carried out in thin sections cut parallel to lineations and normal to foliations, parallel to both, and normal to both. Inspection of inclusions in garnet porphyroblasts indicate their alignment corresponds with the lineation present in the matrix. No evidence for tectonic fabrics preceding those interpreted to be associated with UHP metamorphism and later exhumation has been identified in this sample. This indicates a possible detrital origin for parts of monazite grains that yielded ages ~ 800 Ma and ~1060 Ma metamorphism. Of the two samples examined from the same exposure, the oriented sample does not appear to have any conclusive kinematic indicators to show a sense of motion, but the sample that is not oriented shows asymmetric shear fabrics in the garnet porphyroblasts.