PT HISTORY FROM THE KVALVIKA MANTLE PERIDOTITE, NORWAY: IMPLICATIONS FOR CRUST-MANTLE INTERACTIONS, AND CONDITIONS AND TIMING FOR OLIVINE C-SLIP DURING CONTINENTAL SUBDUCTION

The Kvalvika peridotite occurs in locally eclogitized diorite gneiss and both contain the same foliation and lineation formed during HP metamorphism. Metamorphic overgrowths on zircon in nearby (500 m) eclogitized gabbro with the same fabric yield Scandian (425-375 Ma) ages of 410 ±1 and 408 ±1Ma. The peridotite contains the Scandian HP assemblage Grt + Ol + Opx + Spl ± Cpx. SEM-EBSD study of orthopyroxene from a garnet harzburgite indicates a CPO with {100} subparallel to the foliation and a [001] maximum parallel to the lineation, indicating intracrystalline slip on the (100)[001] slip system commonly observed in upper mantle peridotites. Olivine in the same sample shows alignment of {010} and [001] with the foliation and lineation respectively, indicating dislocation glide on the (010)[001] slip system. Metamorphic conditions and the PT path during UHPM were determined by geothermobarometry and differential thermodynamic modeling in the (N)CFMAS system using the program Gibbs. Geothermobarometry yields conditions of 810-860 °C and 2.9-3.4 GPa for garnet rims. Compositional zoning in relatively coarse-grained Opx and Cpx and zoning of Ca and Cr in garnet provide a record of Scandian metamorphism. Changes in the Fe/(Fe+Mg)ratio and the Al content in Opx were used to determine the PT path. The observed changes of the grossular content in garnet, which serves as an independent monitor, agree with those predicted by the model. The observed changes in Cr/(Cr+Al) in garnet and spinel included in garnet agree with those inferred from experimental studies for this path. The resulting PT path from Grt/Opx shows an increase in P and T from core to rim of 765 °C and 3.5 GPa to 790 °C and 4.0 GPa, followed by decompression and heating to 850 °C and 3.4 GPa. The modeled PT path records a transition from subduction to exhumation and indicates that olivine c-slip was active over a range of conditions during UHP metamorphism. The increasing P and T in the early part of the path indicate that the peridotite lens was incorporated into the crust prior to metamorphism or was part of the subducted mantle lithosphere and emplaced during metamorphism.