THE POTENTIAL FOR DETERMINING THE PT EVOLUTION OF ROCKS AT UHP CONDITIONS USING CR IN GARNET AND SPINEL BEARING KVALVIKA PERIDOTITE, WESTERN GNEISS REGION, NORWAY

Use of changes Cr from garnet and spinel present in mantle peridotite has excellent potential for determining the PT evolution for peridotite bodies that have experienced UHP conditions. In this study, changes in Cr from garnet zoning and spinel inclusions are compared to PT-path determined using more traditional approaches. A PT-path for the Kvalvika Peridotite was determined by applying quantitative geothermobarometry and differential thermodynamic modeling in the NCFMAS system to constrain a P-T path during metamorphism. The geothermobarometry indicates conditions of 810-860 °C and 2.9-3.4 GPa for the garnet rim. Backward modeling of the changes Fe/(Fe+Mg) ratio and Al content in Opx yields a clockwise P-T path indicating a range conditions for olivine c-slip in the peridotite lens of ~740 to 850 ˚C and ~3.0 to 4.0 GPa. The P-T path is restricted to UHP conditions and captures the change from subduction to exhumation without a change in subduction kinematics. The core to rim zoning in garnet and compositions of Scandian inclusions of spinel in garnet indicate increasing pressure followed by a decrease in pressure from core to rim in the garnet which is consisted with the determined PT evolution for the Kvalvika peridotite. These are also consistent with available experimental data CrMAS system. The is path captures the transitions from subduction to exhumation at UHP conditions without a change in shear sense which is most easily explained by ductile flow within the subducting continental crust.