Paper No. 2-2
Presentation Time: 8:25 AM
COMBINED ZIRCON PETROCHRONOLOGY AND MICROSTRUCTURAL ANALYSIS: INSIGHT INTO LONG-LIVED SHEAR ZONE ACTIVITY IN THE WESTERN GNEISS REGION, NORWAY
The deformation history of ultrahigh pressure (UHP) terranes provides great insight into subduction and exhumation processes across depths of >100 km. The Western Gneiss Region (WGR) is one of the largest and best studied UHP terranes on Earth. Previous studies have argued that large portions of the UHP terrane did not deform during exhumation. Numerous small (up to 10s of meters) shear zones are present across the WGR. These shear zones were active during exhumation of the crustal rocks to the near surface and preserve an important record of the evolving pressure–temperature–time–deformation conditions that the terrane experienced during exhumation. We collected EBSD maps and split-stream LA-ICP-MS U-Pb and trace-element data from zircon grains to determine 1) the timing of peak metamorphism, and 2) if the zircon crystals had been affected by deformation during exhumation. We target zircon grains from multiple lithologies and variably deformed samples across multiple shear zones. Inherited zircon cores from all the shear zones yield the well-established protolith ages of ca. 1650, 1200, and 950 Ma. In comparison, the rims and some whole grains, mainly from mafic samples, yield Caledonian ages from ca. 420–395 Ma; REE results indicate that these grains (re)crystallized at eclogite-facies conditions. EBSD maps of zircon from nearly all the studied samples show intragrain distributed dislocations, subgrain boundaries, and/or high-angle boundaries. They are not associated with specific textural locations within the samples, nor are they associated with any obvious intragrain changes in trace element or U-Pb composition. Overall, these microstructures are mainly found in Precambrian grains in which only some have Caledonian rim growth. In some cases, the microstructures crosscut these Caledonian rims. These microstructural characteristics are consistent across multiple shear zones in regions of the WGR that record different peak P–T conditions. These combined results suggest that deformation and shear zone activity began in the Precambrian. The shear zones may have been reactivated during Caledonian exhumation of the WGR. The combined petrochronology and microstructural analyses allows for tracking of shear zone activity in polymetamorphic and polydeformational terranes.