The role of shear zones in the exhumation of the ultrahigh-pressure Western Gneiss Region

Abstract

The study of ultrahigh-pressure terranes informs our understanding of continental collision, subduction, and exhumation in both ancient and modern orogens. However, these studies rarely focus on the metamorphosed continental crust that forms the bulk of ultrahigh-pressure terranes. Here, we document a combined petrologic and structural investigation of mylonite zones in metamorphosed continental crust of the ultrahigh-pressure Western Gneiss Region of Norway that sheds light on the exhumation of ultrahigh-pressure terranes through the study of the continental crust. One of these shear zones, the Salt Mylonite Zone, is a prime example of the importance of the quartzofeldspathic gneiss that is associated with ultrahigh-pressure (coesite-bearing) eclogite. Eclogite in the Salt mylonite zone preserves quartz fabrics formed near UHP conditions (prism <c> fabric) and omphacite fabrics that are consistent with constrictional deformation near UHP conditions. Ti-in-quartz thermobarometry indicates equilibration of eclogite at temperatures > 750 °C and for quartzofeldspathic gneiss indicates quartz deformation/recrystallization took place over a range of temperatures from near UHP conditions to amphibolite facies conditions. Gneiss in the Salt Mylonite Zone preserves two dominant microstructures: phase-separated gneiss and phase-mixed gneiss. Despite preserving the same assemblage and being in close proximity (shared pressure and temperature conditions), fabrics between the two gneiss types differ due to habit. In both gneiss types, reverse zoning in plagioclase (Na-rich cores, Ca-rich rims) is ubiquitous. The compositional zoning records grain growth by grain boundary migration during exhumation-induced decompression. The study of the Salt Mylonite Zone is proof that shear zones in continental crust preserve a vital record of exhumation of ultrahigh-pressure terranes. Preliminary studies were conducted on two additional shear zones, The Finnøya Mylonite Zone and the Svartberget Mylonite Zone. Like the Salt Mylonite Zone, both contain evidence of deformation over a wide range of conditions. The Finnøya Mylonite Zone contains clinopyroxene-garnet gneiss with a composition similar to associated deformed eclogite pods. This shear zone, in particular, suggests that eclogite may have experienced extensive melting and mylonitization during exhumation from peak conditions. The Svartberget Mylonite Zone is composed of paragneiss and was deformed at sillimanite-stable conditions. The associated mafic material was synkinematically deformed as evidenced by elongate, foliation-parallel amphibolite and retrogressed eclogite pods. The abundant shear zones in the Western Gneiss Region of Norway record local exhumation from UHP to amphibolite-facies conditions. Understanding the deformational history of these individual shear zones can contribute to the overall understanding of the exhumation of the WGR.