Abstract
Partial melt in the deforming mid- or lower continental crust causes a strength decrease and drives formation of lithological heterogeneities. However, mechanisms of formation of syn-melt deformation zones and strain partitioning in partially molten rock remain poorly understood. We use field and microstructural observations to unravel the evolution of a partial melt shear zone, Seiland Igneous Province, northern Norway. The Øksfjord shear zone (ØSZ) is one of several paragneiss shear zones present within gabbros of the Seiland Igneous Province, formed by syn-intrusive deep crustal shearing during lithospheric extension related to continental rifting. Microstructures from the ØSZ show evidence for different deformation conditions. The first phase was active pre-melt and involved deformation at high subsolidus temperatures. This was followed by syn-melt deformation of the shear zone causing a relative strength increase towards the shear zone centre upon crystallization. The third phase nucleated two parallel shear zones at the edges of the ØSZ; melt textures are absent and microstructures indicate deformation at lower temperatures and higher stresses. In effect, melt migration towards the shear zone centre ultimately led to strengthening of the shear zone core, with post-crystallization deformation focusing along shear zone margins where significant heterogeneities are present. Supplementary material: Figures illustrating grain boundary dihedral angle data, EBSD maps, grain size distribution and additional pole figures is available at https://doi.org/10.6084/m9.figshare.c.4819494
Highlights
200–500 μm in size, are preserved in the centre of the Øksfjord shear zone (ØSZ), suggesting that it is a peritectic product of biotite dehydration melting (SIP20, Fig. 3f )
We investigate the microstructural signature of a syn-intrusive partial melt shear zone from the Øksfjord peninsula in the Seiland Igneous Province of the North Norwegian Caledonides
In contrast to conventional expectations for melt-free shear zones, a reverse grain size distribution is observed with finer grains at the shear zone edges and coarser grains in the centre
Summary
200–500 μm in size, are preserved in the centre of the ØSZ, suggesting that it is a peritectic product of biotite dehydration melting (SIP20, Fig. 3f ). If the migmatite was in equilibrium during crystallization, peritectic phases and delicate melt textures would be preserved; these features are not visible in shear zone boundary samples, suggesting post-melt deformation.
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