Structural and anisotropy of magnetic susceptibility records of granitoid sheets emplacement during growth of a continental gneiss dome (Central Sudetes, European Variscan Belt)

Abstract

AbstractGranitoid magmatic sheets emplaced syntectonically during growth of the Orlica‐Śnieżnik mantled gneiss dome (Central Sudetes, European Variscan belt) were examined by means of structural geology, quartz, and anisotropy of magnetic susceptibility (AMS) fabric studies. Magmatic emplacement was localized along the eastern transpressive margin of the high metamorphic core and below the low‐grade detached mantle rimming the southern margin of the dome. In the first area, the magmatic sheets were emplaced parallel to the dilated subhorizontal foliation of an anisotropic pre‐orogenic block. The resulting melt‐host rock multilayer localized the transpressive zone along which the bulk of magma was emplaced. The AMS study shows an across‐strike fabric zonation underlying a strongly transpressive solid‐state deformation in the hanging‐wall, magmatic subhorizontal fabric in the foot‐wall and a transitional fabric in the center of the intrusion. In contrast, along the southern dome margin, magmatic sheets intruded along steep foliations of weakly metamorphic mantle rocks, and are affected by recumbent folding and subhorizontal shearing. The bulk of the sheets present a shallow‐dipping magmatic foliation and an along‐strike magmatic lineation. The variations of the quartz and magnetic fabrics are attributed to superimposition of pure shear‐dominated ductile thinning followed by simple shear‐dominated detachment onto the original steep fabric. AMS modeling confirms the role of variation of orientation of pre‐intrusive anisotropy during progressive deformation on the resulting fabric pattern and helps explaining observed variations in fabric orientations and symmetries. This study highlights contrasting mechanical behavior of syntectonically emplaced magmas in different parts of a growing crustal‐scale mantled gneiss dome.