Deciphering the peculiar magnetic fabric of the Gubaoquan doleritic dikes in the southern Beishan Orogenic Belt, NW China: Implications for their emplacement mechanism and regional tectonics

The anisotropy of magnetic susceptibility is widely applied to infer the magma flow direction in dikes and to assess broader geological implications. The “normal” magnetic fabric is characterized by a magnetic lineation parallel to the magma flow and a magnetic foliation that is subparallel to or imbricated against the dike walls. However, interpreting new results from the Gubaoquan doleritic dikes in NW China proves challenging. To elucidate the peculiar magnetic fabrics encountered in the Gubaoquan doleritic dikes and to explore their regional tectonic implications, we conducted a multidisciplinary study on 26 dikes in the southern Beishan Orogenic Belt, NW China. Rock magnetic investigations indicate that multidomain to pseudo-single-domain Ti-poor magnetite is probably the main magnetic carrier. Petrological observations affirm the magnetic carrier and the typical ophitic structure of the dikes; neither the preferred orientations of constituent minerals nor authigenic minerals are detected. Magnetic fabric analyses reveal a peculiar magnetic fabric, characterized by a predominant fabric with an aclinic SW-NE−striking magnetic lineation and a horizontal magnetic foliation, as well as a subordinate fabric comprising a horizontal SE-NW−striking magnetic lineation and a horizontal magnetic foliation. Two emplacement mechanisms are proposed to interpret the observed magnetic fabrics: (1) in near-surface conditions (<300 m), a horizontal intrusion generated the magnetic fabric that was locally disrupted by preexisting and synemplacement subfractures, and (2) at subsurface conditions (>1 km), due to the possible stress balance between overburden load and magma pressure, the vertical component of the magma flow was obscured; in such settings, the subordinate fabric may result from the propagation of cooling joints. Integrated with available geochronological, geological, geochemical, and paleomagnetic data, our findings suggest that the Gubaoquan dikes were emplaced under the influence of the Tarim mantle plume or within a postcollisional extensional setting during the early Permian.