Magma flow pattern of the 1.78 Ga dyke swarm of the North China Craton during the initial assembly of the Supercontinent Nuna/Columbia: Constraints from rock magnetic and anisotropy of magnetic susceptibility studies

摘要

The geodynamic origin of the 1.78 Ga dyke swarm in the North China Craton (NCC) is long debated. It is considered either a post-collision process or paleoplume-related. Constraining its origin is essential for the reconstruction of the NCC position within the supercontinent Nuna (also referred to as Columbia). Here, we report on the rock-magnetism and anisotropy of magnetic susceptibility (AMS) of thirty-three 1.78 Ga dykes, to constrain the magma center of the igneous event. The samples were collected from the entire swarm across the NCC. Magnetite appears to be the dominant magnetic mineral in all the dykes, occasionally with minor maghemite, while the magnetite grain size within the dykes is generally increasing from south to north. AMS results of the dykes portray a magma flow-related fabric. Over half of the studied dykes show a predominance of horizontal to subhorizontal magmatic flow, while a few have been formed by vertical intrusion. These observations imply that the 1.78 Ga dyke swarm may have been generated by sustained magma flow from a magmatic source centered at the south margin of the NCC. During dyke propagation, some localized magma subchambers may have been formed; they could be responsible for the less common vertically intruded dykes and the EW-trending dykes. Therefore, a plume-related tectonic setting for the 1.78 Ga dyke swarm is more compelling. With the eruption center along the margin of the NCC, it may serve as an essential criterion to search for possible counterparts in other tectonic units within Nuna/Columbia to evaluate the proximity of the NCC with them.