Decoupling of Magnetic Fabrics From Magnetic Remanences: Insights From Migmatites in Central Tianshan, NW China

Abstract

The coupling of magnetic fabrics and magnetic remanences is critical in interpreting paleomagnetic data. To estimate whether primary magnetic fabrics imply primary magnetic remanences, and to assess the practicability of metamorphic rocks in magnetic study, we carried out petrographic, geochronological, rock magnetic investigations, and analyses in anisotropy of magnetic susceptibility and paleomagnetism on migmatites in the Central Tianshan, NW China. Petrological observations indicate no significant dynamic recrystallization post to the migmatization. In-situ monazite U-Pb dating suggests that the migmatization happened during ∼314–297 Ma. Rock magnetic results reveal that the magnetic properties of migmatites are dominated by biotites with minor titanomagnetites. Despite the structural and compositional complexities of migmatites, a simple magnetic fabric pattern is observed with concentrated magnetic foliations and dispersed magnetic lineations. The anisotropy degree and shape parameter significantly change from leucosomes, mesosomes to melanosomes, suggesting that the magnetic fabrics should have been acquired during the migmatization. Characteristic remanent magnetization directions were isolated from a quarter of samples with anomalous shallow magnetic inclination. Combined with available geochronological and paleomagnetic results from the Central Tianshan and neighboring blocks, the magnetic remanences preserved in the migmatites were suggested to be obtained at ∼314–303 Ma, later than the acquisition of magnetic fabrics, probably due to thermal remagnetization or resulted from long-term progressive magnetization during tectonic exhumation of migmatites. This study provides an important yet rarely reported example to manifest the decoupling of magnetic fabrics from magnetic remanences. Meanwhile, migmatites are found to be operable materials for magnetic fabric and paleomagnetic studies.