Magmatic-Hydrothermal Alteration Mechanism for Late Mesozoic Remagnetization in the South China Block

Although the late Mesozoic remagnetization of the South China Block (SCB) is well recognized, the mechanism is unclear due to poor constraints on the genesis of secondary magnetic minerals. This issue is addressed via the study of the world's largest Paleozoic Mn deposit, in which both early Paleozoic sedimentation of manganese carbonate minerals and undated hydrothermal overprinting manifested by the presence of Mn silicate minerals with associated magnetite. Laser ablation-inductively coupled plasma-mass spectrometry in situ element analyses and paleomagnetic studies are applied to determine the age of hydrothermal alteration, and the results shed new light on the remagnetization mechanism of the SCB. Laser ablation-inductively coupled plasma-mass spectrometry in situ analyses show that the secondary magnetites have high values of (Ca + Al + Mn) and low values of (Ti + V) and (Ni/Cr)/Ti. This, together with coexisting skarn formation, supports a hydrothermal origin. The postfolding remagnetization is characterized by a stable component and a systematic normal polarity, carried by magnetite. The mean direction has a declination of 22.9° and an inclination of 56.1°, with a statistical parameter α₉₅ of 4.1°. Comparison with previously reported paleopoles indicates that the remagnetization was mostly acquired during the Late Cretaceous. Therefore, our results confirm that the hydrothermal magnetite carries a chemical remanent magnetization as a result of Late Cretaceous skarn formation. From a regional perspective, hydrothermal alteration from the Middle Jurassic to the Late Cretaceous, covering most parts of SCB, is proposed to be one of the important mechanisms for the ubiquitous late Mesozoic remagnetization.