Sensitivity of remagnetization in fingerprinting chemical fluids: insights from paleomagnetism and calcite geochronology of the Ordovician limestones, northern Tibetan Plateau

Remagnetization has proven to be a powerful process in geosciences, and determining the timing of remagnetization is the key to exploring its broader significance. Although carbonate rocks are the most common carriers of secondary magnetization, their remagnetization mechanism and the relationship between remagnetization and the resetting of the calcite U-Pb radiogenic system remain unclear. In this contribution, we conducted in-situ calcite U-Pb dating, scanning electron microscope observations, as well as detailed rock magnetic and paleomagnetic investigations on the Duoquanshan Formation limestones in the Oulongbuluke Terrane, northern Tibetan Plateau. Calcite geochronology reveals an inferred depositional age of ca. 479–466 Ma and three vein generations with estimated ages of ∼479 Ma, ∼462 Ma, and ∼400 Ma. Paleomagnetic results document differential remagnetization: the north limb of the limestone anticline was remagnetized during the Late Ordovician to Devonian, consistent with the ages of late-stage veins, whereas the south limb probably experienced a Mesozoic-Cenozoic remagnetization. Integrating all lines of evidence, we interpret that orogenic fluid infiltration, related to the subduction and closure of the Proto-Tethys Ocean, is the most probable remagnetization mechanism for the north limb limestones. As the chemical remanent magnetization is carried by trace authigenic magnetic minerals that dominate the bulk magnetic signal, while the host calcite matrix retains its primary U-Pb age, we conclude that remagnetization is more sensitive than calcite U-Pb geochronology in fingerprinting chemical fluids at the sample scale.