Ferromagnetism as a pressure proxy for metamorphism?

Throughout geological history, changes in metamorphic conditions plays a crucial role in reflecting the evolution of the tectono-thermal conditions of Earth. The determination of temperature and pressure conditions for evaluating metamorphism typically relies on a limited number of samples, and the testing and analysis process requires significant time and work for thin section preparation, thin section analysis, geochemical analysis, as well as pseudosection calculations based on thin section and geochemical constraints. Following these procedures, it is commonly assumed that localized results can represent thermobaric conditions at regional or even orogenic belt scales. This assumption holds true when metamorphism reflects rock burial depth (i.e., lithostatic pressure). However, on small spatial scales, deviational stresses resulting from tectonic extension or compression can induce structural underpressure or overpressure, thereby rendering this assumption invalid. Therefore, we introduce magnetic susceptibility (k) as a measure of material magnetization in an applied magnetic field. It exhibits two key characteristics: (i) Fe-Ti oxides in mafic rocks display a mineral phase sequence largely associated with changes in pressure, with approximate pressure ranges for ilmenite + magnetite (<6 kbar), ilmenite (6–12 kbar), rutile + ilmenite (~12 kbar), and rutile (>12 kbar); (ii) Magnetic susceptibility can be quickly and extensively measured to reflect variations in Fe-Ti oxide content within mafic rocks. Our findings demonstrate that Fe content decreases while Ti content increases with increasing pressure. Thus, as pressure increases during prograde metamorphism, Ti generally replaces Fe, and this process can be detected as systematic decreases in magnetic susceptibility. This pressure–susceptibility correlation is further corroborated by retrograde metamorphism, where k inversely increases with decreasing pressure during exhumation. Based on our comprehensive petrological and geophysical analysis of mafic rock samples collected from the Paleoproterozoic (ca. 1.8 Ga) Jiao–Liao–Ji Belt of the North China craton, including coupled thin section, geochemical, and thermal susceptibility (kT) fingerprinting of Fe-Ti oxides, we posit that magnetic susceptibility can serve as a reliable indicator for discerning metamorphic pressure. Furthermore, owing to its ease of rapid and extensive data acquisition in fieldwork settings, we contend that it can play a pivotal role in identifying potential deviations from lithostatic stress due to tectonic stress on either small and/or large spatial scales.