Magnetic petrology of high Fe-Ti eclogites from the CCSD main hole: Implications for subduction-zone magnetism

The high Fe‐Ti eclogites with exsolved lamellar in 530–600 m depths from the Chinese Continental Scientific Drilling (CCSD) main hole in the Sulu Ultra‐High Pressure (UHP) metamorphic belt, eastern China, record an anomalously high susceptibility (κ), natural remanent magnetization (NRM) and Köenigsberger ratio Q (NRM/Ji, Ji is induced magnetization). This provides us with a good opportunity to study the effects of magnetic minerals and exsolution lamellae on the magnetic properties of deep rocks. In this paper, we have measured systematically magnetic properties and mineral assemblage and structure for three special samples (No. 83, No.86 and No.89). Results show that these Fe‐Ti‐rich eclogites are the result of the fine‐grained (titano)‐magnetite exsolution in pyroxene and lamellar structure consisting of finely interlayered ilmenite and hematite in exsolved hemo‐ilmenite. We found that the dominant Fe‐bearing oxide minerals in samples studied are ilmenite, hematite (Hem + Ilm up to 25%), little (titano)magnetite and pyrite. The ferromagnetic susceptibility is mainly controlled by fine grained (titano)magnetite and NRM is closely related to the exsolved lamellar structure. We hence propose that the anomalous magnetism of these eclogites observed in our samples is the result of exsolution from homogenous pyroxene and ilmeno‐hematite during cooling and decompression processes. These Fe‐Ti‐rich eclogites might be one of the sources of high‐magnetic anomalies observed in the Sulu subduction zone, eastern China. Key Points Study the effects of lamellar structure and magnetic minerals on NRM High‐ultrahigh pressure (HP‐UHP) rocks with HISS may be strongly magnetic Suggested relations between magnetism and exhumation process of eclogites