Magnetic properties of serpentinized garnet peridotites from the CCSD main hole in the Sulu ultrahigh-pressure metamorphic belt, eastern China

Magnetic properties of 14 serpentinized garnet peridotites from the interval of 603.20-683.53 m of the Chinese Continental Scientific Drilling (CCSD) hole in the Sulu ultrahigh pressure (UHP) metamorphic belt, eastern China, were investigated and compared with those of ultramafic rocks from other tectonic settings, to relate them to the UHP (post-) metamorphic evolution. Serpentinized garnet peridotites are characterized by high susceptibility (chi), natural remanent magnetization (NRM), and Q (Koenigsberger ratio = (NRM/chi x H), where H is the intensity of the local geomagnetic field) values. The average chi, NRM, and Q values are 299.65 x 10(-7) m(3) kg(-1), 19.15 x 10(-3) Am-2 kg(-1), and 16.59, respectively, which are generally higher than those of other serpentinized peridotites in the world. The serpentine concentration for most samples ranges from 40% to 60%, suggesting that the peridotites are of obvious homogeneous serpentinization degree. Micro-Raman and X-ray diffraction spectra of serpentine for representative samples indicate that lizardite is the only serpentine mineral. Opaque minerals in samples are mainly nonstoichiometric magnetite and Cr-spinel with or without minor ilmenite and Fe-sulfide. Magnetite occurs as rims around olivine and garnet as well as in clusters. A chromian garnet further overgrew on the rims of the garnet. The grain size of magnetite in rims is in the pseudo-single-domain region and favorable for development of high NRM. High pressure-UHP metamorphic processes possibly lead to enhanced concentrations of Fe-Ti oxides in garnet peridotites and, thus, increase the magnetism of serpentinized garnet peridotites in the CCSD main hole. Integrated lizardite formation temperature (<450 degrees C) and its relationships with the pressure-temperature-time evolution of the associated UHP rocks permit us to infer that serpentinization of garnet peridotites in the CCSD main hole occurred during late exhumation stages, ca. 200-215 Ma ago, under temperatures of 425 degrees C. However, the exact relationship between magnetic properties of garnet peridotite and the UHP metamorphic process will require further studies.