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[SMP27-18] Origin of antigorite serpentinite lenses in the early Carboniferous and early Triassic accretionary complexes of the Nedamo Belt, Kitakami Mountains, NE Japan
Keywords:antigorite serpentinite, trace elements, antimony, Nedamo Belt, Kitakami mountains
Serpentinite, typically occurring at tectonic plate boundaries, is a unique rock that consists mostly of serpentine minerals. The massive serpentinite and serpentinized mantle peridotite are commonly observed as fragments of accreted oceanic lithosphere (a.k.a. ophiolites) in orogenic belts and as serpentinite mélange derived from serpentinized hydrous mantle wedge beneath the forearc settings. Although the occurrences of serpentinite characterize past and present-day tectonic plate boundaries, serpentinite is less common in unmetamorphosed (or weakly-metamorphosed) accretionary complexes with abundant coherent to semi-coherent sedimentary sequences. A peculiar accretional complex containing numerous small serpentinite bodies occurs in the Nedamo Belt of the South Kitakami Mountains of NE Japan. Recent detrital zircon geochronology subdivided the Nedamo Belt into the Devono-Carboniferous Tsunatori Unit and the Permo-Triassic Takinosawa Unit (Uchino 2021). Interestingly, those two units with different accretionary ages share the same feature of sporadic exposures of lenticular serpentinite bodies within the clastic rock sequence. In this study, we characterized petrological, mineralogical, and geochemical features of serpentinite bodies in two different units of the Nedamo Belt.
Field observations confirm that no metasomatic reaction zones are developed along the serpentinite–country rock contacts; neither tremolite-actinolite rocks nor silicate-carbonate rocks occur. Although the contacts are not clear in the field, lenticular serpentinites seem to distribute along the NNE-SSW structural trend faults system. Centimeter-scale chevron folds are observed in some serpentinite bodies. Mineralogically, there are no significant differences among serpentinites in the two units. The serpentinites in both units lack protolith texture. They consist mainly of serpentine minerals with magnetite, ferritchromite, and chlorite; ferritchromite is commonly corroded by chlorite. Although they can be divided into antigorite-bearing and antigorite-free serpentinites, lizardite and chrysotile in the latter one seem to be retrograded equivalents of antigorite-bearing serpentinites. Relict chromites in both units are characterized by high Cr# >0.6. The compositional trends are quite different from those of the Cr-spinels in lherzolitic residual peridotites (aluminous spinel ultramafic suite: Ozawa et al. 2019) of the western part of the Hayachine Complex. Instead, the trend overlaps with contact aureole metaperidotites (or metaserpentinites) of the eastern part of the complex (Fujimaki and Yomogida 1986). There is also no significant difference in trace-elements patterns of serpentine minerals in both units. However, antigorite in both units is characterized by a low Li/U ratio; Li, La, Th, and Pb concentrations of antigorite are significantly lower than those of lizardite and chrysotile. Compared to trace-elements data of various serpentinites in the Franciscan Complexes (USA) and the Osayama serpentinite mélange (SW Japan), the Nedamo serpentinites are characterized by high Cs/U and low Rb/U ratios. We also found enrichment of Sb and decoupling of Sb and As in the Nedamo serpentinites. These peculiar features are also found in contact aureole metaserpentinite in the Osayama, suggesting that the oxidation states of Sb in dehydrating of completely-serpentinized peridotite can control such geochemical signatures. Considering our results together with geological context, serpentinites in the two different units of the Nedamo Belt might have originated from contact aureole metaserpentinite and have been likely emplaced by the Neogene strike-slip faulting.
Field observations confirm that no metasomatic reaction zones are developed along the serpentinite–country rock contacts; neither tremolite-actinolite rocks nor silicate-carbonate rocks occur. Although the contacts are not clear in the field, lenticular serpentinites seem to distribute along the NNE-SSW structural trend faults system. Centimeter-scale chevron folds are observed in some serpentinite bodies. Mineralogically, there are no significant differences among serpentinites in the two units. The serpentinites in both units lack protolith texture. They consist mainly of serpentine minerals with magnetite, ferritchromite, and chlorite; ferritchromite is commonly corroded by chlorite. Although they can be divided into antigorite-bearing and antigorite-free serpentinites, lizardite and chrysotile in the latter one seem to be retrograded equivalents of antigorite-bearing serpentinites. Relict chromites in both units are characterized by high Cr# >0.6. The compositional trends are quite different from those of the Cr-spinels in lherzolitic residual peridotites (aluminous spinel ultramafic suite: Ozawa et al. 2019) of the western part of the Hayachine Complex. Instead, the trend overlaps with contact aureole metaperidotites (or metaserpentinites) of the eastern part of the complex (Fujimaki and Yomogida 1986). There is also no significant difference in trace-elements patterns of serpentine minerals in both units. However, antigorite in both units is characterized by a low Li/U ratio; Li, La, Th, and Pb concentrations of antigorite are significantly lower than those of lizardite and chrysotile. Compared to trace-elements data of various serpentinites in the Franciscan Complexes (USA) and the Osayama serpentinite mélange (SW Japan), the Nedamo serpentinites are characterized by high Cs/U and low Rb/U ratios. We also found enrichment of Sb and decoupling of Sb and As in the Nedamo serpentinites. These peculiar features are also found in contact aureole metaserpentinite in the Osayama, suggesting that the oxidation states of Sb in dehydrating of completely-serpentinized peridotite can control such geochemical signatures. Considering our results together with geological context, serpentinites in the two different units of the Nedamo Belt might have originated from contact aureole metaserpentinite and have been likely emplaced by the Neogene strike-slip faulting.