[S-MP46_28PO1] Deformed rocks, Metamorphic rocks and Tectonics

[SMP46-P01] Comparison of UHP chromitites from the Higo and Nishisonogi Metamorphic Rocks, Kyushu, Japan.

*Dai SHIOSAKI¹, Yosuke MORIBE¹, Hibiki EGUCHI¹, Tadao NISHIYAMA¹ (1.Department of Earth and Environmental Sciences, School of Science and Technology Kumamoto University)

Keywords:microdiamond, chromitite, UHP, Higo metamorphic rocks, Nishisonogi metamorphic rocks, subduction

We have found microdiamond - bearing ultrahigh-pressure (UHP) chromitites from two metamorphic terranes in Kyushu: the Higo (HMR)¹ and Nishisonogi (NMR)² Metamorphic Rocks. This paper describes the similarity and difference between the two UHP chromitites. The HMR are located in west-central Kyushu with an E-W trend. They have undergone low P /T metamorphism, however, precursor HP or UHP metamorphism of ca. 250 Ma has been inferred³. The protoliths have affinity to continental shelf deposits⁴, consisting mainly of pelitic gneisses and meta-carbonates with minor metabasites and metaperidotites (partly serpentinite). Chromitite occurs very rarely as a nodular form in serpentinized metaperidotites which shows spinifex-texture. The NMR is located in western Kyushu with a N-S trend. They have undergone high P /T metamorphism of epidote-blueschist subfacies. They consists mainly of pelitic and psammitic schists with minor basic schists and serpentinites, some of which show a character of serpentinite melange⁵. Detrital zircon from the pelitic schists show the age of 89-86 Ma⁶, whereas zircon from jadeitites in a serpentinite melange does 136 -126 Ma in the core and 84 - 80 Ma in the rim^7,8. Chromitite occurs as a deformed schlieren-like layer in serpentinite with no reilc minerals. The P-T condition of the HMR has been estimated to be 200 - 600 MPa and 600 - 800 ℃ ^{3,9,10,11,12,13}. Higher pressure and temperature conditions are reported from the following two samples: a sapphirine-bearing granulite^3,10 as a tectonic block in the spinifex-textured metaperidotite (900 MPa and 950 ℃) and a calc-silicate granulite¹³ (900 MPa and 820 ℃) intercalating with garnet - biotite gneiss. We newly estimated the peak P-T condition of Al-spinel and chlorite -bearing metaperidotite as 2.0 GPa and 780 - 990 ℃. In the case of the NMR, the peak metamorphic condition of the crystalline schists is 1.4 GPa and 520 ℃ for a garnet galucophanite¹⁴. Jadeitites¹⁵ as tectonic blocks in the serpentinite melange shows the peak condition of 1.5 GPa and 500 ℃. Chromite from the HMR has the composition (Mg_0.34Fe²⁺_0.75Mn_0.02)(Cr_0.81Al_0.06Fe³⁺_0.04Si_0.05)2O₄, whereas that from the NMR has similar composition (Mg_0.33Fe²⁺_0.65Mn_0.03)(Cr_0.84Al_0.12Fe³⁺_0.04)₂O₄ in the core and Fe-rich composition (Mg_0.06Fe²⁺_0.89Zn_0.02Mn_0.03)(Cr_0.85Al_0.12Fe³⁺_0.04)₂O₄ in the rim. Microdiamonds occur as in situ inclusions in chromite in both chromitites. They are 1 to 10 μm in size in HMR chromite, and those in NMR chromite is much smaller, mostly <1 μm with small number of larger grains. In both chromitites microdiamonds occur in some cases as numerous aligned grains, making diamond - rich zones. Both microdiamonds are identified with Raman spectra. HMR microdiamonds show a broad peak at 1333 cm^-1. NMR microdiamond, also shows a broad peak at 1331 cm^-1 with graphite peak at around 1600 cm^-1, suggesting partial graphitization. Both UHP chromitites will be deep subduction origin. HMR can be an eastern extension of the Dabie-Sulu UHP terrane in China, however, NMR is more problematic. No corresponding UHP terrane of ca. 80Ma is found around Kyushu. Our findings of UHP chromitites require reexamination of micro-tectonics in Kyushu, a peculiar location of an arc-arc junction at the continental margin.References 1:Nishiyama et al., JpGU Meeting, S-MP46, 2014; 2: Nishiyama et al., JpGU Meeting, S-CG08, 2014; 3: Osanai, et al., Gondwana Res.,9, 152-166, 2006; 4: Omori and Isozaki, J.Geogr., 120, 40-51, 2011; 5: Nishiyama, Mem. Geol. Soc. Japan, 33, 237-257, 1989; 6: Kouchi, Y., J. Geogr., 120, 30-39, 2011; 7: Mori, et al., JMG, 29, 673-684, 2011; 8: Yui, et al., EJM., 24, 263-275, 2011; 9: Obata et al., Lithos, 32, 135-147, 1994; 10: Osanai et al., JMG., 16, 53-66, 199