10:45 AM - 11:00 AM
[SMP42-07] New evidence of ultrahigh-pressure metamorphism in the Cretaceous low P/T metamorphic terrane, Higo Metamorphic Rock, Central Kyushu, Japan
Keywords:Higo Metamorphic Rock, ultrahigh-pressure metamorphism, diamond-graphite aggregate, Raman spectrometry, Soft X-ray emission spectrometry
DGA-bearing pelitic gneisses are found at three localities around Mt. Kousa (Sakamoto, Taira, and Kawagoe from east to west). They all belong to the garnet-cordierite zone (Obata et al., 1994; Miyazaki, 2004). Garnets in these rocks show doubly zoned structures consisting of black core surrounded by pink mantle, which are distinct even in naked eyes. This type of garnet occurs ubiquitously in the HMR, however, both simply zoned garnets and doubly zoned garnets occur frequently in the same outcrop, depending on the bulk compositions of the host rocks (Ishimaru et al., 2015). DGA occurs as platy grains of 10 mm in size in the garnet cores, but not in the mantles. We confirmed C peaks for the grains with EDS analysis under Au coating on the sections. DGA shows Raman spectra with 1335 cm-1 diamond-band together with 1580 cm-1 (G band) and 2680 cm-1 (S1 band) of graphite. These features are quite similar to those from the Nishisonogi metamorphic rocks (Nishiyama et al., 2017; this meeting), however, the wave number of the diamond-band is slightly shifted to be higher (1335 cm-1) compared to that (1330 cm-1) in the Nishisonogi DGA. The grain size (10 mm) is larger than that (1 mm) of the Nishisonogi DGA. The SEM-SXES (scanning electron microscope combined with soft X-ray emission spectrometer) analysis confirmed coexistence of sp3 and sp2 structures in the grains, showing that the grains consist of diamond and graphite. The result is consistent with the Raman spectra. Radial alignment of tiny inclusions of rutile is occasionally observed in the garnet cores, however, coesite has not been identified yet from the inclusions. Quartz inclusions occur in the garnet mantles.
The garnet-bearing amphibolite occurs as a block of 50 ×100 cm in size within pelitic gneisses at a stream near Kawagoe, Tomochi Town. It consists mainly of magnesio-hornblende and biotite with small amounts of plagioclase, quartz, and ilmenite. Garnet (Alm64Prp17Grs18) occurs locally as anhedral crystals, several millimeters across, and decomposes into a symplectite consisting of orthopyroxene (En47) + plagioclase (An85) from the periphery. The orthopyroxene contains a small amount of Al2O3 (0.02 apfu based on 3 oxygens) and is frequently altered to chlorite. The garnet-hornblende geothermometry gives a temperature of 450 oC, whereas the garnet-orthopyroxene geothermometry does 640 oC (at 0.5 GPa) - 670 oC (at 1 GPa). This indicates that the rock was initially equilibrated at 450 oC and then heated up by about 200 oC, forming the symplectite. The DGA occurs as granular materials 100 mm across in this symplectite. It does not occur in garnets. EDS analysis of the sample coated with Au reveals C peaks. Raman spectra shows a 1335 cm-1 diamond-band together with 1580 cm-1 (G) and 2680 cm-1 (S1) bands of graphite. Although the diamond band shows a slightly higher wave number, other features are the same as those of DGA in the pelitic gneiss. The SEM-SXES method also shows coexistence of sp2 and sp3 structures, indicating that this material is a mixture of diamond and graphite.
Our new finding of DGA from four localities in two types of rocks confirms that the HMR has undergone the ultrahigh-pressure metamorphism and that the Cretaceous low P/T metamorphism has superimposed on it.