Petrology and Geothermometry of the Clinopyroxene-Garnet Rock from Cape Ryugu, East Antarctica

• Main Authors: Shin-ichi Yoshikura, Yutaka Nakai, Takashi Kano
• Format: Report
• Language: English
• Published: Department of Geology, Faculty of Science, Kochi University 1979
• Subjects: Cape Ryugu; East Antarctica; Ryugu; Antarc*; Antarctica; Memoirs of National Institute of Polar Research; Polar Research
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=1031; http://id.nii.ac.jp/1291/00001031/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=1031&item_no=1&attribute_id=18&file_no=1

In Cape Ryugu, the most abundant rocks are biotite gneiss, garnet-biotite gneiss and hornblende-biotite gneiss, none of which show any obvious indication of their more detail metamorphic condition from their mineral parageneses. Petrology of the clinopyroxene-garnet rock occurring as thin intercalation in biotite gneiss of this area is described to determine the physical condition of metamorphism. Assuming the pressure during the metamorphism to be 5kb, the temperature estimated from the distribution coefficient of Fe and Mg between coexisting garnet and clinopyroxene obtained from microprobe analyses using the Raheim and Green's calibration is about 590℃. Raheim and Green suggested that the distribution coefficient, (Kd)^<Ga-Cpx>_<Fe-Mg> value, varies with grossular content in the garnet and thus with bulk rock composition. As the bulk rock chemistry of the investigated rock is different from the composition of starting materials used in their experiment, Raheim and Green's calibration cannot applied to this rock without reservation. To check the effect on the (Kd)^<Ga-Cpx>_<Fe-Mg> value of Ca content in the garnet, this estimated temperature is compared with the temperature determined on the basis of the (Kd)^<Ga-Bi>_<Fe-Mg> geothermometer and the "plagioclase-garnet-Al_2SiO_5-quartz geothermometer" from staurolite-bearing sillimanite-garnet-biotite gneiss which is considered to have been formed under an isophysical condition. The calculated temperature based on the (Kd)^<Ga-Cpx>_<Fe-Mg> well agrees with those determined by the other methods. The temperauture condition is also consistent with mineral chemistry of low Al_2O_3 content in clinopyroxene coexisting with garnet and with the field observation that this area lacks in migmatization phenomena by partial melting of the metapelites.