Fluid inclusions study of Abukuma metamorphic rocks from the Hanazono district, northeast Japan

• OHYAMA Hiroyuki

  Graduate School of Life and Environmental Sciences (Earth Evolution Sciences), University of Tsukuba

• TSUNOGAE Toshiaki

  Graduate School of Life and Environmental Sciences (Earth Evolution Sciences), University of Tsukuba

抄録

A systematic fluid inclusion study on pelitic and granitic gneisses of the Cretaceous Abukuma metamorphic rocks, exposed in the Hanazono district, northeast Japan, identified three categories of fluid inclusions: primary aqueous inclusions in porphyroblastic garnet, primary to pseudosecondary carbonic inclusions in quartz, and secondary aqueous inclusions in quartz. The primary aqueous inclusions in garnet exhibit melting and homogenization temperatures of −2.5 to −0.3 °C and +168.2 to +360.0 °C, respectively. The dominant primary to pseudosecondary fluid inclusions in undeformed quartz display melting temperatures of −64.2 to −56.6 °C, indicating a CO₂-rich composition with additional CH₄ and/or N₂. The carbonic inclusions can be divided into high-density (1.121-1.153 g/cm³) and medium-density (0.683-1.111 g/cm³) types. The fluid densities of the latter inclusions, when translated into isochores, indicate entrapment of CO₂ at the peak P-T conditions experienced by rocks in the study area (2.8-5.2 kbar at 770-850 °C). In contrast, the isochores of high-density category in quartz and aqueous inclusions in garnet do not intersect the expected P-T trajectory, yet are consistent with the prograde high-pressure condition of the Abukuma metamorphic rocks in the Takanuki district (8-10 kbar at 700-750 °C). Isochores calculated for carbonic inclusions in granitic gneiss intersect the P-T path at 2-3 kbar at 650 °C, suggesting that the fluids were captured during the retrograde stage. It is inferred that H₂O was the dominant fluid species during the prograde stage, while CO₂ was more abundant during the peak to retrograde stages of the Abukuma metamorphic belt. The prograde H₂O-bearing fluid was probably derived from the break down of hydrous minerals. Although the origin of carbonic fluid is not known, the presence of CO₂-rich fluid during peak metamorphism probably lowers the H₂O activity in the rocks. The isochores computed for secondary aqueous inclusions in quartz give very low P-T conditions (300 °C and 1.7-3.5 kbar), suggesting that the fluids are of retrograde origin.

収録刊行物

• Journal of Mineralogical and Petrological Sciences

  Journal of Mineralogical and Petrological Sciences 102 (6), 325-336, 2007

  一般社団法人　日本鉱物科学会

キーワード

• Abukuma metamorphic rocks
• Fluid inclusions
• Geothermobarometry
• Microthermometry
• <i>P-T</i> path

詳細情報

• CRID: 1390001206544940288
• NII論文ID: 10019908518
• NII書誌ID: AA11460926
• DOI: 10.2465/jmps.061113
• ISSN: 13493825
• ISSN: 13456296
• 本文言語コード: en
• データソース種別:
  • JaLC
  • Crossref
  • CiNii Articles
• 抄録ライセンスフラグ: 使用不可