10:15 〜 10:30
[SCG51-06] Zircon-hosted melt and fluid inclusions as a trace of multiple orogenic events:
the Bhimphedian orogeny overprinted by the Himalayan orogeny
キーワード：地質年代学、ジルコン、マグマ-流体、LA-SS-ICPMS、溶解再沈殿、ヒマラヤ
The formation and modification of crustal materials associated with orogeny is a key process in continental crustal evolution. Despite the importance, however, the record of former orogenic event is often overprinted by later event, preserved only in the internal microstructure and inclusions of metamorphic minerals. The orthogneiss widely distributed in the Himalayas is thought to be originated from the granite intruded during the early Paleozoic Bhimphedian orogeny on the north-eastern margin of Gondwana, although details are not clear due to the poor of metamorphic minerals such as garnet [1, 2]. In this study, we focus on the internal microstructure and inclusions of zircons in orthogneiss in the central Himalaya and attempt to decipher the early Paleozoic magma-fluid activity, which was overprinted by high-temperature metamorphism of Cenozoic Himalayan orogeny.
The mineral assemblage of the studied Formation III orthogneiss distributed along the Kali Gandaki River in the central Himalaya is quartz + K-feldspar + plagioclase + muscovite + biotite + tourmaline with accessory minerals of zircon, apatite, monazite, garnet, and ilmenite. Zircon encloses numerous fluid and polyphase inclusions. Following petrological observation and fluid inclusion analysis, U–Th–Pb isotopes and trace element concentrations of zircon were determined by laser ablation split stream inductively coupled plasma mass spectrometry.
Zircon from the Formation III orthogneiss commonly has the internal microstructure consisting of 1) the inner-core, 2) the outer-core, 3) the dark annulus, and 4) the metamorphic rim divided based on the optical microscope and cathodoluminescence (CL) observation. Each of these domains is cut by the outermost domain. 1) the inner-core shows oscillatory or sector zoning in the CL image, with U–Pb date of >1000 Ma. 2) the outer-core shows an oscillatory zoning in the CL image, with U–Pb date between 510 and 460 Ma. In addition to mineral inclusions such as quartz and biotite, the outer-core encloses primary fluid inclusions of medium-salinity (2–7%NaCleq) and polyphase mineral inclusions. Euhedral quartz, K-feldspar, chalcopyrite, fluorite, sphalerite, and native bismuth occur in some of the polyphase inclusions, indicating that the outer-core crystallized from a highly differentiated S-type granitic melt. 3) The dark annulus is recognized in the CL image as a dark annular domain, rich in P, Y, REE, and U, with U–Pb date of 490–440 Ma. The dark annulus is rich in mineral inclusions such as xenotime, monazite, and thorite, as well as high-salinity (10–11%NaCleq) primary fluid inclusions. These internal microstructures and low Gd/Yb ratio of the dark annulus suggest the non-equilibrium relationship with garnet and zircon dissolution-reprecipitation accompanied with high-salinity fluid influx during late stage of magmatism. The results from the outer-core and the dark annulus suggest that partial melting of the crust formed the S-type granitic melt and subsequently intruded to form the S-type granite during the Bhimphedian Orogeny, followed by the high-salinity fluid activity. The U–Pb date of 4) the metamorphic rim (45–17 Ma) suggests that during the Himalayan metamorphism, the dissolution/regrowth of zircon occurred during partial melting of the S-type granite and formed the Formation III orthogneiss. These results suggest that the traces of magma-fluid activity accompanied with the Bhimphedian orogeny are recorded in the outer-core and dark annulus of the zircons and have not been reset completely by the Himalayan high-temperature metamorphism.
[1]: Cawood, P. A., Johnson, M. R., & Nemchin, A. A. (2007)., Earth Planet. Sci. Lett., 255, 70–84.
[2]: Cawood, P. A., Martin, E. L., Murphy, J. B., & Pisarevsky, S. A. (2021)., Earth Planet. Sci. Lett., 568, 117057.
The mineral assemblage of the studied Formation III orthogneiss distributed along the Kali Gandaki River in the central Himalaya is quartz + K-feldspar + plagioclase + muscovite + biotite + tourmaline with accessory minerals of zircon, apatite, monazite, garnet, and ilmenite. Zircon encloses numerous fluid and polyphase inclusions. Following petrological observation and fluid inclusion analysis, U–Th–Pb isotopes and trace element concentrations of zircon were determined by laser ablation split stream inductively coupled plasma mass spectrometry.
Zircon from the Formation III orthogneiss commonly has the internal microstructure consisting of 1) the inner-core, 2) the outer-core, 3) the dark annulus, and 4) the metamorphic rim divided based on the optical microscope and cathodoluminescence (CL) observation. Each of these domains is cut by the outermost domain. 1) the inner-core shows oscillatory or sector zoning in the CL image, with U–Pb date of >1000 Ma. 2) the outer-core shows an oscillatory zoning in the CL image, with U–Pb date between 510 and 460 Ma. In addition to mineral inclusions such as quartz and biotite, the outer-core encloses primary fluid inclusions of medium-salinity (2–7%NaCleq) and polyphase mineral inclusions. Euhedral quartz, K-feldspar, chalcopyrite, fluorite, sphalerite, and native bismuth occur in some of the polyphase inclusions, indicating that the outer-core crystallized from a highly differentiated S-type granitic melt. 3) The dark annulus is recognized in the CL image as a dark annular domain, rich in P, Y, REE, and U, with U–Pb date of 490–440 Ma. The dark annulus is rich in mineral inclusions such as xenotime, monazite, and thorite, as well as high-salinity (10–11%NaCleq) primary fluid inclusions. These internal microstructures and low Gd/Yb ratio of the dark annulus suggest the non-equilibrium relationship with garnet and zircon dissolution-reprecipitation accompanied with high-salinity fluid influx during late stage of magmatism. The results from the outer-core and the dark annulus suggest that partial melting of the crust formed the S-type granitic melt and subsequently intruded to form the S-type granite during the Bhimphedian Orogeny, followed by the high-salinity fluid activity. The U–Pb date of 4) the metamorphic rim (45–17 Ma) suggests that during the Himalayan metamorphism, the dissolution/regrowth of zircon occurred during partial melting of the S-type granite and formed the Formation III orthogneiss. These results suggest that the traces of magma-fluid activity accompanied with the Bhimphedian orogeny are recorded in the outer-core and dark annulus of the zircons and have not been reset completely by the Himalayan high-temperature metamorphism.
[1]: Cawood, P. A., Johnson, M. R., & Nemchin, A. A. (2007)., Earth Planet. Sci. Lett., 255, 70–84.
[2]: Cawood, P. A., Martin, E. L., Murphy, J. B., & Pisarevsky, S. A. (2021)., Earth Planet. Sci. Lett., 568, 117057.