Abstract
Greenschist-facies Al-rich metapelites of the Pyeongan Supergroup in the eastern part of the Taebaeksan Basin, South Korea, contain unusual mineral assemblages, including the associations Mrg + Cld + Ms ± Chl ± Cal, An + Mrg + Ms, and An + Ms ± Grt ± St (all rocks contain Qtz). The X Mrg [Ca/(Ca + Na + K)] and X Pa [Na/(Ca + Na + K)] values of margarite range from 0. 12 to 0. 92 and from 0. 10 to 0. 28, respectively. High-Ca plagioclase porphyroblasts are slightly zoned, from An 85 in the core to An98 in the rim. The Mrg + Cld-bearing assemblage yields P-T conditions of ca. 450-480 °C, 4. 0-5. 0 kbar based on the intersection of isopleths of Fe/(Fe + Mg) of chloritoid and X Pa of margarite in the MnO-Na 2O-CaO-K 2O-FeO-MgO-Al 2O 3-SiO 2-H 2O (MnNCKFMASH) system. The anorthite-bearing mineral assemblages yield P-T conditions of ca. 480-520 °C, 3. 8-4. 4 kbar, and the An contents of plagioclase increased during decompression rather than during heating, based on isopleths of An content and the modal proportion of plagioclase. Phase relations in T-X CO2 pseudosections at 4 kbar in the CASH-CO 2 system indicate that anorthite in the Al-rich metapelites grew during the consumption of margarite at T-X CO2 values of ca. 0. 2-0. 5 during sub-isothermal decompression at temperatures of 480-500 °C. Consequently, the overall P-T path for the formation of chloritoid, margarite and anorthite is characterized by relatively rapid compressional heating up to 480 °C at 5. 0 kbar (geothermal gradient of 27-30 °C/km), followed by sub-isothermal retrograde decompression. This clockwise P-T path in the eastern part of the Taebaeksan Basin reflects the effects of the Late Permian-Triassic Songrim orogeny, which probably occurred at an accretionary forearc-related active continental margin.
Original language | English |
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Pages (from-to) | 207-227 |
Number of pages | 21 |
Journal | Geosciences Journal |
Volume | 16 |
Issue number | 3 |
DOIs | |
Publication status | Published - 2012 Sept |
Externally published | Yes |
Bibliographical note
Funding Information:ACKNOWLEDGMENTS: This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2010-0009429). I thank two anonymous reviewers and Dr. Hadi Shafai Moghadam for helpful and constructive comments on an earlier version of this paper that improved manuscript immensely.
Keywords
- Permian-Triassic orogeny
- Pyeongan Supergroup
- anorthite
- margarite
- sub-isothermal decompression
ASJC Scopus subject areas
- General Environmental Science
- General Earth and Planetary Sciences