High-pressure pelitic granulites from the Helanshan Complex in the Khondalite Belt, North China Craton: Metamorphic P-t path and tectonic implications

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ID: 293157
2015
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Abstract
This paper presents petrographic and mineral chemistry data and an inferred P-T path for the high-pressure pelitic granulites from the Helanshan Complex in the Khondalite Belt, a Palaeoproterozoic collisional belt along which the Yinshan and Ordos blocks joined to form the Western Block in the North China Craton. Petrographic observation indicates that these high-pressure pelitic granulites experienced four metamorphic stages: the early prograde (M~1~), peak high-pressure (M~2~), post-peak decompression (M~3~) and late retrograde (M~4~) stages. The early prograde assemblage (M~1~) is preserved as mineral inclusions within the cores of garnet, represented by plagioclase + biotite + quartz + muscovite + kyanite + ilmenite. The peak high-pressure assemblage (M~2~) is garnet + kyanite + biotite + K-feldspar + plagioclase + quartz + ilmenite in the matrix. The post-peak decompressional stage (M~3~) is represented by sillimanite replacing kyanite and formation of cordierite + sillimanite symplectites (M~3-1~) and cordierite ± spinel coronas replacing garnet porphyroblasts (M~3-2~). The late retrograde stage (M~4~) is indicated by the formation of staurolite + chlorite in the matrix. Pseudosection modeling for a representative sample in the NCKFMASHTO system constrains the P-T conditions of M~1~, M~2~, M~3-1~, M~3-2~ and M~4~ stages at 9.3-9.7 kbar/656-674 °C, 10.2-11.2 kbar/792-805 °C, 5.5-5.7 kbar/810-820 °C, 4.5-5.0 kbar/780-785 °C and 3.4-4.4 kbar/580-610 °C, respectively. These mineral assemblages and their P-T conditions define a clockwise P-T path involving isothermal decompression and isobaric cooling following the peak high-pressure metamorphism, suggesting that the high-pressure pelitic granulites underwent initial crustal thickening (M~1~-M~2~), followed by isothermal exhumation (M~3~) and final retrogression and cooling (M~4~). Such a clockwise P-T path involving near-isothermal decompression is in accord with the P-T paths reconstructed for other complexes in the Khondalite Belt, and, combined with available geochronological data, is considered to record a continent-continent collisional event that led to the amalgamation of the Yinshan and Ordos Blocks to form the Western Block of the North China Craton at ∼1.95 Ga. This is coincident with these global-scale collisional events (2.1--1.8 Ga) leading to the assembly of the Palaeo-Mesoproterozoic supercontinent, further supporting an early proposal that the North China Craton was one of the components of the Palaeo-Mesoproterozoic Columbia (or Nuna) supercontinent.
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Authors Changqing Yin, Guochun Zhao, Min Sun
Journal american journal of science
Year 2015
DOI
10.2475/09.2015.03
URL
https://doi.org/10.2475/09.2015.03
Keywords Keywords not found

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