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

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 (M1), peak high-pressure (M2), post-peak decompression (M3) and late retrograde (M4) stages. The early prograde assemblage (M1) is preserved as mineral inclusions within the cores of garnet, represented by plagioclase + biotite + quartz + muscovite + kyanite + ilmenite. The peak high-pressure assemblage (M2) is garnet + kyanite + biotite + K-feldspar + plagioclase + quartz + ilmenite in the matrix. The post-peak decompressional stage (M3) is represented by sillimanite replacing kyanite and formation of cordierite + sillimanite symplectites (M3-1) and cordierite ± spinel coronas replacing garnet porphyroblasts (M3-2). The late retrograde stage (M4) 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 M1, M2, M3-1, M3-2 and M4 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 (M1-M2), followed by isothermal exhumation (M3) and final retrogression and cooling (M4). 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.