HIGH-PRESSURE PELITIC GRANULITES FROM THE HELANSHAN COMPLEX IN THE KHONDALITE BELT, NORTH CHINA CRATON: METAMORPHIC P-T PATH AND TECTONIC IMPLICATIONS

摘要

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 (M3-1) and cordierite +/- spinel coronas replacing garnet porphyroblasts (M3-2). 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, M3-1, M3-2 and M-4 stages at 9.3-9.7 kbar/656-674 degrees C, 10.2-11.2 kbar/792-805 degrees C, 5.5-5.7 kbar/810-820 degrees C, 4.5-5.0 kbar/780-785 degrees C and 3.4-4.4 kbar/580-610 degrees 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 highpressure 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 similar to 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.