ZIRCON AGES, HF ISOTOPES AND P-T PATH OF THE YISHUI COMPLEX: IMPLICATIONS FOR THE CRUST GROWTH AND TECTONOTHERMAL EVOLUTION OF THE EASTERN BLOCK OF THE NORTH CHINA CRATON

High-grade metamorphic supracrustal rocks and granitoids (TTG suites and charnockites) comprise the Yishui Complex, one of the major Neoarchean complexes in the Eastern Block of the North China Craton. New zircon dating results reveal the magmatic precursors of the supracrustal metapelites and TTG gneisses were generated at 2.58-2.53 Ga and 2.53-2.52 Ga, respectively, and the charnockites were emplaced around 2.58-2.53 Ga. The metamorphic single zircons and overgrowth rims document consistent metamorphic ages at ~2.50 Ga, suggesting the Yishui Complex experienced a regional metamorphism at the end of Neoarchean. Inherited detrital zircons yield ages between 3.0-2.6 Ga with a peak at 2.7-2.6 Ga, indicating the existence of Early Neoarchean magmatism in the Yishui area. Hf isotopic compositions show the Late Neoarchean zircons have positive ε_Hf(t) values from +1.6 to +7.1, with depleted mantle model ages of 2.78-2.59 Ga, close to their crystallization ages. This suggests a significant crustal growth event at 2.8-2.6 Ga and the Neoarchean crust was predominantly derived from juvenile crustal sources with weak contributions of older crustal materials. A metamorphic study on mafic granulite enclaves has reveled three distinct mineral assemblages: the pre-peak assemblage (M1) of Hb + Pl + Qtz + Ilm + Mt occurring as inclusions within garnet and pyroxene, peak assemblage (M2) of Opx + Cpx + Pl + Grt + Hb + Qtz + Ilm + Mt, and post-peak assemblage (M3) represented by Grt + Qtz symplectites. Pseudosection modeling by THERMOCALC in the NCFMASHTO system for a representative mafic granulite constrains the P-T conditions of M1, M2 and M3 stages at 660-730°C/<6.6 kbar, 800-820°C/8.0-8.5 kbar and 686-710°C/7.6-8.6 kbar, respectively. The petrology and quantitative P-T pseudosection modeling define an anticlockwise P-T path involving near-isobaric cooling, suggesting the metamorphism of the Yishui Complex was most likely related to the intrusion and underplating of mantle-derived magmas, which may occur in magmatic arc regions, hot spots, or rift environments. Combined with previous data, it is concluded that the Neoarchean is an important crust growth period in the Yishui Complex, and the Late Archean widespread tectonothermal event is possibly related to a mantle plume activity. Mineral abbreviations are after Kretz (1983).