Neoarchean Basement Rock Assemblage, Crustal Evolution and Crust-Mantle Interactions of Western Liaoning Province

Abstract

The formation and evolution of Archean continental crust as well as related crust-mantle geodynamic evolution history are the major focus of Precambrian studies. As one of the oldest cratons in the world, the North China Craton is dominated by late Neoarchean (~2.6–2.5 Ga) geological events. However, it is still hotly debated about the nature of these major episodes of tectonothermal processes, i.e., whether they represent crustal growth or reworking, and how about the geodynamic regimes (plate tectonics or mantle plume setting)? Systematic studies of lithological assemblage of the basement rocks, petrogenesis, and crust-mantle interactions are key to resolve the above issues. In this chapter, we provide geological, petrological, whole-rock geochemical, and zircon U–Pb and Lu–Hf isotopic data for representative late Neoarchean greenstone metavolcanic rocks and granitoid gneisses in the Western Liaoning Province (WLP) along the northern margin of Eastern Block. It is suggested that magmatic precursors of the metavolcanic rocks in the Fuxin greenstone belt were erupted during ~2640–2534 Ma, and they show dominantly positive zircon εHf(t₂) values of +2.7 to +9.7. These metavolcanic rocks show chemical affinities to Mid-ocean ridge basalts (MORBs), island arc tholeiitic to calc-alkaline basalts, adakite-like and high magnesium andesites, respectively, and they were considered to have been generated by the partial melting of upwelling asthenospheric mantle beneath an Archean spreading ridge and complex interactions between the depleted mantle wedge lithospheric mantle and slab-derived fluids and melts, respectively. Dioritic to TTG gneisses are the major lithologies in the WLP. They were emplaced during ~2532–2506 Ma, and show intrusive relationships with the metavolcanic rocks, with chiefly positive zircon εHf(t₂) values of +1.2 to +8.4. Based on mineral assemblages and chemical features, these granitoid gneisses were subdivided into a high magnesium group (HMG) and a low magnesium group (LMG), which could have been derived from the partial melting of descending oceanic slabs and metabasaltic rocks at the arc root, respectively. Some volume of potassium-rich granitoid gneisses, including granodioritic and monzogranitic rocks with weakly gneissic to massive structures, intruded both the supracrustal metavolcanic rocks and the strongly deformed dioritic to TTG gneisses. They were formed at ~2495 Ma with zircon εHf(t₂) values of +0.9 to +7.6, which were suggested to have been produced by the partial melting of metamorphosed felsic to sedimentary rocks under an extensional setting. Most of the above lithologies were subjected to ~2485 Ma regional peak granulite facies and ~2450–2401 Ma retrograde metamorphism. ~2.4–1.7 Ga metamorphic imprints could be ascribed to the middle to late Paleoproterozoic tectonothermal events prevailed along the northern margin of NCC. Accordingly, the Western Liaoning Province experienced complex late Neoarchean subduction-accretion processes from mid-ocean ridge spreading, through initiation and maturation of an intra-oceanic arc system, to the final arc-continent accretion. It records intense ~2.6–2.5 Ga subduction-related crustal growth, and the Archean lithospheric mantle sources beneath the northern margin of NCC could have been transformed from juvenile oceanic lithospheric mantle that was initially formed under the oceanic spreading ridge, and they were subjected to gradual metasomatism by slab-derived fluids and melts. All the above data indicate that intra-oceanic subduction and arc-continent accretion within an accretionary orogen could have been an important mechanism of continental growth along the northwestern margin of Eastern Block in the Neoarchean.