Crustal growth in the North China Craton at ~2.5Ga: Evidence from in situ zircon U-Pb ages, Hf isotopes and whole-rock geochemistry of the Dengfeng complex

摘要

The Dengfeng complex is located on the southern margin of the North China Craton (NCC) and belongs to the southern portion of the Trans-North China Orogen. This terrane is important to understand the formation and evolution of NCC during late Neoarchean (~2.5Ga). The Dengfeng complex is well exposed in the Junzhao region and comprises two distinct lithologic units: supracrustal assemblage and plutonic rocks. LA-ICPMS magmatic zircon U-Pb age data shows that the rocks formed within the range of 2547-2504Ma. The available Hf isotope data indicate that the majority of ca. 2.5Ga zircons from the Dengfeng complex have high ε_(Hf)(t) values close to the initial Hf isotope ratios of the contemporaneous depleted mantle. These data indicate that the rocks in the Dengfeng represent juvenile crust. The TTG gneisses in the Dengfeng complex display low Mg~# (41-48), MgO (<2wt.%), Cr (6-14ppm), Ni (9-22ppm) contents and low Nb/Ta ratio (6-12), which are interpreted to have been produced by the partial melting of a flatly subducted slab. The metadiorites of the Dengfeng complex are typically characterized by high Mg~# (59-69), MgO (3.5-6.6wt.%), Ni (82-130ppm) and Cr (148-237ppm) abundances, elevated Sr (1759-1927ppm) and Ba (1742-2289ppm) concentrations, and high LREE (La_N=38-487). Such geochemical features are similar to Archean sanukitoids. A two-stage model is applied here to explain the genesis of metadiorites of Dengfeng complex: (1) firstly, the mantle is metasomatized either by melts or by aqueous fluids from a subducted slab; (2) subsequently, sanukitoid magmas were produced by partial melting of the hybridized mantle. Furthermore, the amphibolites of supracrustal rocks have a mixture of MORB- and arc-like geochemical affinities, suggesting the development of a back-arc in the southern NCC at ca. 2.5Ga. The contemporary late Neoarchean TTGs, sanukitoids and MORB-back arc association may represent a late Neoarchean tectonic mélange, implying a Neoarchean subduction-accretion process which would suggest that modern-style plate tectonics processes was probably initiated in the southern NCC by 2.5Ga.