科研之友
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摘要
There are at least four episodes of magmatism in the southern segment of the Trans-North China Orogen (TNCO) in the Late Archean and early Paleoproterozoic (2.85-2.72 Ga, 2.57-2.48 Ga, 2.34-2.30 Ga, 2.20-2.07 Ga) based on the zircon U-Pb geochronology of the Taihua and Dengfeng Complexes. The early Neoarchean magmatism in the Southern Segment of the TNCO predates that in the Central and Northern Segments of the TNCO. Additionally, the early Paleoproterozoic magmatism in the Southern Segment (2.34-2.30 Ga) is much stronger than that in the Central and North Segments. TTG rocks of the first episode (2.85-2.72 Ga) have overall positive ΕHf(t) and ΕNd(t) values, and show weak Hf-Nd isotope decoupling above the Terrestrial Hf-Nd Array, indicative of a genetic relationship with metasomatism mantle. TTG rocks at the end of Neoarchean and the beginning of Paleoproterozoic (2.57-2.48 Ga) have mostly positive ΕHf(t) and ΕNd(t) values. Some samples show weak Hf-Nd isotope decoupling under the Terrestrial Hf-Nd Array, which is attributed to zircon effect. So the protolith of the gneisses is proposed to be the result of partial melting of dominantly juvenile crust with minor ancient continental crust. TTG rocks or the potassium-rich igneous in the early and middle Paleoproterozoic (2.34-2.30 Ga and 2.20-2.07 Ga, respectively) have all negative ΕHf(t) and ΕNd(t) values, and show notable Hf-Nd isotope decoupling under the Terrestrial Hf-Nd Array due to zircon effect. The last two episodes of magmatism were derived from partial melting of dominantly ancient continental crustal materials. Therefore, the Hf-Nd isotope decoupling of the gneisses in the Taihua and Dengfeng Complexes might have been inherited from their source related to the mantle metamorphism or zircon effect rather than produced by garnet effect. This suggests that the geochemical characteristics, such as high Sr/Y and (La/Yb)Nratios and low concentrations of heavy REE in the TTG rocks, could not have been inherited from their sources, but are attributed to magmatism under high-pressure condition. In conclusion, the earlier two episodes of magmatism (early Neoarchean and the end of Neoarchean to beginning of Paleoproterozoic, respectively) recorded two main episodic crustal growth, while the later two episodes of magmatism (early and middle early Paleoproterozoic, respectively) were attributed to crustal reworking.
