华北克拉通中部造山带南段早前寒武纪变质杂岩的Hf-Nd同位素特征及其地壳演化意义

Hf-Nd Isotopes of the Early Precambrian Metamorphic Complexes in the Southern Segment of the Trans-North China Orogen: Implications for Crustal Evolution

太华群和登封群变质杂岩的锆石U-Pb定年结果统计表明,华北克拉通中部造山带南段地区在新太古代-古元古代期间存在至少四阶段岩浆活动（2.85~2.72 Ga,2.57~2.48 Ga,2.34~2.30 Ga和2.20~2.07 Ga）。与中部造山带中、北段地区相比,南段地区的新太古代早期的岩浆活动略早,并具有相对较强烈的古元古代早期（~2.3 Ga）岩浆活动。新太古代早期（2.85~2.72 Ga）的TTG质岩石整体具有正的全岩ε_（Hf）（t）和ε_（Nd）（t）值,并且有εHf（t）值偏高的Hf-Nd同位素解耦现象,其源区形成与地幔交代作用相关。新太古代末期-古元古代初期（2.57~2.48 Ga）的TTG质片麻岩主要为正的全岩εHf（t）和εNd（t）值,部分样品具有ε_（Hf）（t）值偏低的Hf-Nd同位素解耦现象,为锆石效应的结果,其岩石成因以初生地壳物质部分熔融为主,存在少量古老地壳物质的再循环作用。古元古代早期（2.34~2.30 Ga）TTG质岩石和古元古代中期（2.20~2.07 Ga）的富钾岩及TTG质岩石的全岩ε_（Hf）（t）和ε_（Nd）（t）值均为负值,并具ε_（Hf）（t）值明显偏低的Hf-Nd同位素解耦现象,均为锆石效应的结果,其岩石成因以古老地壳物质的再循环作用为主。太华群和登封群片麻岩的全岩Hf-Nd同位素解耦主要与其源区形成过程存在地幔交代作用或者锆石效应有关,而与石榴子石效应关系不大,表明各阶段TTG质岩石的高Sr/Y、（La/Yb）_N比值和低重稀土含量等特征不是继承自源区,而确实反映了较大压力深度的岩浆活动过程。总体上,新太古代早期-古元古代初期为华北克拉通中部造山带南段地区主要的地壳生长阶段,而古元古代早-中期主要为地壳再循环阶段。

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 ε_（Nd）（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 ε_（Hf）（t） and ε_（Nd）（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 ε_（Hf）（t） and ε_（Nd）（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）N ratios 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.