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Ca.2.5 billion year old coeval ultramafic-mafic and syenitic dykes in Eastern Hebei:Implications for cratonization of the North China Craton

【摘要】:正A group of extremely rare coeval ultramafic-mafic and syenitic dykes was discovered in the Eastern Hebei region of the North China Craton.These dykes intrude the 3.8-2.55 Ga old Caozhuang complex.An olivine gabbro dyke and syenite dyke yield,respectively,SHRIMP zircon U-Pb ages of 2516±26 Ma and 2504±11 Ma,interpreted as the magmatic crystallization age.Their zircons have single-stage Hf model ages 2677 Ma and 2705 Ma.The olivine gabbros have Mg~# values of 59-63,similar to high-magnesian tholeiitic basalt.They show relatively LREE-enriched patterns without Eu anomalies(La/Yb_N = 9.28-9.78, Gd/Yb_N = 2.8,δEu = 0.96-1.00),with enrichment in LILEs and depletion in HFSEs.They also contain high Cr and Ni,and have Zr/Hf and Nb/Ta ratios similar to those of the primitive mantle.The syenites are alkaline in composition,with 8.67-8.88 wt.%Na_2O + K_2O,and show high total REE contents(543-854ppm) and strong LREE-enriched patterns with minor Eu negative anomalies(La/Yb_N = 50-101,Gd/Yb_N = 5.2-5.3,δEu = 0.80-0.82).Trace element diagrams show strong LILE-enrichment relative to HFSE.Zr/Hf and Nb/Ta ratios are 47-49 and 26-76,respectively,which are much higher than those of primitive mantle and higher than the average Nb/Ta ratio of post-Archean continental crust.The petrological and geochemical features indicate that these dykes were derived from a deep subcontinental lithospheric mantle source,which implies that the NCC probably had continental crust of considerable thickness at this time.Combined with evidence for~2.5 Ga granite intrusion and metamorphism in the Eastern Hebei region and adjacent areas,we propose that the NCC has been a present scale craton at the end of Archean.

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