怀安麻粒岩锆石U-Pb年代学及Hf同位素: 华北北缘下地壳增生再造过程研究

The Huai' an Complex, an Archean to Proterozoic amphibolite to granulite facies high-grade terrane locates at the northcenter of the North China Craton, recording complicated crustal evolution. This paper is about the generation and implication for reworking of the lower crust according to the researching on the petrography, zircon U-Pb geochronology and Hf isotopic characteristics for the mafic and felsic granulites from the Huai' an Complex. The mafic granulites are mosaic granular blastic structure with mineral assemblage of Cpx + PI + Qz ± Opx ± Hb ± Grt, local kelyphites of orthopyroxene + plagioclase around garnet suggest granulite facies and subsequent near-isothermal decompression metamorphism. The selective Cpx-granulite composites of Cpx + PI + Qz. The concordant magmatic zircons from it yield the formation time of 2461 ±20Ma. The zircon ε_m(τ) values range from + 2. 1 ∼ + 9. 7, the highest ε_m(τ)value is near the contemporaneous depleted mantle and the depleted mantle Hf model age is in the range of 2. 6 Ga to 2. 7Ga, two younger ages are 2412Ma and 2461Ma, similar to the formation time. The felsic granulite is mainly composed of PI + Kfs + Cpx + Qz + Opx, showing granular blastic structure. The zircons analysis intimates the felsic granulite has formed at 2458 Ma. The ε _m(τ)) of all analyses zircons changes from + 2. 9 to +5.3, two stage Hf model ages distribute between 2.8 ∼ 2. 7Ga. These characteristics demonstrate the partial melting of the depleted mantle in Huai' an at ∼ 2. 5 Ga, which provided protolith material for the Cpx-granulite, moreover, setted off the remelting of the previous (≥2. 7Ga) crust, means the crustal reworking accompanied by the accretion.