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

Zircon U-Pb ages and Hf isotopes of granulites from the Huai'an Complex:Implications for the accretion and reworking of the lower crust beneath the north margin of the North China Craton.

怀安杂岩体位于华北克拉通中北部,出露太古代-元古代的角闪岩相-麻粒岩相高级变质岩,记录了华北克拉通复杂的地壳形成和演化史。本文对其中的镁铁质麻粒岩和长英质麻粒岩进行了岩石学及锆石U-Pb年代学和Hf同位素组成研究,并在此基础上对其形成过程及所反映的华北北缘古老下地壳的增生再造过程进行讨论。镁铁质麻粒岩主要矿物组合为Cpx+Pl+Qz±Opx±Hb±Grt,粒状变晶结构,局部发育石榴子石的"白眼圈"现象,表明经历了高压麻粒岩相及后期近等温减压退变质作用过程。选择其中矿物组成为Cpx、Pl和少量Qz的单斜辉石麻粒岩进行锆石定年获得2461±20Ma的原岩形成年龄。这些岩浆锆石的εHf(t)为+2.1～+9.7,最高值接近当时的亏损地幔值,其Hf亏损地幔模式年龄(tDM)集中在2.6～2.7Ga之间,最小为2412Ma和2461Ma,接近原岩形成年龄。长英质麻粒岩主要组成矿物为Pl+Kfs+Cpx+Qz+Opx,粒状变晶结构。锆石测年获得2458±46Ma的原岩形成年龄。这些锆石εHf(t)范围为+2.9～+5.3,锆石Hf两阶段模式年龄(tcrust)集中在2.8～2.7Ga之间。怀安麻粒岩的这些特征显示怀安地区在～2.5Ga发生亏损地幔的部分熔融,为单斜辉石麻粒岩提供了原岩物质,同时可能引起了先存(≥2.7Ga)地壳的重熔,即存在地壳再造,进一步为长英质麻粒岩提供了原岩物质。单斜辉石麻粒岩和长英质麻粒岩原岩都形成于～2.5Ga。

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 ＋ Pl ＋ 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 ＋ Pl ＋ Qz.The concordant magmatic zircons from it yield the formation time of 2461 ± 20Ma.The zircon ε Hf （t） values range from ＋2.1 ～ ＋9.7,the highest ε Hf （t） value is near the contemporaneous depleted mantle and the depleted mantle Hf model age is in the range of 2.6Ga to 2.7Ga,two younger ages are 2412Ma and 2461Ma,similar to the formation time.The felsic granulite is mainly composed of Pl ＋ Kfs ＋ Cpx ＋ Qz ＋ Opx,showing granular blastic structure.The zircons analysis intimates the felsic granulite has formed at 2458Ma.The ε Hf （t） 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 ～ 2.5Ga,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.