冀西北怀安地区中性辉石麻粒岩的锆石U-Pb、Lu-Hf及微量元素组成对区域退变质时代的制约

Zircon U-Pb,Lu-Hf isotope and trace element compositions of intermediat pyroxene granulite in the Huai’an area,Northwest Hebei Province:Constraints on the timing of retrograde metamorphism

冀西北怀安地区位于华北克拉通北缘中段。其区域麻粒岩相变质峰期时间及所代表的碰撞造山时限一直存在不同认识。本文对出露于区内的中性辉石麻粒岩体(变石英闪长岩)进行锆石U-Pb、Lu-Hf及微量元素组成的分析研究,探讨其锆石同位素年龄的地质意义。该类岩石的矿物组合表明它们在中压麻粒岩峰期变质之后经历了角闪岩相退变质作用。锆石阴极发光显示,辉石麻粒岩中存在两种类型锆石:1)具双层结构锆石:核部锆石晶形保存相对完好并具生长环带,幔部为无内部结构的变质增生边;2)内部结构均一,无生长环带,为变质增生的新生锆石。其中具生长环带的核部锆石207Pb/206Pb年龄分布在2542～1902Ma(1σ),并与之相应的Th/U比值(0.10～1.92)、ΣHREE(59.1×10-6～452×10-6)和εHf(t)值呈正相关关系,表明核部锆石曾发生过不同程度的变质重结晶作用。其中谐和度高且年龄大于2400Ma的锆石(Th/U=0.42～1.92)获得了2471±18Ma(2σ,MSWD=6.7)加权平均年龄,该年龄可近似代表原岩的形成时代;相应的单阶段Hf模式年龄(tDM=2550～2621Ma,峰值2586Ma)和两阶段模式年龄(tDMC=2596～2716Ma,峰值2665Ma)以及与较高的εHf(t)值(4.1～6.7),反映其原岩的源区具新生下地壳特征,暗示本区新太古代末发生过地壳增生事件。变质增生锆石具更低的Th/U(<0.10)和176Lu/177Hf比值(<0.0001)以及明显偏低的重稀土含量。结合岩相学分析表明,这些变质锆石应是麻粒岩相向角闪岩相退变质过程中的产物,非峰期麻粒岩相产物。锆石Ti温度计算结果(683～714℃)也进一步支持这种解释。它们获得的1831±7Ma(2σ,MSWD=2.5)加权平均年龄应代表中压麻粒岩相变质晚期至角闪岩相退变质时代。为此,我们认为本区麻粒岩广泛记录的1850～1800Ma的变质年龄应代表古元古代末地壳抬升退变质事件。

The Huai’an area of Northwest Hebei stands on the central-north of the North China Craton （NCC）. Intermediat pyroxene-granulite in this area can provide important clues for the timing of regional metamorphism. The mineral assemblages indicate that it was undergone mid-pressure granulite metamorphism and overlapped by amphibolite facies retrograde. Zircon U-Pb, Lu-Hf isotope and trace elements compositions are analyzed. As a result, zircon cores with oscillatory zoning exhibit various 207Pb/206Pb ages in range of 2542～1902Ma （1σ）. They show a positive relations with Th/U （0.10～1.92） ratios, ΣHREE （59.1×10-6～452×10-6） and εHf（t） values, indicating their different degrees of lattice recrystallization due to the postdated metamorphism. Zircon grains with the most concordant and old ages yield a mean 207Pb/206Pb age of 2471±18Ma （2σ, MSWD=6.7）. This age is interpreted to be the emplacement age of the protolith rocks. Their depleted Hf model ages of tDM and tDMC vary in range of 2550～2621Ma （peak at 2586Ma）, and 2596～2716Ma （peak at 2665Ma）, respectively. Whereas, they have εHf（t） values of 4.1～6.7, indicating that the protolith rocks is dominately derived from the Late Archean juvenile crust. The metamorphic over growth zircons occurring as metamorphic new-single grains or as zircon mantles overgrowth around the magmatic zircon cores, yield U-Pb ages varying from 1865Ma to 1782Ma, with relatively low Th/U （0.10）, 176Lu/177Hf （0.0001）, and HREE abundance. They yield a mean 207Pb/206Pb age of 1831±7Ma （2σ, MSWD=2.5）. We interpret this age to be the age of granulite to amphibolite retrograde metamorphism. The Ti-in-zircon temperatures record their formation temperatures in range of 683～714℃ which are highly consistent with the temperatures from granulite to amphibolite retrograde metamorphism. Therefore, the isotopic ages of 1850～1800Ma might be representative of the stage of regional crust uplift postdating the Late Palaeoproterozoic cratonization.