华北克拉通古老麻粒岩纪录的三期构造-岩浆事件

Three period tectonic-magmatic events recorded in ancient granulite of the North China Craton

华北克拉通中部造山带上广泛发育新太古代-古元古代的基性岩墙。本文选取出露于华北南缘太古代太华群花岗片麻岩中的基性麻粒岩脉体进行研究,进而探讨了克拉通南缘太古代地体所经历的构造-岩浆-变质事件以及其后期演化过程。通过对基性麻粒岩脉体的野外地质、岩石学、锆石的U-Pb年代学、锆石O及Hf同位素地球化学的研究,显示该脉体记录有3期明显的构造-岩浆-变质事件：（1）新太古代晚期岩浆侵位事件（2523±8Ma）。岩浆锆石正的εHf（t）值（2.88～7.16）显示该基性岩脉是由亏损幔源玄武质岩浆侵位于太古代基底而形成的,结合锆石略高于正常地幔的δ18O值（6.12‰～7.47‰）说明岩浆侵位后受地壳混染并不显著。该岩浆事件与华北克拉通新太古代（～2.5Ga）广泛存在的地壳再造和少量的地壳增生事件吻合;（2）古元古代变质作用（1922±6Ma）。麻粒岩中变质锆石纪录的古元古代变质作用事件与华北克拉通中部造山带普遍遭受变质作用的时期（1.85～1.97Ga）相一致。锆石正的εHf（t）值（1.61～5.52）说明变质作用过程中Hf同位素体系保持封闭,因此其Hf同位素组成继承了原岩幔源岩浆的组成。而略低于岩浆锆石的δ18O值（4.85‰～5.76‰）可能是由变质作用过程中发生的高温热液蚀变导致的;（3）渐新世岩浆活动（31.38±0.15Ma）,该期岩浆作用在华北克拉通属首次发现。麻粒岩中部分岩浆锆石给出了非常好的谐和年龄,这些锆石的εHf（t）值（-3.03～1.69）集中分布于原始地幔岩浆库（CHUR）演化线之上,且具有非常接近于原始地幔的δ18O值（5.78‰～6.62‰）,表明该基性岩脉所记录的渐新世岩浆活动也来源于地幔。结合已有研究成果,我们认为华北南缘太华群基性脉体形成于新太古代晚期,侵位于早先的太古代地体之中,并随同古老基底一起俯冲至下地壳深度发生了麻粒岩相的变质作用,不久又一同被抬升至发生角闪岩相退变质,后又经历了渐新世的幔源岩浆活动的扰动,抬升至地表的地质演化过程。

The Neoarchean-Paleoproterozoic mafic dikes are widely distributed in the Trans-North China Orogen（ TNCO）,North China Craton（ NCC）. In order to probe what tectonic-magmatic-metamorphic events the Archean terranes of the southern NCC had undergone and their evolution,we choose mafic granulite dike which intruded Archean granitoid gneisses of the Taihua Group in the southern NCC. Petrological observations,together with the geochronological and oxygen（ O） and hafnium（ Hf） isotopic studies on zircons from the mafic granulite dike,have revealed three tectonic-magmatic events：（ 1） magmatic emplacement happened at the Late Neoarchean（ 2523 ± 8Ma）,which coincided well with an important period of extensive ancient crust reworking with minor crustal growth at the Neoarchean（ ～ 2. 5Ga） in the NCC. Magmatic zircons from the dike have positive εHf（ t） values（ 2. 88 ～ 7. 16）,suggesting that these zircons formed from basaltic magma derived from depleted mantle. The oxygen isotope feature,i. e. slightly higher（ 6. 12‰ ～ 7. 47‰） than normal mantle δ18O values,indicates that the crustal contamination was insignificant during magma emplacement;（ 2） the Paleoproterozoic metamorphism（ 1922 ± 6Ma） recorded in metamorphic zircons of the granulite dike is in alignment to extensive metamorphism（ 1. 85 ～ 1. 97Ga） of the Archean terranes in the TNCO. In addition,metamorphic zircons also have positive εHf（ t） values（ 1. 61 ～ 5. 52）,implying their Hf isotope heritage from its protolith. The δ18O values（ 4. 85‰ ～5. 76‰）,which are slightly lower than their protoliths, could be caused by high temperature hydrothermal alteration during metamorphism;（ 3） magmatic activity occurred in the Oligocene（ concordia age of 31. 38 ± 0. 15Ma）,which period of magmatism was first discovered in the NCC. Both εHf（ t） values（- 3. 03 ～ 1. 69） which mainly distribute on the Chondritic Uniform Reservoir（ CHUR） trend and δ18O values（ 5. 78‰ ～ 6. 62‰） which are close to the primitive mantle reveal that the Oligocene magmatic activity were also derived from the mantle. Based on available data of the southern NCC and new geochronological and isotopic data in this research,we conclude that the mafic dike intruded the Archean terrain of the Taihua Group in the southern NCC at the Late Neoarchean,and then underwent the granulite-facies metamorphism at the Paleoproterozoic together with the subduction of the Archean basement. Subsequently the subducted slab rapidly uplifted and suffered from the retrograde metamorphism when they reached to the amphibolites-facies. Importantly,the mafic granulite dike also recorded another magmatic activity at the Oligocene,after which the Archean terrain finally lifted to the surface and exposed.