华北克拉通始太古代演化——来自冀东38亿年片麻岩锆石Hf-O同位素的记录

陆壳成因的研究对了解地球演化具有重要意义, 但由于古老陆壳岩石出露有限, 目前地球早期陆壳研究还比较薄弱。近期, 冀东地区一处新的始太古代岩石记录的发现, 为研究华北克拉通内部早期陆壳性质提供了极为重要的对象。本文对冀东卢龙喇叭山地区这处新发现的始太古代花岗闪长质片麻岩进行了详细的锆石U-Pb年代学和Hf-O同位素分析。结果显示该样品中锆石具有自形-半自形晶形、振荡环带的内部结构, 较高的Th、U含量以及Th/U比值, 显示岩浆锆石特征。7个SIMS分析点给出的加权平均²⁰⁷Pb/²⁰⁶Pb年龄为3776±11Ma(MSWD=3.1), 代表了花岗闪长质岩浆的结晶年龄, 指示冀东地区存在始太古代结晶基底。这些锆石的O同位素组成(δ¹⁸O=5.81±0.13‰)与地幔锆石在分析误差范围内一致, 其Hf同位素组成(ε_Hf(t)=0.09±0.31)与球粒陨石均一岩浆库类似。这些Hf-O同位素特征显著不同于冀东地区曹庄岩组中发现的始太古代碎屑锆石, 说明其不可能作为曹庄岩组变沉积岩的物源区。而华北克拉通内部鞍山地区的始太古代片麻岩中的岩浆锆石与冀东始太古代碎屑锆石在Hf-O同位素特征上最为类似, 因此前者是冀东始太古代碎屑锆石最可能的源岩, 暗示冀东地区和鞍山地区在始太古代时可能是统一的整体。与全球始太古代岩石记录对比发现, 华北克拉通主体与North Atlantic克拉通、Pilbara克拉通和塔里木克拉通等主要克拉通类似, 其内部目前发现的最古老岩石都具有与球粒陨石一致的锆石Hf同位素组成, 指示大部分始太古代陆壳可能源自一个未发生明显分异的相对原始地幔源区。

Eoarchean evolution of the North China Craton: Zircon Hf-O isotopic evidence of the ca. 3.8Ga gneiss from eastern Hebei Province

The formation of continental crust is the key to understanding the evolution of the Earth. However, the genesis of continental crust on early Earth has been poorly constrained due to the scarcity of ancient continental crustal remnants. A new Eoarchean outcrop was recently discovered in eastern Hebei Province, which provided a critical object for understanding the nature of the early continental crust of the North China Craton. In this study, we carried out an integrated study of zircon U-Pb age and Hf-O isotopes on the newly discovered Eoarchean granodioritic gneiss from the Labashan area, Lulong City, eastern Hebei Province. The zircons from the granodioritic gneiss show euhedral to subhedral shapes and oscillatory zoning in CL images, and have relatively high Th and U contents and Th/U ratios, which coincide with the characteristics of magmatic zircons. Seven SIMS U-Pb dating analyses on these zircons gave a weighted mean 207Pb/206Pb age of 3776±11Ma (MSWD=3.1), which was interpreted as the crystallization age of the granodioritic magma. This indicates that there were Eoarchean continental basements distributed in eastern Hebei Province. The studied zircons had a weighted mean δ18O value of 5.81±0.13‰, consistent with that of normal mantle zircon within the analytical error, and gave a weighted mean εHf(t) value of 0.09±0.31, similar to that of the chondrite uniform reservoir (CHUR). These Hf-O isotopic features were evidently different from those of the Eoarchean detrital zircons in the Caozhuang Formation, eastern Hebei Province, suggesting that the Labashan granodioritic gneiss did not provide the detritus for the Caozhuang Formation. The Eoarchean rocks in the Anshan gneissic complex were the most probable source rocks for the Eoarchean detrital zircons in the Caozhuang Formation due to their similar Hf-O isotopic features. This implies that there was a unified basement beneath both eastern Hebei Province and the Anshan area during the Eoarchean. The Eoarchean rocks detected in the North China Craton are comparable with those exposed in other main cratons like North Atlantic Craton, Pilbara Craton, and Tarim Craton in terms of zircon Hf-O isotopes. The presence of these ancient rocks suggests that the oldest continental crust originated from a primitive and undifferentiated mantle source.