在对华北克拉通太古宙基底作简要介绍的基础上,本文总结了华北克拉通新太古代晚期(主要为2.55~2.5 Ga期间)岩浆岩的锆石年龄分布模式、地球化学和Nd-Hf-O同位素组成特征。华北克拉通新太古代晚期变质基底的主要特征如下:(1)新太古代晚期岩石在华北克拉通广泛分布,但许多地区都存在中太古代晚期—新太古代早期地质记录。(2)岩浆锆石年龄主要变化于2.55~2.5 Ga,年龄峰值约为2.52 Ga。(3)与新太古代早期以前(>2.6 Ga)的英云闪长岩-奥长花岗岩-花岗闪长岩(TTG)相比,新太古代晚期英云闪长岩和花岗闪长岩比例明显增大。富钾花岗岩、闪长-辉长岩(包括闪长岩、辉长岩及两者之间过渡岩石)、赞岐岩(Sanukite,主体为富镁闪长岩)分布范围和规模也明显增大。大规模富钾花岗岩主要分布于华北克拉通东部,构成新太古代晚期双岩浆岩带的富钾花岗岩带。(4)在新太古代晚期变质基底分布区,几乎都有变质表壳岩存在。它们以较小规模存在于TTG和富钾花岗岩中。岩石类型包括变玄武质岩石、变质安山质-英安质火山岩/火山碎屑岩和变质碎屑沉积岩。一些地区存在变质超基性岩。(5)总体上,新太古代晚期地质事件存在如下演化规律:首先是表壳岩形成,然后是TTG侵入,最后是变质变形和富钾花岗岩形成。2.6~2.55 Ga为华北克拉通岩浆构造的“寂静期”。(6)新太古代晚期TTG岩石的Sr/Y和La/Yb比值存在很大变化,中-高压TTG岩石大量形成表明新太古代晚期陆壳规模、厚度发生了明显增大。至少部分富钾花岗岩在形成过程中有沉积物参与。(7)不同类型TTG岩石具有类似的全岩Nd同位素和岩浆锆石Hf同位素组成,Nd-Hf同位素亏损地幔模式年龄主要分布在3.0~2.5 Ga,与中太古代晚期—新太古代早期岩石的模式年龄相近或稍偏年轻。富钾花岗岩Nd-Hf同位素组成特征受物源区早期形成演化历史制约。岩浆锆石O同位素组成与全球太古宙岩浆锆石类似,但显示更大的变化范围。结合其他研究,上述证据表明:(1)中太古代晚期—新太古代早期是华北克拉通陆壳增生最重要时期,这与全球其他许多克拉通类似,不同之处在于华北克拉通遭受了新太古代晚期构造岩浆热事件强烈改造;(2)在华北克拉通,类似于现代板块运动的构造体制在新太古代晚期开始启动;(3)规模最大的BIF(条带状铁建造)沿华北克拉通东部古老陆块西缘的双岩浆岩带分布,鞍本和冀东之间、冀东和鲁西之间是BIF找矿重要靶区;(4)华北克拉通在新太古代晚期完成初始克拉通化。

Following a brief introduction to the Archean basement of the North China Craton (NCC), this paper summarizes the age distribution pattern, geochemistry and Nd-Hf-O isotopic compositions of late Neoarchean (mainly 2.55-2.5 Ga) magmatic rocks in the NCC. The late Neoarchean basement has the following features: 1) Late Neoarchean rocks are widespread while late Mesoarchean-early Neoarchean strata occur in many areas. 2) The magmatic zircon ages are mainly between 2.55-2.5 Ga and has a peak around 2.52 Ga. 3) Compared with pre-early Neoarchean (> 2.6 Ga) TTG (tonalite, trondhjemite, granodiorite), late Neoarchean tonalite and granodiorite are much more abundant, so are potassic granite, diorite-gabbro and sanukitoid with increased distribution range and scale. Voluminous potassic granites are mainly distributed in the east, forming a potassic granite belt-one of the double magmatic rock belts of the late Neoarchean (another is a TTG belt in the west). 4) Late Neoarchean supracrustal rocks are present in almost every area of basement outcrop, but occurring on a small scale when associated with TTG and potassic granite. The supracrustal rock types include metabasalt, meta-andesite, metadacite and clastic metasedimentary rocks, with meta-ultramafic rocks occurring in some areas. 5) In general, geological evolution of the late Neoarchean basement rocks began with supracrustal rock formation, then TTG intrusion, followed by metamorphism, deformation and emplacement of crust-derived potassic granite. The 2.6-2.55 Ga interval was a “quiet period” of magmatic and tectonothermal activity. 6) The late Neoarchean TTG rocks show large variations of Sr/Y and La/Yb ratios, consistent with medium-high formation pressures; their high abundances indicate significant continental crustal thickening in the late Neoarchean. Potassic granites were mostly derived from re-working of continental crust, with some, at least, involving sedimentary rocks. 7) All TTG rock types have similar whole-rock Nd and magmatic zircon Hf isotopic compositions, and the Nd-Hf isotope depleted mantle model ages are mainly between 3.0-2.5 Ga, similar to or slightly younger than that of late Mesoarchean-early Neoarchean rocks. The Nd-Hf isotopic composition of potassic granites is mostly constrained by the formation and evolutionary history of the source region on early Earth. Magmatic zircon has similar but more variable O isotopic composition compared to Archean magmatic zircon worldwide. Combined with results from other studies we arrive at the following conclusions: i) Similar to many other cratons, the late Mesoarchean-early Neoarchean was the most important period of rapid production of continental crust in the North China Craton, however, the NCC underwent strong magmatic and tectonothermal modification during the late Neoarchean. ii) “Modern-style” plate tectonic regimes began to form in the late Neoarchean in the NCC. iii) BIFs (banded iron formations) are mostly distributed along the double magmatic rock belts along the western margin of the Eastern Ancient Block of the NCC, thus the important exploration targets for BIF-hosted Fe resource should be between Anshan-Benxi and eastern Hebei and between eastern Hebei and western Shandong. iv) The initial cratonization of the NCC had completed by the end of the late Neoarchean.