华北克拉通太古宙研究若干进展

摘要

The North China Craton has experienced a long geological history which can be traced back to ca. 3.8 Ga ago. This paper reports some new progress made in recent years in the study of North China Craton in such areas as Anshan-Benxi, eastern Hebei, western Shandong, eastern Shandong and Yinshan. In Anshan, the Shengousi Complex provides evidence of magmatic events occurring at ~3770 Ma, 3660–3600 Ma, ~3450 Ma, 3330–3310 Ma, and ~3120 Ma. These data are similar to data from the Baijiafen and Dongshan complexes in showing protracted, episodic Archaean crustal evolution. In eastern Hebei, detrital and metamorphic zircons from para-amphibolite and garnet-biotite gneiss of the Caozhuang Complex gave ages of 3.8–3.4 Ga and ~2.5 Ga, respectively. Combined with previous work, the new geochronological results suggest that a significant amount of Eoarchean crustal fragments may occur in eastern Hebei and that the sedimentary protoliths of the paragneisses were deposited between 3.4 and 2.5 Ga. In western Shandong, the Archean basement can be divided into three belts: a late Neoarchean crust-derived granite belt in the northeast that consists predominantly of 2525–2490 Ma monzogranite and syenogranite and banded gneisses (Belt A), a middle to early Neoarchean belt in the center which is mainly composed of 2.75–2.60 Ga TTGs and supracrustal rocks (Belt B), and a late Neoarchean belt of juvenile rocks in the southwest that is dominated by granodiorite, gabbro, quartz diorite and tonalite, with some monzogranite and syenogranite (Belt C). Western Shandong is the only area where both early and late Neoarchean supracrustal rocks were identified. In eastern Shandong, a major magmatic event was identified at 2.9 Ga, which resulted in a major period of juvenile crustal addition from the depleted mantle. Together with 2.7 Ga rocks, the 2.9 Ga rocks underwent strong metamorphism at 2.5 Ga, probably as a result of magmatic underplating. In the Yinshan Block, there are 2.7 Ga tonalites and 2.6 Ga granites and older xenocrystic zircons in 2.5 Ga granitoid rocks. Data obtained in recent years indicate that the Yinshan Block is similar in early Precambrian geology to the Eastern Block. The authors also summarized zircon U-Pb dating and Nd-Hf isotopic features of Archean basement of the North China Craton. Combined with previous work, the authors discussed general features of the Archean crust formation and evolution of the North China Craton. It is concluded that the most important production of continental crust occurred during the late Mesoarchean to early Neoarchean period, but the most intensive and widespread tectono-thermal event took place at the end of the Neoarchean. In this new tectonic model, the authors have defined and outlined three ancient terranes (>2.6 Ga), namely Eastern Ancient Terrane, Southern Ancient Terrane and Central Ancient Terrane.%华北克拉通存在3.8 Ga以上的地质演化历史。本文对近年来我们在鞍本、冀东、鲁西、胶东和阴山地区研究取得的进展作了简要介绍。在鞍山,深沟寺杂岩获得~3770 Ma,3600–3660 Ma,~3450 Ma,3310–3330 Ma和~3120 Ma锆石年龄,与白家坟杂岩和东山杂岩的岩浆事件十分类似。在冀东,对曹庄杂岩副变质斜长角闪岩和石榴黑云片麻岩定年,发现大量3.5–3.8 Ga碎屑锆石,并获得~2.5 Ga变质锆石年龄,冀东地区很可能存在始太古代岩石,可把曹庄杂岩形成时代限定在2.5 Ga和3.4 Ga之间。在鲁西,把太古宙基底从东北到西南依次划分为A、B、C三个岩带： A带主要为新太古代晚期的壳源花岗岩, B带主要为新太古代早期的岩石, C带主要为新太古代晚期的新生岩浆岩。鲁西是华北克拉通新太古代早期岩石分布最广泛的地区,也是华北克拉通确认新太古代早期和晚期表壳岩系共存的唯一地区。在胶东,规模巨大的2.9 Ga岩浆热事件被识别出来,它是强烈地幔添加作用的产物。可能作为岩浆板底垫托作用的结果,2.9 Ga岩石与2.7 Ga岩石一道,在新太古代晚期(~2.5 Ga)遭受强烈变质改造。在阴山地块,除2.5 Ga表壳岩和侵入岩外,还存在2.7 Ga英云闪长岩和2.6 Ga花岗岩。新获得的资料表明阴山地块与华北克拉通东部陆块具有类似的早前寒武纪地质演化历史。还对华北克拉通太古宙变质基底的锆石年龄和 Hf同位素组成及全岩 Nd同位素组成进行了统计研究。结合前人工作,初步总结了华北克拉通太古宙地质演化特点。认为华北克拉通与其它克拉通类似,新太古代早期—中太古代晚期是陆壳形成增生的主要时期,但最强烈广泛的构造热事件存在于新太古代晚期。在我们新提出的构造区划中,划分出了三个古陆块(>2.6 Ga),即东部古陆块、南部古陆块和中部古陆块。