华北克拉通南部太古宙大陆地壳的生长和演化

太古宙约占地球已有演化历史的三分之一强,这一时期涉及到大陆地壳起源、陆壳的巨量生长和稳定以及板块构造作用的启动、建立等诸多最根本的全球性重大地质事件。太古宙岩石在华北克拉通南部的涑水、登封、太华、霍邱和五河等杂岩中广泛出露,这为解析上述重大科学问题提供难得的素材。近十年来,在华北克拉通南部古生代-中生代火山岩或早前寒武纪变沉积岩中陆续发现冥古宙-古太古代的捕获/碎屑锆石,暗示南部地块依然尚存地球形成最初期的陆壳物质。根据华北克拉通南部太古宙岩石年龄统计结果显示有2850~2700Ma和2580~2480Ma两个突出年龄区间,对应的峰值年龄分别为~2.76Ga和~2.52Ga。其中~2.76Ga的岩石主要出露于南部的鲁山、霍邱、五河和中条山地区。此外,在华北克拉通诸多地区,诸如怀安、阜平、五台、中条等地区的花岗质片麻岩和变质沉积岩中也均发现年龄为~2.76Ga的碎屑锆石或者继承锆石,暗示华北克拉通2.85~2.70Ga岩石的分布似乎比现今出露范围更为广泛。与整个华北克拉通类似,2.58~2.48Ga岩石亦在克拉通南部广泛分布,尤其是嵩箕地区的登封杂岩几乎全部是由新太古代晚期的岩石组成。~2.52Ga是华北克拉通南部,乃至整个克拉通太古宙地壳演化最突出、最重要的岩浆-构造事件,明显有别于全球其它诸多典型克拉通。已有的同位素资料研究表明华北克拉通南部,乃至整个克拉通在太古宙经历了两期明显的地壳生长事件:一期发生在2.85~2.70Ga左右,以形成于此时期的涑水杂岩中花岗质岩石和鲁山太华片麻岩系中深成侵入岩和斜长角闪岩为代表;另一期发生在2.58~2.48Ga,以登封杂岩、涑水杂岩以及小秦岭地区太华杂岩中~2.52Ga各类花岗质岩石和变基性岩为代表。华北克拉通正是经过这两期陆壳巨量生长事件之后完成初始的克拉通化。我们在登封杂岩中识别出形成于俯冲汇聚环境的TTG质片麻岩、类似于赞岐岩的变闪长岩和具有N-MORB地球化学特征的变基性火山岩,提出其构成"新太古代构造混杂岩",标志着新太古代末期具有现代体制的板块构造在华北克拉通南部已经开始启动。最近,在登封杂岩中识别出的新太古代双变质带也支持上述观点。

Crustal growth and evolution of Archean continental crust in the southern North China Craton

The Archean covers more than one third of the earth''s history. It is a key period that related to many global fundamental geological issues, such as the origin, growth and stabilization of the continental crust as well as the initiation and establishment of plate tectonics. Archean rocks have been widely exposed in the Sushui, Dengfeng, Taihua, Huoqiu and Wuhe complexes in the southern North China Craton (NCC), which is served as excellent areas for investigating the above important scientific issues. In the last decade, Hadean-Eoarchean captured/detrital zircons have been successively found in the Paleozoic or Mesozoic volcanic rocks or Early Precambrian metasedimentary rocks in the southern NCC, suggesting that the crustal materials of the beginning earth still exist in there. According to statistical analysis of the geochronology of the Archean rocks in the southern NCC, there are two prominent age intervals of 2850~2700Ma and 2580~2480Ma, with corresponding peak ages of ~2.76Ga and ~2.52Ga respectively. Rocks with ages of ~2.76Ga are mainly exposed in the Lushan, Huoqiu, Wuhe and Zhongtiaoshan regions. In addition, ~2.76Ga detrital zircons or inherited zircons have also been found in granitoid gneisses or metasedimentary rocks in many areas of the NCC, such as Huai''an, Fuping, Wutai and Zhongtiao areas, suggesting that the 2.85~2.70Ga rocks were likely more widely distributed than today. Similar to other typical regions in the NCC, ~2.52Ga rocks are also widely occurred in its south, especially for the Dengfeng Complex that almost entirely composed of Late Neoarchean rocks. The ~2.52Ga tectonothermal events are the most prominent and important period of the whole NCC, which make the craton obviously different from many other typical cratons worldwide. Hf-O isotopic data show that the southern as well as whole craton have experienced two prominent periods of crustal growth during the Archean. The earlier occurred around 2.85~2.70Ga, minor granitoid rocks of the Sushui Complex and nearly all of plutonic rocks and amphibolites in the lower part of the Taihua Complex in the Lushan area are formed at that time; the later happened at 2.58~2.48Ga, the Dengfeng Complex, majority of the Sushui Complex, and minor ~2.52Ga granitoid rocks in Xiaoqinling area have formed at the time interval. The NCC has accomplished its firstly cratonization via the above two stages of crustal growth events. We have identified contemporary TTGs, sanukitoids and MORB-back arc association in the Dengfeng Complex, which likely represent a Late Neoarchean tectonic mélange, implying for a Neoarchean subduction-accretion process and the modern-style plate tectonics processes likely initiated in the southern NCC at end of the Neoarchean. A Neoarchean paired metamorphic belt are recently recognized from the Dengfeng Complex, which also support the operation of plate tectonics at the end of the Archean.