Paleoproterozoic basin evolution in the Trans-North China Orogen, North China Craton

Abstract

华北克拉通中部造山带被认为是由东、西部陆块碰撞而产生的陆陆碰撞带。然而,关于两个陆块碰撞的时间以及方式还存在着争议。其中一个模式认为俯冲方向是向西的,两个陆块最终碰撞的时间是在2.5Ga左右,而另一个模式则认为俯冲方向是向东的,并且存在着2.1Ga和1.85Ga两期碰撞事件,第三个模式则认为俯冲方向是向东的,而最终的碰撞拼合发生在1.85Ga左右。近几年来,对于中部造山带中浅变质表壳岩系的研究取得了很大进展,为解决以上争议提供了进一步的资料。根据岩石组合和表壳岩中的不整合接触关系,位于中部造山带中段五台杂岩中的滹沱群和吕梁杂岩中的野鸡山群被分成了上下两个部分。其中,两个群的下部以及位于中部造山带南段中条杂岩中的绛县群和下中条群主要由类似于弧后盆地沉积的变质碎屑岩、碳酸盐岩和火山岩组成,而滹沱群和野鸡山群的上部以及上中条群和担山石群的岩石组合则是类似于山前磨拉石建造的变质砾岩和砂岩。来自于这些浅变质碎屑岩中碎屑锆石的年龄为我们提供了源区以及最大沉积年龄的信息。滹沱群和野鸡山群中的碎屑锆石具有两个峰值年龄,2.5Ga和2.15Ga,它们分别对应于中部造山带中段两期重要的岩浆事件。中条杂岩中的碎屑锆石年龄图谱则较为复杂,除了具有2.5Ga和2.15Ga这两个峰值,还具有2.7Ga这个较低和较老的峰值,这些较老的锆石可能来自中部造山带最南端太华杂岩中的古老岩石。弧后盆地沉积中最年轻的碎屑锆石年龄在2.1Ga左右,而山前磨拉石中最年轻的碎屑锆石在1.85Ga左右。结合前人的研究成果,我们给出了一个华北克拉通中部造山带早元古代沉积盆地的演化模式。从2.1Ga左右开始,一系列弧后盆地在'安第斯型'大陆边缘弧之后产生,在之后的东西陆块碰撞过程中他们成为了中部造山带的一部分。从1.85Ga左右开始,东西陆块沿着中部造山带碰撞,导致了陆壳加厚以及随后的快速抬升和剥蚀,从而形成了周缘前陆盆地中的山前磨拉石建造。发生在早元古代晚期的由弧后盆地向周缘前陆盆地的转化支持了华北的最终克拉通化发生在1.85Ga的构造演化模式。

The Trans-North China Orogen (TNCO) has been recognized as a continent-continent collisional belt along which the Eastern and Western blocks amalgamated to form the North China Craton. However, controversy has surrounded the timing and tectonic processes involved in the collision of the two blocks, ranging from the westward-directed subduction with final collision at ~2.5Ga, through the west-dipping subduction with two collisional events at ~2.1Ga and ~1.85Ga, to the eastward-directed subduction with final collision at ~1.85Ga. In the past few years, detailed lithostratigraphic, geochronological and isotopic analyses for the low-grade supracrustal successions in the TNCO has been carried out, which help us to examine current models. Lithostratigraphic data indicate that the Jiangxian and Lower Zhongtiao groups and lower parts of the Hutuo and Yejishan groups are composed of metaclastic rocks, carbonates and metavolcanic rocks, interpreted as back-arc basin deposits, whereas the Upper Zhongtiao, Danshanshi groups and the upper parts of the Hutuo and Yejishan groups consist only of metaconglomerates and metasandstones, interpreted as foreland basin deposits. U-Pb ages for detrital zircons from them place constraint on the provenance and maximum depositional ages for these low-grade supracrustal successions. For the Hutuo and Yejishan groups, we found major age peaks at ~2.5Ga and ~2.15Ga, which are consistent with ages of the lithological units in the middle sector of the TNCO. Besides the age peaks of ~2.5Ga and~2.15Ga, detrital zircons from the Lower Zhongtiao, Upper Zhongtiao and Danshanshi groups also gave an older age peak of 2.7Ga, which is comparable with ages of the lithological units in the Taihua Complex. For the back-arc basin deposits, their maximum depositional ages were constrained at ~2.1Ga, whereas the presence of ~1.85Ga detrital zircons from the foreland basin deposits indicates that they were deposited after this time. Taken together, we present a brief scenario for the evolution of the sedimentary basins in the TNCO. At ~2.1Ga, a series of back-arc basins developed behind an "Andean-type" arc that were subsequently incorporated into the TNCO during the collision of the Eastern and Western blocks. At ~1.85Ga, the two blocks collided along the TNCO, resulting in the crustal thickening followed by rapid exhumation/uplift, which shifted the back-arc basins to foreland basins. Such a shift in the Late Paleoproterozoic supports the model that the collision between the Eastern and Western blocks occurred at ~1.85Ga.