早期陆壳在全球岩浆"静寂期" (2.45-2.20 Ga) 的构造演化：来自华北南缘岩浆热事件的制约

Recent studies suggest that the current Palaeoproterozoic record does not favor the global magmatic shutdown (2.45-2.20 Ga) hypothesis, as magmatism at this period are found in many cratons, though less frequent than those in the 100 Ma prior to the quiet interval. The scenario could involve subduction cessation following the global Archean cratonization, with contemporary initiation of subduction in the exterior ocean or within restricted small-scale ocean basins. But current data of tectono-magmatic events has not reached a sufficient mass to verify the model. During the quiet interval, magmatism are more widely distributed in North China Craton (NCC) than previously known, especially in the southern margin, making it an ideal place to study the crustal evolution of this important period. However, whether the magmatism occurred episodically or in terms of small frequency successive pulse is still controversial. Their temporal-spatial distribution, rock properties and petrogenesis are also unclear. This proposed project plans to conduct systematic geochronology and geochemical analyses on the Paleoproterozoic igneous rocks in the Lushan, Xiong’er Shan and Xiaoqinling areas, southern margin of the NCC, to investigate their petrogenesis and genetic connections. The ultimate aim is to explore the geodynamic background recorded in the magmatism during the period of global magmatic shutdown and provide a better understanding of the early Paleoproterzoic crustal evolution in the NCC.

最新资料显示，全球“静寂期”(2.45-2.20 Ga)的岩浆作用和初生物质添加并未停止，只是相比其他时期少，这可能是由太古宙全球克拉通化后构造活动停滞造成，但仍然存在大洋一侧或小型闭合洋盆内的俯冲构造过程。然而这些认识还未从构造-岩浆演化角度进一步验证。华北克拉通在所谓“静寂期”岩浆活动活跃，尤其是南缘该时期岩浆岩分布广泛，类型丰富，在时代和成因上又有所差异，因此是开展该时期陆壳演化研究和对比的理想场所。但目前对这些岩浆作用是“幕式”发生，还是小频率持续脉冲发生，它们的属性、时空分布和成因还缺乏了解。本申请选择华北南缘鲁山、熊耳山和小秦岭为典型研究区，通过详细的岩石学、元素-同位素地球化学和年代学研究，以2.45-2.20 Ga岩浆岩反映的深部动力学过程为切入点，解析其成因及内在演化联系，探讨它们记录的构造-热事件期次和性质，为认识岩浆“静寂期”的构造转换和动力学过程提供依据。

地球进入2.45-2.20 Ga，岩浆活动骤减，地幔组分和热状态发生巨大变化，如地幔温度快速降低，基性岩高场强元素含量迅速增加。之后，岩浆活动重新活跃，并在~2.10 Ga达到峰期，紧随其后的则是全球规模的造山(~1.85 Ga)，指示热驱动由太古宙地幔柱为主向大规模板块运动演变。然而，地幔状态和壳幔作用机制目前并不清楚。基性-中酸性侵入岩蕴含着地幔性质和壳幔作用的重要信息。因此，本研究选取了华北克拉通南缘太华杂岩内2.45-2.20 Ga辉长岩、闪长岩、TTG和钾质花岗岩，开展了详细的锆石/斜锆石U-Pb同位素定年；综合全岩组分及其Sm-Nd和锆石Hf-O等多元同位素体系，厘定了岩石源区特征和成岩过程，获得的主要认识如下：(1)确定华北南部太华杂岩内广泛发育古元古代早期所谓岩浆“静寂期”的岩浆作用，它们形成的构造背景均为板内的拉伸环境。(2)解析出该时期的TTG质岩石不同于太古宙典型高铝TTG，具有相对高温低压特征，形成于非岛弧环境；镁铁质岩石具有碱性-拉斑特征，总体与板内裂谷岩浆作用类似；高钾花岗岩具有A型花岗岩特征。整套岩石总体形成于板内伸展(裂谷)环境。(3)各类岩石均具有低δ18O特征(0-5‰)。对于中酸性低δ18O岩石，其源区可能在同伸展阶段与地表水发生了高温水岩反应，而幔源镁铁质岩石由于表现出显著的陆壳物质混染，我们认为其低18O特征可能与混染了低δ18O的陆壳物质有关。(4)通过对比太古宙晚期和古元古代早期阶段TTG岩石特征，发现前者所形成的机制既包括俯冲板片的部分熔融，也包括俯冲背景下幔源岩浆底侵基性下地壳部分熔融形成。到了古元古代早期阶段，华北克拉通整体处于板内拉伸环境，TTG岩石基本属于古老基性下地壳的再造产物。