中国东部岩石圈上地幔的组成特征及其地质意义

无论是在岩石学领域,还是在地质学的其它研究领域,地幔的岩石学特征都具有十分重要的意义。本文试图把统计学方法引入区域岩石学研究,从中国东部新生代玄武质岩石中超镁铁岩包体的成分特征入手,应用统计学方法阐明中国东部岩石圈上地幔整体上为弱亏损上地幔的观点。并且认为,在中国东部,岩石圈上地幔具有东西分带的特点。地幔岩石的带状分布方向为NNE向,与中国东部上地幔隆起的长轴方向一致。从东向西可以分出3个带,即东南沿海带、长白山—郯庐断裂带及大兴安岭—太行山带。另一方面,岩石圈上地幔岩石成分的这种分带性又受到其它因素的影响,以致在东西分带的基础上显现南北分块的特征,大致划分出了东北、华北、华南三大地块,体现了中国东部岩石圈上地幔的南北差异。在这个基础上,进一步探讨了我国东部岩石圈上地幔的组成特征在玄武岩岩浆起源、裂谷作用及地质演化等方面的意义。     

The Liaonan Metamorphic Core Complex： Constitution, Structure and Evolution

The Liaonan metamorphic core complex (mcc) has a three-layer structure and is constituted by five parts, i.e. a detachment fault zone, an allochthonous upper plate and an supradetachment basin above the fault zone, and highly metamorphosed rocks and intrusive rocks in the lower plate. The allochthonous upper plate is mainly of Neoproterozoic and Paleozoic rocks weakly deformed and metamorphosed in pre-Indosinan stage. Above these rocks is a small-scale supradetachment basin of Cretaceous sedimentary and volcanic rocks. The lower plate is dominated by Archean TTG gneisses with minor amount of supracrustal rocks. The Archean rocks are intruded by late Mesozoic synkinematic monzogranitic and granitic plutons. Different types of fault rocks, providing clues to the evolution of the detachment fault zone, are well-preserved in the fault zone, e.g. mylonitic gneiss, mylonites, brecciated mylonites, microbreccias and pseudotachylites. Lineations in lower plate granitic intrusions have consistent orientation that indicate uniform top-to-NW shearing along the main detachment fault zone. This also provides evidence for the synkinematic characteristics of the granitic plutons in the lower plate. Structural analysis of the different parts in the mcc and isotopic dating of plutonic rocks from the lower plate and mylonitic rocks from detachment fault zone suggest that exhumation of the mcc started with regional crustal extension due to crustal block rotation and tangential shearing. The extension triggered magma formation, upwelling and emplacement. This event ended with appearance of pseudotachylite and fault gauges formed at the uppermost crustal level. U-Pb dating of single zircon grains from granitic rocks in the lower plate gives an age of 130±5 Ma, and biotite grains from the mam detachment fault zone have 40Ar-39Ar ages of 108-119 Ma. Several aspects may provide constraints for the exhumation of the Liaonan mcc. These include regional extensional setting, cover/basement contact, temporal and spatial coupling of extension and magmatism, basin development and evolution of fault tectonites along detachment fault zone. We propose that the exhumation of the Liaonan mcc resulted from regional extension and thinning of crust or lithosphere in eastern North China, and accompanied with synkinematic intrusion of granitic plutons, formation of detachment fault zone, uplifting and exhumation of lower-plate rocks, and appearance of supradetachment basin.     

从深部背景看燕山陆内造山作用的主因

中生代华北燕山造山带是陆内造山作用的典型,但长期来对造山作用的动力学机制存在多种分歧观点。本文尝试将岩石圈不同深度的构造整合在一起,探索陆内造山作用的主因。本文拟从燕山带中生代构造运动的基本特征入手,选择集宁-承德基底岩石圈断裂带作为先存构造薄弱带,结合不同深度层次的地质作用:①地表的断陷盆地及其火山-沉积岩建造;②断块构造的差异隆升;③固态流变和地热活动;④幔源或者壳幔混溶的火山岩,论证先存构造薄弱带的活化现象。在此基础上,通过幔源熔-流体的交代作用讨论了岩石圈强度的弱化,结合岩石圈结构的层析成像研究,得到初步结论:地幔底辟体上涌是华北克拉通中生代构造活化的重要背景。不排除不同板块间相互作用的影响。最后,从与国外典型的陆内造山带的对比中,进一步认识到:深部地幔物质上涌和上部岩石圈物质的均衡调整,是陆内造山与陆缘造山的本质区别。     

中国东部大陆岩石圈地幔置换作用的内外原因

克拉通大陆通常有古老、巨厚且难熔的岩石圈地幔。这种地幔高度亏损玄武质组分,有密度低、刚性程度高的特点,能长期漂浮于软流圈之上而稳定存在。中国东部大陆主要由华北和华南两个古老地块在古生代—早中生代沿中央造山带拼合形成,在晚中生代时强烈活化,表现为构造变形、盆地形成、岩浆活动、巨量成矿等,其深部原因是什么？在分析东部大陆形成过程和岩石圈地幔属性基础上发现:块体初始规模小且发育薄弱带,后期容易受改造;特别是显生宙以来中国大陆受周边多个构造域夹持,板块俯冲作用会引起软流圈物质扰动和上涌并沿薄弱带侵蚀和改造上覆岩石圈,使之发生有效减薄、明显再富集和最终地幔置换。改造和置换后的岩石圈地幔富含玄武质组分,有较高密度和较低刚性程度,容易发生变形和部分熔融,使克拉通大陆活化。因此,块体规模大小并发育薄弱带以及周边构造环境是大陆稳定性控制重要的内、外在因素;中国东部大陆岩石圈显生宙强烈活化和地幔置换是由于块体规模较小而且周边多体系俯冲作用等内、外在有利因素协同作用下的结果。