【作者】 田志伟；

【导师】 李红艳；

【作者基本信息】 中国地质科学院 ， 地球化学， 2009， 硕士

【摘要】 地壳的两大组成部分——洋壳和陆壳,由于在物质成分上的巨大差异,致使在板块汇聚边界的俯冲消减作用中,二者呈现截然不同的地球动力学特征。我们已经清楚,洋壳俯冲以上覆地幔楔部分熔融、形成岛弧岩浆岩为特征。然而对陆壳的俯冲过程,我们尚处于探索之中。自二十世纪八十年代以来,由于在西阿尔卑斯和挪威西部的变质表壳岩中,发现了超高压变质矿物柯石英,说明大陆地壳曾被俯冲到至少80km深的地幔内部,然后又折返至地表。到目前为止,地质学家已在全球二十二条变质带中发现了柯石英、金刚石和其它超高压指示矿物。大陆深俯冲以及由此引发的陆壳岩石超高压变质作用,是最近二十余年来固体地球科学领域的研究热点。大别一苏鲁造山带出露有世界上规模最大、保存最好的超高压变质地体,是三叠纪时扬子板块向北俯冲进入华北板块之下,而形成的大陆碰撞造山带。是国际超高压变质研究的经典地区。大量的研究表明,与大洋地壳相比,大陆地壳除上部含少量的水之外,其余深部圈层均较“干”,因此在大陆深俯冲过程中无大规模的流体活动,难以形成类似于洋壳俯冲的岛弧岩浆作用。也就是说,缺乏同俯冲岩浆活动是大陆深俯冲最重要的特征之一。随着超高压变质作用后的流体活动及与折返过程近同时代的岩浆岩陆续被识别,探索超高压变质岩石在折返退变过程中是否存在岩浆活动或部分熔融过程,已成为大陆深俯冲作用研究的一个活跃方向。在大别一苏鲁超高压变质带中,广泛发育有较晚期的混合岩(约130Ma)。一般认为,这些主要分布在北大别的混合岩,与大陆深俯冲作用没有直接的动力学联系。然而,我们在苏鲁超高压变质地体的威海、荣成地区,以及大别超高压变质地体的碧溪岭地区均发现呈现类似于部分熔融特征的混合岩,却在空间上和时间上与超高压变质作用具有密切的关系。本文选取碧溪岭地区和荣成地区的两处典型混合岩为研究对象,通过较详细的野外调研,以及矿物化学、同位素地球化学和同位素定年等室内综合研究,探讨了两工作区混合岩的形成时代,成因及其与超高压变质作用的关系,并进一步讨论陆壳超高压变质地体中是否存在部分熔融作用,以及它们对大陆深俯冲带后期构造和物质组成演化的意义。碧溪岭岩体东北端外侧混合岩中角闪岩和奥长花岗岩均发育两期角闪石,早期(Ⅰ型)均为阳起石,后期(Ⅱ型)均为绿钠闪石。尽管角闪岩和奥长花岗岩的化学成分差别极大,但两者的Ⅰ型角闪石成分很相近,Ⅱ型角闪石成分基本相同,但均与该区新鲜富镁铝榴辉岩和退变富铁钛榴辉岩中角闪石的成分相差甚远。结合本人导师李红艳研究员前期元素-同位素地球化学研究结果,从矿物化学的角度进一步证明碧溪岭岩体混合岩是一期发生于780Ma的部分熔融事件的产物。荣成超高压变质地体中,发育由浅灰色片麻岩和肉红色片麻岩构成的混合岩。锆石SHRIMP U-Pb定年结果表明,两类片麻岩的原岩均形成于780Ma左右。锆石Hf同位素研究表明,肉红色片麻岩和浅灰色片麻岩具有完全一致的Hf同位素组成,表明它们的原岩为同一岩浆事件的产物。该区超高压变质作用的峰期变质发生于242Ma左右,而220Ma代表的是一次部分熔融事件。该部分熔融事件使肉红色片麻岩发生塑性流动,从而在宏观上形成了该混合岩化的现象。总体来看,根据大别山碧溪岭地区和苏鲁荣成地区的研究结果,超高压变质地体中可能存在明显的部分熔融作用,一期发生于780Ma左右,即超高压变质岩的原岩形成时代;另一期发生于220Ma左右,处于超高压变质岩的折返退变阶段。更多还原

【Abstract】 During the subduction processes at the plate convergent boundary, the continental crust and oceanic crust, due to their remarkable differences in composition, show striking different dynamic characters. We already well understand that the subduction of oceanic crust induces partial melting of the mantle wedge and produces island arc magmatic activities. However, many important facts of the continental subduction processes still remain unknown.In the 1980s, continental crust was first proved to have subducted to the depth of more than 80 km and thereafter come back rapidly to the surface, since the discoveries of coesite in supracrustal rocks in the West alpines and West Norway. Up to now, geologists have identified ultra-high pressure (UHP) index minerals, such as coesite and diamond, in 22 metamorphic belts on the global earth. Continental deep subduction and related ultra-high pressure metamorphism has been the highlight field in the earth science research in the recent two decades. The Dabie-Sulu orogenic belt, formed by the collision of the Yangtze craton and the North China craton in the late Triassic, has been know as the largest and best-preserved UHP terrane all over the world. Therefore, it becomes the ideal area for the UHP metamorphic research.Numerous studies show that most parts of the continental crust are "dry" compared to the oceanic crust. No large scale fluid activities occurred during the continental deep subduction and hence island arc magmatism similar to the oceanic subduction could not be generated. Therefore, lack of syn-collisional magmatism is one of the most important characters of the continental deep subduction. However, more and more evidences have been identified showing post-UHP metamorphic fluid activities and contemporary magmatism related to the exhumation process. It has become an active topic in the continental deep subduction research to disclose whether it exist partial melting or even magma activities during the retrogression and exhumation processes of the UHP metamorphic rocks.In the Dabie-Sulu UHP metamorphic belt, post orogenic migmatite (130 Ma) is widely distributed. It is commonly understand that these migmatite, which are most reside in the Northern Dabie terrane, have no direct tectonic connection with the continental deep subduction. However, we identified certain migmatite showing partial melting characters in the Weihai and Rongcheng area, Northern Sulu UHP metamorphic terrane, and in the Bixiling area, Central Dabie UHP terrane. Evidences indicate that they have close temporal-spatial relationship with the UHP metamorphism.In this study, we choose two typical migmatite in the Bixiling area and Rongcheng area to carry out detailed field investigation, mineral chemistry, isotope chemistry and isotopic dating, to explore their geological age, origin, as well as their relations to the UHP metamorphism. We further discuss whether partial melting has ever occurred in the UHP metamorphosed supracrustal rocks, and try to understand the significance to the evolution of post-collisional structure and composition of the continental deep subduction belt.The migmatite to the Northeast of the Bixiling complex, Dabie terrane is composed of amphibolite and trondhjemite. Both of the rocks develop two types of amphibole - typeⅠis actinolite, and typeⅡis hastingsite. Despite the striking difference of the amphibolite and trondhjemite in bulk chemical composition, the typeⅠamphibole in both rocks is similar and the typeⅡamphibole in both rocks has nearly same composition. The compositions of these amphiboles are far from those in the MgAl-riched fresh eclogite and the retrograde FeTi-riched eclogite. Combined with the earlier isotopic and geochemical research work of Li H.Y., we further demonstrate that the migmatite in Bixiling represents the products of a 780 Ma partial melting event.In the Rongcheng area, Sulu UHP terrane, migmatite are composed of pale grey gneiss and pink gneiss. Zircon SHRIMP U-Pb dating shows that the protolith of the two types of gneiss were formed in ca. 780 Ma. Analysis of Hf isotopic data indicates that they obtain consistent Hf isotopic character at that time. This implies the two types of gneiss might be originated from a same magmatic source. The zircon dating also shows a 242 Ma event, which represents the peak UHP metamorphism, and a 220 event, which represents the partial melting. The later attributed to the pink gneiss with the rheological property and formed the migmatitic structure.In summary, according to the research work on the migmatite in the Bixiling area, Dabie UHP terrane and Rongcheng area, Sulu UHP terrane, partial melting events might have occurred in two stages. The earlier stage was around 780 Ma, i.e. the time of the protolith of the UHP metamorphic rocks; the later stage was around 220 Ma, which coincident with the exhumation of the UHP metamorphic rocks.更多还原