麻粒岩相变质作用的PTt演化及其地质动力学意义

麻粒岩地体和基性火山岩中的麻粒岩包体是研究过去和现在下部地壳的窗口,通过它们的研究可了解下地壳的物质组成,物理和化学状态,构造变形,岩浆作用和变质作用的特征,并进而探讨大陆地壳形成和改造的地质动力学机制。当前研究中涉及许多重要问题,如麻粒岩相变质作用的PTt演化和成因模式及其构造环境、变质流体的含量、成分和作用,岩浆作用的性质和机制及其对麻粒岩相变质作用的贡献,构造变形作用的样式及其所反映的构造体制,麻粒岩中LILE亏损等地球化学特征的成因意义等等。这些问题互相联系,都是麻粒岩地体成因的约束条件。本文将讨论麻粒岩相变质作用的PTt演化特征,阐述大地构造环境和地质动力学机制对其所起的约束作用。     

麻粒岩相带变质作用

1 概述麻粒岩是前寒武纪麻粒岩相带的重要岩石组成。麻粒岩相带是地壳地质演化重大转变的一种标志。麻粒岩的研究对探索早期地球形成历史以及下地壳的特征具有重要意义。“麻粒岩”一词是变质岩名称;“麻粒岩相”指的是高级变质程度;而“麻粒岩区、麻粒岩相带、麻粒岩地体、高级区、麻粒岩—片麻岩区”等则是反映了大地构造特征的一些综合性术语。在阅读文献时,有时出于省略仅用麻粒岩一词表示,则要分辨是指岩石、变质相,还是大地构造单元,以免弄混。     

华北麻粒岩相岩石的主要特征及今后研究中值得注意的几个问题

华北前寒武纪麻粒岩与国外同类组合相比有特殊性,如(1)具面状分布的特点;(2)麻粒岩类型缺少斜长岩-淡色辉长岩组合以及下部堆晶相辉长岩组合,灰色片麻岩比例少,并普遍受到钾质交代;(3)多为不亏损型麻粒岩,未发现熔融残留型麻粒岩;(4)变质压力为低—中压过渡型,堇青石在变质沉积岩中普遍存在;(5)华北克拉通经历了多期麻粒岩相变质作用。这些特征对于深入研究华北麻粒岩的成因、下部地壳的性质以及克拉通的形成和演化都是必须加以考虑的。     

前寒武纪麻粒岩与活动的下地壳

地球表面前寒武纪地壳(出露的和埋藏的)总共106×10~6km~2,即大陆地壳总面积的72％。以构造年龄划分前寒武纪地壳所占比例为:太古界(+2.5Ga)=15％;早元古界(到1.8Ga)=19％;中元古界(到1.0Ga)=23％;晚元古界(到0.57Ga)=43％。如果排除了大面积平坦的和近平坦的,那些没变质到浅变质的,象刚果盆地那样的晚元古界的台地盖层,世界上前寒武纪地体包含平均13％的绿片岩或不足点,62％的角闪岩相和25％的麻粒岩相变质作用。这暗示了上覆地壳被剥蚀了12～15km,麻粒岩地质相应得剥露17～31km。虽然也清楚地反映了下地壳,然而出露的最基性的麻粒岩是否代表最下部的地壳,还是有疑问的。下地壳在整个前寒武纪时期已明显地是活动的了。     

辽宁阜新两类变质地区的构造关系

根据太古宙地壳在后寒武纪被改造的特点,可以将研究区－－阜新地区划分为西部稳定区和东部线性杂岩带。而西部稳定区又可依据变质作用强度进一步划分为西部麻粒岩相区和东部角闪岩相区。本文通过西部麻料岩相区和东部角闪岩相区岩石组合特征、变质作用演化的动力学以及发育于其中的韧性剪切带运动学、动力学和地球物理学特征的对比研究,研究麻粒岩相区为角前岩相区的根部带,后期构造运动的右行拉伸作用使上部的角闪岩相区与下部的     

胶北地区早前寒武纪孔兹岩系的变质演化

在胶北地区出露大面积早前寒武纪孔兹岩,主体岩石为富铝片岩-片麻岩,夹有大理岩等变沉积岩。岩相学、成因矿物学和变质反应性质的研究以及温压条件估算结果表明,研究区孔兹岩系经历了四个阶段的变质演化。早期进变质阶段(M1)的矿物组合为石榴石及其内部的黑云母+斜长石+石英;峰期中-高压麻粒岩相变质阶段(M2)的矿物组合以含蓝晶石的麻粒岩相矿物组合为标志,该阶段最高的温压条件可达760℃、10.0kb;峰后近等温减压麻粒岩相退变质阶段(M3)发生一系列典型减压反应,以新生堇青石的出现为标志,形成了含堇青石的中-低压麻粒岩相矿物组合,稳定的温压条件为750～800℃、4.0～5.4kb;晚期降温退变质阶段(M4)以石榴石转变形成细小黑云母等退变矿物为特征,该阶段的温压条件为550～650℃、4.0kb。孔兹岩系变质演化P-T轨迹具有顺时针型式,先后经历了近等温减压(ITD)和近等压冷却(IBC)的P-T演化,标志着胶北地区早前寒武纪孔兹岩系曾经历了地壳加厚-构造隆升的动力学过程。     

华北克拉通两类早前寒武纪麻粒岩(HT-HP和HT-UHT)及其相关问题

华北克拉通广泛存在高级变质的早前寒武纪岩石,其中高温高压(HT-HP)麻粒岩和高温超高温(HT-UHT)麻粒岩近年来备受人们的关注,并将它们视为探讨早前寒武纪大陆演化的关键课题.HT-HP麻粒岩主要是含石榴石的基性麻粒岩,它们以透镜体或强烈变形的岩墙状出露于片麻岩中.HT-UHT麻粒岩主要是富铝的变质沉积岩系,俗称孔兹岩系,其中有含假蓝宝石和尖晶石等矿物组合,指示部分岩石的变质温度高于900～1000℃.本文探讨了这两类麻粒岩的出露状况、变质过程与变质历史、原岩与变质年龄.初步研究表明:①两类麻粒岩在变质的峰期温度和压力上有很大的重叠区间,都经历了一个近等温-略升温的降压变质;②两类麻粒岩很有可能在峰期和随后的降压变质阶段是同时的或有关联的;③HT-HP和HT-UHT麻粒岩的分布特征是线状或面状分布仍有待进一步查明;④高级变质的麻粒岩代表了华北克拉通的最下部地壳,它们变质的温压体系、岩石的刚性程度、分布特征、岩石组合以及抬升速率等具有与显生宙明显的不同.本文从而指出两类麻粒岩是否有成因联系是今后研究中非常值得注意的问题,深入研究将会对华北克拉通早期地壳演化、构造和动力学机制提供丰富的资料和证据.     

冀西北麻粒岩区早前寒武纪主要地质事件的年代格架

本文通过年代学资料和其它地质依据建立了冀西北及邻区麻粒岩地体早前寒武纪主要地质事件的年代格架。早期的基性火山喷发事件发生在2868—2932Ma期间,形成本区的早期地壳。在2761Ma左右发生了大规模的TTG岩浆侵位事件,在2650Ma时发生了基性岩浆侵位,使地壳加厚。在2561—2503Ma期间,花岗闪长质岩浆在本区广泛侵入,使地壳进一步加厚。2477—2461Ma期间,紫苏花岗岩以岩株形式侵入,同时发生区域麻粒岩相变质,早期地壳受到改造。大约在2300Ma时发生第二阶段的麻粒岩相变质。此后,在2144—2087Ma期间红色花岗岩侵位,形成花岗岩带。     

太古宙下部地壳流体研究现状

太古宙下部地壳流体的研究对于阐明麻粒岩相变质作用成因及深成地质作用过程具有重要意义。下部地壳流体的性状，成因，运移方式及其与麻粒岩相变质作用成因联系等方面取得了一定的研究成果。采取流体包裹体研究与脱水变质反应平衡热力学计算，矿物对地质温压计估算等相结合的方法才能获得更可信的结果，实验研究是今后重要的发展方向。     

琼中麻粒岩的成因及地质意义

对琼中前寒武纪高级变质杂岩中的麻粒岩进行了较系统的岩石学和矿物学研究,并利用不同的矿物温压计估算了麻粒岩形成的温压条件。结果表明,本区麻粒岩形成于高温低压(低p/T)环境,属于典型的低压麻粒岩,其峰期变质温度为824±15℃,压力小于0.5GPa,晚期岩石发生了退变冷却,终止温度为705℃左右,压力与峰期时的压力相近;麻粒岩相变质作用具有顺时针的p-T轨迹,早期可能经历了一个缓慢降压升温的过程,峰期之后的退变则表现出近等压冷却(IBC)的特点。结合岩石变形作用较弱的特点及区域大地构造背景,认为琼中低压麻粒岩形成于陆缘拉张环境,其可能的机制是,由于地壳拉伸减薄,导致软流圈减压上隆产生岩浆,上覆地壳岩石(包括已经结晶的岩浆)受到下部岩浆热源的影响而发生进变质,随后受拉伸扰动的地热梯度向正常地热梯度恢复,从而形成具有降压升温过程和IBC过程的顺时针p-T轨迹。     

中国东南新生代玄武岩中麻粒岩相捕虏体的新发现及其意义

本文报道了在中国东南地区女山、桂子山、西垄和麒麟等地新生代玄武岩中新发现的麻粒岩相捕虏体。研究显示女山存在多种岩石类型的捕虏体,其中本次研究新发现的石榴子石麻粒岩与二辉麻粒岩在矿物化学和形成条件上明显不同。石榴子石麻粒岩的次透辉石（富Al2O3,Na2O)、紫苏辉石（富Al2O3) 形成于较深（＞40km)的下地壳,厚的地壳和较低的地温梯度（20－24℃/km)表明当时女山处于相对稳定的克拉通环境;女山二辉麻粒岩的次透辉石和紫苏辉石（贫Al2O3,Na2O)形成于较高地温梯度（31－34℃/km）的构造背景,指示当时女山处于活动大陆边缘或裂谷环境。复杂的岩石类型和不同的形成条件表明女山的下地壳是由多期岩浆活动的产物组成。广东麒麟和雷州的二辉麻粒岩的次透辉石以低Al2O3和低Na2O为特征,温压估算显示它们形成于较浅部（23－27km)和高地温梯度的地质背景,指示当时华南处在强烈的拉张减薄的构造环境。浙江西垄和新昌的麻粒岩捕虏体的矿物学特征和形成条件介于女山和广东的麻粒岩捕虏体之间。女山麻粒岩捕虏体的岩石组合和形成条件与华北太古代麻粒岩地体和汉诺坝麻粒岩捕虏体相似,而与华南的明显不同,表明女山的下地壳隶属于华北板块。在华南沿海从北到南基性麻粒岩的发现,显示中生代基性岩浆的底侵作用普遍存在。     

Geochemical Characteristic of Charnockites in North Margin of North China Craton: Indicating the Significiant of the Neoarchean Tecnotic Event

The Neoarchean charnockites of North margain of North China Craton(NCC) has become a hot topic into understanding the Early Precambrian basement. Although there is a broad consensus that charnockite is usually related to granulite facies metamorphism, whether its petrogenesis and tectonics characteristics remains controversial. Inclusions within hypersthene and garnet in charnockite are used to identify the peak granulite facies mineral assemblage, with the formation of Magnesian–charnockite attributed to anatexis of the protolith associated with this granulite facies metamorphism. The distribution of major and trace elements in charnockite is very uneven, significant depleted in LILEs(eg. Cs, U, Th) and HFSEs(eg. Nb, Ta, P and Ti), riched in Sr. Raising to the coexistence of Eu–enrichment and Eu–depletion type of REE patterns that influenced by the content of plagioclase and the remnants minerals of zircon and apatite. Comparative the petrography, geochemistry and geochronology data of Magnesian–charnockite indicate that the ratios of mafic pellites and basalts involved in anatectic melting are different by the upwelling of mantle magma, also resulting in the Eu anormals characteristics. The formation of the Magnesian–charnockite is closely connected with the subduction of the NCC oceanic crust(About ～2.5 Ga). However, Ferroan–charnockite may be the formed by the crystallization differentiation of the upwelling of mantle–derived shoshonitic magma(About ～2.45 Ga), with the lower crust material addition.     

Ultrahigh-pressure eclogite transformed from mafic granulite in the Dabie orogen, east-central China

Although ultrahigh‐pressure (UHP) metamorphic rocks are present in many collisional orogenic belts, almost all exposed UHP metamorphic rocks are subducted upper or felsic lower continental crust with minor mafic boudins. Eclogites formed by subduction of mafic lower continental crust have not been identified yet. Here an eclogite occurrence that formed during subduction of the mafic lower continental crust in the Dabie orogen, east‐central China is reported. At least four generations of metamorphic mineral assemblages can be discerned: (i) hypersthene + plagioclase ± garnet; (ii) omphacite + garnet + rutile + quartz; (iii) symplectite stage of garnet + diopside + hypersthene + ilmenite + plagioclase; (iv) amphibole + plagioclase + magnetite, which correspond to four metamorphic stages: (a) an early granulite facies, (b) eclogite facies, (c) retrograde metamorphism of high‐pressure granulite facies and (d) retrograde metamorphism of amphibolite facies. Mineral inclusion assemblages and cathodoluminescence images show that zircon is characterized by distinctive domains of core and a thin overgrowth rim. The zircon core domains are classified into two types: the first is igneous with clear oscillatory zonation ± apatite and quartz inclusions; and the second is metamorphic containing a granulite facies mineral assemblage of garnet, hypersthene and plagioclase (andesine). The zircon rims contain garnet, omphacite and rutile inclusions, indicating a metamorphic overgrowth at eclogite facies. The almost identical ages of the two types of core domains (magmatic = 791 ± 9 Ma and granulite facies metamorphic zircon = 794 ± 10 Ma), and the Triassic age (212 ± 10 Ma) of eclogitic facies metamorphic overgrowth zircon rim are interpreted as indicating that the protolith of the eclogite is mafic granulite that originated from underplating of mantle‐derived magma onto the base of continental crust during the Neoproterozoic (c. 800 Ma) and then subducted during the Triassic, experiencing UHP eclogite facies metamorphism at mantle depths. The new finding has two‐fold significance: (i) voluminous mafic lower continental crust can increase the average density of subducted continental lithosphere, thus promoting its deep subduction; (ii) because of the current absence of mafic lower continental crust in the Dabie orogen, delamination or recycling of subducted mafic lower continental crust can be inferred as the geochemical cause for the mantle heterogeneity and the unusually evolved crustal composition.     

Metamorphic petrology and geology in China: A review

The development of metamorphic petrology to metamorphic geology in China has a long history. Ancient basement metamorphic rocks are distributed primarily in the North China Craton, the Yangtze Block and Tarim Craton. They are mainly made up of plutonic gneiss and metamorphosed supercrust rock, transformed to granulite facies through Archean Paleoproterozoic. Many of the Paleoproterozoic metamorphic rocks have undergone high-pressure granulite facies metamorphism with a clockwise metamorphic evolution path. The ultrahigh temperature (UHT) granulites from the Late Paleoproterozoic are found in North China Craton. Many high-precision chronological data have allowed preliminary construction of the formation and evolutionary framework of different metamorphic basements. Primarily there are low-temperature and high-pressure blue schist, high-temperature and high-pressure granulite and ultrahigh-pressure (UHP) eclogite facies metamorphic rocks in the Phanerozoic orogenic belt. The discovery of eclogite in the Sulu orogen and a large quantity of coesite in its country rocks show that there was a deep subduction of voluminous continental materials during the collision process between the Yangtze block and the North China Craton in the Early Mesozoic phase. From the studies of, for instance, organic matter vitrinite reflectance, illite crystallinity, illite (muscovite) polytype and illite (muscovite) b dimension, the Late Paleozoic strata in the eastern region of Inner Mongolia and the north-central region of NE China have only experienced diagenesis to an extremely low-grade metamorphism. The discovery of impact-metamorphosed rocks in Xiuyan area of Liaoning province has enriched the type and category of metamorphic rocks in China. The phase equilibrium method has been widely used in the study of metamorphism of middle and high-grade metamorphic rocks. On the basis of existing geologic surveys and monographic study results, different scholars have respectively compiled 1:1500000 Metamorphic Geological Map and Specifications of Qinghai Tibet Plateau and its Adjacent Areas, 1:2500000 Metamorphic Tectonic Map of China, and the 1:5000000 Metamorphic Geological Map and Specifications of China, among others repectively, which have systematically summarized the research results of metamorphic petrology and metamorphic geology in China.     

变质流体研究新进展

变质流体是变质过程的主要动力学因素之一。目前变质流体研究主要集中在下部地壳麻粒岩相变质流体，俯冲带高压－超高压变质流体和接触变质流体等方面。研究的主要问题是流体流动机制和元素迁移，流体－岩石相互作用和流体来源。下部地壳麻粒岩相变质流体以ＣＯ２为主，具有较低的ａＨ２Ｏ。δ１３Ｃ研究表明大约２／３ＣＯ２是深成的。富ＣＯ２流体流动是紫苏花岗岩形成和热扰动的原因之一，也是麻粒岩形成和大离子亲石元素亏损的主要因素。俯冲带是高压、超高压变质作用发生和流体活动最活跃的场所。流体富含Ｈ２Ｏ、ＣＨ４和ＣＯ２，可以诱导部分熔融反应和岛弧岩浆作用。高压变质条件下的矿物稳定性也与流体有关。同位素研究表明，在超高压变质期间没有化学上完全相同的流体大规模循环。流体－熔体系统模式能更有效地解释下插板片的元素再循环。接触变质流体研究主要集中在含有易于发生流体－岩石反应的不纯碳酸盐岩地区。硅灰石带中流体／岩石比率高达４０∶１，表明接触变质岩石中有大量流体存在。接触变质过程流体成分有较大差异，主要取决于流体来源、原岩性质和侵入体特征。流体流动和循环模式受控于构造变形，岩浆作用和变质过程的动力学条件及流体成分。     

惠东─连县地学断面的地壳结构及性质研究

本文利用地球物理－地质地球化学相结合的研究方法，分析了惠东─连县地学断面的地球物理特征及其速度密度模型，探讨了断面地壳的物质组成、结构和深部地壳的形成时代，认为该断面地壳的物质组成以长英质为主，断面深部存在太古代地壳，断面的地壳分层主要由变质相相变产生，由麻粒岩相下地壳、高绿片岩相－角闪岩相中地壳，低绿片岩相及沉积盖层组成上地壳的三部分．     

下地壳麻粒岩包体波速测定及其地质意义

下地壳岩石的波速测定可以提供有关下地壳物质组成及物理性质的重要信息。但是，由于下地壳难于直接采样，因此常常利用目前出露地表的古老麻粒岩地体样品来代表下地壳，但所获结果常常与实际情况不符，由新生代玄武岩携带到地表的下地壳包体则为我们提供有关下地壳的直接信息。本文对采自河北汉诺坝武岩中的下地壳包体进行了密度和高温高压波速测定，下地壳由不同类型的麻粒岩相岩石组成。其密度和波速有较大的差异，因此所获波速不能直接与地震测深得到的下地壳波速进行对比，而需要进行波速的温度、压力校正，并进行具体的分析，本文得到下地壳包体的原地波速6.13-6.90km/s与该区地震资料得到的结果十分接近，并探讨了下地壳低速层的可能成因。     

The geochemistry of Precambrian granulite facies rocks from the Lewisian complex of Tiree,Inner Hebrides,Scotland

The metamorphism and geochemistry of the major components of a small area of granulite facies rock are described and discussed, and a chemical model for the evolution of anomalous trace element distributions in such materials is suggested. The local complex was subjected to medium to high pressure granulite facies metamorphism between 2,900 and 2,600 m.y. All the analysed granulite facies rocks from Tiree; acid to intermediate gneisses, basic metamorphic rocks, and granitic rocks, have anomalous chemistries, being depleted in K, Rb, Nb, Y and Th, and have high K/Rb, Ba/Rb and Ca/Y ratios, and very low K/Ba and Rb/Sr ratios relative to normal portions of the upper continental crust. The gneisses seem to have been enriched in Ba and Sr.The chemical features of the rocks are considered to reflect their stable mineral assemblages in the granulite facies, and to be representative of deep-level crustal materials. The geochemical peculiarities of the complex may have been largely controlled by an upward intergranular diffusion, or “degassing” caused by high-grade metamorphism. It is suggested that such diffusion may have been active at the crust/upper mantle interface, some diffused material of mantle origin accounting for certain chemical oddities typical of Lewisian and some other Precambrian granulite facies rocks.     

Structural development and petrofabrics of eclogite facies shear zones, Bergen Arcs, western Norway: implications for deep crustal deformational processes

Caledonian eclogite facies shear zones developed from Grenvillian garnet granulite facies anorthosites and gabbros in the Bergen Arcs of western Norway allow direct investigation of the relations between macroscopic structures and crystallographic preferred orientation (CPO) in lower continental crust. Field relations on the island of Holsnøy show that the eclogites formed locally from granulite facies rocks by progressive development of: (1) eclogite adjacent to fractures; (2) eclogite in discrete shear zones (> 2 m thick); (3) eclogite breccia consisting of >80% well-foliated eclogite that wraps around rotated granulite blocks; and (4) anastomosing, subparallel, eclogite facies shear zones 30–100 m thick continuous over distances > 1 km within the granulite terrane. These shear zones deformed under eclogite facies conditions at an estimated temperature of 670 ± 50°C and a minimum pressure of 1460 MPa, which corresponds to depths of >55 km in the continental crust. Detailed investigation of the major shear zones shows the development of a strong foliation defined by the shape preferred orientation of omphacite and by alternating segregations of omphacite/garnet-rich and kyanite/zoisite-rich layers. A consistent lineation throughout the shear zones is defined by elongate aggregates of garnet and omphacite. The CPO of omphacite, determined from five-axis universal stage measurements, shows a strong b-axis maximum normal to foliation, and a c-axis girdle within the foliation plane with weak maxima parallel to the lineation direction. These patterns are consistent with deformation of omphacite by slip parallel to [001] and suggest glide along (010). The lineation and CPO data reveal a consistent sense of shear zone movement, although the displacement was small. Localized faulting of high-grade rocks accompanied by fluid infiltration can be an important mode of failure in the lower continental crust. Field relations show that granulite facies rocks can exist in a metastable state under eclogite facies conditions and imply that the lower crust can host differing metamorphic facies at the same depth. Deformation of granulite and partial conversion to eclogite, such as is exposed on Holsnøy Island, may be an orogenic-scale process in the lowermost crust of collisional orogens.     

内蒙古中部凉城地区紫苏斜长麻粒岩锆石U-Pb年龄及地质意义

对内蒙古中部凉城地区的紫苏(二辉)斜长麻粒岩进行了同位素地质年代学、岩石学和岩石地球化学研究,探讨了其形成时代和构造背景。利用LA-ICP-MS锆石U-Pb法测得紫苏斜长麻粒岩岩体的年龄为(1935±9)Ma,属古元古代;研究表明该岩体形成于板内构造环境,在19亿年左右,研究区有地幔上涌并伴随玄武质岩浆的底侵作用,底侵玄武质岩浆作为岩体在下地壳就位,并发生麻粒岩相变质作用。