Dynamic Mechanism of Migmatization in the Dabie Complex,Northeastern Hubei,China1

Abstract On the basis of the detailed field work, compositions and contents of plagioclase and K-feldspar, determination of ordering degree, statistical analysis of plagioclase elongation index, mass-balance calculation and mineral spatial distribution and geochemistry, it is concluded that the migmatites in the Dabie complex are characterized by the presence of thermocenters. There are regular changes in mineral character in the migmatites from the centers outwards. The dominant genetic mechanism is anatexis and metasomatism, whose intensities decrease from the centers outwards. Finally, according to the simulated experiment on Liesegang' s rings and non-linear dynamics (dissipative structure theory), the dynamic mechanism of migmatization is profoundly expouded as consisting of the early-stage metasomatism induced by the thermal anomaly, the cardinal-stage anatexis induced by the early-stage matasomatism and finally the last-stage post-anatexis metasomatism.     

大别杂岩中混合岩的矿物空间分布研究

介绍了矿物空间分布研究的基本原理及两种统计方法(接触频数法和线切法)。作者对混合岩矿物空间分布的研究表明:(1)前人提出的统计方法存在方法上的缺陷和应用上的局限性,作者推导出矿物接触类型的概率公式;(2)部分浅色体中矿物显示聚集分布的特征,而绝大部分的浅色体中矿物具有分散分布的特点。结合质量平衡和地球化学研究认为:大别杂岩中主体混合岩成因机制是重熔和交代作用。     

鄂东北大别杂岩中深熔混合岩存在的地质地球化学证据

基于地质地球化学研究结果提出识别大别杂岩中深熔混合岩的证据：①浅色体粗大可横切面理，伴有复杂褶皱，帮助发育；②浅色体和古成体中斜长石牌号有明显差异；③矿物成分和组合指示曾达到深熔条件；④浅色体中富含Ａｌ２Ｏ３、Ｆｅ２Ｏ３、ＴｉＯ２和不活动、不相容元素，如ＬＲＥＥ、Ｔｈ、Ｈｆ、Ｚｒ。最结合混合岩矿物空间分布和质量平衡研究结果得出结论：大别核心杂岩中混合岩的主导成因机制是深熔。     

The genesis of wadi Magrish migmatites (N-E Sinai)

The migmatite complex of the Magrish area is part of a large crystalline massif south of Elat. The mineralogical composition of the migmatites is very uniform. The components of the melanosome are biotite, quartz and plagioclase, with small amounts of garnet and very rarely sillimanite and those of the leucosome — quartz and plagioclase. On the basis of chemical composition of the migmatites and possible premigmatitic parent rocks, absence of orthoclase in the leucosome, similar composition of plagioclase in the leucosome and in neighbouring melanosome, and Qz:Plag values which do not plot around a cotectic line, it is concluded that migmatisation occurred in a nearly closed system, without the presence of a melt phase. Thus, injection of granitic material, metasomatism or partial anatexis as possible main formation mechanisms are rejected and metamorphic differentiation is favoured.     

Geological and Geochemical Evidence for Discriminating Anatectic and Subsolidus Migmatites in the Dabieshan Complex,Northeastern Hubei Province

Based on the geological and geochemical information about migmatites,the following lines of evidence have been proposed for discriminating the anatectic leucosome in the Dabieshan Complex:(1)its width is larger than that derived from the subsolidus genesis,cutting across regional foliation,thus giving rise to complicated folds and wider selvages;(2)it is composed of melanic and accessory minerals in addition to quartz and feldspars;(3)the significant dfference in anorthites of plagioclase between paleosome and leucosome;(4)temperature and pressure(P/T) conditions revealed by the mineral compositions and assemblages are over those for the onset of anatexis;and (5)it is enriched in the major elements(e.g.Al2O3.Fe2O3and TiO2)and immobile and incompatible elements(e.g.LREE,Th,Hf and Zr).Finally,by combining the geological and geochemical features with the statistical data for the spatial distribution of minerals and mass-balance in the migatites,it is concluded that anatexis is the cardinal mechanism of migmatization in the Dabieshan Complex.     

Oligocene HP metamorphism and anatexis of the Higher Himalayan Crystalline Sequence in Yadong region,east-central Himalaya

The Higher Himalayan Crystalline Sequence (HHCS) provides an excellent natural laboratory to study continental subduction, crustal melting and tectonic evolution of orogenic belt generated through the collision of India with Eurasia. Our petrological study and phase equilibrium modeling reveal that the pelitic migmatites in the HHCS of Yadong region, east-central Himalaya, preserve an early mineral assemblage garnet, kyanite, biotite, quartz, plagioclase, K-feldspar, rutile and ilmenite, and a late sillimanite- and/or cordierite-bearing assemblage, and underwent the high pressure (HP) and high temperature (HT) granulite-facies metamorphism and associated partial melting under P–T conditions of ca. 12 kbar and 825–845 °C, followed by nearly isothermal decompression and isobaric cooling. The anatexis of the migmatites occurred dominantly through dehydration-melting of both muscovite and biotite during the prograde metamorphism. The melt produced in the peak metamorphic conditions is about 20 to 30 vol.% of the rocks, and a significant amount of melt has been extracted from the source leading to the formation of Himalayan leucogranites. The zircon U–Pb dating data shows that the migmatites probably witnessed a prolonged melting episode that began at ca. 30 Ma and lasted to ca. 20 Ma. These results show that the thickening lower crust of the Himalayan orogen experienced long-lived and continued HP and HT metamorphism and pervasive anatexis, supporting the models on channel flow.     

大陆造山带深熔垮塌的岩石学、地球化学证据:以北大别深熔混合岩为例

北大别位于大别造山带的核部,分布着大量的造山带垮塌时期形成的混合岩,其于理解大别造山带的形成和演化有着重要的意义。北大别混合岩的原岩为TTG（D）岩石,因黑云母和角闪石的脱水熔融诱发深熔作用产生。顺层产出的为富斜长石浅色体,主要矿物组成为斜长石+石英+黑云母±钾长石±角闪石。伟晶岩脉或团块为富钾长石浅色体,主要矿物组成为钾长石+石英±斜长石±黑云母±角闪石。暗色体为变晶结构,主要矿物组成为角闪石+黑云母+斜长石+石英±单斜辉石;其中,暗色矿物角闪石和黑云母常常定向排列,具有明显的溶蚀结构;暗色体中浅色矿物颗粒较小,以斜长石和石英为主,指示部分熔融的残余产物。全岩地球化学特征表明,碱金属元素（Na、K等）、大离子亲石元素（Ba、K、La等）和LREE等优先进入酸性熔体,而相容元素和中-重稀土元素等残留在残余体中。浅色体与本区花岗岩相比,二者都有右倾的稀土配分模式,富集LREE,亏损HREE。但浅色体具有明显的Eu正异常,δEu值为2.48~6.55,而花岗岩则有弱的Eu负异常,并且浅色体中大颗粒斜长石相互构成框架结构,含量明显高于正常花岗岩熔体,表明浅色体更可能是熔体早期结晶的产物。     

辽吉东部层状混合岩的成因

辽吉东部的层状混合岩与上、下围岩始终保持整合接触,反映它的体系基本上是封闭的,没有外来组份的加入。其组构、岩石化学、微量元素和稀土资料等表明层状混合岩不足岩浆型的,而是重熔岩浆型的,其原岩是沉积变质岩。     

Origin of migmatite granite and its relations to iron deposits—An experimental study

Experiments have been conducted over a wide range of temperatures and pressures (300°–950°C, 500–2000 bars) in an attempt to gain some better understanding of the geological features of Precambrian migmatite granites and banded iron formation (BIF) widespread in Northeast China. Results indicate that the BIF of Anshan type is unstable under the action of sufficient alkaline solutions with some iron or aluminum silicates formed at the expense of quartz. Rock melting experiments show that the initial melting temperatures of phyllite, biotite plagioclase gneiss and migmatite granite range from 630° to 640°C, but those of BIF and plagioclase amphibolite are 150°–200° C higher. The authors suggest that migmatites in this region have resulted from partial melting and metasomatism. With respect to the relationship between migmatization and iron deposits, it has been experimentally revealed that the migmatites will not “digest” the BIF if they are separated by other wall rocks. But melting to various extents will take place in the BIF where migmatites are in direct contact with it. Additionally, the BIF will even become richer in iron under favorable conditions. This information may probably help guide our efforts to prospect for Precambrian iron deposits in regions with extensive occurrence of migmatites.     

Muscovite dehydration melting in Si-rich metapelites: microstructural evidence from trondhjemitic migmatites,Roded, Southern Israel

Making a distinction between partial melting and subsolidus segregation in amphibolite facies migmatites is difficult. The only significant melting reactions at lowpressures, either vapour saturated or muscovite dehydration melting, do not produce melanocratic peritectic phases. If protoliths are Si-rich and K-poor, then peritectic sillimanite and K-feldspar will form in scarce amounts, and may be lost by retrograde rehydration. The Roded migmatites of southern Israel (northernmost Arabian Nubian Shield) formed at P = 4.5 ± 1 kbar and T ≤ 700 °C and include Si-rich, K-poor paragneissic paleosome and trondhjemitic leucosomes. The lack of K-feldspar in leucosomes was taken as evidence for the non-anatectic origin of the Roded migmatites (Gutkin and Eyal, Isr J Earth Sci 47:117, 1998). It is shown here that although the Roded migmatites experienced significant post-peak deformation and recrystallization, microstructural evidence for partial melting is retained. Based on these microstructures, coupled with pseudosection modelling, indicators of anatexis in retrograded migmatites are established. Phase diagram modelling of neosomes shows the onset of muscovite dehydration melting at 4.5 kbar and 660 °C, forming peritectic sillimanite and K-feldspar. Adjacent non-melted paleosomes lack muscovite and would thus not melt by this reaction. Vapour saturation was not attained, as it would have formed cordierite that does not exist. Furthermore, vapour saturation would not allow peritectic K-feldspar to form, however K-feldspar is ubiquitous in melanosomes. Direct petrographic evidence for anatexis is rare and includes euhedral plagioclase phenocrysts in leucosomes and quartz-filled embayments in corroded plagioclase at leucosome-melanosome interfaces. In deformed and recrystallized rocks muscovite dehydration melting is inferred by: (1) lenticular K-feldspar enclosed by biotite in melanosomes, (2) abundant myrmekite in leucosomes, (3) muscovite–quartz symplectites after sillimanite in melanosomes and associated with myrmekite in leucosomes. While peritectic K-feldspar formed in melanosomes by muscovite dehydration melting reaction, K-feldspar crystallizing from granitic melt in adjacent leucosome was myrmekitized. Excess potassium was used in rehydration of sillimanite to muscovite.     

有关混合岩和混合岩化作用的一些问题——对半个世纪以来某些基本认识的回顾

本文首先论述了混合岩和混合岩化作用的一些基本特征(四大特征)。提出了混合岩和有关岩石的混合岩化程度的分类,即三分为部分混合岩化的岩石、混合岩和混合花岗质岩石三大类。论述了确定混合化岩石下界的五个综合标志及混合花岗质岩石的八大特征等,还讨论了部分混合岩化岩石的命名问题。最后从混合岩化作用的地质背景将其分为三类:区域性混合岩化作用、边缘混合岩化作用及断裂带混合岩化作用,并对各自的特征进行了讨论。     

豫西小秦岭地区晚古元古代地壳深熔作用及构造意义:岩石学和锆石U-Pb年代学证据

太华群出露于华北中部造山带南段,是研究华北克拉通南缘演化的关键地区,对其研究不仅有助于深入了解华北克拉通古、中元古代构造演化特征,对于华北克拉通东西陆块碰撞时间的厘定也具有重要意义.通过对豫西小秦岭地区太华群深熔作用形成的3类混合岩开展岩相学研究,发现深熔作用对岩石的改造是不均匀的,明显受到了原岩的物质成分、岩石的结构构等性质的制约.通过对岩浆锆石及深熔锆石进行LA-ICP-MS定年,确定小秦岭地区太华群混合岩原岩年龄为早古元古代,深熔作用发生时间约为1.8 Ga.研究区混合岩形成于华北克拉通东、西陆块碰撞后的伸展阶段,与区域同时期形成的大量基性岩墙群及A型花岗岩共同记录了华北克拉通在1.8 Ga前后发生的大规模伸展拉张事件.      

On the Origin of Migmatites in Yunlu, Western Guangdong

Partially migmatized rocks, banded migmatites, augen-banded migmatites and gneissic migmatites are developed successively from Xindong to Yunlu, Gaozhou, Guangdong Province in the Yunkai Caledonian orogenic belt at the border between Guangdong and Guangxi. Mass-balance calculations, statistical analysis of the textural relations and mineralogical and geochemical studies of the migmatites and the study of the metamorphlc setting of the Yunlu area indicate that the migmatites in the study area were primarily formed by anatexis without remarkable introduction of foreign components such as K, Na and Si and removal of Ca, Fe, Mg, etc.     

Petrogenesis of Migmatites in Maine, USA: Possible Source of Peraluminous Leucogranite in Plutons?

In Maine, Siluro-Devonian turbidites were metamorphosed underhigh-T–low-P facies series conditions during deformationwithin a Devonian crustal-scale shear zone system, defined bykilometer-scale straight belts of apparent flattening strainthat anastomose around lozenges of apparent constrictional strain.At upper amphibolite facies grade, metapelites are partiallymelted, the onset of which is recorded by a migmatite front.The resulting migmatites are stromatic or heterogeneous, andsmaller-volume granites form sheets or cylinders according tothe structural zone in which they occur, suggesting that migmatitesand granites record syntectonic melt flow through the deformingcrust. Common leucogranite of the nearby coeval Phillips pluton,which was emplaced syntectonically, was sourced from crustalrocks with geochemical characteristics similar to those of thehost Siluro-Devonian succession. Migmatites have melt-depletedcompositions relative to metapelites. Leucosomes are peraluminousand represent the cumulate products of fractional crystallizationand variable loss of evolved fractionated liquid. Among theheterogeneous migmatites are schlieric granites, the geochemistryof which suggests melt accumulation before fractional crystallizationand loss of the evolved liquid. Smaller-volume granites areperaluminous with a range of chemistries that reflect variableentrainment of residual plagioclase and biotite, accumulationof products of fractional crystallization and loss of most ofthe evolved liquid. Common leucogranite of the Phillips plutonand larger granites in the migmatites have compositions thatsuggest crystallization of evolved liquids derived by fractionalcrystallization of primary muscovite dehydration melts. We inferthat the leucogranite represents the crystallized fugitive liquidfrom a migmatite source similar to that exposed nearby. Watertransported through the shear zone system dissolved in meltwas exsolved at the wet solidus to cause retrogression in sub-solidusrocks and retrograde muscovite growth in migmatites. KEY WORDS: anatexis of pelite; Maine; migmatite; peraluminous granite; plutons     

Textural evolution in the transition from subsolidus annealing to melting process, Velay Dome, French Massif Central

Petrographic analysis is a useful, but underused tool to aid in distinguishing between subsolidus and anatetic-related textures in migmatites. This study focuses on assessing the relative contributions of these two processes in the development of migmatitic orthogneiss textures in the Velay Massif, French Massif Central. The results of this study show that subsolidus processes are more important in the development of migmatitic textures in the orthogneiss than anatectic leucosome development. Four textural stages are identified from the mylonitic non-anatectic orthogneiss, annealed, migmatitic orthogneiss to diatexite. The monomineralic K-feldspar and plagioclase–muscovite banding was transformed with increasing temperature to polymineralic plagioclase–quartz–muscovite and K-feldspar–quartz–muscovite layers by the wetting of feldspar boundaries during heterogeneous nucleation of quartz from a fluid phase at high surface energy triple points. A further increase of temperature led to the growth of K-feldspar probably related to production of small amounts of melt in plagioclase rich aggregates, controlled by muscovite abundance. Solid state annealing processes in conjunction with incipient anatexis resulted in the formation of apparent granitic-like textures in plagioclase dominated aggregates. By contrast, in K-feldspar dominated aggregates exclusively subsolidus processes prevail, leading to the development of coarse grained leucosome. With the onset of biotite dehydration melting the plagioclase-dominated aggregates are destroyed by the melt whereas the K-feldspar aggregates may be preserved.     

Rb-Sr and U-Pb isotope studies on migmatites from the Schwarzwald (Germany): constraints on isotopic resetting during Variscan high-temperature metamorphism

Mineral and isotope studies were undertaken on migmatites from the Schwarzwald, Moldanubian zone of the Variscan belt. The aims of the study were to date the migmatite formation and to determine the processes involved in migmatization in order to evaluate their influence on isotopic resetting. Textural evidence and the comparison of mineral compositions from leucosomes and mesosomes of two centimetre-scale migmatite profiles, respectively, suggest that migmatitic textures and mineral assemblages were formed by metamorphic segregation (deformation-enhanced mass transport) rather than by partial melting (anatexis). The results of Rb-Sr thin-slab dating on these profiles indicate that Sr isotopes were not completely reset during migmatization. No true isochron ages, but ages of approximate isotopic homogenization were obtained on the thin slabs by calculating ⁸⁷Sr/⁸⁶Sr ratios back to various stages in their evolution. The coincidence of these Rb-Sr data with U-Pb ages of monazites from migmatites and non-migmatitic gneisses shows that gneisses and migmatites were formed during the same high-temperature event in the Carboniferous (330-335 Ma). The observation that high-temperature metamorphism failed to equilibrate Sr isotopes on the centimetre-scale imposes limitations on the use of conventional whole-rock isochron techniques in dating migmatites.     

Major element composition of the Loch Coire migmatite complex,Sutherland, Scotland

Analyses of Moine metasediments and migmatites in the Loch Coire area of northern Scotland are used to delineate the fields of composition of the major rock groups. An assessment is made of the relative merits of the hypotheses of isochemical metamorphism and metasomatism in the genesis of the migmatites. Within the migmatites the Fe/Fe ratio is constant, a feature not shown by the metasediments. This is interpreted as evidence of permeation of the migmatites by metasomatising fluids. Granitic sheets which are common in the migmatites have a very variable and predominantly sodium-rich composition more easily reconciled with metasomatism than a simple magmatic or anatectic origin. Attention is also drawn to the sodic nature of the large mass of Loch Coire granite which occurs in the centre of the migmatite area and the possibility is considered that the uprise of sodic magma provided sodium-rich fluids for permeation of the rest of the complex.     

Origin of CO2-rich fluid inclusions in leucosomes from the Skagit migmatites, North Cascades, Washington, USA

CO₂–CH₄ fluid inclusions are present in anatectic layer-parallel leucosomes from graphite-bearing metasedimentary rocks in the Skagit migmatite complex, North Cascades, Washington. Petrological evidence and additional fluid inclusion observations indicate, however, that the Skagit Gneiss was infiltrated by a water-rich fluid during high-temperature metamorphism and migmatization. CO₂-rich fluid inclusions have not been observed in Skagit metasedimentary mesosomes or melanosomes, meta-igneous migmatites, or unmigmatized rocks, and are absent from subsolidus leucosomes in metasedimentary migmatites. The observation that CO₂-rich inclusions are present only in leucosomes interpreted to be anatectic based on independent mineralogical and chemical criteria suggests that their formation is related to migmatization by partial melting. Although some post-entrapment modification of fluid inclusion composition may have occurred during decompression and deformation, the generation of the CO₂-rich fluid is attributed to water-saturated partial melting of graphitic metasedimentary rocks by a reaction such as biotite + plagioclase + quartz + graphite ± Al₂SiO₅+ water-rich fluid = garnet + melt + CO₂–CH₄. The presence of CO₂-rich fluid inclusions in leucosomes may therefore be an indication that these leucosomes formed by anatexis. Based on the inferences that (1) an influx of fluid triggered partial melting, and (2) some episodes of fluid inclusion trapping are related to migmatization by anatexis, it is concluded that a free fluid was present at some time during high-temperature metamorphism. The infiltrating fluid was a water-rich fluid that may have been derived from nearby crystallizing plutons. Because partial melting took place at pressures of at least 5 kbar, abundant free fluid may have been present in the crust during orogenesis at depths of at least 15 km.     

麻山杂岩的变质-混合岩化作用和花岗质岩浆活动

在黑龙江佳木斯地块麻山杂岩中可分别有高级和中级变质作用部分,两处均可见混合岩化作用。通过混合岩浅色脉体或花岗质脉体中的长石自形晶、斜长石周围的钠长石净边、黑云母向角闪石的转化等现象表明混合岩化作用主要表现为深部岩浆的注入,而不仅是高级变质之后的近原地深熔作用所致,高级变质与相关的深熔作用所致混合岩化在区域上的分布是有限的,集中于西麻山的高级区;而注入式混合岩化是普遍的,其产出可遍布所有麻山杂岩的出露区,在麻粒岩相和角闪岩相部位均可出现。早期麻粒岩相变质与后期混合岩化作用应是相互独立的构造或热事件;注入式混合岩化引起了中级变质作用和高级区的退变质作用,注入混合岩化作用的时代约为500Ma;中级变质作用是注入混合岩化的结果,而不是混合岩化的原因。与注入混合岩相关的花岗岩虽然表现出一些S型花岗岩的特征,但根据矿物组合、地化性质的综合分析,更可能是富钾及钾长石斑晶的钙碱性花岗岩类,属于I型花岗岩,形成于挤压向引张转化的过程中。麻山杂岩的变质与混合岩化特征表明,以西伯利亚古陆为中心的南部边缘发生了与冈瓦纳陆块内泛非事件类似的构造活动,只不过这里的规模略小,在变质之后迅速发生了构造体系的转换,而形成大量花岗质岩浆活动。     

Genetic Model for the Stromatic Migmatites of the Rantasalmi-Sulkava Area, Finland

Metasediments of the Rantasalmi-Sulkava area (Finland) showprogressive regional metamorphism with migmatization. The metasedimentsare represented by various types of metapsammites (plagioclase-rich,quartz-rich, and layers of granitic compositions—somerich in microcline and others in plagioclase) and metapelites(dark and light layers). The migmatites of this area are of stromatic type. They consistof leucosomes, mesosomes, and light-coloured plagioclase-richlayers which do not fit the definition of leucosome. Melanosomes,which usually separate leucosomes and mesosomes in stromaticmigmatites, are almost absent. The leucosomes are of three types: (i) quartz-rich; (ii) cordierite-rich;and (iii) granitic. The quartz-rich leucosomes formed firstat subsolidus temperatures through recrystallization. The graniticleucosomes are considered to have developed via partial melting.The cordierite-rich leucosomes are formed—like the graniticones—at supersolidus conditions, but the role of partialmelting is not clear. The mesosomes are the metamorphic portions of the migmatiteswhich are not transformed into leucosomes. They include metapsammiticlayers and light-coloured metapelitic layers, both rich in plagioclase. Besides mineral reactions resulting in new assemblages duringregional metamorphism, the main process changing the protolithsinto migmatites is the conversion of some of the protolith layersinto leucosomes, through (as we believe) an almost isochemicalpartial melting. The migmatites of the Rantasalmi-Sulkava area differ from othermigmatites investigated by the authors in having two differentgenetic types of leucosomes: one formed via partial meltingand the other through subsolidus recrystallization as mentionedabove. The process of migmatization is described and modelledin three steps. Reprint requests to W. Johannes