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1.
Orogeny, migmatites and leucogranites: A review 总被引：13，自引：0，他引：13
Michael Brown 《Journal of Earth System Science》2001,110(4):313-336
The type ofP-T-t path and availability of fluid (H2O-rich metamorphic volatile phase or melt) are important variables in metamorphism. Collisional orogens are characterized
by clockwiseP-T evolution, which means that in the core, where temperatures exceed the wet solidus for common crustal rocks, melt may be
present throughout a significant portion of the evolution. Field observations of eroded orogens show that lower crust is migmatitic,
and geophysical observations have been interpreted to suggest the presence of melt in active orogens. A consequence of these
results is that orogenic collapse in mature orogens may be controlled by a partially-molten layer that decouples weak crust
from subducting lithosphere, and such a weak layer may enable exhumation of deeply buried crust. Migmatites provide a record
of melt segregation in partially molten crustal materials and syn-anatectic deformation under natural conditions. Grain boundary
flow and intra-and inter-grain fracture flow are the principal grain scale melt flow mechanisms. Field observations of migmatites
in ancient orogens show that leucosomes occur oriented in the metamorphic fabrics or are located in dilational sites. These
observations are interpreted to suggest that melt segregation and extraction are syntectonic processes, and that melt migration
pathways commonly relate to rock fabrics and structures. Thus, leucosomes in depleted migmatites record the remnant permeability
network, but evolution of permeability networks and amplification of anomalies are poorly understood. Deformation of partially
molten rocks is accommodated by melt-enhanced granular flow, and volumetric strain is accommodated by melt loss. Melt segregation
and extraction may be cyclic or continuous, depending on the level of applied differential stress and rate of melt pressure
buildup. During clockwiseP-T evolution, H2O is transferred from protolith to melt as rocks cross dehydration melting reactions, and H2O may be evolved above the solidus at lowP by crossing supra-solidus decompression-dehydration reactions if micas are still present in the depleted protolith. H2O dissolved in melt is transported through the crust to be exsolved on crystallization. This recycled H2O may promote wet melting at supra-solidus conditions and retrogression at subsolidus conditions. The common growth of ‘late’
muscovite over sillimanite in migmatite may be the result of this process, and influx of exogenous H2O may not be necessary. However, in general, metasomatism in the evolution of the crust remains a contentious issue. Processes
in the lower-most crust may be inferred from studies of xenolith suites brought to the surface in lavas. Based on geochemical
data, we can use statistical methods and modeling to evaluate whether migmatites are sources or feeder zones for granites,
or simply segregated melt that was stagnant in residue, and to compare xenoliths of inferred lower crust with exposed deep
crust. Upper-crustal granites are a necessary complement to melt-depleted granulites common in the lower crust, but the role
of mafic magma in crustal melting remains uncertain. Plutons occur at various depths above and below the brittle-to-viscous
transition in the crust and have a variety of 3-D shapes that may vary systematically with depth. The switch from ascent to
emplacement may be caused by amplification of instabilities within (permeability, magma flow rate) or surrounding (strength
or state of stress) the ascent column, or by the ascending magma intersecting some discontinuity in the crust that enables
horizontal magma emplacement followed by thickening during pluton inflation. Feedback relations between rates of pluton filling,
magma ascent and melt extraction maintain compatibility among these processes. 相似文献
2.
山东雁翎关地区雌山混合花岗岩地球化学特征及其成因 总被引：1，自引：0，他引：1
一、地质背景雁翎关地区位于山东泰安东南四十余公里处(图1)。本区出露泰山群变质岩系,受到中压相系的角闪岩相区域变质作用,变质时代大于24亿年。雌山混合花岗岩带分布于东石棚—马家雌山西南和磨石山香水河东北的狭长地带(图1)。岩体走向北西—南东、与区域构造线方向一致,围岩为泰山群的山草峪组黑云变粒岩。构成 相似文献
3.
萨勒巴斯推覆体中发育一套深层次变形构造组合和倒转递增变质带,其中糜棱岩、混合岩、混合花岗岩的成因关系对研究挤压造山背景下,地壳深层次变形作用和成岩作用具有特殊的意义。研究表明:在大型滑脱推覆系统中,存在糜棱岩—混合岩—混合花岗岩成岩系列;成岩过程为:韧性变形—剪切加热—部分熔融;控制成岩过程的主导因素为构造动力条件。这一成岩过程能导致稀土元素发生分异,出现重稀土元素有规律地亏损,变质作用pTt轨迹显示造山过程中逆冲推覆作用导致的地壳叠置加厚和剪切加热的典型热演化模式,变形、变质高峰期后,高角度逆冲作用导致变形岩石经历减压、降温的退变质作用,形成由南向北的倒转递增变质带。 相似文献
4.
Crustal reworking in a shear zone: transformation of metagranite to migmatite 总被引：1，自引：0，他引：1 
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下载免费PDF全文 This study uses field, microstructural and geochemical data to investigate the processes contributing to the petrological diversity that arises when granitic continental crust is reworked. The Kinawa migmatite formed when Archean TTG crust in the São Francisco Craton, Brazil was reworked by partial melting at ~730 °C and 5–6 kbar in a regional‐scale shear zone. As a result, a relatively uniform leucogranodiorite protolith produced compositionally and microstructurally diverse diatexites and leucosomes. All outcrops of migmatite display either a magmatic foliation, flow banding or transposed leucosomes and indicate strong, melt‐present shearing. There are three types of diatexite. Grey diatexites are interpreted to be residuum, although melt segregation was incomplete in some samples. Biotite stable, H2O‐fluxed melting is inferred via the reaction Pl + Kfs + Qz + H2O = melt and geochemical modelling indicates 0.35–0.40 partial melting. Schlieren diatexites are extremely heterogeneous; residuum‐rich domains alternate with leucocratic quartzofeldspathic domains. Homogeneous diatexites have the highest SiO2 and K2O contents and are coarse‐grained, leucocratic rocks. Homogeneous diatexites, quartzofeldspathic domains from the schlieren diatexites and the leucosomes contain both plagioclase‐dominated and K‐feldspar‐dominated feldspar framework microstructures and hence were melt‐derived rocks. Both types of feldspar frameworks show evidence of tectonic compaction. Modelling the crystallization of an initial anatectic melt shows plagioclase appears first; K‐feldspar appears after ~40% crystallization. In the active shear zone setting, shear‐enhanced compaction provided an essentially continuous driving force for segregation. Thus, Kinawa migmatites with plagioclase frameworks are interpreted to have formed by shear‐enhanced compaction early in the crystallization of anatectic melt, whereas those with K‐feldspar frameworks formed later from the expelled fractionated melt. Trace element abundances in some biotite and plagioclase from the fractionated melt‐derived rocks indicate that these entrained minerals were derived from the wall rocks. Results from the Kinawa migmatites indicate that the key factor in generating petrological diversity during crustal reworking is that shear‐enhanced compaction drove melt segregation throughout the period that melt was present in the rocks. Segregation of melt during melting produced residuum and anatectic melt and their mixtures, whereas segregation during crystallization resulted in crystal fractionation and generated diverse plagioclase‐rich rocks and fractionated melts. 相似文献
5.
变泥质岩递进部分熔融作用的构造物理学效应 总被引：1，自引：0，他引：1
在南内华达岩基中,晚中生代花岗岩的侵位导致表壳岩广泛的变质及部分熔融,形成混合岩杂岩体。对伊萨贝拉湖南羊圈混合岩杂岩体构造的野外观测和应变测量表明：①变泥质混合岩和鹅卵石砾岩记录了类似强度的应变;②变泥质岩发生了递进部分熔融,表现为离羊圈花岗闪长岩岩体的距离越远,部分熔融程度越低;③随部分熔融程度的变化,变泥质岩的应变承载构造也逐渐从混合岩带的弱相承载构造（IWL）往强相承载构造（LBF）过渡;④在同岩浆构造作用中,浅色体的流变学性质与鹅卵石砾岩中泥质组分相当,为应变的主要承载体。该结果表明：在高级变质岩区中,部分熔融程度是否足够高及熔体能否形成互相链接的网络,是高级变质岩的流变学强度发生突降、深部岩石发生侧向流动的前提。 相似文献
6.
Microstructural, electron backscatter diffraction (EBSD), and misorientation analyses of a migmatitic granulite-facies orthogneiss from the exhumed lower crust of a Cretaceous continental arc in Fiordland, New Zealand show how deformation was accommodated during and after episodes of melt infiltration and high-grade metamorphism. Microstructures in garnet, omphacite, plagioclase, and K-feldspar suggest that an early stage of deformation was achieved by dislocation creep of omphacite and plagioclase, with subsequent deformation becoming partitioned into plagioclase. Continued deformation after melt infiltration resulted in strain localization in the leucosome of the migmatite, where a change of plagioclase deformation mechanism promoted the onset of grain boundary sliding, most likely accommodated by diffusion creep, in fine recrystallized plagioclase grains. Our results suggest three distinctive transitions in the rheology of the lower crust of this continental arc, where initial weakening was primarily achieved by deformation of both omphacite and plagioclase. Subsequent strain localization in plagioclase of the leucosome indicates that the zones of former melt are weaker than the restite, and that changes in deformation mechanisms within plagioclase, and an evolution of its strength, primarily control the rheology of the lower crust during and after episodes of melting and magma addition. 相似文献
7.
D. W. MOGK 《Journal of Metamorphic Geology》1992,10(3):427-438
A high-grade Archaean gneiss terrane in the northern Gallatin Range, south-western Montana, USA, contains a trondhjemite–tonalite gneiss (TTG) sequence that was migmatized during pervasive ductile shearing. Metamorphism of these rocks is in the upper amphibolite to granulite facies at temperatures of 680–735°C, pressures in excess of 8 kbar, and a 'clockwise' P–T–t path is inferred. Ductile shearing occurred in metre-scale anastomosing bands of high strain throughout the area. The TTGs have been extensively migmatized via vapour-present melt reactions involving the incongruent melting of biotite-bearing TTG to produce hornblende and granitic melt. The granitic melt is produced in narrow envelopes adjacent to ductile shear zones in response to infiltration of water-rich solutions. Melt migration occurred on a local scale, and extraction of melt from the system left behind a plagioclase–hornblende residuum with minor interstitial microcline. Ductile shearing and migmatization in the TTG operated in a positive feedback mechanism; the entire volume of gneiss was chemically and mechanically reworked through the cyclical infiltration of aqueous solutions, vapour-present melting and melt-enhanced deformation. The proposed melt reaction may be an important crustal differentiation process considering that (1) many collisional orogens do not attain temperatures high enough to permit vapour-absent melting, (2) pervasive networks of ductile shear zones at mid-crustal levels may serve as channels for fluid ingress and melt extraction, and (3) the large volumes of TTGs in Archaean and Phanerozoic orogens may constitute a significant source reservoir for certain types of high-level granites. 相似文献
8.
9.
We compare melting of potassic alteration zones in metamorphosed gold deposits with that of unaltered rocks of the same protolith
to examine their relative contributions to crust-derived magmas and to investigate the implications for ore genesis. Potassic
hydrothermal alteration, at the crustal levels where orogenic gold deposits form, stabilizes a higher proportion of muscovite
and biotite than is possible in unaltered rocks at high metamorphic grades. Because these micas contain water, they control
the melt fraction generated through dehydration melting in that a greater proportion of micas permits more extensive melting.
Orogenic gold deposits, in which mineralization is typically encapsulated by potassic alteration, form at deep-enough crustal
levels to survive repeated tectonic activity, which can lead to their being metamorphosed. In the vicinity of this metamorphosed
gold mineralization, the greatest proportion of felsic melt is generated in the more metal- and sulfur-rich rocks because
of the associated potassic alteration. Ore minerals dissolve and are physically incorporated into the resulting felsic melt,
which thereby becomes metal- and sulfur-enriched. Since melt fraction is the dominant control on strain partitioning and melt
mobilization, increased melting in K-altered mineralized rocks implies that these sites will be the first to experience melt
escape and will continue to be the focus of melt escape during ongoing metamorphism. This strain partitioning promotes shear
zone development, and once shearing is localized to K-altered mineralized domains, it may attract external magma, allowing
extension and linking with nearby active shear zones. In this way, mineralized zones may connect to a regional network of
magma transfer, allowing metal enrichment of migrating magmas. Terrains that underwent widespread K alteration associated
with mid-crustal gold enrichment are likely, when metamorphosed, to produce significant volumes of reduced, relatively metal-
and sulfur-enriched felsic magma. The ages and relative tectonic preservation potential of different K alteration-associated
ore types implies that Au, Ag, As, Sb, Bi, Te, and W may be recycled within the crust through this mechanism, whereas Cu and
Mo are unlikely to be recycled and require mantle sourcing to form new intrusion-related ores. Silicate melt derived from
preexisting zones of gold enrichment in the lower crust may contribute significantly to the metal budget of intrusion-related
gold systems, and possibly some gold-rich porphyry deposits. 相似文献
10.
U. Altenberger G. Prosser A. Grande C. Günter A. Langone 《Contributions to Mineralogy and Petrology》2013,166(4):975-994
Pseudotachylyte veins frequently associated with mylonites and ultramylonites occur within migmatitic paragneisses, metamonzodiorites, as well as felsic and mafic granulites at the base of the section of the Hercynian lower crust exposed in Calabria (Southern Italy). The crustal section is tectonically superposed on lower grade units. Ultramylonites and pseudotachylytes are particularly well developed in migmatitic paragneisses, whereas sparse fault-related pseudotachylytes and thin mylonite/ultramylonite bands occur in granulite-facies rocks. The presence of sillimanite and clinopyroxene in ultramylonites and mylonites indicates that relatively high-temperature conditions preceded the formation of pseudotachylytes. We have analysed pseudotachylytes from different rock types to ascertain their deep crustal origin and to better understand the relationships between brittle and ductile processes during deformation of the deeper crust. Different protoliths were selected to test how lithology controls pseudotachylyte composition and textures. In migmatites and felsic granulites, euhedral or cauliflower-shaped garnets directly crystallized from pseudotachylyte melts of near andesitic composition. This indicates that pseudotachylytes originated at deep crustal conditions (>0.75 GPa). In mafic protoliths, quenched needle-to-feather-shaped high-alumina orthopyroxene occurs in contact with newly crystallized plagioclase. The pyroxene crystallizes in garnet-free and garnet-bearing veins. The simultaneous growth of orthopyroxene and plagioclase as well as almandine, suggests lower crustal origin, with pressures in excess of 0.85 GPa. The existence of melts of different composition in the same vein indicates the stepwise, non-equilibrium conditions of frictional melting. Melt formed and intruded into pre-existing anisotropies. In mafic granulites, brittle faulting is localized in a previously formed thin high-temperature mylonite bands. migmatitic gneisses are deformed into ultramylonite domains characterized by s-c fabric. Small grain size and fluids lowered the effective stress on the c planes favouring a seismic event and the consequent melt generation. Microstructures and ductile deformation of pseudotachylytes suggest continuous ductile flow punctuated by episodes of high-strain rate, leading to seismic events and melting. 相似文献
11.
The formation of SiO<Subscript>2</Subscript>-rich melts within the deep oceanic crust by hydrous partial melting of gabbros 总被引：5，自引：0，他引：5
Juergen Koepke Jasper Berndt Sandrin T. Feig Francois Holtz 《Contributions to Mineralogy and Petrology》2007,153(1):67-84
Small amounts of felsic, evolved plutonic rocks, often called oceanic plagiogranites, always occur as veins or small stocks within the gabbroic section of the oceanic crust. Four major models are under debate to explain the formation of these rocks: (1) late-stage differentiation of a parental MORB melt, (2) partial melting of gabbroic rocks, (3) immiscibility in an evolved tholeiitic liquid, and (4) assimilation and partial melting of previously altered dikes. Recent experimental data in hydrous MORB-type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust. Experiments show that TiO2 is a key parameter for the discrimination between different processes: TiO2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low TiO2 contents of the protolith, due to the depleted nature of typical cumulate gabbros formed in the oceanic crust. On the other hand, TiO2 is relatively high in those melts generated by MORB differentiation or liquid immiscibility. Since the TiO2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple MORB differentiation or immiscibility, these rocks may be regarded as products of anatexis. This may indicate that partial melting processes triggered by water-rich fluids are more common in the deep oceanic crust than believed up to now. At slow-spreading ridges, seawater may be transported via high-temperature shear zones deeply into the crust and thus made available for melting processes. 相似文献
12.
The Papoose Flat pluton in the White-Inyo Range, California, is one of the best examples of forcefully emplaced plutons within an arc crust, having internal fabrics and a contact aureole that deformed in a ductile manner. A 2-D numerical model for the thermo-rheological evolution of the pluton–wall rock system is proposed. We explore how the frequency of magma input, from instantaneous, episodic to continuous, affects magma chamber crystallinity and rheology of both the pluton and its contact aureole. We model pluton growth in the depth range of 10–13 km, which is at the brittle–ductile transition of the crust, and in the 7–4 km depth range, where the host rocks are initially brittle. For incremental growth (episodic and continuous), the pluton begins as a sill. With subsequent injections to the bottom, the pluton grows into a laccolith. Results of mid-crustal models show that the ductile region above the Papoose Flat pluton is related to thermal weakening. The ductile region during incremental growth is 100–150 m thick, matching the observed thickness. It is ten times thinner than in the instantaneous growth model. In episodic and continuous models, the upper part of the pluton is fully or quasi-crystalline throughout growth. During continuous growth, it is likely to remain ductile with potential for the development of solid-state fabrics. During episodic growth, strain rates within the pluton during each injection may become sufficiently high to cause embrittlement of magma. In no case a ductile aureole develops above the pluton at the upper-crustal level, but may develop below the pluton, which serves as thermal insulator. Thus, the pluton’s floor may sag. During incremental growth, most of the pluton is below the solidus and brittle. The results suggest that large volcanic eruptions are unlikely to occur by slow pressurization of magma chambers; instead they require rapid injections of large melt volumes. 相似文献
13.
综合分析录井、测井、地震、岩心与分析化验等资料,对大民屯凹陷中央构造带太古宇变质岩储层岩性类型、空间分布、岩性对储层的控制作用进行了研究。研究认为：本区太古宇变质岩岩性分为三大类18种,主要有混合花岗岩、混合片麻岩、混合岩、角闪斜长片麻岩、浅粒岩、角闪岩、变粒岩,夹丰富不均一的基性脉岩和碎裂岩。有效储层岩性为碎裂化混合花岗岩与碎裂化变粒岩、浅粒岩、片麻岩;变质岩在垂向上（由下至上）分为多个岩性段,从混合花岗岩至浅粒岩与风化壳,垂向上可进行类似于沉积岩层序的地层对比,岩性在平面上也呈现分区性。变质岩体在岩性上是不均一的、非均质的块体。矿物种类和岩石类型对研究区变质岩裂缝型储层起重要作用,碎裂化变粒岩、浅粒岩、混合花岗岩与片麻岩可作为优质储层。 相似文献
14.
The role of partial melting and extensional strain rates in the development of metamorphic core complexes 总被引：1，自引：0，他引：1
Orogenic collapse involves extension and thinning of thick and hot (partially molten) crust, leading to the formation of metamorphic core complexes (MCC) that are commonly cored by migmatite domes. Two-dimensional thermo-mechanical Ellipsis models evaluate the parameters that likely control the formation and evolution of MCC: the nature and geometry of the heterogeneity that localizes MCC, the presence/absence of a partially molten layer in the lower crust, and the rate of extension. When the localizing heterogeneity is a normal fault in the upper crust, the migmatite core remains in the footwall of the fault, resulting in an asymmetric MCC; if the localizing heterogeneity is point like region within the upper crust, the MCC remains symmetric throughout its development. Therefore, asymmetrically located migmatite domes likely reflect the dip of the original normal fault system that generated the MCC. Modeling of a severe viscosity drop owing to the presence of a partially molten layer, compared to a crust with no melt, demonstrates that the presence of melt slightly enhances upward advection of material and heat. Our experiments show that, when associated with boundary-driven extension, far-field horizontal extension provides space for the domes. Therefore, the buoyancy of migmatite cores contributes little to the outer envelope of metamorphic core complexes, although it may play a significant role in the internal dynamics of the partially molten layer. The presence of melt also favors heterogeneous bulk pure shear of the dome as opposed to the bulk simple shear, which dominates in melt-absent experiments. Melt presence affects the shape of P-T-t paths only slightly for material located near the top of the low-viscosity layer but leads to more complex flow paths for material inside the layer. The effect of extension rate is significant: at high extension rate (cm yr− 1 in the core complex region), partially molten crust crystallizes and cools along a high geothermal gradient (35 to 65 °C km− 1); material remains partially molten in the dome during ascent. At low strain rate (mm yr− 1 in the core complex region), the partially molten crust crystallizes at high pressure; this material is subsequently deformed in the solid-state along a cooler geothermal gradient (20 to 35 °C km− 1) during ascent. Therefore, the models predict distinct crystallization versus exhumation histories of migmatite cores as a function of extensional strain rates. The Shuswap metamorphic core complex (British Columbia, Canada) exemplifies a metamorphic core complex in which an asymmetric, detachment-controlled migmatite dome records rapid exhumation and cooling likely related to faster rates of extension. In contrast the Ruby Mountain-East Humboldt Ranges (Nevada, U.S.A.) exhibits characteristics associated with slower metamorphic core complexes. 相似文献
15.
连山关地区位于华北克拉通北缘铀成矿省辽东铀成矿带,已知铀矿床（点）均发育在韧性剪切带附近。为了解韧性剪切带运动学、几何学构造变形机制及与铀矿的关系,本文以连山关岩体周缘韧性剪切带为研究对象,通过野外宏观调查和室内微观研究相结合的研究方法,探讨构造变形期次、韧性剪切带形成机理及其对铀成矿的控制作用。研究表明：连山关岩体周缘发育的韧性剪切带与近南北向挤压构造变形有关,其右行韧性剪切带应变类型为压扁应变,属于一般压缩-平面应变范围,Flinn指数K值在0.19~0.69之间,岩石类型属于S/SL型构造岩。研究区内铀矿体均为隐伏盲矿体,主要赋存于沿着连山关岩体和辽河群接触带右行剪切作用形成的背斜褶皱核部,和北东东向断裂关系密切。综合分析认为,连山关岩体南缘北西向韧性剪切带为一级控矿构造,是区内铀矿热液运移的通道,而剪切带边部的晚期北东东向断裂则是铀矿储存空间;铀源可能来自于太古宙古风化壳,并在大型韧性剪切活动（提供热液运移通道）和基性脉岩侵入（提供热源和还原剂）等综合因素作用下运移、富集成矿。 相似文献
16.
Insights into the complexity of crustal differentiation: K2O‐poor leucosomes within metasedimentary migmatites from the Southern Marginal Zone of the Limpopo Belt,South Africa 下载免费PDF全文
下载免费PDF全文 G. Nicoli G. Stevens J‐F. Moyen A. Vezinet M. Mayne 《Journal of Metamorphic Geology》2017,35(9):999-1022
Differentiation of the continental crust is the result of complex interactions between a large number of processes, which govern partial melting of the deep crust, magma formation and segregation, and magma ascent to significantly higher crustal levels. The anatectic metasedimentary rocks exposed in the Southern Marginal Zone of the Limpopo Belt represent an unusually well‐exposed natural laboratory where the portion of these processes that operate in the deep crust can be directly investigated in the field. The formation of these migmatites occurred via absent incongruent melting reactions involving biotite, which produced cm‐ to m‐scale, K2O‐poor garnet‐bearing stromatic leucosomes, with high Ca/Na ratios relative to their source rocks. Field investigation combined with geochemical analyses, and phase equilibrium modelling designed to investigate some aspects of disequilibrium partial melting show that the outcrop features and compositions of the leucosomes suggest several steps in their evolution: (1) Melting of a portion of the source, with restricted plagioclase availability due to kinetic controls, to produce a magma (melt + entrained peritectic minerals in variable proportions relative to melt); (2) Segregation of the magma at near peak metamorphic conditions into melt accumulation sites (MAS), also known as future leucosome; (3a) Re‐equilibration of the magma with a portion of the bounding mafic residuum via chemical diffusion (H2O, K2O), which triggers the co‐precipitation of quartz and plagioclase in the MAS; (3b) Extraction of melt‐dominated magma to higher crustal levels, leaving peritectic minerals entrained from the site of the melting reaction, and the minerals precipitated in the MASs to form the leucosome in the source. The key mechanism controlling this behaviour is the kinetically induced restriction of the amount of plagioclase available to the melting reaction. This results in elevated melt H2O and K2O and chemical potential gradient for these components across the leucosome/mafic residuum contact. The combination of all of these processes accurately explains the composition of the K2O‐poor leucosomes. These findings have important implications for our understanding of melt segregation in the lower crust and minimum melt residency time which, according to the chemical modelling, is <5 years. We demonstrate that in some migmatitic granulites, the leucosomes constitute a type of felsic refractory residuum, rather than evidence of failed magma extraction. This provides a new insight into the ways that source heterogeneity may control anatexis. 相似文献
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Partial melting of ultrahigh-pressure metamorphic rocks at convergent continental margins: Evidences,melt compositions and physical effects 总被引：2，自引：0，他引：2
Ultrahigh-pressure(UHP) metamorphic rocks are distinctive products of crustal deep subduction,and are mainly exposed in continental subduction-collision terranes. UHP slices of continental crust are usually involved in multistage exhumation and partial melting, which has obvious influence on the rheological features of the rocks, and thus significantly affect the dynamic behavior of subducted slices. Moreover,partial melting of UHP rocks have significant influence on element mobility and related isotope behavior within continental subduction zones, which is in turn crucial to chemical differentiation of the continental crust and to crust-mantle interaction.Partial melting can occur before, during or after the peak metamorphism of UHP rocks. Post-peak decompression melting has been better constrained by remelting experiments; however, because of multiple stages of decompression, retrogression and deformation, evidence of former melts in UHP rocks is often erased. Field evidence is among the most reliable criteria to infer partial melting. Glass and nanogranitoid inclusions are generally considered conclusive petrographic evidence. The residual assemblages after melt extraction are also significant to indicate partial melting in some cases. Besides field and petrographic evidence, bulk-rock and zircon trace-element geochemical features are also effective tools for recognizing partial melting of UHP rocks. Phase equilibrium modeling is an important petrological tool that is becoming more and more popular in P-T estimation of the evolution of metamorphic rocks; by taking into account the activity model of silicate melt, it can predict when partial melting occurred if the P-T path of a given rock is provided.UHP silicate melt is commonly leucogranitic and peraluminous in composition with high SiO_2,low MgO, FeO, MnO, TiO_2 and CaO, and variable K_2 O and Na_2 O contents. Mineralogy of nanogranites found in UHP rocks mainly consists of plagioclase + K-feldspar + quartz, plagioclase being commonly albite-rich.Trace element pattern of the melt is characterized by significant enrichment of large ion lithophile elements(LILE), depletion of heavy rare earth elements(HREE) and high field strength elements(HFSE),indicating garnet and rutile stability in the residual assemblage. In eclogites, significant Mg-isotope fractionation occurs between garnet and phengite; therefore, Mg isotopes may become an effective indicator for partial melting of eclogites. 相似文献
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Syn-deformational migmatites and magmatic-arc metamorphism in the Xolapa Complex, southern Mexico 总被引：1，自引：0，他引：1
The Xolapa Complex (XC) is the largest plutonic and metamorphic mid‐crustal basement unit in Mexico and represents an ancient continental magmatic‐arc. A complete range from metatexite to diatexite migmatitic structures has been produced during a single high‐grade metamorphic event. However, structural relics reveal the existence of early Cpx + Pl + Qtz ± Opx and Grt + Opx + Pl + Qtz ± Cpx pre‐migmatitic metamorphic assemblages. Field relationships and microstructural observations allow us to constrain five pre‐, syn‐ and post‐migmatitic deformational phases. It is argued that migmatitic structures and minor anatectic granites were developed during ductile recumbent folding and shear structures related to the D2–D3 phases. Late post‐migmatitic ductile‐brittle deformation is evidenced by the development of NNE trending transpressional thrusting (D4), and E–W left‐lateral mylonitic shear zones (D5). Biotite‐breakdown melting in felsic rocks and amphibole‐breakdown melting in mafic rocks, as well as geothermobarometric results, indicate that metamorphism took place at temperatures from 830 to 900 °C and pressures ranging from ≥6.3 to 9.5 kbar. Late migmatitic assemblages equilibrated in the highest temperature range along a clockwise P–T path. The relationships between the large diversity of migmatitic structures and the progressive production of melt suggest that feedback relations prevailed as a time‐marker during a contractional regime. Deformation, metamorphism, and plutonism of the XC show that this terrane evolved as a north‐east‐verging thrust system with synkinematic metamorphism and partial melting, during the Late Cretaceous – Palaeogene. The tectonothermal history of XC is analogous to a Cordilleran metamorphic magmatic‐arc formed in an accretionary tectonic framework. This new model provides constraints on the exhumation mechanism and thermal evolution of southern Mexico. 相似文献
19.
大青山高级变质岩不仅记录华北克拉通早期大陆形成演化历史,也保留了中下部地壳岩石流变信息,它们经历了下部地壳构造层次高角闪岩相-麻粒岩相条件变质变形、深熔作用改造,形成了复杂构造样式和构造要素组合.韧性剪切带是高级变质岩中主要构造形迹,控制着早前寒武纪高级变质岩主体构造格架.依据野外地质产状、变形特征与构造要素叠加改造关系,韧性剪切带划分为早期近水平顺层伸展型和晚期陡倾韧性剪切带.近水平顺层伸展韧性剪切带呈残留状保留在后期变形改造较弱部位上,主要沿着不同地质单元或者岩性层界面上发育,是在伸展变形体制下形成的.晚期陡倾韧性剪切带呈近东西方向展布,规模较大,叠加和改造早期构造形迹,形成于晚期造山挤压构造环境中,以左行滑移为主.这两种韧性剪切带都形成于地壳中深部构造层次高角闪岩相-麻粒岩相条件下,变形机制主要为熔体增强颗粒边界扩散和颗粒流动,使岩石发生大规模的塑性流动.在宏观上形成了不对称流动组构、条纹条带构造、熔融线理、层内流动褶皱等构造形迹,在微观上矿物晶体没有发生明显塑性变形,均匀消光,晶体为三边平衡结构,与静态变质结构相似,形成了地壳深部构造层次上变质构造岩-构造片麻岩. 相似文献
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The role of regional extension on the rise and emplacement of granites in the crust is still debated. Pluton ascent and emplacement widely occurred in Tuscany (Italy) since late Miocene during the post-orogenic collapse of the inner Apennines, and are presently occurring in the geothermal areas of Amiata and Larderello. Tuscany offers a preferred test site to study the role of regional extension on pluton ascent and emplacement at different crustal levels. Ductile extension enhanced the segregation and ascent of granitic melts in the lower crust, controlling pluton emplacement in correspondence with the brittle–ductile transition. In the brittle crust, magma ascent occurred through subvertical faults and fractures compatible with the regional extension direction; pluton emplacement mainly occurred by means of roof lifting. The case of Tuscany suggests that the extensional structures enhance melt segregation and ascent in the ductile crust, but are not efficient alone to provide a pathway for the ascent of granitic magmas in the brittle-extending crust. The estimated magmatic strain rates due to pluton emplacement in the geothermal areas are much larger than the regional tectonic strain rates. This suggests that regional tectonics did not control magma emplacement in the brittle crust and explains why nontectonic processes (roof lifting) accommodated the space required for pluton emplacement. 相似文献