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1.
High‐strain zones are potential pathways of melt migration through the crust. However, the identification of melt‐present high‐strain deformation is commonly limited to cases where the interpreted volume of melt “frozen” within the high‐strain zone is high (>10%). In this contribution, we examine high‐strain zones in the Pembroke Granulite, an otherwise low‐strain outcrop of volcanic arc lower crust exposed in Fiordland, New Zealand. These high‐strain zones display compositional layering, flaser‐shaped mineral grains, and closely spaced foliation planes indicative of high‐strain deformation. Asymmetric leucosome surrounding peritectic garnet grains suggest deformation was synchronous with minor amounts of in situ partial melting. High‐strain zones lack typical mylonite microstructures and instead display typical equilibrium microstructures, such as straight grain boundaries, 120° triple junctions, and subhedral grain shapes. We identify five key microstructures indicative of the former presence of melt within the high‐strain zones: (a) small dihedral angles of interstitial phases; (b) elongate interstitial grains; (c) small aggregates of quartz grains with xenomorphic plagioclase grains connected in three dimensions; (d) fine‐grained, K‐feldspar bearing, multiphase aggregates with or without augite rims; and (e) mm‐ to cm‐scale felsic dykelets. Preservation of key microstructures indicates that deformation ceased as conditions crossed the solidus, breaking the positive feedback loop between deformation and the presence of melt. We propose that microstructures indicative of the former presence of melt, such as the five identified above, may be used as a tool for recognising rocks formed during melt‐present high‐strain deformation where low (<5%) volumes of leucosome are “frozen” within the high‐strain zone.  相似文献   

2.
Quantitative analysis of the structural evolution of jadeite‐quartzite, a rare ultra‐high pressure (UHP) rock type from the Dabie Mountains of eastern China, sheds light on the formation and evolution of UHP orogenic belts worldwide. Geological mapping of the Shuanghe area, where jadeite‐quartzites crop out, was carried out to determine the spatial relationships between different UHP rocks within this orogen. The deformation mechanisms of jadeite‐quartzite, geodynamical parameters (stress, strain, strain rate), and microstructure including lattice preferred orientation (LPO) were determined from six jadeite‐quartzite samples from the Shuanghe area. LPOs of clinopyroxene (jadeite and omphacite), garnet, rutile and quartz from these jadeite‐quartzite samples are compared with those of three eclogites preserving different degrees of deformation from the Shuanghe area. Microstructural LPOs of jadeite, omphacite, garnet, rutile and quartz were determined using electron backscattered diffraction (EBSD) analysis. Quartz fabrics were largely recrystallized during late, low‐grade stages of deformation, whereas garnet shows no strong LPO patterns. Rutile fabrics show a weak LS fabric along [001]. Jadeite and omphacite show the strongest eclogite facies LPO patterns, suggesting that they may provide important information about mantle deformation patterns and control the rheology of deeply subducted continental crust. Microstructural data show that the jadeite LPO patterns are similar to those of omphacite and vary between L‐ and S‐types, which correlate with prolate and oblate grain shape fabrics (SPO); quartz LPOs are monoclinic. Microstructural analysis using TEM shows that the dominant slip systems of jadeite in one sample are (100)[001], (110)[001] and (1 1 0)1/2[110], while in another sample, no dislocations are observed. Abundant dislocations in quartz were accommodated by the dominant slip system (0001)[110], indicating basal glide and represents regional shearing during the exhumation process. This suggests that dislocation creep is the dominant fundamental deformation mechanism in jadeite under UHP conditions. The protoliths of jadeite‐quartzite, metasedimentary rocks from the northern passive continental margin of the Yangtze craton, experienced the same deep subduction and were deformed under similar rheological conditions as other UHP eclogite, marble and paragneiss. Experimental UHP deformation of quartzo‐feldspathic gneiss with a chemical composition similar to the bulk continental crust has shown that the formation of a jadeite–stishovite rock is associated with a density increase of the host rock similar to the eclogite conversion from basaltic protoliths. The resulting rock can be denser than the surrounding mantle pyrolite up to depths of 660 km (24 GPa). Thus, processes of deep continental subduction may be better‐understood through understanding the rheology and mechanical behaviour of jadeite. Jadeite‐quartzites such as those from the Shuanghe may be exhumed remnants of deeply‐subducted slabs of continental crust, other parts of which subducted past the ‘depth of no return’, and remain in the deep mantle.  相似文献   

3.
Localized shear zones along low-angle normal faults have been identified in regions of extension at the brittle-ductile transition of the continental crust. The possibility of the strain localizing at a depth of 10 km is interpreted here as a consequence of an increase in the equivalent shear stress applied to the flow of the lower crust. This enhancement of the flow stress is seen as a prerequisite for the triggering of brittle deformation mechanisms leading to strain localization. The lower crust rheology used to examine this stress increase is strain-rate, temperature and grain-size dependent, due to the coupling of dislocation and diffusion creep. The model structure proposed consists of a top layer, the upper crust, gliding rigidly above a bottom layer, the lower crust, which deforms in simple shear. During a short time interval (1400 years), the equivalent shear stress is found to increase by a factor of up to 3 (67 MPa for anorthite and 17 MPa for quartz). For anorthite, this stress could explain the activation of a Mohr-Coulomb failure with a friction coefficient of 0.2, which is reasonable at the depth of 10 km. Dislocation creep is activated during a rapid change in the prescribed velocity, whereas diffusion creep dominates if the velocity is held constant, highlighting the importance of grain-size sensitivity for lower crustal rheology.  相似文献   

4.
岩石扩散蠕变及其地质意义   总被引：3，自引：1，他引：3  
总结了近十年来岩石扩散蠕变显微构造鉴别特征，研究方法的最新进展，矿物颗粒大小和流体（包括熔体和水）等是影响扩散蠕变的重要因素。扩散蠕变与位错蠕变和超塑性变形有密切关系，它的研究对大陆裂谷化过程，大陆碰撞带中岩石圈地幔强度弱化，中下地壳韧性剪切应变局限化以及与相变有关的矿物粒度细小化作用的分布有重要应用意义。  相似文献   

5.
大陆中部地壳应变局部化与应变弱化   总被引：1，自引：1，他引：0  
刘俊来 《岩石学报》2017,33(6):1653-1666
大陆岩石圈流变学研究是构造地质学学科发展的必然,也是发展板块构造理论、探索大陆板块内部变形与动力学演化的核心问题。大陆中部地壳是大陆岩石圈中一个具有特殊性的圈层,其主要成分以花岗质岩石为代表,位于岩石脆-韧性转变域。在中部地壳层次上,岩石既具有脆性变形特点,又具有韧性变形属性,而且常常表现出多种流变强度。研究成果显示,中部地壳岩石流变具有许多特殊性:1)应变局部化是中部地壳流动最为典型表现形式;2)存在大陆地壳多震层:多震与强震,显示出中部地壳既弱又强的流变学属性;3)液/岩反应强烈,流体相直接影响着岩石的流变性;4)在许多地区存在有地球物理异常体(低速高导体)。大陆中部地壳应变局部化是板块相互作用过程中地壳层次上应变积累与集中的重要表现。在宏观尺度、中小型尺度和微观尺度上都有着重要的构造特点。地壳岩石的应变弱化,是诱发应变局部化的主要机制。多种形式的水致弱化(包括液压致裂、反应弱化、水解弱化等)与结构弱化(包括细粒化、晶格取向、成分分带性等)对于应变局部化具有重要的贡献。大陆地壳岩石流变学、中部地壳弱化与应变局部化研究,是未来岩石圈流变学研究的重要方向。  相似文献   

6.
刘同君  李伟民  刘永江  金巍  邵弋伦 《岩石学报》2020,36(11):3309-3330
内蒙古大青山地区是典型的早前寒武纪造山带根部岩石出露区，大量下地壳高级变质-深熔岩石中保留了丰富的流变构造样式，是开展部分熔融岩石流变行为研究的天然实验室。本文对内蒙古大青山地区雪海沟和大庙-忽鸡沟构造带内变形岩石开展了宏观、微观构造解析、EBSD组构以及锆石U-Pb年代学研究，并结合前人研究成果，考查了不同构造层次、不同熔融程度岩石的流变特征及其与造山过程的耦合关系。研究结果表明，雪海沟至大庙-忽鸡沟地区体现了造山带地壳结构层次由深至浅的特点。深部构造层次（雪海沟）高级深熔条件下，宏观上以各种形式的浅色脉体参与岩石流变，深熔熔体夹矿物颗粒流形成网状的断续条带状构造。微观上斜长石受扩散蠕变、颗粒边界滑移和颗粒流动影响，一般没有明显的晶内变形组构，CPO（晶格优选方位）发育不明显。在中深部构造层次（大庙-忽鸡沟）深熔作用相对较弱，宏观上形成平行片麻理的较规则条纹/条带构造。微观上斜长石发生位错蠕变，CPO发育明显。此外，角闪石在不同构造层次下均表现出位错蠕变特征，结晶优选方向受深熔作用影响不大。上述变形样式和机制，体现了造山带根部高级变质-深熔岩石在固-液两相介质中，发生水平层状粘-塑性流动变形特点。随后深熔岩石的塑性流动变形伴随加厚地壳的伸展减薄-隆升过程而逐渐硬化并遭受剥露。  相似文献   

7.
The formation of Ca-rich myrmekites is described in syntectonic syenites crystallized and progressively deformed under granulite facies conditions. The syenites are found in high- and low-strain zones where microstructure and mineral composition are compared. Heterogeneously distributed water-rich, late-magmatic liquids were responsible for strain partitioning into dry and wet high-strain zones at outcrop scale, where contrasting deformation mechanisms are reported. In dry high-strain zones K-feldspar and clinopyroxene are recrystallized under high-T conditions. In wet high-strain zones, the de-stabilization of clinopyroxene and pervasive replacement of relatively undeformed K-feldspar porphyroclasts by myrmekite and subordinate micrographic intergrowths indicate dissolution-replacement creep as the main deformation mechanism. The reworking of these intergrowths is observed and is considered to contribute significantly to the development of the mylonitic foliation and banding. A model is proposed for strain partitioning relating a positive feedback between myrmekite-forming reaction, continuous inflow of late-magmatic liquids and dissolution-replacement creep in the wet zone at the expenses of original mineralogy preserved in the dry zones. Melt-assisted dissolution-replacement creep in syntectonic environments under granulite-facies conditions may extend the field of operation of dissolution-replacement creep, changing significantly the rheology of the lower continental crust.  相似文献   

8.
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   

9.
《Journal of Structural Geology》2002,24(6-7):1101-1107
Flow laws for high-temperature creep of olivine, plagioclase, and diabase are used to place constraints on the rheology of partially molten lower oceanic crust. This analysis is motivated by the observation of olivine lattice preferred orientations and subgrain microstructures in oceanic gabbros that lack evidence for dislocation creep in coexisting plagioclase and pyroxene. Extrapolation of experimental flow laws indicates that at temperatures above 1100°C and stresses less than 10 MPa, olivine may be the weakest phase in rocks with gabbroic composition. By accounting for variations in the melt fraction (0–10%) and grain size of partially molten plagioclase aggregates we can constrain the rheological conditions where olivine deforms by dislocation creep while plagioclase deforms by diffusion creep. Calculated effective viscosities range from 1015 to 1019 Pa s; based on observations of the geometry of the partially molten zone beneath the East Pacific Rise and the microstructural and experimental constraints we favor a value of ∼1018 Pa s. This value approaches estimates for the viscosity of the upper mantle beneath ridge axes, but is significantly higher than previously suggested for the partially molten lower crust. Such high viscosities are inconsistent with ridge evolution models that require large amounts of lower crustal flow to accommodate melt redistribution. However, the results are compatible with recent models that favor local magma replenishment from the mantle at closely spaced intervals along the spreading center axis in a 2D, ‘sheet-like’ fashion.  相似文献   

10.
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.  相似文献   

11.
Quantifying crustal deformation is important for evaluating mass balance, material transfer, and the interplay between tectonism and magmatism in continental arcs. We present a dataset of >650 finite strain analyses compiled from published works and our own studies with associated structural, geochronologic, and geobarometric information in central and southern Sierra Nevada, California, to quantify the arc crust deformation. Our results show that Mesozoic tectonism results in 65% arc-perpendicular bulk crust shortening under a more or less plane strain condition. Mesozoic arc magmatism replaced ∼80% of this actively deforming arc crust with plutons requiring significantly greater crustal thickening. We suggest that by ∼85 Ma, the arc crust thickness was ∼80 km with a 30-km-thick arc root, resulting in a ∼5 km elevation. Most tectonic shortening and magma emplacement must be accommodated by downward displacements of crustal materials into growing crustal roots at the estimated downward transfer rate of 2–13 km/Myr. The downward transfer of crustal materials must occur in active magma channels, or in “escape channels” in between solidified plutons that decrease in size with time and depth resulting in an increase in the intensity of constrictional strain with depth. We argue that both tectonism and magmatism control the thickness of the crust and surface elevation with slight modification by surface erosion. The downward transported crustal materials initially fertilize the MASH zone thus enhancing to the generation of additional magmas. As the crustal root grows it may potentially pinch out and cool the mantle wedge and thus cause reduction of arc magmatism.  相似文献   

12.
Ultra high-pressure (UHP) eclogites from Sulu region (China) represent mafic components of the continental crust, which were first subducted to mantle depths greater than 100 km and then exhumed to the earth's surface. Detailed investigation of microstructures, chemical compositions, petrofabrics and seismic properties of the UHP eclogites can provide important information on the operating deformation mechanisms and rheology of subducted continental crust and on the origin of seismic reflections within the upper mantle. We present here results from field, optical and TEM observations, electron back-scattered diffraction (EBSD) measurements and numerical computations of the seismic properties of UHP eclogites collected from fresh surface outcrops at the drill site (Maobei, Donghai County, Jiangsu Province) of the Chinese Continental Scientific Drilling Program (CCSD). Two types of eclogites have been distinguished: Type-1 (coarse-grained) eclogites deformed by recovery-accommodated dislocation creep at the peak metamorphic conditions, and Type-2 (fine-grained) eclogites which are composed of reworked Type-1 materials during recrystallization-accommodated dislocation creep in shear zones which were active during the exhumation of the UHP metamorphic rocks. Both garnet and omphacite in these eclogites deformed plastically and the flow strength contrast between these two constituent minerals is apparently much less than an order of magnitude under the UHP metamorphic conditions. Plasticity of eclogites under UHP conditions can effectively facilitate channeled flow along the interplate shear zone. The preservation of the relict crustal materials within the continental lithosphere may produce regionally extensive, strong, seismic reflections in the upper mantle. This may explain the origin of mantle reflections observed in many areas of the world.  相似文献   

13.
超高压变质岩提供了研究大陆俯冲隧道中岩石的变形机制和流变差异性的窗口。文章使用电子背散射衍射技术分析了大别山超高压变质带的榴辉岩和长英质片麻岩的显微构造。榴辉岩中的石榴子石基本呈无序分布，绿辉石发育较强烈的晶格优选定向，[001]轴的极密平行或近平行于拉伸线理，(100)面的法线近垂直于面理，退变榴辉岩中角闪石的(100)[001]组构可能继承了绿辉石的晶格优选定向。退变榴辉岩和长英质片麻岩中的石英记录了(0001)低温底面滑移和{1010}中温 柱面滑移，反映了超高压变质岩折返到中地壳的韧性变形；而斜长石的(001)<110>和(010)[100]组构形成于折返到下地壳的角闪岩相变质条件（>600℃）。根据主要矿物的流变律计算了俯冲与折返过程中无水矿物的有效黏度变化。俯冲过程中，钠长石=硬玉+石英的分解反应以及石英-柯石英相变导致长英质片麻岩的有效黏度和密度都显著增高，有利于陆壳深俯冲。但是折返过程中由于温度较高，这两个反应带来的有效黏度变化较小。>80 km深度，石榴子石的流变强度>硬玉>绿辉 石>柯石英，俯冲上地壳的流变由柯石英和硬玉控制，下地壳的流变由绿辉石和石榴子石控制。超高压变质岩流变强度的差异有助于上—下地壳力学解耦，使相对低密度、低黏度的上地壳物质在俯冲隧道内快速折返。  相似文献   

14.
大陆地壳结构具不均一性，由此表现出分层性。深成构造作用是应变局部化的过程，表现为韧性剪切变形作用。地壳中近水平或网络状反射体韧性剪切带的产生，为深成构造作用的结果。地壳结构的特点和深成构造作用都与大陆岩石圈流变性有密切的关系。另外，熔融作用对深成构造作用及岩石流变性也有重要的影响。  相似文献   

15.
A microstructural and metamorphic study of a naturally deformed medium‐ to high‐pressure granitic orthogneiss (Orlica–?nie?nik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction‐type apparent thermal gradient (～10 °C km?1). The granitic orthogneisses develops three distinct microstructural types, as follows: type I – augen orthogneiss, type II – banded orthogneiss and type III – mylonitic orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I orthogneiss is composed of partially recrystallized K‐feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II orthogneiss is defined by plagioclase‐ and K‐feldspar‐rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III orthogneiss shows relicts of quartz and K‐feldspar ribbons preserved in a fine‐grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K‐feldspar + muscovite + biotite + garnet + sphene ± ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a prograde P?T path ranging from <15 kbar and <700 °C (type I orthogneiss) to 19–20 kbar and >700 °C (types II and III orthogneisses). The deformation types thus do not represent evolutionary stages of a highly partitioned deformation at constant P?T conditions, but reflect progressive formation during the burial of the continental crust. The microstructures of the type I and type II orthogneisses result from the dislocation creep of quartz and K‐feldspar whereas a grain boundary sliding‐dominated diffusion creep regime is the characteristic of the type III orthogneiss. Strain weakening related to the transition from type I to type II microstructures was enhanced by the recrystallization of wide myrmekite fronts, and plagioclase and quartz, and further weakening and strain localization in type III orthogneiss occurred via grain boundary sliding‐enhanced diffusion creep. The potential role of incipient melting in strain localization is discussed.  相似文献   

16.
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.  相似文献   

17.
Adakites have a distinct chemistry that links them to melting of a mafic source at high pressure. They have been attributed to melting of subducted oceanic crust or melting of the mafic crustal roots of thick continental arcs, and are an important contrast to mantle wedge melting as a means of generating continental crust. We report the first direct evidence for the generation of adakitic melts in mafic lower continental crust, in an exhumed Cretaceous arc in the South Island of New Zealand. The lower crustal Pembroke Granulite has the bulk chemistry and partial melting textures involving peritectic garnet appropriate for a source region for an adakitic melt. The melt migrated from the area through a fracture network now filled with trondhjemitic veins. Emplacement of the melt was in the upper crust of the Cretaceous section, illustrated by the presence of coeval adakites in the upper crustal Nelson-Westland region.  相似文献   

18.
赵中岩  方爱民 《岩石学报》2005,21(4):1109-1116
超高压变质岩是大陆深俯冲作用的产物。超高压变质岩在深俯冲和快速折返过程中,经历了长距离地构造搬运和构造力的作用。其构造变形主要集中在韧性剪切带中,并发生强烈地塑性流变。研究超高压变质构造岩的显微构造及其变形机制对于深入了解大陆壳岩石在深俯冲过程中的流变学行为有十分重要的意义,山东仰口的超高压韧性剪切带中榴辉岩质和花岗质糜棱岩记录了超高压变形的历史。在超高压条件下的稳定矿物绿辉石、多硅白云母、兰晶石和钾长石具有不规则波状消光、亚晶界、核幔构造和动态重结晶等显微构造特征,TEM 研究揭示了大量的位错构造,表明位错蠕变是其主要的变形机制。在花岗质糜棱岩中,金红石在刚性矿物的压力影中沉积,细粒的石榴石条带平行片理延伸,都说明超高压变形过程中有流体存在,流体助力的物质扩散迁移是又一个重要的变形机制。依据现有的流变学定律估算的流变应力应该在几十兆帕以上。  相似文献   

19.
大陆下地壳麻粒岩的流变学研究进展   总被引：1，自引：0，他引：1  
大险下地壳麻粒岩的流变学研究可以解释地壳变形、壳幔物质交换以及岩石圈深部动力学过程等科学问题.前人通过研究各矿物的显微构造变形特征与变形机制,运用广义混合流变律探讨多矿物复合岩石的流变性质,结合水与流体对岩石变形强度的弱化作用,阐明在大陆下地壳变形环境下复矿麻粒岩的塑性变形和韧性流变性质.目前人们致力于对天然变形岩石和...  相似文献   

20.
嵇少丞  黎乐  许志琴 《地质学报》2021,95(1):159-181
地球是一动态系统,其各层圈的构造运动归根究底就是多矿物复合岩石及其中各主要造岩矿物在变化的物理条件(例如,温度、围压、差应力、应变速率、应变方式等)下和化学环境(例如,氧逸度和水含量)中的形变。岩石流变学是一门研究岩石力学性质和变形行为的科学,现已成为定量大陆动力学和构造地质学发展的一个瓶颈,超越这个瓶颈,学科才能大踏步前进。本文对过去四十年来岩石流变学的实验和韧性变形域内古应力研究成果做了简明扼要的总结,特别关注尚存的问题与急需克服的困难。强调运用现代材料学、地球物理学和地球化学的新理论和新方法,改进与完善高温高压实验设备,提高其力学测量的灵敏度和准确度。而且必须采用大应变的实验途径解决稳态蠕变与稳态显微构造的问题,保证实验所获流动律外延至自然界的合理性与稳定性。鼓励那些有坚实积累、开阔视野和科学思维的青年学者,开拓进取,在岩石圈流变学与大陆动力学领域做出经得起时间淘洗、实践检验的原创性成果来。  相似文献   

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