Experimental deformation of partially melted granitic aggregates

Abstract The effects of varying amounts of partial melt on the deformation of granitic aggregates have been tested experimentally at conditions (900°C, 1500 MPa, 10^-4 to 10^-6/s) where melt-free samples deform by dislocation creep, with microstructures approximately equivalent to those of upper greenschist facies. Experiments were performed on samples of various grain sizes, including an aplite (150 μm) and sintered aggregates of quartz-albitemicrocline (10–50 and 2–10 μm). Water was added to the samples to obtain various amounts of melt (1–15% in the aplite, 1–5% in the sintered aggregates). Optical and TEM observations of the melt distribution in hydrostatically annealed samples show that the melt in the sintered aggregates is homogeneously distributed along an interconnected network of triple junction channels, while the melt in the aplites is inhomogeneously distributed. The effect of partial melt on deformation depends an melt amount and distribution, grain size and strain rate. For samples deformed with ? 1% melt, all grain sizes exhibit microstructures indicative of dislocation creep. For samples deformed with 3–5% melt, the 150 μm and 10–50 μm grain size samples also exhibit dislocation creep microstructures, but the 2–10 μm grain size samples exhibit abundant TEM-scale evidence of dissolution-precipitation and little evidence of dislocation activity, suggesting a switch in deformation mechanism to predominantly melt-enhanced diffusion creep. At natural strain rates melt-enhanced diffusion creep would predominate at larger grain sizes, although probably not for most coarse-grained granites. The effects of melt percentage and strain rate have been studied for the 150 μm aplites. For samples with ? 5 and 10% melt, deformation at 10^–6/s squeezes excess melt out of the central compressed region allowing predominantly dislocation creep. Conversely, deformation at 10^-5/s produces considerable cataclasis presumably because the excess melt cannot flow laterally fast enough and a high pore fluid pressure results. For samples with 15% melt, deformation at both strain rates produces cataclasis, presumably because the inhomogeneous melt distribution resulted in regions of decoupled grains, which would produce high stress concentrations at point contacts. At natural strain rates there should be little or no cataclasis if an equilibrium melt texture exists and if the melt can flow as fast as the imposed strain rate. However, if the melt is confined and cannot migrate, a high pore fluid pressure should promote brittle deformation.     

Fabric development in brittle-to-ductile shear zones

Brittle-to-ductile shear zones from two separate geological settings are shown to have nucleated on zones of predominantly brittle deformation. The shear zones are not simply foliated cataclasites, since they contain abundant evidence of dynamic recrystallization of constituent minerals. A small quartz diorite lens in the Borrego Springs shear zone, southern California, contains centimeter-scale cataclasite zones that exhibit a gradual transition into foliated rock. Alteration of magnesiohornblende to actinolite, feldspar to white mica plus quartz, and biotite to chlorite, produced elongate minerals that define the foliation. During the later stages of deformation, intracrystalline slip and dynamic recrystallization of quartz and feldspar were important deformation mechanisms.The widespread occurrence of mineralized dilatant cracks predated the development of meter-to-decimeter-scale ductile shear zones in the Striped Rock granite, southern Virginia. Again, important deformation mechanisms in the final stages of deformation were intracrystalline slip and dynamic recrystallization of quartz.In both field areas the role of fluids has been important from the onset of brittle deformation. Fluids may have enhanced early fracturing in addition to causing the alteration and hydrolytic weakening of host rock minerals and the introduction of new mineral species. Each of these processes is thought to have contributed to the later localization of crystal plastic deformation in the rocks.     

深部地壳镁铁质岩石(斜长角闪岩)叶理的组成及形成机制分析--以辽东海城地区变质镁铁质岩石为例

高级变质镁铁质岩石叶理形成于地壳深部环境，对其组成及形成机制的深入研究有助于讨论在深部地壳(下地壳)环境中岩石的表现与流动过程。以辽宁海城马风地区广泛出露的斜长角闪岩为例，对变质镁铁质岩石中几种不同类型叶理的组成及形成机制进行初步探讨。显微构造分析结果表明，变质斜长角闪岩中主要发育三种具有不同特点的叶理构造型式：条带状、糜棱状和片麻状。作为岩石叶理的主要组成矿物。角闪石对叶理的形成贡献最大。斜长石在不同类型岩石中对叶理的形成作用不同。组成叶理的斜长石和角闪石矿物对的温压测算结果表明，它们形成于426～532MPa和58l℃～639℃之间。变质基性岩中叶理的形成是这种温度和压力条件下多种机制(包括晶体的定向重结晶作用、应力作用下的变质分异、晶质塑性变形、亚颗粒旋转动态重结晶)共同作用的结果。     

Kyanite deformation in whiteschist of the ultrahigh-pressure metamorphic Kokchetav Massif, Kazakhstan

Microstructures in minerals from ultrahigh‐pressure metamorphic (UHPM) terranes are keys to understanding the rheological properties and the exhumation mechanisms of rocks from subduction zones. Kyanite‐bearing whiteschist, associated with eclogite lenses, is part of UHPM unit II located south‐west of Lake Zheltau in the Kulet region of the Kokchetav Massif. The equilibrium assemblage is kyanite + garnet + talc + phengite + coesite/quartz. Previously reported peak pressure–temperature (P–T) conditions are ～3.5 GPa at 750 °C. A strong foliation is defined by the talc and phengite, with a corresponding weak shape preferred alignment of kyanite. Crystallographic orientation maps and analysis of kyanite blades were performed using electron backscatter diffraction methods. The data are consistent with a (100)[001] slip system for the formation of undulose extinction and kink bands in kyanite. Rotations measured across individual kink bands are 10–50° about <010>, and rotations along kyanite with undulose extinction are up to 50° about <010> with variations between adjacent points typically <2°. The undulose extinction is interpreted to have developed through crystal plastic deformation by dislocation creep. Kink bands mark the development of high‐angle grain boundaries by dislocation climb. The deformation of kyanite occurred in the fault‐bounded terrane during the exhumation of the Kokchetav Massif.     

Microstructurally controlled trace element (Zr,U–Pb) concentrations in metamorphic rutile: An example from the amphibolites of the Bergen Arcs

As a common constituent of metamorphic assemblages, rutile provides constraints on the timing and conditions of rock transformation at high resolution. However, very little is known about the links between trace element mobility and rutile microstructures that result from synmetamorphic deformation. To address this issue, here we combine in situ LA-ICP-MS and sensitive high-resolution ion microprobe trace element data with electron back-scatter diffraction microstructural analyses to investigate the links between rutile lattice distortions and Zr and U–Pb systematics. Furthermore, we apply this integrated approach to constrain further the temperature and timing of amphibolite facies metamorphism and deformation in the Bergen Arcs of southwestern Norway. In outcrop, the formation of porphyroblastic rutile in dynamically hydrated leucocratic domains of otherwise rutile-poor statically hydrated amphibolite provides key contextual information on both the ambient conditions of hydration and deformation and the composition of the reactive fluid. Rutile in amphibolite recorded ambient metamorphic temperatures of ~590–730°C during static hydration of the granulitic precursor. By contrast, rutile from leucocratic domains in the directly adjacent shear zone indicates that deformation was accompanied by a localized increase in temperature. These higher temperatures are recorded in strain-free rutile (~600–860°C) and by Zr concentration measurements on low-angle boundaries and shear bands (620–820°C). In addition, we also observe slight depletions of Zr and U along rutile low-angle boundaries relative to strain-free areas in deformed grains from the shear zone. This indicates that crystal–plastic deformation facilitated the compositional re-equilibration of rutile upon cooling to slightly below the peak temperature of deformation. Cessation of deformation at mid-crustal conditions near ~600°C is recorded by late stage growth of small (<150 µm) rutile in the high-strain zones. U–Pb age data obtained from the strain-free and distorted rutile grains cluster in distinct populations of 437.4 ± 2.7 Ma and c. 405–410 Ma, respectively. These different ages are interpreted to reflect the difference in closure for thermally induced Pb diffusion between undeformed and deformed rutile during post-deformation exhumation and cooling. Thus, our results provide a reconstruction of the thermochronological history of the amphibolite facies rocks of the Lindås Nappe and highlight the importance of integration of microstructural data during application of thermometers and geochronometers.     

Image‐based modelling and analysis of microstructures for two‐scale problems in geomechanics

This paper focuses on the geometry modelling and numerical analysis of microstructures of geomaterials employing the concept of image‐based engineering. The novel modelling and analysis techniques with digital images are incorporated with the mathematical homogenization method to study the interaction between individual phases, each of whose shape and spatial distribution are irregular. Owing to the distinctive features of these computational techniques, the evaluation of homogenized properties for geomaterials provides the reliable information about the micro‐ or macroscopic mechanical behaviours for engineering practice. It is, naturally, inevitable that engineers' demands on safety and efficient design place emphasis on quantitative estimates for these values. Thus, calibration accompanied with actual measurements comes within the scope of this study so that these properties would be realistic and practical from the engineering viewpoints. Copyright © 2002 John Wiley & Sons, Ltd.     

Initiation Timing of the Jiamusi–Yitong Fault Zone in NE China

The NE–striking Jiamusi–Yitong fault zone(JYFZ) is the most important branch in the northern segment of the Tancheng–Lujiang fault zone. The precise shearing time of its large–scale sinistral strike–slip has yet to determined and must be constrained. Detailed field investigations and comprehensive analyses show that strike–slip faults or ductile shear belts exist as origination structures along the western region of Yitong Graben. The strike of the shear belts trend to the NE–SW with steep mylonitic foliation. The zircon U–Pb dating result for the granite was 264.1±1 Ma in the ductile shear belt of the JYFZ. The microstructural observation(rotated feldspar porphyroclasts, S–C fabrics, and quartz c–axis fabrics, etc.) demonstrated the sinistral shearing of the ductile shear zones. Moreover, the recrystallized quartz types show a transitional stage of the subgrain rotation toward the recrystallization of the grain boundary migration(SR–GBM). Therefore, we suggest that the metamorphic grade of the shear zone in the ductile shear zones should have reached high greenschist facies conditions, and the deformation temperatures should approximately 450–500°C, which is obviously higher than the blocking temperature of muscovite(300–400°C). Hence, the ~(40)Ar/~(39)Ar isochron age of muscovite from ductile shear zones should be a cooling age(162.7±1 Ma). We infer that the sinistral strike–slipping event at the JYFZ occurred in the late Jurassic period, and it was further inferred from the ages of the main geological events in this region that the second sinistral strike–slip age of the Tancheng–Lujiang fault zone occurred during the period of tectonic movements in the Circum–Pacific tectonic domain. This discovery also indicates the age of the Tancheng–Lujiang fault zone that stretches to northeastern China. The initiation of the JYFZ in the late Jurassic is related to the speed and direction of oblique subduction of the west Pacific Plate under the Eurasian continent and is responsible for collision during the Jurassic period.     

中国的伤齿龙类恐龙化石

伤齿龙类是手盗龙类中较为独特的一支, 时代从中侏罗世到晚白垩世, 主要分布于亚洲(东部和中部)以及北美地区。本文基于前人的研究成果, 结合新标本, 对中国伤齿龙类的系统分类、骨骼形态、骨组织、系统发育关系和古生物地理等方面进行了全面深入的评估与探讨。     

Deformation and seismic anisotropy of the lithospheric mantle in the southeastern Carpathians inferred from the study of mantle xenoliths

Peridotite xenoliths with a broad range of textures provides evidence for consistent microstructural evolution in a vertical transect of the shallow lithospheric mantle (35–55 km depth) beneath the Persani Mountains, SE Carpathians, Romania, due to ongoing plate convergence in the Carpathian Arc nearby. The recrystallized grain size, crystal preferred orientations strength, and resulting seismic anisotropy vary continuously and display a strong correlation to equilibrium temperatures, suggesting a continuous change in deformation conditions with depth. The shallowmost xenoliths have microstructures typical of high stress deformation, marked by strong recrystallization to fine grain sizes, which results in weak crystal preferred orientations and anisotropy. The deepest xenoliths have coarse-grained porphyroclastic microstructures and strong crystal preferred orientations. Replacive orthopyroxene structures, consuming olivine, and high H₂O concentrations in the pyroxenes are observed in some xenoliths indicating limited percolation of fluids or volatile-rich melts. Despite the high stress deformation and high H₂O contents in some of the studied xenoliths, analysis of olivine crystallographic orientations indicates that [100] slip systems, rather than “wet” [001] accommodate most of the deformation in all samples. Seismic anisotropy estimated from the measured olivine and pyroxene crystal preferred orientations suggests that the strike-parallel fast SKS polarization directions and ~ 1 s delay times measured in the SE Carpathians are likely the consequence of convergence-driven belt-parallel flow in the lithospheric mantle.     

岩石变形机制与流变学研究的近期发展--显微构造、变形机制与流变学国际会议简介

结合显微构造,变形机制与流学国际会议讨论的有关为形机制与流学的几个主要方面,阐述了目前在该学科领域与研究方向上的研究现状与进展;地壳岩石脆－韧性转变变形机制,强调脆性变形机制与韧性变形机制之间的反应及由此引起的岩石强度降低;应变局部化机理与主要影响因素,应变局部化可以由是软化或硬化机制引起的。