Amphibolite Mylonites Research Articles

Analysis of the Submicrostructural Deformation of Amphibole in a Ductile Shear Zone Based on the TEM Technique.

Deformed amphibole in the plagioclase amphibolite mylonite of the Guandi Complex, Xishan, Beijing, is the research object in this study. The amphibole nanodeformation under the middle crust was analyzed using microstructural analysis and high-resolution transmission electron microscopy (TEM). Microscope observations show that the amphibolite deformations in the plagioclase amphibolite mylonite are δ and σ type porphyroclasts, and the porphyroclast tail is composed of new long-columnar crystals. Using transmission electron microscopy (TEM, and this acronyms would be defined only once), the authors observed the nanodeformation characteristics of the amphibole porphyroclast core and mantle. Dislocation tangles are dominant in the porphyroclast core, and inside the new crystal, there is little or no dislocation. Swelled new crystals surrounded by dislocation can be observed in the transition zone between the porphyroclasts and new crystals. The deformed amphibole microstructure and submicrostructure represent typical brittle-ductile transitional deformation. The deformation process can be divided into two stages: the disordered dislocation increment stage and the dislocation reduction and ordering stage. Crystalline plastic deformation occurs in the amphibole in the plagioclase amphibolite mylonite of the Xishan area, Beijing. The crystalline plastic deformation temperature in amphiboles is higher than that in plagioclase.

Orientation-related deformation mechanisms of naturally deformed amphibole in amphibolite mylonites from the Diancang Shan, SW Yunnan, China

Sheared amphibolite rocks from Diancang Shan high-grade metamorphic complex along the Ailao Shan-Red River shear zone, southwestern Yunnan, China, show typical mylonitic microstructures. The mylonites are characterized by porphyroclastic microstructures and the ultramylonites are highly lineated with alternating amphibole- and quartzofeldspathic domains. Microstructural analysis and P/T estimation suggest that the amphibole grains in the mylonitic rocks are deformed and dynamically recrystallized at amphibolite facies. In the mylonitic amphibolites, there are two types of amphibole porphyroclasts, i.e. type I “hard” and type II “soft” porphyroclasts. They have their [001] crystallographic orientations subnormal and sub-parallel to the stretching lineation of the rocks, respectively. The two types of porphyroclasts show distinct deformation microstructures and sub-microstructures formed by various deformation mechanisms, which contribute in different ways to the generation of the fine-grained matrix. Shape preferred orientation analysis, misorientation analysis of the two types of porphyroclasts and new fine grains around them further prove the generation of the fine grains in matrix from the type II porphyroclasts. The type I “hard” porphyroclasts are deformed mainly by mechanical rotation, work hardening and intragranular microfracturing. In contrast, the deformation of the type II “soft” porphyroclasts is mainly attributed to crystalline plasticity, i.e. twinning, dislocation creep and dynamic recrystallization. During the deformation of the type II porphyroclasts, the (100) [001] slip system plays a dominant role during deformation and grain size reduction of amphibole. Twinning along the active (100) slip system, in combination with dislocation creep (gliding and climbing) governs the nucleation of subgrains and formation of dynamically recrystallized fine grains, a process here named Twinning Nucleation Recrystallization.

Textures and microstructures of naturally deformed amphibolites from the northern Cascades, NW USA: methodology and regional aspects

Abstract Neutron texture analyses of quartz-bearing and quartz-free amphibolite mylonites from the Windy Pass thrust, Cascades Crystalline Core (Washington/USA) reveal pronounced textures of plagioclase and clino-amphiboles (hornblende, cummingtonite) but no preferred orientation of quartz. A reliable strategy for amphibolite fabric analysis is presented by a systematic analytical approach to the experimental diffraction data processing. Clino-amphiboles show transitional textures between ideal single crystal orientations and axial symmetric great circle distributions. Plagioclase reveals a -axes distributions scattering along a great circle approximating the foliation plane as well as a -axes maxima close to the macroscopic lineation. Correlation of the textures with grain shape anisotropies of horn-blende and plagioclase and comparison with data from the literature suggest that the texture variations are due to different strain regimes rather than due to different crystallographic reorientation mechanisms. The kinematic directions deduced from the microfabric correlate well with the regional tectonic interpretations. In contrast, individual deformation paths are not yet established for the different tectonic units, as the significance of the separating Windy Pass thrust requires further structural analysis and fabric studies.

Transformation of Archean structural inheritance at the Grenvillian Foreland Parautochthon Transition Zone, Chibougamau, Quebec

The easternmost portion of the Archean Abitibi greenstone belt adjacent to the Grenville Front in the Chibougamau area displays distinctive structural features related to crustal-scale contraction during the Grenvillian orogeny. The "Foreland parautochthon transition zone," (FPTZ) a 3–15 km wide zone of Archean rocks, shows a Grenvillian deformation and metamorphic history that overprint fabrics inherited from the Archean Kenoran orogeny. The deformation history of this zone can be divided into two stages. During stage 1, a major crustal-scale deformation zone is responsible for a first uplift episode of deep Archean crust. During this stage, the Grenvillian deformation reactivated and emphasized the Archean fabrics. Amphibolite-grade mylonitic fabrics are interpreted to be an enhancement of the Archean regional schistosity, which remains axial planar to the regional folds. Archean rocks with well-developed former fabrics are strongly affected by Grenvillian non-coaxial flow associated with southeast-dipping stretching lineations. The northwestward transport direction, deduced from shear-sense indicators, is compatible with the signature of the Grenvillian orogeny. Stage 2 is responsible for the formation of the N015°-trending, late Grenvillian faults that have dissected the stage 1 deformation zone. Steep east-dipping narrow zones of retrograde low-temperature mylonites indicate a reverse, east-side-up movement that has contributed to the uplift of deeper Archean structural levels. In a later stage of brittle deformation, pseudotachylite injections and cataclasis, resulting from sinistral lateral offsets in a dominantly brittle regime, are superimposed on the mylonitic structures in response to a modification of the transport direction at the end of the Grenvillian orogeny. Even if high-grade metamorphism can be partly explained by the uplift of deep Archean crust, the structural signature associated with amphibolite mylonites within the FPTZ is interpreted as a product of the Grenvillian orogeny.