Holger Stünitz

Deformation of granitoids at low metamorphic grade. I: Reactions and grain size reduction

Abstract Deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones: two from the Alps (Corvatsch Granodiorite and Aiguilles Rouges Granite, Switzerland) and one from Eastern Australia (Wyangala Granite). When examined by light and electron microscopy, all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite: quartz deformed plastically, while both K-feldspar and plagioclase fractured and recrystallized in conjunction with chemical change, which had, as its end point, an assemblage albite + quartz ± white mica ± CaAl-silicates. K-feldspar and calcic plagioclase (depending on fluid chemistry) from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid. Since Ca-bearing plagioclase was not stable in these environments, myrmekite did not replace K-feldspar in any of the rocks examined. Recrystallization of feldspar, which should perhaps be termed neocrystallization, was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets. In old feldspar grains, particularly in plagioclases, dislocations exist in wall-like structures that are commonly associated with voids, suggesting origins in microfractures. These dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization. We conclude that recrystallization mainly occurred via a classical nucleation mechanism, with minor contributions from twin boundary migration. We argue that the 450–550°C lower limit for recrystallization of feldspar, referenced throughout the literature, is not applicable to the rock systems under investigation and should be discarded as a universal limit. Microfracturing, microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades (see part II of this paper). Neocrystallization, fluid access and nucleation appear to have accompanied, or immediately followed, the first deformation increments and were primarily responsible for continued grain size reduction. Transgranular fracture was comparatively important only at the onset of deformation. Since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade, in the presence of aqueous fluids in the Earths crust, breakdown reactions and nucleation-recrystallization in feldspars may be important influences upon localization of deformation in granitoids.

Dynamic recrystallization of quartz: correlation between natural and experimental conditions

Abstract Quartz veins in the Eastern Tonale mylonite zone (Italian Alps) were deformed in strike-slip shear. Due to the synkinematic emplacement of the Adamello Pluton, a temperature gradient between 280°C and 700°C was effected across this fault zone. The resulting dynamic recrystallization microstructures are characteristic of bulging recrystallization, subgrain rotation recrystallization and grain boundary migration recrystallization. The transitions in recrystallization mechanisms are marked by discrete changes of grain size dependence on temperature. Differential stresses are calculated from the recrystallized grain size data using paleopiezometric relationships. Deformation temperatures are obtained from metamorphic reactions in the deformed host rock. Flow stresses and deformation temperatures are used to determine the strain rate of the Tonale mylonites through integration with several published flow laws yielding an average rate of approximately 10−14s−1 to 10−12s−1. The deformation conditions of the natural fault rocks are compared and correlated with three experimental dislocation creep regimes of quartz of Hirth & Tullis. Linking the microstructures of the naturally and experimentally deformed quartz rocks, a recrystallization mechanism map is presented. This map permits the derivation of temperature and strain rate for mylonitic fault rocks once the recrystallization mechanism is known.

Deformation of granitoids at low metamorphic grade. II: Granular flow in albite-rich mylonites

Abstract The high strain deformation at low metamorphic grades of three investigated granitoids is dominated by the granular flow of albite-rich polyphase aggregates. These aggregates formed by retrograde breakdown reactions from intermediate plagioclase and K-feldspar. The predominant deformation mechanism changes in granitoids as the grain size is reduced: coarse-grained (low strain) examples are deformed by a combination of intracrystalline plasticity (quartz) and fracturing (feldspar). In the mylonites, intracrystalline plasticity of quartz plays only a minor role and the dominant deformation mechanism is a non-cataclastic granular flow of polyphase aggregates, consisting largely of albite and quartz. Deformation appears to be stable, probably because grain growth in albite-quartz mixtures is inhibited. These fine-grained aggregates are mechanically weaker than pure quartz aggregates. Thus, the change in deformation mechanism, mainly due to feldspar breakdown reactions, appears to be important for the localization of high shear strain deformation at low metamorphic grades in granitoids and other lithologies modally dominated by feldspar. The rheological behaviour of albite-dominated mineral aggregates may have two consequences for middle to upper crustal deformation of modally feldspar-dominated lithologies: (1) feldspar, in the presence of aqueous fluids, can be an important mineral controlling the rheology at low metamorphic grades, that is, below amphibolite facies P–T conditions; (2) the occurrence of granular flow of fine-grained polyphase aggregates in low-grade granitoids is probably common and calls for great care in the modelling of middle to upper crustal rheology based on flow laws for intracrystalline plasticity of single minerals such as quartz.

Deformation mechanisms and phase distribution in mafic high-temperature mylonites from the Jotun Nappe, southern Norway

Abstract Shear zones and mylonites in anorthositic to gabbroic rocks from the Jotun Nappe in southern Norway have been studied as an example of high-temperature deformation in polyphase rocks. The deformation has taken place at approximately 700°C and maximum confining pressures of 900 MPa. The constituent phases amphibole, pyroxene, and plagioclase have recrystallized synkinematically, reducing the grain size of the rock to about 20 to 70 μm. Two different microstructures develop, depending on the recrystallization mechanism. Dynamic recrystallization of plagioclase, hornblende and pyroxenes produces monophase layers, which extend from porphyroclasts. Heterogeneous nucleation of phases due to chemical disequilibrium produces phase mixtures of hornblende and plagioclase or clinopyroxene and plagioclase. These phase mixtures extend from hornblende or clinopyroxene porphyroclasts. The product of the deformation and simultaneous recrystallization is a well laminated microstructure of some monophase hornblende, pyroxene and plagioclase layers, which alternate with abundant layers of phase mixtures. Different deformation mechanisms operate in the different types of microstructures. Dislocation substructures, dynamic recrystallization microstructures and a crystallographic preferred orientation (measured by Ji and Mainprice (1988) on samples of the same shear zone) indicate crystal plastic deformation in monophase plagioclase layers. Dynamic recrystallization in monophase hornblende layers indicates also crystal plastic deformation. The phase distribution of plagioclase and hornblende, grain shapes, and lower dislocation densities in the phase mixtures suggest that the deformation mechanism is not crystal plasticity but rather a granular flow (grain boundary sliding, probably accommodated by diffusional mass transfer). In the plagioclase–hornblende mixtures, the phases show an anticlustered, non-random distribution. Grain boundary sliding may contribute to phase mixing but cannot be the main reason for the anticlustering of phases and the complete phase mixing directly at the boundaries of the porphyroclasts. The reason for the anticlustered distribution is not clear but could be explained by preferential heterogeneous nucleation of hornblende at plagioclase grain boundaries.

Dislocation generation, slip systems, and dynamic recrystallization in experimentally deformed plagioclase single crystals

Abstract Three samples of gem quality plagioclase crystals of An60 were experimentally deformed at 900 °C, 1 GPa confining pressure and strain rates of 7.5–8.7×10−7 s−1. The starting material is effectively dislocation-free so that all observed defects were introduced during the experiments. Two samples were shortened normal to one of the principal slip planes (010), corresponding to a “hard” orientation, and one sample was deformed with a Schmid factor of 0.45 for the principal slip system [001](010), corresponding to a “soft” orientation. Several slip systems were activated in the “soft” sample: dislocations of the [001](010) and 〈110〉(001) system are about equally abundant, whereas 〈110〉{111} and [101] in (131) to (242) are less common. In the “soft” sample plastic deformation is pervasive and deformation bands are abundant. In the “hard” samples the plastic deformation is concentrated in rims along the sample boundaries. Deformation bands and shear fractures are common. Twinning occurs in close association with fracturing, and the processes are clearly interrelated. Glissile dislocations of all observed slip systems are associated with fractures and deformation bands indicating that deformation bands and fractures are important sites of dislocation generation. Grain boundaries of tiny, defect-free grains in healed fracture zones have migrated subsequent to fracturing. These grains represent former fragments of the fracture process and may act as nuclei for new grains during dynamic recrystallization. Nucleation via small fragments can explain a non-host-controlled orientation of recrystallized grains in plagioclase and possibly in other silicate materials which have been plastically deformed near the semi-brittle to plastic transition.

Deformation and recrystallization of plagioclase along a temperature gradient: an example from the Bergell tonalite

Abstract Syn- to post-mylonitic tilting of the Bergell tonalite allows investigation of the deformation and recrystallization of plagioclase grains along a temperature gradient from the lower to the upper amphibolite facies. At the lowest temperatures recrystallization occurs by nucleation and growth of new grains having a different composition from the old grains. In contrast, at the highest temperatures recrystallization occurs by subgrain rotation associated with grain boundary migration. The temperature increase is inferred to induce this transition in two ways. First, at higher temperatures the anorthite content of the new grains is higher and thus the comositional difference between old and new grains is lower. Hence, the chemical driving potential is small compared with recrystallization at lower temperatures. Second, higher temperatures facilitate climb of dislocations and thus subgrain formation as well as grain boundary mobility. The temperature increase is also associated with a transition in the dominant deformation mechanism. Intracrystalline plasticity dominates deformation at lower temperatures, whereas at higher temperatures deformation occurs primarily by diffusion-accommodated grain boundary sliding, as indicated by the weakening of the indicatrix preferred orientation and the formation of a mixed plagioclase–biotite matrix with increasing deformation. Therefore, the strength of the mid to lower crust may be overestimated by dislocation creep flow laws for plagioclase. Lower crust rheology for polyphase rocks is better approximated by constitutive relationships involving diffusion accommodated grain boundary sliding.

DEFORMATION MECHANISMS AND REACTION OF HORNBLENDE : EXAMPLES FROM THE BERGELL TONALITE (CENTRAL ALPS)

Abstract Hornblendes in the Bergell tonalite begin to crystallize relatively early during the crystallization history of the pluton and become resorbed during the late stages of crystallization. During the crystallization of the magma deformation commences by magmatic flow. In this stage the hornblendes behave as rigid particles in a viscous matrix (melt). The rotation and alignment of hornblende as elongate, rigid particles have produced a strong preferred orientation of both, particle long axes and crystallographic directions. With progressive solidification of the melt, there is a gradual transition from magmatic flow to solid-state deformation in the Bergell tonalite. The crystallographic and shape preferred orientation that have developed during solid-state deformation are identical to those of magmatic flow. This identical fabric development during solid-state and magmatic flow deformation can be explained by the same deformation mechanism that has prevailed in hornblends during both deformation periods, i.e., hornblendes have always acted as rigid particles in a viscous matrix. TEM observations show no evidence for intracrystalline plasticity in hornblende grains. The modal abundance of hornblende decreases progressively with increasing solid-state deformation to yield more biotite, epidote and quartz. Small hornblende matrix grains form by cataclastic processes. The observed changes in grain size, shape and abundance of hornblende occur mainly by fracturing, dissolution and reaction, so that the solid-state deformation of the tonalite is a complex process, to which fracturing, dissolution of hornblende and metamorphic reactions all contribute. The relationship between deformation and reaction represents an example of incongruous pressure solution or diffusive mass transfer involving reaction. It is inferred that generally, hornblende does not appear to deform significantly by intracrystalline plasticity at temperatures below 650–750°C in the presence of an aqueous fluid. Crystal plasticity could become dominant at higher temperatures and/or lower aqueous fluid activities.

Strain localization by fracturing and reaction weakening — a mechanism for initiating exhumation of subcontinental mantle beneath rifted margins

Abstract Rift-related strain localization in spinel lherzolite from an exhumed passive continental margin in the Southern Alps involved two stages. (1) Critical fracturing coincided with heterogeneous nucleation of plagioclase, olivine, and hornblende aggregates to form discrete, ultrafine-grained (0.5–0.6 µm) shear zones oriented at high angles to the pre-existing foliation in the host rock. The syntectonic replacement of spinal lherzolite by lower pressure, plagioclase-hornblende lherzolite documents extensional exhumation under high temperature (700–900 °C) conditions accompanied by limited fluid infiltration. Deformation involved a combination of dislocation creep (ol) and diffusion-accommodated viscous granular flow (plag, ol, hbl aggregates). (2) Hydrous deformation at lower temperatures (200–400 °C) involved the formation of serpentine-chlorite mylonite and cataclasite along discrete, anastomozing shear zones oriented at low angles to the pre-existing foliation. Both stages involved drastic weakening, particularly once the shear zones coalesced subparallel to the extensional shearing plane. The top of the lithospheric mantle was initially strong, but is inferred to have become weaker than both the underlying mantle and the overlying mafic lower crust. The interconnection of such strong-then-weak delamination zones to form trans-lithospheric extensional shear zones accelerated rifting and led to the exhumation of subcontinental mantle during the late stages of continental breakup.

Folding and shear deformation in quartzites, inferred from crystallographic preferred orientation and shape fabrics

Abstract Folding and simple shear deformation frequently occur together on all scales. The kinematic information from crystallographic preferred orientation (CPO) and microstructural information from shape fabrics are used to investigate the relationships of simple shear and folding in small-scale folds. Three fold samples were analysed: sample 1 with the fold axis perpendicular; sample 2 oblique; and sample 3 essentially parallel to the shear direction. All folds have formed by buckling in a simple shear deformation regime. The principal kinematic directions for each sample lie in a single plane normal to the fold axial plane and at variable angles to the fold axis. The CPOs allow a distinction to be identified between coaxial and simple shear deformation components in different parts of each fold. The shear senses determined by CPOs and shape fabrics indicate that the shear deformation and the buckling of the layers occurred approximately simultaneously in all samples. CPO analysis of the flexural-slip components of the folding suggests that the fold axes have not rotated substantially towards the extension direction since their initiation. The variable orientations of the fold axes are explained by variable original orientations of the anisotropy with respect to the shear direction.

Composition of ibotenic acid-induced calcifications in rat substantia nigra

Agonists of the excitatory neurotransmitter glutamate have neurotoxic properties and are, therefore, frequently used to place locally circumscript brain lesions. In certain vulnerable brain areas, especially the substantia nigra and globus pallidus, the ensuing neurodegeneration is accompanied by the formation of calcium deposits. In the present study, we investigated the structure and chemical composition of calcium deposits formed in rat substantia nigra upon local application of ibotenic acid. Using scanning and transmission electron microscopy in combination with X-ray analysis and analysis of the electron diffraction patterns, we demonstrate that the inorganic components of the calcifications consist of calcium and phosphate. The calcium phosphate is deposited in a polycrystalline manner in degenerating neurons and in a matrix surrounding the degenerated complexes. New matrix is continuously added around the enlarging calcium deposits. Content of inorganic material is always higher in the center of the deposits than in the margin, but in every case the diffraction pattern reveals that the calcium phosphates are present in the form of hydroxyapatite. Thus, organic and inorganic components of the calcifications are subject to a continuous process of growth and maturation. The ibotenic acid-induced calcium deposits in rat substantia nigra provide a reliable model system to study the pathogenesis of non-arteriosclerotic calcifications.