Mylonite Belt Research Articles

Deformation naturelle par cisaillement ductile d'un granite de grande kabylie occidentale (algerie)

A mylonitic belt trending N70° E is found in the western part of the crystalline massif of Great Kabylia in Algeria. Late during a last phase of deformation, a granite was emplaced into this belt. A last pulse of deformation has locally mylonitized the granite itself. The regional deflection of the schistosity in the belt from the margin toward its center and the existence of numerous, small, right- and left-handed, zones of ductile shearing in the granite, together seem to allow an interpretation of the belt as resulting from a deepseated, right-handed shear zone. This paper concentrates on the evolution of minerals with increasing deformation in the small-scale shear zones in the granite. The newly developing mineral assemblages allow one to postulate a temperature of approximately 600°C and low pressure. The granite turns into mylonite by (1) progressive reduction of the grain size and (2) the rapid disappearance of the original biotite, which is replaced by a different biotite and/or muscovite. On the other hand, some of the original muscovite persists to the final stages. (3) Phyllosilicates increase considerably in relative importance at the expense of the feldspars. (4) The rocks become banded in alternating layers which are richer in mica or in quartz. Various minerals react differently and sometimes not uniformly under deformation. The feldspars start out brittle and become broken up into progressively smaller fragments. These fragments behave then as rigid bodies, and strain shadows develop along the plane of schistosity. The strain shadows can be S-shaped and relatively long. Early stages of polygonization into subgrains, implying dislocations, glide and climb, can be observed in feldspar grains in the most strongly deformed portions of the rock. Depending on their original orientation, phyllosilicates are either affected by kinkbands and/or fracturation, or they rotate by slip on (001). Slip on (001) is dominant when there is large strain. Biotite becomes unstable early, whereas muscovite persists in lens-shaped bodies. Garnet behaves as a brittle and fragile material and soon becomes chemically unstable. In early stages, quartz deforms predominantly by basal slip. Dynamic recrystallisation rapidly becomes dominant. New, strain-free grains then concentrate in ribbons where they again show evidence of basal slip, then of recovery. When most of the grains have a size of about 10μm, the main flow mechanism may be boundary sliding, which suggests superplasticity. Preferred orientation of [0001] axes increases with deformation by basal slip and shows patterns characteristic of a rotational strain history. With the help of these patterns one can determine the sense of shearing. However, when boundary sliding is dominant, small-size grains have a random orientation.

The effects of folding on the C-axis fabrics of a quartz mylonite

The progressive c-axis fabric development in quartz-mylonites from the Cap de Creus (NE Spain) shear zones, is briefly summarized and the subsequent effects of fold development on these fabrics is analyzed in more detail. A stable fabric pattern consisting of a girdle lying in, or almost in, a plane perpendicular to both the mylonitic foliation and the lineation and with two maxima slightly inclined to the mylonitic foliation plane develops at relatively low shear strains. Folds in the mylonite belts have their axes disposed at variable angles to the stretching lineation. Generally the inclination of the axial planes to the plane of foliation decreases as the fold axes become parallel to the lineation. Similar folds develop as the axial planes become parallel to the foliation. The modification of the stable c-axis fabric pattern by folding can be achieved by means of two processes: bulk pseudo-passive rotation or intragranular deformation. The stable fabric pattern rotates about the Y direction of the incremental strain ellipsoid if only pseudo-passive rotation occurs. Where grain elongation is marked, a new girdle symmetrically arranged about the axial plane of the fold appears. Where both occur and if grain elongation is slight, a crossed-girdle pattern appears consisting of two elements; a weakened rotated old pattern and a stronger new one related to the axial plane. Their relative intensity depends on the degree of intragranular deformation indicated by grain elongation. Transmission electron microscopy studies were made of folded specimens and also unfolded mylonites which exhibited the stable c-axis microfabric. The dislocation sub-structures in most grains in all specimens were indicative of recovery. However in some grains high densities of dislocations which did not show any effects of recovery were present. These are thought to reflect slight straining during uplift whereas the former are more likely to be associated with the deformation which produced the microfabrics. Burgers vectors were determined by invisibility criteria, and slip planes mainly by the trace analysis of loops. The recovered dislocations indicated slip in an a direction along {101̄0}, (0001) and {101̄2} planes and in a c direction along {101̄0} and {112̄0}planes in both folded and unfolded specimens. The unrecovered dislocations indicated that the post-tectonic slip occured mainly in an a direction along the (0001) plane.

Geochronology of the Canadian Shield in Northeastern Alberta II. Charles–Andrew–Colin Lakes Area

The broad outline of the tectonic–metamorphic history of this area is reasonably clear. Whole rock U–Pb, mineral U–Pb, whole rock Rb–Sr, mineral Rb–Sr, and mineral K–Ar data have resulted in the recognition of a narrow discontinuous N–S zone of Archean (> 2550 m.y.) granitic material which may predate the surrounding widespread migmatitic–gneissic complex. Other N–S belts of younger granitic to dioritic plutons (1900 m.y.) intrude the gneissic complex. A last severe thermal metamorphic event sharply 'reset' mica K–Ar dates for all bodies in the area to 1790 ± 40 m.y. The recently identified Archean belt is located in the Allan fault zone: a complex major zone of weakness which has undergone repeated activation involving deep seated folding, mylonitization, and multiple intrusion, with late plastic deformation and recrystallization of the mylonite belts. The unraveling of the tectonic–metamorphic history of the area is greatly complicated by multiple metamorphic effects and the compositional heterogeneity of the para– and ortho–gneissic materials of the migmatitic–gneissic complex. Local uranium mineralization in this area is geochronologically related to intrusion of the younger granites and is essentially contemporaneous with the main early phase of uranium deposition in the Uranium City area of Saskatchewan.