Classification of cleavage

There are many classification and naming schemes for cleavage. Here we only introduce two different classification schemes that are currently commonly used, namely the traditional scheme and the structural classification scheme.

1. Traditional classification

This is a scheme that is still widely used but needs to be improved. This plan divides cleavage into flow cleavage, breaking cleavage and sliding cleavage based on its structure and origin.

(1) Flow cleavage

Flow cleavage is the most common secondary permeable planar structure in metamorphic rocks. It is composed of flakes and plates. Or it is composed of parallel arrangements of oblate minerals or their aggregates, and has the ability to split rocks into countless thin slices (Figure 7-7).

The meaning of liuli is not yet completely unified. Some people believe that flow cleavage is a synonym for plate cleavage, which is only used for shallow metamorphic rocks. Its basic feature is that the degree of recrystallization of minerals on the cleavage plane is very low or insignificant. If the recrystallization is obvious and the parallel arrangement of flaky minerals can be identified with the naked eye, it should be called schistosity or even schistosity. However, more and more people believe that slab cleavage, schistosity, and gneissiness are just different manifestations of flow cleavage in different metamorphic rocks.

The term flow cleavage should generally refer to the new parallel plane structure of rocks during the process of metamorphic solid rheology. It is the flattening, elongation, rotation and reshaping of the internal components of the rock during rock deformation. The final result of crystallization is that the flow cleavage is perpendicular to the direction of the maximum principal compressive stress, so the flow cleavage is regarded as the "squeezing strain surface" in mechanical analysis.

(2) Cleavage

The original meaning of cleavage refers to a group of dense shear fracture surfaces in rocks. The orientation of the fracture surfaces has nothing to do with the arrangement of minerals in the rock. The interval between cracks is generally several millimeters to several centimeters (Figure 7-8). According to this concept, the splitting principle is only distinguished from the shearing joints by its denseness and parallelism. When the spacing exceeds a few centimeters, it is called shearing joints. Therefore, from the original intention, the splitting principle is different from the shearing joints. There is no clear boundary between them.

Figure 7-8 Cleavage in sandstone and siltstone

(According to Beach, photo sketch, 1982)

The upper part is siltstone, and the middle part is long cleavage. Chlorite sandstone, the lower part is quartz sandstone; the width of the cleavage domain and microcleavage stone changes from small to the widest and smallest accordingly

Figure 7-9 Pressure solution fractionation cleavage in the quartz-rich rock layer

(Based on B.E. Hobbs et al., 1976, negative film)

S0—bedding of sedimentary layer; S1—looks like cleavage, but is actually a mica-rich domain Microcleavage is composed of fine and narrow cleavage domains (light-colored bands), and microcleavage is composed of quartz (dark bands), with an average interval of 3mm.

Cleaved cleavage is mainly developed in slightly metamorphic or non-metamorphic rocks. Regarding the nature of the cleavage mechanism, in the past, based on the intensive ruptures developed on the surface fold wings and parallel faults, the cleavage mechanism was considered to be a shear plane. However, "cleavage" is also developed at the fold turning ends, which cannot be explained by the shear rupture mechanism. Although the term cleavage is widely used in field geological work, its origin has been controversial. Due to differences and differences in understanding, even some pressure solution fractionation cleavages are described as breaking cleavages (Figure 7-9).

(3) Slip cleavage

Slip cleavage or strain slip cleavage is also a controversial term. It is called slip cleavage because it is regarded as the result of shear sliding.

Slip cleavage is developed in rocks with pre-existing foliation. It is a set of differential parallel sliding surfaces that cut through the pre-existing foliation. The sliding surface is actually a sliding belt. In the slip zone, the minerals have new orientations. This arrangement can be the result of pre-existing flake minerals being rotated to be parallel or nearly parallel to the sliding surface, or it can be the result of the directional arrangement of new minerals recrystallized along the sliding surface. The pre-existing foliation in microcleaved stones with slip cleavage generally bends and forms various wrinkles (Figure 7-10), so this cleavage is also called pleat cleavage.

2. Structural classification of cleavage

In recent years, practice has proved that the traditional classification scheme of cleavage has a genetic meaning. Due to the complexity of the cause, the naming results often do not conform to the actual geology. situation, and it is difficult to name accurately. Therefore, in recent years, geologists have gradually abandoned the genetic meaning of cleavage classification and instead emphasized classification based on structure. The scheme of classifying according to structure is currently more popular in European and American countries. This classification scheme was first proposed by G. McA. Powell (1979), and later systematically elaborated and supplemented by G.E. Borradaile and G.H. Davis.

Figure 7-10 Slip cleavage in the Carboniferous slate of Dahichang, Beijing

(Sketch based on Song Honglin’s thin section photo)

Early flow cleavage S1 It is S-shaped due to shear sliding, and the minerals near the cleavage plane are pulled and twisted to be nearly parallel to the slip cleavage plane S2

This classification scheme is mainly based on the internal structure and ability of cleavage rocks to be identified The scale divides cleavage into continuous cleavage and discontinuous cleavage. Whenever the minerals in the rock are evenly distributed and all oriented, or the intervals between cleavage domains are so small that the cleavage domains and microcleavage can only be distinguished under a microscope, they are called continuous cleavage. On the contrary, if the cleavage domain has obvious intervals in the rock, and the cleavage domain and the micro-cleavage domain can be distinguished at the naked eye scale, it is called discontinuous cleavage.

(1) Continuous cleavage

Continuous cleavage is developed in metamorphic rocks. According to its deformation characteristics and recrystallization status, it can be divided into tabular cleavage and phyllite. , Katari and Katamori. Accordingly, the aforementioned flow cleavage should belong to continuous cleavage.

Slab cleavage Slab cleavage is mainly developed in mud-rich low-grade metamorphic rocks, and the internal particles of the rock are very fine. Well-developed slab cleavage has good splitability, making the rock split into very flat stone slabs. However, at the microscopic scale, the cleavage domains of plate cleavage are arranged in an intertwined manner, appearing as mica-rich layered silicate cleavage domains and quartz-feldspar-rich lenticular domains. It can be seen from Figure 7-11 that within microcleaved rocks, they are mainly composed of granular minerals or their aggregates, lacking an obvious preferred orientation. In contrast, within the cleavage domain, the fabric of the original rock is almost completely transformed into strongly oriented phyllosilicate mineral bands.

Figure 7-11 Microstructure of cleavage

(According to B.E. Hobbs et al., 1976)

Schisted schist is characterized by its relatively high crystalline minerals. Large and visible to the naked eye, it is distinguished from the cleft texture. The splitting ability of schist is not as good as that of slate, but it is still very significant. Rocks with developed schistosity are generally medium-grained (1 to 10 mm), and mica flakes can be seen in hand specimens. The grain size is slightly larger than that of slate, mostly reflecting the strong recrystallization accompanying metamorphism. The most common outcrop feature of schist is the parallel, planar arrangement of flake minerals, including muscovite, biotite, sericite and chlorite. Schistosity usually occurs in intermediate to high-grade metamorphic rocks, where most of the rock minerals have recrystallized and new minerals have developed. The structure of schist (metamorphic rock with a schistose texture) is determined by the preferred orientation of large layered silicate crystals. There are three types of schistosity (Borladaile et al., 1982): ① The schistosity domain is characterized by nearly parallel mica particle domains. Mica mostly forms thin films that surround lens domains composed of other minerals in a network shape ( Figure 7-12A); ②Type 1 continuous schistosity is characterized by coarse mica particles with very obvious dominant orientation. There is no lenticular microcleavage that can be directly seen (Figure 7-12B), but it can still be seen under a microscope. to microcleaved stones; ③ Type 2 continuous cleavage has a planar structure (phyllosilicates are flat, stretched particles), with only one preferred orientation, and is distributed throughout the rock rather than concentrated in bands, so this kind of flakes Management cannot be defined by any domain structure (Figure 7-12C).

Figure 7-12 Schissology types

(Photographed by Marshak S. et al., 1998, Mitra S.)

A—Schissology domain, Mica flakes surround lenticular quartz; B-1 type continuous schistosity is sketched under the microscope, the dominant direction of mica is obvious; C-2 type continuous schistosity, elongated and flattened quartz in quartzite

Thousands of pieces Cleavage is a structure with particle size and properties between cleavage and schistosity. On outcrops, senari is softer and has a pearly or silky sheen that sparkles in the sun. But there are no clear, individual mica grains in the schistosity. Thousand millimeters have good splitting properties.

Gnemise Gnemesis is another continuous foliation widely found in deep metamorphic rock areas. It is the product of highly recrystallized cleavage rock and is composed of dark and light mineral bands (Figure 7-13).

Figure 7-13 Gneissic structure

Single-mineral amphibolite, pyroxenite and plagioclase also develop gneiss, which is composed of columnar or plate-like minerals The crystals are arranged in parallel. Gnemesis is often spread out in layers in deep metamorphic rock areas, where most of the traces of early structural deformation have disappeared, forming a new regional geological surface.

(2) Discontinuous cleavage

The intervals between discontinuous cleavage domains can be determined at the naked eye scale, on outcrops or hand specimens

Showing its discontinuous structural features. Discontinuous cleavage can be divided into interval cleavage and fold cleavage according to the structure of micro-cleavage stone domain. According to this, the aforementioned broken cleavage and sliding cleavage should belong to discontinuous cleavage.

Spaced cleavage Spaced cleavage is composed of cleavage domains arranged in parallel, reticular, suture-like, smooth and crack-like arrangements. Cleavage domains are often filled with argillaceous and carbonaceous materials. Spaced cleavage is usually developed in folded but not metamorphosed sedimentary rocks, especially in impure limestones and marls, and also in some impure sandstones. The distance between cleavage domains (i.e., domain spacing) is 1 to 10 mm, and microcleavage stone is very thick compared with other cleavages.

In the past, many interval cleavages were classified as broken cleavages. When cleavage cuts off fossils, it is difficult to find the truncated parts in adjacent cleavage domains and microcleavage stones (Figure 7-14A, B). Therefore, it has been suggested that some cracking mechanisms in the past have essentially nothing to do with sliding. This kind of spaced cleavage composed of insoluble residues forming a cleavage domain is caused by pressure solution. There is also a new explanation for the bedding dislocation, which was used to demonstrate the cause of shear fracture in the past. It is considered to be the result of pressure solution sealing. False dislocation (Figure 7-14C, D).

Figure 7-14 Two essentially different ooid dislocations (A, B) and pseudo displacements (C, D) caused by pressure solution interaction

(C , D diagram is based on G.J.Borradaile et al., 1982)

x—width of pressure solution; d—close back-sight distance; φ—angle between cleavage and bedding

< p>According to the size of the cleavage intervals and the shape of the cleavage domain, the interval cleavages can be further classified. Figure 7-15A provides a series of scales to accurately describe the cleavage domain intervals, and Figure 7-15B provides a cleavage domain morphology diagram to describe the cleavage domain morphology.

Figure 7-15 Interval cleavage

(According to Engelder & Marshak, 1985)

Pleat cleavage The pleat cleavage cuts through the first continuous one at certain visible intervals. Characterized by cleavage, the spacing size is 0.1mm~1cm. Early continuous cleavage occurs with deflection or fine wrinkles. According to the width of the cleavage domain and its relationship with the internal fabric of the microcleavage on both sides, the cleavage can be subdivided into gradient cleavage, banded cleavage and discrete crenulatin cleavage (discrete crenulatin cleavage) as shown in the figure. 7-16.