Vugs, Veins and Pseudotachylites

Pseudotachylite, first described by Lapworth (1885) and Clough (1888) and named by Shand (1916), refers to a dark colored glassy rock apparently resembling the volcanic glass tachylite. These rocks are formed along domains of high momentary strain such as fault planes or at large meteorite impact sites. Such instantaneous high strain rate leads to localized high heat generation and flow whereby the rocks in the immediate vicinity undergo frictional melting followed by rapid quenching. The quickly congealed dark colored glassy material along the fault is called fault vein whereas the veins of pseudotachylite generated at high angles to the fault plane are called injection veins. Although pseudotachylites are normally associated with brittle faulting regime (Jeffreys, 1942; Sibson, 1975; Grocott, 1981; Pas chier et al., 1990; Maddock, 1983; 1992; Spray, 1985) or meteorite impact (Shand, 1916; Thompson and Spray, 1994), they can also develop in domains of rock ductility and crystal plastic deformation where local strain hardening can lead to sudden failure, fracturing and localized melting (Sibson, 1980; Passchier, 1982; Maddock, 1992; Takagi et al, 2000; Roy et al, 2008; Mahapatro et al., 2009). Such pseudotachylites in ductile domains are subsequently mylonitized and it is generally difficult to identify them after moderate to intense deformation and recrystallization. Local melting and pseudotachylite development may also be associated with large landslides (Masch et al, 1985; Legros et al, 2000).

A problem with pseudotachylite identification is the fact that pseudotachylite, as well as ultramylonite, microbreccia and microcataclasite are all extremely fine grained, dark colored rocks and are frequently associated with each other. Also pseudotachylites tend to get crystallized/ devitrified and easily altered. Passchier (1982) and Krikpatrick and Rowe (2013) have provided a number of criteria to differentiate pseudotachylites from other ultrafine deformation related rocks or to identify recrystallized equivalents of pseudotachylites. The following is a list of criteria that may help in identification of pseudotachylite:

Ultracataclasites may also appear dark and near aphanatic with flow bands of different compositions formed due to differences in degree of communion of minerals (rheology dependent) or due to subsequent mylonitization of the cataclasite (fig 8.1m). Observations under higher magnifications (SEM) help in identification of cataclastic texture at submicroscopic scale. Also the size distribution of crushed rock fragments vs frequency of occurrence of different size fractions in pseudotachylite follow the ‘modified power distribution law’ which is different from the pattern observed in cases of microbrecciation (Behera et al., 2017; Morozov et al, 2019). Similarly rounding factors of the clasts can be calculated (Behera et al., 2017). Lin (1999) has documented that roundness < 0.4 are indicative of cataclastic crushing (fig 8.1L) while roundness > 0.4 is suggestive of melting origin pseudotachylte

Psedotachylites that are later subjected to mylonitization will yield to crystal plasticity in the clasts but the degree of internal deformation will be much less than the host mylonites. Also biotite preferred orientation are generally developed in such mylonitized pseudotachylites. Metamorphosed pseudotachylites may be identifiable as long as degree of alteration or metamorphism is low. Due to their higher proportion of ferromagnesian and calcic components, the minerals developed in pseudotachylite veins will initially be different from the surrounding rocks and the vein shapes may still be discernible. However with progressive metamorphism, the chemical gradient between the melt-vein and adjoining rocks will tend to homogenize the system ultimately obliterating the vein itself

Hydrothermal breccias usually form at shallow crustal levels (<1 km) when seismic activity causes a void to open along a fault underground. The void draws in hot water/fluid and as pressure in the cavity is less than the surrounding rocks, the water violently boils. In addition, the sudden opening of a cavity causes rock at the sides of the fault to destabilize and implode inwards. The broken rock gets caught up in a churning mixture of rock, steam and boiling fluid. Rock fragments collide with each other and with the sides of the void and the angular fragments become more rounded. Minerals are precipitated in between the angular breccia fragments and cements the later into a compact rock. Botroidal structures are frequently developed in such cases. Often the left over spaces between the breccia fragments are occupied by drussy growth of the precipitating mineral. Breccia-hosted ore deposits are quite common.

The vesicles formed due to gas release from congealing magma form pathways for circulation of late fluids. Similarly hydrothermal solutions can form cavities to caverns in limestone or dolostone formations or in duricrast. Depending upon the circulating fluids and adjoining rock compositions, a variety of minerals including zeolite, quartz, calcite, epidote, gypsum etc are deposited in these open spaces. The depositing minerals can wholly or partly fill up the available space. Euhedral crystal terminations are often observed within the partially filled vugs or amygdules. These are called geoids

Vugs and Geoids can range in size from microscopic scale through every size range up to gigantic crystal caves like the Crystal Cave of Sierra Nevada, Cueva de las Espadas cave of Natica Mine at a depth of 120 metres, the giant geode of Pulpí, SE Spain, Ojo de la Reina Cave of Natica mine that was found in April 2000 and the Cueva de los Cristales, also discovered in 2000, that hosts the largest gypsum crystals known to date (~ 11 meters long). The crystals were formed from hypersaline brines in hypogenic systems. The hollows or open spaces in case of Natica mines were created due to movement along active fault systems. Research is being carried out at present on fluid inclusion microthermometry, mineralogy, spectroscopic signatures and isotopic constitution of these crystals to understand their implications in microenvironment reconstructions, palaeoenvironment study, age dating, planetary geology and astrobiological researches (fig 8.3e; Garquez et. al, 2016, Despain and Stock, 2005 and references therein)

Stalactites and stalagmites are often encountered in limestone cave systems. Stalagmites provide high resolution palaeoclimate records describing both local and global environmental conditions. Stalagmites are well suited for U-Th dating, δO18 analyses (which is inversely related to proportion of monsoon precipitation) and δC13 analyses suggesting soil productivity

Veins are common in all types of rocks including igneous, sedimentary and metamorphic. Fluid is generally associated with metamorphic processes unless the grade of metamorphism is very high and the system is dry. Differences in response of different minerals and rocks to stress lead to the formation of fractures and dilation sites. These low-pressure openings suck-in the percolating fluids. When solution enter such spaces, they undergo a fall in pressure, that leads to boiling, saturation of dissolved chemical components and their consequent precipitation leading to vein formation (Vernon, 2004). Increase in fluid pressure through progressive dehydration or decarbonation can even make Pfl ≥ σ3 so that fluid filled spaces or hydrofractures can open at any depth. Even under granulite facies condition, dehydration melting may lead to generation of a melt phase in the system.

Mineral-scale repeated incidences of cracking, infiltration and sealing can affect rocks and veins often leading to development of planar or curvilinear surfaces within minerals consisting of arrays of secondary mineral or fluid inclusions (fig 8.4a-b). However, massive veins can form at relatively low temperature through growth of minerals in open spaces. Botroydal structures or comb structure or geoid formation are common in such open space filling minerals (fig 8.4c).

The different categories of fibrous veins are as follow:

1.Syntaxial veins : Syntaxial veins are formed when crystals/ mineral fibers grow from wall rocks towards the median line of the vein. As the vein opens more, new materials are deposited on the existing crystals from solutions in the percolating fluid. This results in a symmetric or asymmetric vein structure with a median line. The median line is often marked by small opaque grains or a discontinuity in fiber fabric. In syntaxial veins, the fibers will always be perpendicular to the vein walls and the youngest growth is closest to median line while the oldest material is at the vein-wall rock contact (Durney and Ramsay, 1973, Vernon, 2004 and references therein). Syntaxial veins are more common when the minerals in host rock are same as those in veins (fig 8.4d).

2.Antitaxial veins: Vein minerals that are different from the wall rocks commonly grow from the contact of fibers and wall rocks, on two growth surfaces, towards the walls and away from the vein center. Thus the oldest material, in this case, is at the vein centre while the youngest material is at the contact of the vein with the wall rock (Durney and Ramsay, 1973). This type of growth is known as antitaxial growth. Complex veins with both synthetic and antithetic growth can also be observed.

3.Ataxial Veins: Fibrous veins may also form by repeated fracturing and growth at alternating different sites in the vein. Such non-localized ataxial cracking and sealing may produce veins with jogged or smooth fibers without a median line. Often fractured segment of a wall rock mineral grain can be identified on either walls of the vein while the elongated crystal in the vein is in optical continuity with both the fragments. This type of texture in ataxial vein is called stretched crystals.

4.Blocky texture in veins: A blocky texture is a texture in which grains are roughly equidimensional and randomly oriented. Blocky textures can be primary, if, during vein growth, nucleation of new grains continues. Blocky textures can, however, also be secondary and due to recrystallisation of a primary fibrous texture.

In greenschist to amphibolites facies metamorphic rocks, the veins generally are recrystallized and have intergrowth textures. Quartz and carbonate veins are common. But there may be veins of epidote, kyanite, sillimanite or andalusite suggesting that the apparently immobile alumina may, under suitable circumstances, be mobilized in veins (fig 8.4f,g). Quartz tourmaline veins are also common suggesting boron mobility in fluid (fig 8.4i). Random alignment of tourmaline suggests free growth in fluid medium. Fluid and other dissolved components in veins may react with wall rocks forming metasomatic zones. Mineralizing fluids veins can form mineral veins in rocks (fig 8.4j,k)

5.Elongate blocky texture: Crystals in an ‘elongate blocky texture’ (Fisher & Brantley 1992) are typically moderately elongate (length/width ratio generally in the order of 10) and the long axes of crystals are aligned (fig. 8.4L). This texture forms when nucleation of new grains does not occur during vein growth, and all growth is by crystallographically continuous overgrowths on existing grains and growth occurs at the tips of existing crystals. Elongate blocky textures show evidence for crystallographically controlled growth competition between grains (Mügge 1928). Crystals growing into a fluid typically show faceted morphologies as some crystal faces grow faster than others. Some grains, which are crystallographically oriented favourably with respect to the general growth direction, will outgrow unfavourably oriented grains. The faster growing 'winner' grains not only grow faster, but also wider, at the expense of the 'looser' grains.

6.Partial Melt Veins & Shock melt veins: Partial melting and migmatization can lead to development of a variety of melt pools and melt veins. Fig 8.4m and fig 8.4n are field photos of such partial melt pools and veins.

Shock Melt Veins are at times observed in meteorites. Fig 8.4o is such a shock melt vein of silicate glass with droplets of opaques (troilite, magnetite and Fe-Ni metals). The vein shows sharp contacts with the host meteorite. Fig 8.4p is a shock melt pool from the same sample which contains metal-sulfide droplets in a silicate glass mosaic.

Vein like tubular structure also develops in duricrust due to upward movement of solutions through capillary action at extremely shallow depth.

Foliation tends to swerve over preexisting rigid bodies like porphyroblast or porphyroclast leading to development of pressure shadows. Minerals dissolved through pressure solution from adjoining domains of compression are deposited in the pressure shadows that are effectively shielded from stress due to presence of the porphyroblast. Figures 6.6.2c-g present a set of pressure shadows developed around porphyroblasts. Similar pressure shadows can also develop around porphyroclasts

During deformation under low temperature near a rigid object within a more ductile rock material, increased pressure solution may occur adjacent to the rigid object on the side of shortening ISA, while extensional gashes may open at the contact of the object and the matrix on the side of extensional ISA. Materials dissolved from zones of compression will permeate into these dilational zones and crystallize as fibrous material at the contact of rigid body and the matrix. As deformation and dilation continues, new materials are added at this zone in continuation with older fibers already crystallized which now moves away from the contact line. Thus progressive addition to the fibers always occurs at the interface with the rigid body and the youngest parts of the fibers are always in this domain (Passchier and Trouw, 1998). Such structure is known as pressure fringe. Fibers may be syntaxial, like calcite fibers on crinoidal stems. But more commonly the fibers are antitaxial like quartz fibers on rigid clasts of magnetite or pyrite. The geometry (shape and orientation) of the fibers depend on the shape of the core object, its initial orientation with respect to kinematic axes, roughness of its surface, whether growth is face controlled or displacement controlled, whether pure or simple shear is dominant and whether the fibers have been deformed (Passchier and Trouw, 1998, Chatalov, 2014). While face-controlled fibers grow perpendicular to the growth surface, displacement-controlled fringes track the separation of the matrix from the core object irrespective or the orientation of the later (Ramsay and Huber, 1983). If the core object surface is smooth, face controlled fibers tend to develop growing towards to object surface, independent of the relative motion of fringe and object. Fringe structures with both face-controlled and displacement-controlled fibers grow preferentially on rough core objects (Koehn et. al., 2000). Possible geometries of face-controlled and displacement controlled pressure fringe for both co-axial and non-coaxial domains have been diagrammatically produced in details in Passchier and Trouw (1998) and have been reproduced from their work in fig 8.6a and 8.6b.

Tension gashes are zones of dilation formed at an angle of 135ᵒ to the extensional ISA. They are later rotated and new tension gashes may form transecting the older ones. In low temperature deformation, pressure solution may remove materials from surroundings and redeposit them in these dilation domains. Depending on scale, such features may be mesoscopic or microscopic

Solutions or melts may flow into boudin necks and precipitate / crystallize in these dilational domains.

This chapter deals with short descriptions of vugs and veins that are often encountered in rocks. The sections are as per present status of knowledge in this subject as worked out by many workers over time. The material has been contributed by scientists both from within and outside GSI. We thank them all for their excellent contributions. In this connection, we would specially like to mention an wonderful set of microphotographs of strain fringe adjoining pyrite porphyroclasts contributed by Professor Dipak C Pal, and Professor Susanta K Samanta from Jadavpur University. We thank them for the wonderful set of text book type photos. We express our sincere gratitude to Dr. Abhinaba Roy who has not only carried out a thorough review of the chapter, but has himself contributed his life collection of excellent microphotographs for the benefit of young learning officers, students and researchers.