Petrography of Shock-Metamorphosed Crystalline Rocks and Impactites

Masaitis, Victor L.; Raikhlin, Anatoly I.; Selivanovskaya, Tatjana V.; Mashchak, Mikhail S.; Naumov, Mikhail V.

doi:10.1007/978-3-319-77988-1_4

Victor L. Masaitis⁵,
Anatoly I. Raikhlin⁵,
Tatjana V. Selivanovskaya⁵,
Mikhail S. Mashchak⁵ &
…
Mikhail V. Naumov⁵

Part of the book series: Impact Studies ((IMPACTSTUD))

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Abstract

The gneisses of the crystalline basement, mainly garnet-biotite varieties including graphite-bearing ones were transformed under the influence of a powerful shock wave and carry numerous shock metamorphic features. They represent the initial substrate for the appearance of large volume of an impact melt induced by shock. This melt solidified in the form of fragment-bearing continuous masses (tagamites) or in the form of bombs, lapilli and ash particles mixed with crushed material of altered to a variable degree crystalline and sedimentary rocks (suevites). Tagamites and suevites, among which, according to petrographic features, a number of varieties are distinguished, represent the main types of rocks containing impact diamonds. These rocks were partially altered during cooling by the action of meteoric water.

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4.1 Crystalline Target Rocks and Their Shock Metamorphic Features

Petrography of shock-metamorphosed rocks and impactites in the Popigai astrobleme was first characterized by the authors in a number of papers (Masaitis and Selivanovskaya 1972; Masaitis et al. 1971, 1972, 1975, 1980 etc.); they are also described in some publications of other researchers (Vishnevsky and Montanary 1999; Whitehead et al. 2002).

As noted above, various features of shock metamorphism are recorded in crystalline rocks of peak ring, and, partly, within the floor of the annular trough. Shock metamorphism is much more distinct in the clastic material comprised in allogenic lithic breccias, suevites, and tagamites. At the same time, in the two last-mentioned rock varieties, features of shock metamorphism of the therein-enclosed fragments and blocks are often vague due to the subsequent impact of high temperatures, as well as superimposed hydrothermal alterations. Sedimentary rocks occurring in allogenic breccias as blocks and fragments, display shock alteration, which is much weaker as compared to gneiss; it is mainly manifested as jointing, crushing, mortar structures, and shatter cones.

4.1.1 Brief Characteristic of Gneisses

As shown above, the rocks making up the true bottom of the impact structure and studied mainly in its western part, are predominantly represented by high-alumina gneisses and plagiogneisses, often graphite-bearing (biotite-garnet, biotite-garnet-sillimanite, biotite-garnet-sillimanite-cordierite) and calcareous-silicate garnet-bearing gneisses and plagiogneisses (biotite-hypersthene, biotite-bipyroxene, and biotite-salite). There are also lenses of high-calcareous rocks (marbles, calci phyres, salite-scapolite rocks). Charnokites occur in migmatization and granitization zones. Allogenic lithic breccias and impactites contain crystalline rock fragments of all above varieties; however, shocked fragments are dominated by garnet-bearing gneisses and plagiogneisses. Only tagamites in the southeastern sector are noted for the predominance of pyroxene gneisses and plagiogneisses among crystalline rocks inclusions.

Gneisses and plagiogneisses are leuco-, meso-, and melanocratic rocks with the grain size of 0.5–2 mm and gneissic structure. Commonly, they are of the granoblastic texture combining with poikiloblastic texture. Garnet gneisses are frequently noted for the porphyroblastic texture; sillimanite gneisses, for the nematoblastic texture; and cordierite gneisses, for the symplectic one. The main rock-forming minerals are plagioclase (35–70%), quartz (10–35%), and biotite (2–17%). Orthoclase constitutes up to 30% of the gneiss volume; in granitized varieties, microcline develops. In pyroxene gneisses and plagiogneisses, hypersthene constitutes up to 25 vol%, while salite, up to 45 vol%. In melanocratic varieties, the summary content of pyroxenes reaches 80 vol% or even more. In biotite-garnet gneiss, garnet takes up about a half of the rock volume in places, and sillimanite in biotite-garnet-sillimanite gneiss, to 20 vol%. Cordierite in the corresponding rocks makes up their essential portion, i.e. up to 30–45%. High-alumina rocks also contain the first percents of graphite, spinel, and hypersthene. Calcic-silicate lithologies contain garnet and titanite. Tenths of a percent are made up by magnetite, ilmenite, pyrrhotite, apatite, zircon, rutile, monazite; in cordierite-bearing gneisses, chromite occurs in addition.

Plagioclase is represented by acid and intermediate andesine (Table 4.1), sericitized locally. In melanocratic plagiogneisses, it corresponds to the basic andesine An_46–48. There are poikiloblasts with quartz, hypersthene, and salite inclusions. Quartz occurs irregularly forming lenticular segregations aligned with banding. Alkali feldspar is represented by the orthoclase granoblasts with microperthite plagioclase growths (Table 3.1), corresponding by its composition to orthoclase-microperthite from pyroxene granulites (Dobretsov et al. 1971). Porphyroblasts of latticed microcline also occur on granitization areas.

Table 4.1 Average compositions of main minerals from gneiss (EMP data)

Full size table

Salite forms irregular or oblong prismatic grains of a light green colour. In crystalline schists, it is the main rock-forming mineral and is represented by poikiloblasts with frequent growths of basic plagioclase. It often localizes as bands along banding of the rock. The composition of salite (Table 4.1) is characterized by a high and stable magnesium content, a low iron content and a low CaO/(CaO+MgO+FeO) ratio, which corresponds to the intermediate salite-augite, characteristic of granulite complexes (Dobretsov et al. 1971). In the bipyroxene gneisses, composition of salite is noted for a marked reduction of iron content and increase of calcium content, along with a constantly high magnesium content, which is directly dependent on the composition of initial rocks. In granitization areas, salite is partly or fully replaced by the brown-green hastingsite hornblende (Table 4.1).

Hypersthene most often occurs as relatively large prisms with a sinuous outline. There are frequent poikilocrysts to 2–3 mm with plagioclase and quartz inclusions. The characteristic feature of the composition (Table 4.1) is a constantly high content of alumina, a low content of calcium, and magnesium to iron ratio, which is typical of the granulite complex of the Anabar Shield (Vishnevsky 1978). The observed differences in the content of iron, magnesium, and alumina are directly related to the concentration of these components in host rocks. Primarily hypersthenes from parageneses with spinel are noted for a high alumina content (6.13% Al₂O₃). The highest magnesium content (26.44%) is recorded in hypersthene from bipyroxene plagiogneisses. Along fissures and around the periphery of grains, and, in rare cases, totally, hypersthene is replaced by the finely structured serpentine-chlorite aggregate of a light brown colour, possibly, bastite (Table 4.1).

Biotite commonly occurs as bands in the rock and is represented by yellow-brown scales, oriented along banding. Its composition (Table 4.1) is characterized by a constantly high titanium content. In the rock series from calcareous-silicate to high-alumina, the content of titanium, alumina, and magnesium in biotite increases, and that of silica, iron, and manganese decreases. Such a change in the composition might be due not only to the composition of host rocks, but also to the increasing metamorphic grade; there occurs a reduction of Mn, Fe content and a simultaneous increase of Ti, Mg, Cr, and V amounts. Biotites in rocks of a low metamorphic grade are characterized by a higher Si content and a lower Al content as compared to biotites in rocks of the intermediate and high grades (Perchuk et al. 1985).

Garnet is represented by rounded, in places faced crystals or porphyry poikiloblasts up to 6–8 mm of the pyrope-almandine series (Table 4.1). Its composition varires insignificantly; it is characterized by a high-alumina and iron content and a rather low content of the grossular constituent. Such features are typical of garnets from granulite complexes (Vishnevsky 1978).

Sillimanite forms oblong colourless prisms with rounded edges and commonly with distinct transverse cleavage. Locally, certain individuals are replaced by the low-birefringent micaceous aggregate. This mineral is often intergrown with garnet. The composition of sillimanite (Table 4.1) is noted for the low silica content, increased alumina content, and an admixture of abundant constituents, which are more typical of kyanite.

Cordierite commonly forms large grains with a composite polysynthetically sectorial twinning. In places, pleochroic halos around small zircon, rutile, and monazite grains are observed. Often particularly in granitization areas, it is replaced by serpophite or talc-serpentine aggregate. The composition of cordierite (Table 4.1) is noted for a slightly elevated total content of iron and magnesium. It constantly contains small simplectite growths of brown-green spinel and, in places, chromite of different shape. By its composition (Table 4.1), spinel corresponds to magnesioferrite with chromium content of 1.3–4.1%.

Graphite prevails as oblong laminae with the sides ratio from 1:4 to 1:10 (Fig. 4.1). In rare instances, there occur relatively isometric plates. There were intergrowths of several differently oriented scales; in quartzitic gneisses graphite segregations reach several millimetres. The predominant size of the latter is 0.4–0.8 mm; however, there are also scales, which are up to 2 mm long. Graphite is mostly confined to the quartz-feldspar areas, where it mainly occurs as intergrowths in quartz and feldspars; in these cases, such segregations might be up to the first centimeters across. There are quartz and feldspar granoblasts, containing accumulations of differently oriented graphite scales. The highest graphite contents are characteristic of biotite-garnet-sillimanite-cordierite gneisses.

The compositions of the main rock-forming minerals (Table 4.1) and the observed parageneses from the abyssal (Hy+Cord+Ort, Sill+Gr+Bt, Gr+Hy+CPx) to the medium-depth (Sill+Gr+Cord+Ort) subfacies (Vishnevsky 1978; Bucher and Grapes 2011) indicate that gneisses and plagiogneisses formed under granulite facies of moderate and elevated pressures.

Marbles and calciphyres are medium-, more frequently coarse-grained rocks of yellow, cream, or greenish-grey colour with the prevailing heterogranoblastic texture and gneissoid, or, in places, massive structure. Monomineral marbles are rare; there are frequent mica, graphite, in places diopside scales. Most of calciphyres are composed of calcite (70–80%) and subsidiary diopside; in places, wollastonite, feldspars, and quartz occur in addition. There are calciphyres with a more diverse composition, where silicate minerals exceed the amount of calcite. Along with the constantly present calcite, feldspars and quartz, they also comprise different shares of diopside, wollastonite, scapolite, phlogopite, talc, serpentine, forsterite, or more often its replacement products, as well as spinel, titanite, and apatite.

Scapolite-salite rocks form lenses or separate bands among calciphyres or bipyroxene and salite plagiogneisses. There are inequigranular meso-melanocratic rocks of greenish-grey colour with diablastic or granoblastic texture and massive structure. They are made up of approximately equal amounts of salite and scapolite or scapolitized plagioclase, which are in intergrowth. Salite has a bright green colour, Inferred from its optical properties corresponds to of ferruginous diopside. Scapolite is represented by high-calcium meionite; and plagioclase, by andesine or labrador. The rock also constantly contains calcite as granoblasts and titanite as accumulations of small, often irregular grains; their content can reach 3–5%. Ore minerals and apatite are rare.

Gneissoid hypersthene granite (charnokites) are particularly widespread among pyroxene gneisses. These are medium- or coarse-grained inequigranular rocks of a brownish-yellow, less frequently yellowish-red colour. Their texture is granoblastic or blastocataclastic; less frequently allotriomorphic granular and porphyroid. The main rock volume is taken up by plagioclase, potassium feldspar and quartz, with the general predominance of plagioclase. Hypersthene, biotite, plus, in places, salite and hornblende do not exceed 15 vol%. Among the accesories, magnetite, pyrrhotite, allanite, and apatite are characteristic. Plagioclase is often represented by sericitized oligoclase or andesine; and alkali feldspar, by orthoclase, or, in migmatization zones, by microcline. Migmatites accompanying charnokites have a coarse crystalline texture and an irregular distribution of the constituent minerals, represented by quartz, microcline, and plagioclase in different proportions, with rare biotite scales and single magnetite and zircon grains.

4.1.2 Mineralogical Criteria of the Shock Metamorphic Parameters

Data on alteration of tectosilicates are commonly used as the basic criteria of the shock compression degree of crystalline rocks and the constituent minerals (Stöffler 1971; Val’ter and Ryabenko 1977; Ryabenko 1982; Basilevsky et al. 1983; Feldman 1990; Stöffler and Langenhorst 1994; Grieve et al. 1996; French and Koeberl 2010, etc.).

There were repeated attempts to determine the value of the loads applied with a very high precision, for example, to tenths of GPa. In this connection, it is necessary to make a few remarks. Firstly, the precise evaluations have certain sense only if the samples are taken from the autochthon or parautochthon (i.e. target rocks), which can allow to restore the zonation of the character of their attenuation in space, etc. Secondly, the differences in the initial temperature of rocks, subject to shock compression, which are difficult to access and which affect the intensity of alterations under the same load, should be taken into account. Thirdly, account should be taken of a significant dynamic inhomogeneity of rock masses, of a complex action of the stress waves on them, particularly in case of large-scale impact processes, influence of fluid phases, etc. The last two circumstances, which are not easily registered precisely, can result in the shift of estimates, amounting to 5–10 GPa. In most cases, the use of the integral averaged estimates of the shock alteration parameters in rock volumes about n × 10⁶ to n × 10⁸ m³ is commonly quite sufficient for the practical purposes.

Another feature of the shock wave alterations in rock masses, which is not always taken into account in the analysis of these processes, should be emphasized. These are the shear stresses, which are accompanying or directly following the rarefaction wave, and can result in brittle destruction of rocks even under a relatively weak shock compression; and in case of significant shock loads, to flow of the molten material. This leads to crushing, cataclasis, mixing of rock substance and, eventually, to generation of homogenized impact melt masses.

In the present section, the evaluations of the shock compression amplitude are given, proceeding from the main criteria, established using experimental evidence (Stöffler and Langenhorst 1994; Grieve et al. 1996) and generally presented in Table 4.2.

Table 4.2 Progressive shock metamorphism of crystalline feldspar-bearing rocks (Grieve et al. 1996)

Full size table

Though there are not distinct boundaries between the distinguished alteration stages, they should be considered within certain intervals of the shock loads, applied to the rocks and constituent minerals.

4.1.3 Weakly and Moderately Shock-Metamorphosed Rocks

Gneisses and other crystalline rocks, subject to weak (to 10–20 GPa) and moderate (20–35 GPa) shock loads, are characterized by different diaplectic altrerations, including brittle deformations of quartz and feldspars, to a lesser extent biotite and other rock-forming minerals. In cases, when shock compression is accompanied by shear stresses, impact cataclastites form, which have a breccia, banded or lenticular fluidal structure and are often made up of turned, displaced fragments of different size, separated by bands and lenses of cataclastic material.

Irregular jointing develops in quartz under weak alteration conditions and the displacement of certain parts of crystals occurs, resulting in mosaic attenuation and block character. Under a more intense impact, planar fractures (PF) become widespread and, mainly planar deformation features (PDF), their number in quartz grains ranging from one to 5–7 (Fig. 4.2). Their orientation relative to the crystallographic axis C can be used as a geobarometer (Grieve et al. 1996). Quartz crystals, where the number of PDF systems exceeds three and, particularly, if there are five or six systems are characterized by a noticeable change of some physical properties, including the decrease of the mean refractive index from 1.548 to 1.536 and below, and picnometric density from 2.65 to 2.635 and even 2.615 g/cm³. Certain grains display slightly birefringent (to 0.002) or isotropic areas, in which PDF seem to be attenuating. Certain PDF systems are decorated by stishovite in the form of the finest (several mm) needles and laminae. Stishovite, clearly diagnosed by X-ray techniques (Vishnevsky et al. 1975), is mainly generated under loads of 12–20 GPa or more.

Plagioclase in rocks of the considered group is characterized by jointing, but, which is more typical, by the appearance of deformation lamellae; to a lesser extent, PDE (one or two systems, which are predominantly parallel to the albite twin systems). In plagioclases of the intermediate composition (An_27–35), the refractive index is decreased by 0.017–0.019; respectively, there is a density reduction from 2.66 to 2.62 g/cm³. Often, plagioclase grains display certain spots and irregular isotropized areas, as well as partly or fully isotropized system of twins, with birefringence being retained in the other system. The last-mentioned effects, judging by experimental evidence, point to a moderate shock compression to 35 GPa.

Alkali feldspars (orthoclase etc.) under weak and moderate shock compression are characterized by development of the deformation bands and one to three PDE systems; the refractive index is decreased by approximately 0.006–0.009; birefringence drops to 0.003–0.004 up to the partial isotropization; density decreases from 2.56–2.58 to 2.52 g/cm³ (Raikhlin et al. 1979).

Biotite, altered under the above loads, is noted for the development of kink bands, as well as one to three PDF systems, oriented parallel to (111), (111), (112), (112), etc. In certain cases, the reduction of birefringence and loss of pleochroism are recorded.

Garnet bears almost no traces of alteration, except a slight jointing, which is enhanced in rocks, subject to moderate alterations under shock compression to 20–35 GPa. Grains are cut into microblocks by the network of irregular fissures; their transparency decreases, and the intensity of the colour increases, which asquires brownish hues.

Pyroxenes, similar to garnet, are resistant to the moderate shock compression. Hypersthene and salite, subject to loads below 30 GPa, are characterized by an irregular jointing, more pronounced in rocks, undergoing more intense alterations; in this case, crystals appear to be cut by abundant coarse fissures.

Other rock-forming minerals (sillimanite, cordierite, spinel and some others) do not reveal any noticeable mechanical and optical alterations within the considered shock load ranges.

4.1.4 Intensely Shock-Metamorphosed Rocks

Crystalline rocks, subject to intense shock compression, are noted for marked changes in the character of minerals, which are particularly pronounced in thin sections, as well as a certain reduction of density, increase of porosity, etc. Tectosilicates are commonly transformed to diaplectic glass, and a simultaneous dynamic action results in generation of cataclasites.

The main rock-forming minerals of gneisses, compressed to 35–45 GPa, can be characterized as follows.

Quartz transforms into diaplectic glass under the pressure above 35 GPa; glass has a brownish colour and the characteristic shagreen surface. In places, domains of low-birefringent quartz are discernible in it; and relics of the PDF systems are visible in transmitted light. A significant feature of the impact of high pressures is the transformation of a part of quartz and diaplectic quartz into coesite in the rocks under consideration (Masaitis et al. 1974). Coesite forms kidney-shaped accumulations, intersecting veins, and grain aggregates up to 100–200 mm and more (Fig. 4.3). It is colourless, or locally has a slightly brownish colour and is slightly birefringent: 2V = 57°–61°, Ng = 1.602 ± 0.002, Np = 1.597 ± 0.002.

Plagioclase in gneisses (An_27–35), subject to intense shock compression, transforms into diaplectic glass (maskelynite) with the refractive index of 1.525, though, in certain cases, vaguely outlined areas with a weak birefringence can be distinguished in its isotropic mass.

Orthoclase and orthoclase-perthite undergo alterations, similar to those of plagioclase, when subject to loads above 35 GPa. Disordered orthoclase represents an amorphous phase, retaining the shape of the initial crystal and with the refractive index of 1.518–1.520 and the density of 2.5–2.48 g/cm³ (Raikhlin et al. 1979).

Biotite laminae are characterized by weakening or complete loss of pleochroism; there occurs a partial thermal decomposition with generation of fine-grained opaque aggregate accumulations of ilmenite, hypersthene, alkali feldspar, and glass.

In garnet, abundant fissures develop within the considered interval of alterations including planar fractures; it becomes opaque and asquires a brounish and dark grey colour at the expense of new phases, appearing in the course of its thermal decomposition, i.e. hypersthene, hercynite, and glass (Gnevushev et al. 1982; Kaminskaya et al. 1986; Kozlov et al. 1987; Feldman 1990). Locally, spots of non-decomposed garnet are preserved.

Pyroxene is noted for the slightly decreased birefringence, wavy extinction, appearance of planar fractures.

Cordierite in the considered group of crystalline rocks is transformed into colorless or brownish diaplectic glass, composition of which being given in Table 4.3.

Table 4.3 Chemical composition of homogeneous (“momomineral”) fusion glasses and products of their devitrification from vitrified gneisses (“protoimpactites”)

Full size table

Sillimanite, forming prismatic grains, is characterized by coarse transverse fracturing; its colouring is intensified in places.

Graphite, which is present in rocks, is also subjected to alteration and transforms to diamond; the specific features of this alteration are considered below, as well as under Chap. 6.

4.1.5 Very Intensely Shock-Metamorphosed Rocks

Gneisses subjected to very strong shock transformations, commonly above 45 GPa, are of utmost interest. If the rocks assigned to this group appearing to be enclosed into impactites as blocks and fragments and undergo additional heating, recrystallization of minerals and different glass results to essential changes in the rock habit and, in places, to an almost complete disappearance of the shock metamorphic features. This leads to significant difficulties in distinguishing the effects caused by the shock load proper (including the post-shock temperature) and resulting from subsequent annealing. In this sense, for analysing the shock effects proper, it would be preferable to study the fragments, subject to chilling or quick cooling after the pressure dropping.

Under shock compression above 45–50 GPa and residual temperature above 1,100°, there occurs a partial or complete melting of quartz, which after cooling solidifies to form quartz glass or lechatelierite with the refractive index 1.460–1.462. Lechatelierite often encloses rounded pores, has a fluidal texture resulting from the starting flow of the material. During slow cooling, lechatelierite recrystallizes, often forming cristobalite and α-quartz. The former is easily diagnosed by the characteristic ball-texture, while the latter commonly replaces cristobalite and also forms mosaic aggregate. Lechatelierite, partly transformed, often occurs in suevites and lithic microbreccias as minor independent bombs and fragments.

Under strong compression, plagioclase fusion glass appears (Table 4.3), it commonly being characterized by porosity and fluidal texture. Refractive indices of this glass (which is of oligoclase-andesine composition), are lower as compared to those of maskelynite (about 1.520–1.523). During plagioclase transition into fusion glass, the supply of potassium and silica, and the removal of calcium and, particularly, sodium occurs (Sazonova and Korotayeva 1989). Slow cooling leads to recrystallization of both fusion glass and maskelynite with generation of spherulitic and radially-fibrous aggregates of neogenetic plagioclase, which is commonly more high-temperature (Table 4.4).

Table 4.4 Representative EMP analyses of products of thermal alteration of some original minerals

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Orthoclase transforms into monomineral fusion glass (Table 4.3) with porosity and fluidal texture. Its density falls to 2.35–2.27 g/cm³; refractive index, to 1.502–1.505. Chemical changes show up as the increasing silica content, and to a lesser extent, aluminium and calcium contents, and an abrupt reduction of potassium (Sazonova and Korotayeva 1989). Similar to plagioclase fusion glass, these data point to a significant role of the volatile transport. Fusion glass after orthoclase during annealing undergoes recrystallization with generation of radially fibrous sanidine aggregates (Table 4.4).

Thereby, the main constituents of intensely compressed gneisses, i.e. quartz and feldspars, are, to a major degree, transformed into commonly vesicular fusion glass. The latter in most cases is similar by composition to mineral precursors and may be regarded as “monomineralic”. Mixing of these melts along the differential moving of transforming rocks led to the appearance of “polymineralic” i.e. homogenized impact melt. Original rocks partly consisting of “monomineralic” glass, usually retain the initial structural and textural features. This allowed once calling them “protoimpactites” (Masaitis 1983), unlike tagamites, which are also mainly made up of impact glass (however, totally homogenized) and have quite different textural and structural features. Though the term “protoimpactite” is not quite strict, it will be applied below when necessary. The chilled specimens of such rocks occur in vitroclastic and vitrolithoclastic suevites, which are relatively cool at the moment of deposition. Depending on the amount of glass, different protoimpactite varieties can be distinguished, containing 10–30, 30–80 and over 80% of products of the shock melting of mineral species.

In the first variety, an essential part of tectosilicates is molten, though there are areas of preserved diaplectic plagioclase glass, but to a major degree, diaplectic quartz. The general initial structure is not disturbed. Coloured minerals usually show shock deformations, and features of thermal decomposition, particularly biotite. With increasing melting degree against the background of the relict initial texture, lenticular banded areas appear, made up of porous fusion glass after orthoclase, and to a lesser extent, after plagioclase. Feldspar glass often encloses isometric, angular rounded particles of diaplectic glass and lechatelierite. Garnet retains the initial shape of grains; however, it is transformed into the black mass of decomposition products. Biotite appears as brownish semitransparent isotropic substance (Table 4.3). In certain cases, cordierite fusion glass is recorded (Table 4.3). In protoimpactites, characterized by the maximum melting degree, salic constituents are fully molten, and dark-coloured minerals are completely altered as noted above, and cut into blocks subsided into monomineral fusion glass. Locally, areas of mixing of such glass occur, which might be joined by the substance of molten cordierite or coloured minerals. Garnet and pyroxenes are transformed into opaque structureless masses, commonly recrystallized. Detailed research shows that after garnet, hercynite, ferrogortonolite, hypersthene, and cordierite can form in different proportions (Table 4.4), as well as glass. (Masaitis 1978). Recrystallization commonly proceeds under the actions of heat of the surrounding rocks or melt. Biotite is transformed into pink-brown glass, whith encloses very small idiomorphic spinel crystals. Commonly, quartz and feldspar glass are also recrystallized with generation of mosaic or granoblastic quartz, plagioclase microlites and spherulites, high-temperature feldspar, respectively.

In certain cases, differential movements of this molten material result in generation of fluidal structures; porphyroblasts of altered garnet are deformed and extended. Along certain planes, where such movements were most intense, thin veinlets, minor spots of black glass corresponding in its composition to the mixture of all the initial constituents and essentially representing the initial generation phase of large masses of impact melt, form in rocks. Evaluations of shock loads and temperatures, under which the above alterations occur and protoimpactites appear, are, respectively, within 45–60 GPa and 900°–1300°, or, less frequently, more.

Gneiss fragments containing initial graphite transformed in impact diamonds to a certain degree, often occur as inclusions in suevites and tagamites. In addition to shock alterations, they are commonly strongly recrystallized. Such inclusions were studied, especially to establish the character of parasteresis association of the transformation products of rock-forming minerals and evaluating the parameters of graphite transformation to diamond.

The main coloured minerals presented in the described group of graphite- and diamond-bearing gneisses, are garnet, biotite, and, less frequently, sillimanite, all these constituting 20% of the rock volume or more. All of them are, to a major extent, altered, apart from quartz and feldspars, the shock alteration products of which being represented by fine-grained aggregate accumulations of new-formed minerals. At the same time, in certain cases, when the protoimpactites underwent quick chilling, they are totally made up of fresh pumiceous monomineral glass after quartz and feldspars. In places, the latter are subject to mixing to a minor extent and enclose opaque palimpsest porphyroblasts of garnet transformed (inferred from X-ray patterns) into the mixture of very small (to 10 mkm) hypersthene and hercynite grains.

These rocks enclose in places diamonds, which were recorded in them not only by studying of the residues of their termochemical decomposition, but also by direct observations in petrographic thin sections under the microscope (Fig. 4.4). Relatively larger diamond particles are disintegrated during preparation of thin sections (they in places leave distinct scratches on the surface of the rock section), but flat tabular grains less than 0.03 mm thick are preserved. In some cases, several small diamond grains can be seen simultaneously within the thin section. They are enclosed into the recrystallized diaplectic glass or fusion glass fragments of predominantly plagioclase composition, diamonds also occur among recrystallization products of quartz glass, as well as within altered garnet in places. Such rocks also commonly contain, apart from diamond, laminae and scales of shock-metamorphosed graphite.

All these observations, as well as the reconstructions of the state of tectosilicates altered in the shock wave prior to their annealing in the intensely heated environment, show that the crystalline rocks, in which impact diamonds occur, were subject to the shock load of about 40–60 GPa. The possible lower limit of the evaluation of pressure, at which graphite starts to transform to diamond, is indicated by the presence of the latter in gneisses, where only plagioclase is fully transformed to maskelynite (about 35 GPa). In most cases, diamonds and shock-metamorphosed graphite, which commonly coexists with them, are associated with diaplectic glass and melting glass both after feldspar and after quartz. These gneisses, possibly, belong to the III and, partly, to the IV shock metamorphic grades.

4.2 Tagamites

4.2.1 Mineral Composition and Texture

Tagamites of the Popigai impact structure are characterized in a number of publications (Masaitis et al. 1975, 1980, 1983; Selivanovskaya 1977, 1987; Masaitis 1983, 1994; Raikhlin et al. 1983, 1987; Mashchak and Selivanovskaya 1988; Vishnevsky and Montanary 1999; Whitehead et al. 2002, etc.). They are massive, less frequently porous or ataxic rocks of aphanitic image, that have almost black, dark grey, grey-lilac or light grey colour. They are made up of glassy or, to a certain extent, crystallized matrix, into which the fragments mainly of crystalline rocks and their minerals are subsided (Fig. 4.5). Relatively larger inclusions (from the first cm to several metres) commonly contribute no more than 3–5% and are regularly distributed, there are some areas where blocks constitute 40% of tagamite volume or more. Minor inclusions (tenths of a mm—the first centimetres) take up from 5–10 to 25–30% and often form shlieren-like and band-like accumulations. Porous tagamites contain to 15–20% or more rounded or ellipsoid-like oblong pores from 1–2 to 5–8 mm across, or, less frequently, larger; in places, there are caverns to 5–8 cm. Tagamite matrix is holohyaline, hemicrystalline or holocrystalline; it contains plagioclase, orthopyroxene, lesser amounts of other minerals, as well as glassy or crystallized groundmass.

Inclusions in tagamites are represented by diverse crystalline (to 90%) and sedimentary (to 10%) target rocks (lithoclasts) and their minerals (crystalloclasts). Crystalline rocks are dominated by biotite-garnet, biotite-pyroxene, bipyroxene and other gneisses and plagiogneisses (including shock metamorphosed ones); dolerites and some other rocks are less frequent. Among the sedimentary rocks, quartzites, siltstones, sandstones, shales, coaly mudstones, limestones, and dolomites are frequent. Crystalloclasts, formed due to destruction of all these rocks, represented by quartz and feldspars (prevail); garnet (almandine), hypersthene, salite, ilmenite, magnetite. Hornblende, sillimanite, and other minerals are rare. There are also graphite and impact diamonds. The heavy fraction of tagamites is dominated by pyroxene (mainly hypersthene); almandine, ilmenite, magnetite and pyrrhotite spherules, and limonite occur; there are minor amounts of sillimanite, zircon, apatite, rutile, mullite, kyanite, epidote, titanite, hornblende, biotite, rarely other sulphides (pyrite, sphalerite, galena, chalcopyrite), as well as moissanite. The structures and mineral composition of rock inclusions from tagamites (as well as from suevites) carry many expressive features of their transformation and interrelation with host impactite (Masaitis 1976).

For the purpose of subdivision of tagamites, primarily microscopic textural features are used, reflecting the temperature regime of impact melt generation and its cooling. Proceeding from these features, tagamites are classified under the high-temperature (HT) and low-temperature (LT) varieties, forming from the impact melt fractions with different initial temperature. As noted above, the bodies, made up of these varieties, are often in complex geological relationships; peculiar heterotaxial tagamites form, they made up of areas, spots, blocks, etc., of both varieties, commonly with abrupt contacts between them (Figs. 4.6 and 4.7).

Petromagnetic studies revealed significant differences between HT- and LT-tagamites, resulting from the domain structure of ferromagnetic substance in these rocks (Gorshkov and Starunov 1981, etc.). HT-tagamites mainly contain finely dispersed grains of ferromagnetic pyrrhotite in supermagnetic state, whereas in LT-tagamites, this mineral is represented by multi- and uni-domain grains. Results of measurements on natural residual magnetization (I_n) and magnetic susceptibility (κ) demonstrated that I_n of HT-tagamites is 1–2 orders lower than that of LT-tagamites, whereas κ value for these rocks is of the same order (Fig. 4.8); these features are not caused by alteration processes, but are due to the generation and crystallization regime of different impact melt fractions.

4.2.2 Low-Temperature Tagamites

These rocks occur ubiquitously within the Popigai structure; they form bodies of different size and shape. LT-tagamites were studied in most detail within the Majachika Upland and in the Balagan-Yuryage River Basin, as well as in some areas in the southern, northeastern, and northern sectors of the impact structure.

Most of holohyaline tagamites are represented by significantly altered glass (from 70 to 90%), which has massive, fluidal, breccia-like ataxitic structure and contains about 25–30% minor inclusions. These rocks are characteristics of the marginal parts of large bodies and also form small isolated bodies. The glassy matrix is brownish or brownish-grey under the microscope, which is commonly caused by its alteration. The greatest contribution into the general content of clasts of all fractions (60–70% of the total volume of clasts) is made by microinclusions (less than 0.5 cm) and, to a lesser extent (about 10%) fragments of the fraction of 3–5 cm (Fig. 4.9). Crystalloclasts mostly display no features of significant shock alterations, there are inclusions of monomineral fusion glass and partly crystallized diaplectic glas, though.

Hemicrystalline tagamites are characterized by a partial crystallization of the matrix glass, they contain to 10–15% rock and mineral fragments. These rocks occur in the inner parts of sheeted bodies, they also make up minor independent bodies. Hemicrystalline tagamites with microlitic and cryptocrystalline texture of the groundmass can be distinguished. The former are characterized by development of small (0.01–0.02 mm, less frequently larger) prisms of orthopyroxene in the glassy, mostly altered opaque matrix (Table 4.5) and microlites of acicular and prismatic andesine-labradorite (Table 4.6); the content of plagioclase is approximately @at 1.5 times higher, than that of pyroxene. Residual glass in the considered rocks is enriched in silica, and to a lesser extent, in alkalies (Table 4.7). Quartz and feldspar clasts often bear traces of shock metamorphism, as well as recrystallization; their contours are vague. Hemicrystalline tagamites, which form cement of megabreccia, are characterized by hyalopilitic or pilotaxitic texture; acicular plagioclase microlites peaking 0.1–0.2 mm long in places, can constitute up to 40% of the rock volume. Small isometric hypersthene crystals occur in interstices together with non-crystallized semitransparent basis.

Table 4.5 Average compositions (wt%) of pyroxenes from tagamites (EMP analyses)

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Table 4.6 Average compositions (wt%) of minerals from tagamite matrix as measured by EMP analyses

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Table 4.7 Average compositions (wt%) of residual glass from tagamites and buchites as measured by EMP analysis

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Hemicrystalline tagamites with cryptocrystalline texture are commonly characterized by irregularly mottled structure, which is due to crystallization in the basis of the finest hundredths and thousandths of a mm) isometric quartz and cristobalite grains and their accumulations. Plagioclase microlites, enclosed within this basis, are represented by labradorite, frequently zonal and more acid in the marginal part of the grains; pyroxene is represented by hypersthene, which also displays zonation (the centre, Fs₄₀; margins, Fs₆₀, where titanium content also increases).

Holocrystalline tagamites, occurring in the central parts of thick sheeted bodies, contain not more than 5–10% minor inclusions. The matrix (to 90%) is made up of the aggregate of andesine-labrador (An_50–54) and hypersthene (Fs_40–45) microlites and prisms of 0.02–0.1 mm in size, which determining microophitic, micro-pismatic-granular, and micropanallotriomorphic granular texture (Fig. 4.10). There are minor amounts (fractions of a %) of cordierite plates, sanidine, biotite lamelae, ilmenite needles, and magnetite (Table 4.6). Interstitial glass (Table 4.7) is, as a rule, devitrified, has a cryptofelsitic texture resulting from the development of the finest quartz and trydimite grains, along with biotite and sanidine. Relationship of the clasts with the matrix has a distinct reactionary character and shows up as partial melting of quartz, around which a rim of minor hypersthene crystals forms, making up accumulations in the groundmass in places (Fig. 4.11). Inclusions of quartz and diaplectic glass are commonly recrystallized.

All low-temperature tagamites are characterized by extensive development of secondary minerals, i.e. smectites, chlorite, zeolites, calcite, less frequently, quartz. They fill pores and caverns, replace the minerals of inclusions and microlites from reaction rims, and also develop as spots and irregular areas. In places, there are thin stringers made up of quartz, calcite, zeolite aggregates with participation of pyrite.

LT-tagamites are generally characterized by a comparatively low value of low-temperature water losses (0.8–1.5%), its volatilization occurring at temperature to 250°. DTA curves clearly display the effects, typical of smectite.

4.2.3 High-Temperature Tagamites

High-temperature tagamites were discovered within a composite sheeted body in the Balagan-Yuryage River Basin. They were also recorded on the surface in several places in the southern sector of the impact structure (Kysym River Basin, the Upper Chordu-Daldyn River, and the Kygam area in the upper course of Daldyn River), where they form relatively small sheeted and irregular bodies, in places in complicatred relationships with the low-temperature tagamites (Figs. 4.6 and 4.7). Among the considered rocks, tagamites with holohyaline and hemicrystalline texture of the matrix can be distinguished. The former are characteristic for the near-roof parts of thick sheets; they also make up minor bodies. Rocks contain from 10–15 to 25–30% of frequently irregularly distributed inclusions, predominantly of quartz and feldspar clasts, which have abrupt boundaries, or, less frequently, shock-metamorphosed and partly molten with generation of a very thin (thousandths of a mm) rim of coloured glass, in places in combination with the finest pyroxene grains. The glassy matrix is rather homogeneous, despite a certain degree of alteration (in this case it is brown and opaque) or devitrification, in places, there are areas of fresh semitransparent glass (Fig. 4.10). Generally, these rocks look more fresh than their low-temperature equivalents. The analysis of derivatograms shows, that in the HT-tagamites the loss of low-temperature water is significant (2–3%) and occurs within a broad range of temperatures (60–600 °C).

Hemicrystalline tagamites, predominantly occurring within thick sheeted bodies, are easily diagnosed even from the outward appearance displaying a greasy lustre and a darker colour. Inclusions in these rocks constitute 2–5%; these are almost exclusively crystalline rocks and their minerals, though sedimentary ones are also occur in places. Quartz and plagioclase crystalloclasts often bear features of intense shock alterations and recrystallization.

The matrix of these rocks has a microlithic, or, less frequently, intersertal texture. The matrix is composed of colourless or light-coloured residual glass and microlites. Glass constitutes 10–20 to 30–40% of the volume and has the refractive index of n = 1486–1502; its composition is given in Table 4.7. It is enriched in silica and potassium. Often, perlitic jointing and the finest crystallites of trichite type are observed there. In the most entirely crystallized rocks, glass is partly transformed into the aggregate of sanidine (Table 4.6) and quartz. Prismatic plagioclase and hypersthene microlites (their quantitative ratios being evaluated on the average as 1.5:1) have the mean size of 0.02–0.03 mm, the maximum ones are of 0.08–0.1 mm. Besides, cordierite plates, ilmenite laminae, magnetite and pyrrhotite also occur, the latter forming spherules in places. Hypersthene in tagamites of the large sheeted bodies has the average composition Fs₄₂ (Table 4.5); in places, it displays zonation: the central parts of microlites are more magnesian; the marginal ones, more ferruginous. In hypersthene microlites from minor bodies, where it is less ferruginous (Fs₃₂), the role of wollastonite constituent is increased; however, this hypersthene is poorer in alumina. There are also relatively more ferruginous and magnesian varieties, also differing in the content of alumina. Proceeding from the optical properties of hypersthene (2V = −60°…−74°) and its composition variations, it might be inferred, that hypersthene microlites from tagamites of the small bodies are the least ordered ones and crystallized under an abrupt change in the temperature regime as compared to large bodies (Mashchak et al. 1992).

Plagioclase in high-temperature tagamites is represented by labradorite An_55–60 (Table 4.6), in places also with normal zonation. Cordierite more often occurs near contacts with gneiss inclusions; its plates are characterized by the sectorial and polysynthetic twins and inclusions (Table 4.6). Ilmenite often forms thin hexagonal, slightly translucent laminae (Table 4.6).

The most pronounced petrographic characteristic of the considered rocks are the features of intense interactions of the matrix with inclusions of rocks and minerals. Clasts of molten or recrystallized quartz are surrounded by relatively broad (up to 0.2–0.5 mm) zonal reaction rims, made up of pyroxene prisms (on the average, about 0.06–0.07 mm, in places up to 0.15 mm) and surrounding glass. In the large sheeted bodies these rims are composed exclusively of hypersthene, which has the composition Fs₄₂ (Table 4.5). Commonly, in places of the entierly absorbed inclusions, aggregate accumulations of relatively large hypersthene prisms remain. Such accumulations can constitute 5–7% of the rock volume; in places, their amount is 10–15% (Fig. 4.12); their size reaches 0.2–0.5 mm.

In tagamites from minor bodies, the composition and ratios of minerals in reaction rims around quartz clasts are more complicated. These rims can be made up of pigeonite and augite, around which hypersthene also develops (Table 4.5). Ferrosilite constituent and alumina content in the latter are lower than in tagamites from larger bodies; there is also a much higher content of the wollastonite constituent. All these differences point to the heteromorphism phenomena during crystallization caused by different cooling conditions of the melts, which generally had the same composition (Mashchak et al. 1992).

Plagioclase clasts inclusions in tagamites have a cribrate habit (“chess texture”), which is resulted from their selective melting and generation of glass with n = 1.527. Often, regeneration rims form around clasts; usually, they are of a more basic composition. Similar regeneration rims are observed around clasts of orthpyroxene and clinopyroxene originated from various gneisses. Iron and aluminium content in the neogenetic rim decrease, whereas the content of the wollastonite constituent grows as compared to the composition of the nucleus preserved (Table 4.5). Pigeonite rims are observed around salite (augite) inclusions in rocks from minor bodies (Table 4.5).

Noteworthy are the phenomena of pyrometamorphic melting of gneiss inclusions in high-temperature tagamites of the thick sheeted bodies in the Balagan-Yuryage River Basin. These inclusions are of 10–20 cm across. They were recorded frequently at depths from several dozens of meters to 500 m or more from this roof; they also occur elsewhere. Glass and the therein contained microlites constitute 50% of these inclusions; they result from crystallization of the pyrometamorhpic melt. Taking into account their predominantly quartz-feldspar composition, these pyrometamorphosed rocks can be called buchites.

Buchites have a diatectic texture resulting from generation of the pyrometamorphic melt at the boundaries of the corroded quartz and feldspar grains, in which the relics of non-molten initial gneiss minerals seem to be floating (Fig. 4.13). These primary minerals commonly bear traces of shock metamorphism and shock melting, as well as the subsequent thermal action of the surrounding impact homogenized melt. The corresponding alterations of these minerals were characterized in the chapter 4.1.3. Pyrometamorphic melt in buchites comprises hypersthene microlites (Fs₄₉ with variable alumina contents (0.3–4.3%) and elevated concentraion of TiO₂); hypersthene commonly forms rims around quartz and its recrystallization products. Less frequently, plagioclase (An_45–55) microlites occur. Anorthoclase was recorded in certain instances; in places, cordierite and ilmenite laminae also develop. The volumetric content of microlites in the melt varies from the first to per cents 10%; the rest of melt is made up of coloured glass. The pores in it are often filled by zeolites, smectites, chlorite, etc. The glass is essentially enriched in silica and alkalies (Table 4.7).

Observations on the well core show that at high degree of pyrometamorphic melting of crystalline rock inclusions in tagamites, buchites disintegrate into the constituent relic minerals and the pyrometamorphic anatectic melt. In this case, the latter does not mix with matrix impact melt of tagamites and localizes as small (to 1 mm) spheroids concentrated on areas of the first tens of cm³ (Masaitis and Raikhlin 1985). At such areas, the composition and textural features of different spheroids are similar. At the same time, the composition of spheroids, which are spaced significantly and occur at different depths, is different. Spheroids are composed of glass, which encloses a certain amount of hypersthene, plagioclase, anorthoclase, tridymite, ilmenite microlites; in places, they are recrystallized into the quartz-feldspar aggregate (Fig. 4.14).

Comparison of the residual glass composition in buchites and spheroids demonstrates their similarity (Table 4.7), whereas glass of spheroids differs markedly from residual glass of tagamites by a lower silica, higher alkalies, an essential predominance of potassium, over sodium, and the two times higher Fe/Mg ratio. Glass in buchites also display similar features. All these observations generally point to the immiscibility of the matrix impact melt, by crystallization of which tagamites originate, and the pyrometamorphic melt, generated at a later stage of formation of the former. At the same time, melting products of inclusions, captured by the impact melt at the earlier stage of ejection, were practically completely absorbed by it.

As noted above, the thick tagamite sheet in the Balagan-Yuryage River area is made up of the series of simple bodies, formed by LT- and HT-tagamite. Tracing of changes in the petrographic features of tagamites through the section of this composite body, conducted in the course of detailed studies on core, revealed a certain pattern (Raikhlin et al. 1983, 1987). In the central part of the composite body (Figs. 4.15 and 4.16), similar to the central parts of simple bodies, of a significant thickness (over 70–100 m), at the depth of 200–300 m and below, rocks with the moderately and well crystallized matrix and a comparatively small number of inclusions are observed (hemicrystalline and holocrystalline mioclastic tagamites with a small number of inclusions). Towards the roof and the base of the composite body the matrix crystallization degree is gradually decreasing; the size of microlites decreases, while the number of inclusions increases. Near the base and the roof, tagamites with non-crystallized or slightly crystallized matrix and a great number of inclusions (to 25–30% of the rock volume) occur.

A gradual transition from slightly crystallized to moderately and well crystallized rocks indicates that the composite body was cooling as a single whole. This is also confirmed by the lack of chilled contacts between HT- and LT-tagamites, features of their mixing, along with a simultaneous immiscibility, etc. Data on the distribution of inclusions throughout the section of the composite body show that at the depth of 200–300 m, the boundary between the rocks differing in their content, is drawn: down in the section, the number of inclusions decreases abruptly. This might indicate that tagamites occurring above and below of this boundary formed from different melt fractions, distinguished by the extent of inclusion trapping.

4.3 Suevites

4.3.1 Composition, Structure and Texture

Suevites of Popigai structure made up of fragments, bombs, and lumps of impact glass of different size (vitroclasts) and target rock fragments (lithoclacts), cemented by the same finely crushed (less than 0.1 mm) material, partly lithified by sintering or other processes (Engelhardt et al. 1969; Masaitis et al. 1975, 1980; Selivanovskaya et al. 1990; Masaitis 1999, 2005; Stöffler et al. 1977, 2013; French 1998; etc.). Suevites, as a rule, lack in bedding and characterized by poor sorting of clastic material (Fig. 4.17). The size of vitro- and lithoclasts ranges from millimetres to 0.5–1.0 m, less frequently more (bombs and blocks). The crushed lithic material of crystalline and sedimentary rocks, represented by crystalloclasts and rounded mineral grains, have the same size, about 0.1–2.0 mm, less frequently more. In some cases the volume of this fine-grained material in lithovitroclastic suevites may be significant, up to 30%. This glassy material may be partly transformed into mixture of various alteration minerals. Suevites differ by a number of lithological and petrographic features: ratio of vitroclasts, lithoclasts and fine-grained material as well as by their grain size and cementation character (Masaitis et al. 1975, 1978, 1983, 1992; Raikhlin and Selivanovskaya 1979; Vishnevsky 1992, etc.).

Depending on the ratio between the main constituents, making up suevites, i.e. impact glass fragments, (vitroclasts) and rock fragments (lithoclasts), three families of these rocks are distinguished: vitroclastic (70–90% glass), lithovitroclastic (40–70% glass) and vitrolithoclastic (10–40% glass).

Suevites with the prevailing size of fragments of 0.1–2.0 mm are assigned to ash; suevites 2.0–50 mm, to lapilli; ones and >50 mm, to agglomerate and block suevites, similar to volcanic tuffs (Petrographic code 2009). If the fragments of a certain size occur approximately equally, ash-lapilli or lapilli-agglomerate suevites can be distinguished. In the generalized section of the crater fill (Raikhlin 1996), ash, less frequently lapilli, vitrolithoclastic suevites, and, to a lesser extent, vitroclastic suevites make up its upper part where abundant lithic microbreccia lenses, often with gradual transitions to vitrolithoclastic suevites, are observed. Lithovitroclastic and vitroclastic suevites are widespread in the lower part of the section, where they are closely associated with tagamite bodies and are connected with them by gradual transitions. The main lithological features of suevites in the generalized section are given in Table 4.8.

Table 4.8 Principal lithological features of suevite from the upper and lower parts of the general sequence

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The content and ratio of certain constituents of suevites in different sectors of the impact structure were evaluated selectively using the quantitative calculation of plots covered by fragments of certain rocks and vitroclasts exceeding 0.5 cm in size, directly at some exposures (areas of 0.5–1.0 m²) and from core of a number of boreholes (areas of 0.3 m²); in addition, the distribution of clastic materials in fractions <0.5 cm has been studied in large petrographic thin sections. The calculation results are given in Table 4.9. As it seen from the table, suevites in the lower part of the section are enriched in glass, particularly in the NW and SW sectors (Buordakh and Balagan-Yuryage River sections). Lithoclasts of lapilli and agglomerate size prevail here; and among the lithoclasts, crystalline rock fragments of 0.5–20 cm or more across dominate. The cementing mass exceeds 10% of the rock volume in rare cases. In suevites of the upper part of the section, glass content is lower (predominantly less than 50%); most of the clastic material has ash, less frequently lapilli size (<0.5 cm); a rather high content of the cementing matrix (over 20%) is characteristic.

Table 4.9 Lithology of suevites from certain sectors of the Popigai impact structure

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4.3.2 Suevites Enriched by Lithoclasts

Suevites, in which sedimentary rock fragments and fine-grained material prevails, are predominantly represented by moderately and slightly cemented ash-sized, less frequently, lapilli-sized vitrolithoclastic varieties of grey, greenish-grey, and brownish-grey colour (Fig. 4.18). Ratio between the lithoclasts, vitroclasts and cementing mass is wide-ranging. Among the sedimentary rock fragments, which commonly constitute 7–10 to 20–30% or more of the suevite volume, the Permian and Cretaceous siltstones, mudstones, and sandstones, the Cambrian limestones and dolomites, the Mesoproterozoic quartzite, the Cretaceous coalified wood, etc., prevail. Characteristically, these rock fragments are predominantly less than 0.5 cm in size; most of them are disintegrated into the fraction of psammitic size (0.1–0.2 mm or, less frequently, more).

Fine-grained material (10–20% of the volume or more) is mainly represented by fragments of quartz, plagioclase, microcline grains, which often have a rounded, or, less frequently, angular shape. There are commonly no shock features, except brittle deformations (jointing, crushing). The source of these grains were incoherent sediments. Shock effects are observed only in crystalloclasts (0.2–2.0 mm, less frequently larger) of quartz, feldspars, biotite, garnet, their content commonly being not high (to several %).

Crystalline rock fragments, which are of subsidiary significance (1–5% of the volume, or, less frequently, more), are represented by gneisses of different composition, subject to the shock of predominantly low and moderate intensity; shock vitrified gneisses are less frequent. In places, large (to 5–10 cm or more) crystalline rock fragments are coated by rims (to 0.5–1.0 cm wide) of dark grey impact glass.

The predominant size of sedimentary and crystalline rock fragments is 0.2–10 mm, less frequently 2–3 cm or more. Some blocks are several dozens of cm to 1 m across.

Vitroclasts are composed of both fresh impact glass of different colour and porous, pumiceous glass, which is altered to a great extent. Fresh and altered glass fragments can occur simultaneously. The size of their fragments ranges within 0.5–2.0 mm, less frequently 5–10 mm or more; certain glass bombs are to 10–15 cm across or more. A detailed description of glass is given below.

The groundmass of described suevites, which commonly amounts to 15–20% of the rock volume or more, is mainly composed of finest (<0.1 mm) mineral fragments, a small amount (the first %) of glass particles and alteration clay minerals, calcite, limonite, zeolites, etc. Cement matter is porous or, less frequently, basal.

Vitrolithoclastic suevites contain in places the accumulations of accretionary lapilli (Balagan-Yuryage, Parchanai, Chordu-Daldyn Rivers, etc.). They appear as rounded zonal masses of 1–1.5 cm across, constituting 20–30% of the rock volume and made up of tiny mineral fragments (70–80%) and glass (10–15%) of silty and psammitic size, cemented by the same psammitic material.

Suevites, dominated by crystalline rock fragments and characteristic of the lower part of the crater fill (Table 4.8), are penetrated by numerous boreholes in Mayachika Upland and Balagan-Yuryage River areas, where they are widespread. They are also exposed in northwestern, western, and southwestern sectors of the impact structure (basins of Buordakh, Kybyttygas, Parchanai, creeks etc. ). These are dense rocks characterized by the dark brownish-grey and dark greenish-grey colour. By the grain size of the clastic material, they are mainly assigned to the lapilli, lapilli-agglomerate, less frequently, to agglomerate varieties (Fig. 4.19). Clasts are predominantly represented by crystalline rock fragments (15–30% of the rock volume or more) and fragments of the constituent minerals, contributing more than 10–15%. The size of the fragments is from the first mm to 3–5 cm, less frequently more. Their shape is similar to isometric, often angular. They are commonly characterized by moderate and intense degree of shock alterations. Subsidiary amounts (1–3% or more) of sedimentary rock fragments occur, as well as their destruction products.

Crystalloclasts, as noted above, are represented by disintegrated gneiss fragments, often bearing shock metamorphic features (diaplectic minerals and glass, frequently recrystallized). Xenomorphic and authomorphic vitroclasts have a massive or fluidal structure, sinuous, less frequently vague boundaries. Impact glass often forms authomorphic lumps, varves (Fladen), and bombs from fractions of a cm to several dozens centimeters and even 0.7–1.0 m in size.

Cementing mass of a pore character represents a mixture of the finest (less than 0.1 mm) ash particles and mineral fragments. Cement content varies from the first % to 7–10%, or, less frequently, more. In glass-enriched suevites the groundmass is partly recrystallized. Montmorillonite, chlorite, calcite and other alteration minerals arose after this material.

4.3.3 Suevites Enriched by Vitroclasts

Vitroclastic suevites predominantly develop in the lower part of the general impactite and impact breccia section (Table 4.8). They are penetated by boreholes in the annular trough and on slopes of the peak ring in the northwestern and southwestern sectors of the structure (Mayachika Upland, Balagan-Yuryage River Basin); they are also exposed at Buordakh, Kybbyttygas, Parchanai creeks and some other areas. These rocks predominantly occur in close association with lithovitroclastic suevites and often occur together with tagamite bodies.

Vitroclastic suevites are dense, dark brownish-grey rocks with a massive structure and lapilli, lapilli-agglomerate and agglomerate size of fragments (Fig. 4.20). Vitroclasts often have a fluidal structure; they are authomorphic or xenomorphic, and contain to 5–10% or more of crystalloclasts. Impact glasses often form sinuous bands, figured lumps and bombs, in places with a “ropy” surface, from fractions of a cm to 10–15 cm or more. Clasts of sedimentary and crystalline rocks constitute from 3–5% to 15–25% of the volume of these suevites; the latter rocks prevail and are characterized by a rather high level of shock and thermal alteration.

The cement of vitroclastic and sintered suevites that constitutes to 7–10%, is dominated by the finest ash glass particles. Close to lenses and, particularly, thick tagamite bodies, this cement is partly or fully recrystallized. Small (<0.5 cm) clasts of crystalline rocks also undergo intense recrystallization. Alteration and secondary minerals, i.e. montmorillonite, chlorite, calcite, zeolites, and limonite, develop after the cementing mass. Impact diamonds also frequently occur in suevites (particularly in vitroclastic ones).

4.3.4 Impact Glass from Suevites and Lithic Microbreccias

Among impact polymineral glass occurring as bombs and fragments in suevites and lithic microbreccias, two varieties can be distinguished: (1) fresh and (2) devitrified.

Fresh or slightly altered glass of a tar-black and dark green (“chrysolite”) colour is most characteristic of vitrolithoclastic suevites. Tar-black glass has a glassy lustre on the fresh fracture, it is semitransparent in thin section. Its structure is massive, less frequently fluidal, the texture is holohyaline; there is a slight porosity (Fig. 4.21). In thin sections, these are light brown or cream-coloured transparent glass with the refractive index ranging within 1.541–1.546, less frequently, to 1.553. The inclusions making up to 1–3 vol.%, are represented by the finest (hundredths and tenths of a mm) quartz and feldspar fragments, as well as ilmenite, magnetite, and sulfide spherules. As irreguarly shaped patches, monomineral fusion glasses, in the first place lechatelierite, occur. Slightly altered glass fragments display a zonal colouring, from brown and yellowish-brown to light brown and cream of variable intensity; contours of these zones often repeat the boundaries of fragments (Fig. 4.22). The central part of the fragments is often characterized by microspherulitic texture; and the marginal part, by cryptocrystalline texture. Secondary alterations mainly show up as the development of montmorillonite in pores and marginal parts of glass fragments.

In vitrolithoclastic suevites forming lenses in microdreccias or gradual transitions to the latter, the most distributed are fragments of fresh glass of a dark green (“chrysolite”) colour, which most frequently occur in microbreccia. They form both xenomorphic fragments 0.5–5 mm in size, and authomorphic bombs, lapilli from 0.3–0.5 to 1–3 cm in size. Under the microscope, these glass is transparent, colourless or slightly coloured to yellow-greenish. The refractive index n = 1.532–1.534. They are characterized by holohyaline texture and massive, locally porous and, in places, fluidal structure. The inclusions, i.e. diaplectic quartz and feldspars, diaplectic and monomineral glasss, are rare (up to 1–3%). Often, ilmenite spherules occur, and less frequently, magnetite spherules up to fractions of a mm across; there are also pyrrhotite and troilite spherules. An alteration show up as the development of smectites along the boundaries of fragments and pore walls.

The results of studies of the chemical composition of impact glass are given in Chap. 5. Features of their microstructure were investigated using different spectroscopic techniques (Raikhlin et al. 1981, 1982, 1986, 1987, 1991; Kozlov and Raikhlin, 1989; Reshetnyak and Raikhlin 1988). According to IR-spectroscopy, fresh glass is noted for vague spectra, a great width of maxima, slightly pronounces bands corresponding to crystalline phases, which points to the absence of the long-range order, i.e. products of glass crystallization or secondary alteration. Data of IR-spectroscopy point to an almost complete lack of water in the structure of fresh glass, which is confirmed by a weak absorption within 3000–3600 cm⁻¹ range. Very low water content is also attested by the results of thermal and microchemical analyses.

A characteristic feature of the chemical composition of impact glass (see Chap. 5) is a high reduction degree of iron, which is confirmed by Mössbauer spectroscopy and EPR. The analysis of NGR spectra demonstrates that only bivalent iron in tetrahedral and octahedral coordination is present in fresh glass. A similar, markedly broadened asymmetric doublet, pointing to the absence of structures with long-range order bonds in glass, is also characteristic of tektites. EPR tecnique was applied to study the occurrence of ferrous iron in the structure of glass. A low absolute content of Fe³⁺ (0.001–0.01%) determines an abruptly reducing environment of generation of the glass. Ranges of the contents of different forms of iron are similar for tektites and chilled green glass from suevites and microbreccia of the Popigai structure. Fe³⁺ in the structure of the impact glass from Popigai being the lowest.

Structural features of the aluminosilicate glass framework are revealed in Raman dispersion and IR-reflection spectra. The appearance of a low-frequency shoulder about 950 cm⁻¹ in Raman spectra is associated with depolymerization of the glass network and the growing concentration of non-bridge oxygen bonds in its structure, partly frozen under the abruptly chilled cooling environment.

Qualitatively similar conclusions were also obtain from IR-reflection spectra. Tar-black and, particularly, green glass, are characterized by the highest defectiveness of the microstructure (local framework deformations, a high concentration of non-bridge oxygen bonds). There, similar to tektites (moldavites and indochinites), non-equilibrium disturbances of the glass network are most expressed. Glass is characterized by the minimum size of micro-inhomogeneities, revealed from low-angle X-ray dispersion spectra.

Characteristic features of fresh glass considered pointing to the high temperature of the initial melt and its quick chilling are: (1) holohyaline texture, lack of crystallization products and reaction rims around inclusions; (2) small number of inclusions (less than 1–3%); (3) presence among the inclusions of molten diaplectic quartz, lechatelierite, ilmenite, magnetite, and sulphide spherules, as well as native iron aggregations (Vishnevsky 1975); (4) lack of unbound oxygen in fluid inclusions and a high CO/CO₂ ratio in them, exceeding 1 (Dolgov et al. 1975); (5) a marked predominance of bivalent iron as compared to trivalent one, along with a low water content; (6) a high textural-structural and chemical homogeneity.

The spectroscopic and other studies conducted on fresh glass demonstrated that this glass formed under an abruptly reducing environment during quick chilling of strongly overheated impact melts. We consider them as a high-temperature impact glass, unlike glass of other varieties altered to a variable degree that is described below and is assigned to relatively low-temperature formations.

Devitrified, highly altered glass fragments, often occurring in suevites, are grey and brounish-grey, have a xenomorphic shape and vary in size from fractions of a mm to 0.5–1.0 cm, or, less frequently, more. The glass is dull, opaque, its structure is massive or, less frequently, fluidal, locally porous and vesicular; the texture is cryptocrystalline. Inclusions of mineral fragments, mainly quartz and feldspars, constitute from 3–5 to 10–15%. Among the alteration minerals, particularly developed in porous and pumiceous glass, illite, limonite, zeolites, clay minerals, and carbonates occur. Crystalloclasts often bear traces of high shock loads (diaplectic glass, monomineral melt glass, mostly recrystallized). They are irregularly distributed in the glass matrix; in places, they form shlieren-like accumulations. In slightly porous and porous glass, oblong pores and inclusions form chains empasizing the fluidal structure, which points to the plastic state of glass during transportation.

Along with relatively homogenous dark-coloured glass (n = 1.551–1.565) characterized by holohyaline or cryptocrystalline texture and massive, slightly porous structure, greenish- and brownish-grey, light brown and brownish, slightly porous, porous, and vesicular non-homogenized - heterogeneous glass particles are widespread; The refractive index of this glass varies from 1.54 to 1.72. The glass fragments are mostly devitrified. In vitroclastic and, particularly, sintered suevites, glass is crystallized to a certain extent; its texture is hypohyaline, spherulitic, hyalopilitic. Microcrystals are represented by globospherites, globulites, and cumulites. Alteration is common including the appearance of chlorite, limonite, smectites, etc. The most intensely devitrified glass acquires dark grey or black colouring.

Vitrolithoclastic suevites and, to a lesser extent, other varietes, contain light-coloured (light grey, yellowish-grey) xenomorphic porous and vesicular glass (n = 1.540–1.560, commonly 1.550) containing from 3–5 to 10–15% of mineral inclusions. Fluidal structure or plication are not recorded in them. There are altered glass fragments, the number of alteration mineral is the same that in above glass.

The study of magnetic properties of impact glass (Gorshkov and Starunov 1981; Raikhlin et al. 1983; Starunov et al. 1984) confirms the distinguishing of two contrasting groups among them. Ferromagnetics from high-temperature varieties (fresh dark green and tar-black glass) are represented mainly by dispersed particles in a supermagnetic state, similar to those in high-temperature tagamites, whereas in glass of other varieties, which are assigned to the low-temperature formations, by multidomain particles with a minor amount of unidomain particles, which is also typical of ferromagnetics from low-temperature tagamites.

Impact diamonds are also present in impact glass particles.

4.4 Hydrothermal Alteration of Impactites and Impact Breccias

Although alteration minerals are present almost in all impact lithologies from different parts of the impact structure, the post-impact hydrothermal mineralization in the Popigai structure is characterized by the low intensity. It is developed mainly in tagamites, suevites, and impact glass while within allogenic breccia and, especially, in parauthochtonous rocks of the peak ring and crater edge, it is much rarer. The distribution of hydrothermal altered rocks is irregular on the crater area. It depends on location of massifs of highly heated rocks, permeability of their surrounding and water supply penetrating mostly from the surface. Most probably, there appeared numerous limited circulation cells, especially under the floor of small lakes, which sporadically arose on the surface. So, the hydrothermal mineralization is characterized by the relatively wide compositional difference of minerals due to the variation of crystallization environment.

Various alteration minerals are typical especially for suevite. Hydrous phyllosilicates locally replace impact glass, but much more frequently occupy pores and fissures. Calcite, zeolites (predominantly mordenite), minor sulphides (mostly pyrite), quartz, cristobalite, and opal are common hydrothermal minerals in vugs and amygdules. From interrelations between minerals infilling cavities, the following generalized order of the mineral formation is established: silica modifications (quartz, cristobalite, opal), hydrous phyllosilicates (smectites and chlorites), zeolites–calcite–pyrite (Naumov 2002).

The spatial distribution of post-impact hydrothermal mineral assemblages across the whole structure indicate that some of their associations are characteristic for certain structural elements of the crater or for certain parts of vertical section. Cristobalite and mordenite, together with saponite and mixed-layered phyllosilicates, dominate among alteration phases in tagamites, whereas in underlying crystalline megabreccia and in the uppermost suevite filling the central depression, calcite, quartz, pyrite, and low-silica zeolites are prevailing.

In microbreccias and suevites of the upper part of the crater fill dioctahedral smectites (beidellite or montmorillonite) dominate, whereas in lithovitroclasic and vitroclastic suevites of the middle part, Ca-saponites (d₀₀₁ = 14.7–15.3 Å), mixed-layered chlorite-saponites, and Fe-chlorites are developed; these data indicate the increase of Fe and Mg contents and the decrease of Si, Al, Na, and K contents in clay minerals downward in the vertical section (Fig. 4.23). However, in thick tagamite sheets both trioctahedral and dioctahedral (both montmorillonite and beidellite) smectites occur often together. This indicates that local compositional variations of solutions, which reflect a local inhomogeneity, are more significant than temperature gradients within a thick impact rock sequence.

Thus, a general trend for variations of clay minerals composition from high-aluminia varieties (beidellite) to Mg–Fe varieties (saponite) from top to bottom in a generalized crater fill sequence is established. The diversity of conditions of hydrothermal alteration is manifested also in the wide range of zeolite compositions. Two zeolitic associations are distinguished (Naumov 2001): (1) mordenite associated with cristobalite, and, in rare cases, with minor chabazite, occurs in cavities of tagamite and suevite bodies, as well as in cataclasites within the polymict megabreccia; (2) stilbite, Na-chabazite, and, more rarely, heulandite, together with calcite, form zonal veins and geodes within the deep-seated breccia of crystalline rocks and tagamite sheets (predominantly in their near-bottom parts enriched in rock clasts), and in vitrolithoclastic suevites of the upper part of the general sequence. All zeolite species are enriched by silica relatively their stoichiometric formulae (Naumov 2001).

Epigenetic sulfides formed during the hydrothermal circulation are represented by the major pyrite and subordinate chalcopyrite and sphalerite. Pyrite (associated with calcite and zeolite) forms veins in tagamites and aggregate pseudomorphs after fragments of carbonaceous shale, argillite, siderite, and others in suevites and allogenic breccia. The youngest pyrite generation is developed in subvertical joints in tagamites. The pyrite is enriched in Ag, As, and Zn and depleted in Ni, Cu, and Co as compared with syngenetic sulfides. Some pyrites contain up to 1.4% Ni and up to 0.3% Co (Naumov et al. 2004). The sulfur isotope composition of epigenetic sulfides varies within a narrow range (from −0.7 to 3.2‰ CDT) to correspond the same values for target metamorphic rock.

Three main features of the hydrothermal mineralization are: (a) occurrence of low-temperature minerals alone; (b) the spatial distribution between different impact lithologies, which have a definite location within crater; a decrease of the Al^IV content from the base to top of thick tagamite bodies is the main alteration trend of compositions of both clay minerals and zeolites. It shows the decrease of temperature and of pH of the mineral-forming solutions in this direction; (c) some common features (e.g., enrichment in silica of hydrothermal minerals, higher Fe content in Mg–Fe phyllosilicates) of the chemical composition.

In geochemical aspect, the hydrothermal alteration in the Popigai is expressed in the concentration of weak basic elements and the depletion of alkali and of high-valency amphoteric elements, which precipitate in cavities as zeolites (Naumov 2005). The extent of chemical alteration of impact rocks under post-impact hydrothermal circulation is determined by the intensity of interactions between hydrothermal solutions and substrate, i.e. the amount of available water.

Inferred from the composition of hydrothermal mineralization, physical-chemical parameters of the impact-induced hot-water circulation system are modelled to be the followings. (a) The superficial meteoric and ground water, products of dehydration and degassing of minerals under shock are the sources for hot-water solutions. (b) Shocked target rocks are sources for the mineral components of hot-water solutions. No evidence of any deep heat and mass addition in the post-impact hydrothermal activity is found. (c) Fluid temperatures vary from ambient to 350–400 °C; (d) high rates of filtration (10⁻⁴–10⁻³ ms⁻¹) are assumed by analogy with a modern Pauzhetka hydrothermal system (Rychagov et al. 1994); this is indicated by intense development of high-aluminia smectites. (e) In general, the succession of mineral crystallization confirm the uniform regressive course of the hydrothermal process.

The chemical parameters of the mineral formation vary within a narrow interval of pH and correspond to weakly alkaline and near-neutral environments (pH = 6 ÷ 8). This feature is very close to composition of host rocks, which consist mainly of shock-disordered aluminosilicates and fusion glass. The intense leaching of this material providing the above-mentioned properties of solutions. The common presence of sulfides together with rare occurrence of iron oxides, indicates that the solutions maintained Eh values of >−0.5 V (for neutral environment) during the course of the hydrothermal process.

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A.P. Karpinsky Russian Geological Research Institute, Sredny prospekt, 74, 199106, Saint Petersburg, Russia
Victor L. Masaitis, Anatoly I. Raikhlin, Tatjana V. Selivanovskaya, Mikhail S. Mashchak & Mikhail V. Naumov

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Victor L. Masaitis
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Tatjana V. Selivanovskaya
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Masaitis, V.L., Raikhlin, A.I., Selivanovskaya, T.V., Mashchak, M.S., Naumov, M.V. (2019). Petrography of Shock-Metamorphosed Crystalline Rocks and Impactites. In: Masaitis, V. (eds) Popigai Impact Structure and its Diamond-Bearing Rocks. Impact Studies. Springer, Cham. https://doi.org/10.1007/978-3-319-77988-1_4

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Petrography of Shock-Metamorphosed Crystalline Rocks and Impactites

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Keywords

4.1 Crystalline Target Rocks and Their Shock Metamorphic Features

4.1.1 Brief Characteristic of Gneisses

4.1.2 Mineralogical Criteria of the Shock Metamorphic Parameters

4.1.3 Weakly and Moderately Shock-Metamorphosed Rocks

4.1.4 Intensely Shock-Metamorphosed Rocks

4.1.5 Very Intensely Shock-Metamorphosed Rocks

4.2 Tagamites

4.2.1 Mineral Composition and Texture

4.2.2 Low-Temperature Tagamites

4.2.3 High-Temperature Tagamites

4.3 Suevites

4.3.1 Composition, Structure and Texture

4.3.2 Suevites Enriched by Lithoclasts

4.3.3 Suevites Enriched by Vitroclasts

4.3.4 Impact Glass from Suevites and Lithic Microbreccias

4.4 Hydrothermal Alteration of Impactites and Impact Breccias

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Petrography of Shock-Metamorphosed Crystalline Rocks and Impactites

Abstract

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Transformations of Crystalline Rocks of the Central Uplift and Its Deep-Seated Zones

Shock Metamorphic Features in the Archean Simlipal Complex, Singhbhum Craton, Eastern India: Possible Remnant of a Large Impact Structure

Logoisk, Belarus

Keywords

4.1 Crystalline Target Rocks and Their Shock Metamorphic Features

4.1.1 Brief Characteristic of Gneisses

4.1.2 Mineralogical Criteria of the Shock Metamorphic Parameters

4.1.3 Weakly and Moderately Shock-Metamorphosed Rocks

4.1.4 Intensely Shock-Metamorphosed Rocks

4.1.5 Very Intensely Shock-Metamorphosed Rocks

4.2 Tagamites

4.2.1 Mineral Composition and Texture

4.2.2 Low-Temperature Tagamites

4.2.3 High-Temperature Tagamites

4.3 Suevites

4.3.1 Composition, Structure and Texture

4.3.2 Suevites Enriched by Lithoclasts

4.3.3 Suevites Enriched by Vitroclasts

4.3.4 Impact Glass from Suevites and Lithic Microbreccias

4.4 Hydrothermal Alteration of Impactites and Impact Breccias

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