(1) Types and characteristics of pyroclastic materials
Pyroclastic refers to all kinds of clastic materials produced by volcanic eruption, mainly from underground molten magma or solidified lava, which is crushed or broken into various cuttings, crystal chips, glass chips and so on due to volcanic eruption. Sometimes it can also be mixed with debris from the surrounding rocks on both sides of the volcanic passage, overlying or basement, which has the characteristics of endogenous origin. On the other hand, pyroclastic materials are transported, deposited or settled in water basin or air after being ejected, which has the characteristics of sedimentary construction. Generally speaking, the content of pyroclastic materials is above 50%, and sometimes a series of transitional rock types appear because of mixing some normal sediments or lava.
Because pyroclastic materials are the main components of pyroclastic rocks, before studying the lithologic characteristics and classification of pyroclastic rocks, we must first understand the types and characteristics of pyroclastic materials. From the physical properties, pyroclastic materials can be divided into rigid, semi-plastic and plastic (Table 5- 1). According to the structural characteristics of internal components of pyroclastic materials, they can be divided into cuttings (rock chips), crystal chips (crystal chips) and glass chips (volcanic glass chips). They are formed by the volatile-rich molten slurry rising from the deep underground to the shallow surface, the pressure plummeting and the volume expanding and exploding. Therefore, molten blocks often break into various strange shapes, which is also an important feature of pyroclastic materials.
1. Gravel
(1) rigid debris: including volcanic blocks (aggregates) with particle size > 2 mm, volcanic breccia and debris with particle size < 2 mm (Table 5- 1, 5-2). It is the surrounding rock of volcanic basement and volcanic passage, including the lava formed first, which was broken into angular fragments of different sizes by volcanic eruption. It is already a consolidated rigid body at the time of fracture, and it is also a rigid state at the time of accumulation and diagenesis. There are angles, secondary angles or irregular polygons, and the appearance is arc-shaped explosion surface, and a few cuttings are eroded by deep molten slurry. Generally, the structures of their respective rocks are preserved in it (photos 5-27, 28).
Solidification degree is solidified, semi-solidified, non-solidified, pyroclastic, hard, semi-plastic, plastic debris, lava that solidified earlier (including all kinds of glass lava), surrounding rock of volcanic tunnel, and volcanic basement debris. It is mainly angular, and a few edges are plastic when ejected by molten magma. During the flight, various forms of volcanic bombs are often formed. Most of them have been consolidated when piled up, and most of them are in a plastic state when ejected. It is still hot and not solidified, but it can be flattened and stretched to form a lenticular, ribbon and flame shape. Sometimes there are spots, pores, almonds and other crystal chips inside, most of which are crystals formed by early underground magma. When the volcano erupted, it was taken out and broken into angular shapes. Due to the sudden cooling during ejection, most of them have cracks, and some of them have molten harbor-like glass fragments. When the magma with high viscosity is ejected, it quickly cools into plastic porous glass, and the gas quickly overflows from the pores and is blown into pumice, concave corners and tears. There are also volcanic tears, volcanic hair and so on. After the magma with high viscosity is ejected, it is cooled into porous glass and blown into glass fragments. When piled up, it is in a thermoplastic state, with rounded edges, and is often flattened and elongated, generally less than 2 mm, which is called plastic glass chips.
Firestone: From the Italian word "fiamme", which means flame, pyroclastic rocks are described as flame.
Particle size range /mm Hard plastic and semi-plastic > 64 Volcanic aggregate (rock block) Volcanic pyrophyllite * 2 ~ 64 Volcanic breccia plastic debris 0.05 ~ 2 Volcanic sand (debris, crystal chips) Coarse pozzolana (glass chips) Coarse plastic glass chips < 0.05 Fine pozzolana (volcanic dust).
(2) Semi-plastic debris: loose lava with basic-neutral composition. Its particle size is larger, generally > 2 mm. They are formed by cooling when sprayed into the air. Its shapes are spindle, pear and oval. Due to rapid cooling, there are often thin glass shells at the edges and developed pore structures. The stomata at the edge are small and numerous; The intermediate individuals are large and few in number. Because it stays in the air and rotates during the spraying process, its surface often produces rotating lines. It can be divided into volcanic bombs (particle size > 64 mm, mostly distributed near the crater) (photo1-25,26) and volcanic gravel (particle size 2-64 mm).
(3) Plastic debris: also known as slurry debris. Due to the high viscosity of molten slurry, when the gas is ejected, the height of the ejected material is not large, and plastic debris is formed when it falls near the surface. After tearing, splashing, flattening and stretching, this thermoplastic melt can take on various shapes, such as strip (photos 5-7 and 8), lenticular (photos 5-6 and 33), flame (usually called flame stone fiamme, photos 5-8, 40 and 46), branched and pie (photo 5 has various colors, such as white and meat). Common phenocrysts (photos 5-8, 38, 4 1, 42), pores, almonds and rhyolite structures in plastic cuttings. Under the cross polarizer, various devitrification phenomena can be seen, such as comb devitrification structure and spherulite devitrification structure.
2. Crystal fragments
Most of the crystal fragments come from porphyritic crystals deposited in early magma. During the eruption of molten magma, it was separated from the semi-solidified state of molten magma and fractured. A few crystal fragments are xenolith fragments formed by crystal rupture in volcanic basement or volcanic pipeline. The common crystal chips are quartz, alkali feldspar and plagioclase, followed by biotite, amphibole, pyroxene and epidote. Generally speaking, crystal pieces are of different sizes. Different from phenocrysts, they are irregular in shape, often broken incompletely, often angular and sub-angular, and have developed cracks. Most of the crystal fragments can see the original crystal morphology (photo 5- 1, 2 1). Generally speaking, the surface of seasonal crystal is clean, with irregular cracks criss-crossing, mostly in the form of sharp corners, polygonal, triangular and arc sections, and the edges are often eroded into harbors and perforations (photo 5-23). Although feldspar crystal chips are angular, they are generally partially crystalline (cylindrical or bottom), and sometimes even remain intact plate (photo 5-3 1). Because of the perfect cleavage of feldspar, it is easy to crack along cleavage, and its cross section is stepped and staggered. It can be seen that crystal fragments melt into fragmentation and honeycomb along cleavage or crack. Sometimes you can see the dark edges of biotite and amphibole chips, and biotite is generally darker when you can't see the dark edges. As crystal fragments, biotite is usually bent (photo 5-42), twisted and broken, while amphibole is generally angular and clastic, and sometimes some crystal forms (column and bottom) can be seen.
3. Glass fragments
According to the physical properties of glass fragments, they can be divided into semi-plastic and plastic. Semi-plastic is generally called glass sheet, and plastic is called plastic glass sheet.
(1) Glass shards: molten slurry rich in volatiles. When a volcano erupts, the volcanic glass filled with gas or liquid formed by rapid condensation expands rapidly, causing it to burst and break. Therefore, glass chips often keep the arc shape of the hole wall, showing various shapes such as arc angle (polygon), bow shape, chicken bone shape, cancellous bone needle shape, sickle shape, wedge shape, etc. (photo 5- 1 ~ 4), and some glass chips can also see bubbles (pumice, photo 5- 1 left), usually with a particle size of 0.60.
(2) Plastic transparent fragments: In the hot volcanic ash flow formed by volcanic eruption, glass fragments are accumulated, compacted and melted in the overheated state of incomplete consolidation. Therefore, plastic glass fragments have the characteristics of elongation, squashing and bifurcation at both ends, and the edges and corners are often eroded and rounded, and no longer have the bending edges and corners (rigidity) of glass fragments. Their shapes are elongated lenticular, intestinal (photos 5-29 and 39), pod-like, earthworm-like (photos 5-5 and 30) and filamentous (photos 5-32 and 39). When plastic glass fragments meet hard rock fragments and crystal fragments, they are bent, thinned and narrowed, forming a false flow structure. The other is broken glass sheet, which has a flat appearance and uneven edges, and has parallel fine lines similar to streamline inside. It is formed by pulling off molten slurry in a plastic state, and its particle size can reach several millimeters to several centimeters.
In practical work, plastic glass chips and plastic chips are often confused, but the particle size of plastic glass chips is generally smaller than that of plastic chips. Secondly, there are sometimes spots, pores and amygdala in plastic fragments, but not in plastic glass fragments. There is often a continuous transitional relationship between the two.
According to the particle size and natural form, pyroclastic materials can be divided into volcanic aggregate, volcanic pebble, volcanic pebble, volcanic bomb, volcanic sand, volcanic ash and volcanic dust. At present, there is no uniform regulation on the limitation of particle size. Table 5-2 gives the particle size range (national standard) and main types of pyroclastic materials in China, and Table 5-3 gives the classification of pyroclastic materials by Sun Shanping (200 1).
Particle size mm Homologous heterogeneous plastic semi-plastic rigid heterogeneous rigidity > 64 slurry, plastic clastic volcanic rock block 2 ~ 64 slurry, plastic clastic volcanic breccia heterogeneous volcanic breccia 27 ~ 2 plastic glass clastic volcanic sand heterogeneous volcanic sand < 27 volcanic dust volcanic dust heterogeneous volcanic dust (II) Structure of pyroclastic rocks 1. Structure of pyroclastic rocks
Firstly, according to the composition of pyroclastic materials, it can be divided into pyroclastic structures, clastic lava structures, fusion structures and sedimentary pyroclastic structures (Table 5-4). The pyroclastic structure is mainly composed of rigid product fragments, rock fragments and other volcanic lava fragments. The structure of clastic lava is composed of pyroclastic (mainly rigid) and lava: the fusion structure is mainly composed of various plastic deformation pyroclastic; Sedimentary pyroclastic structure is composed of pyroclastic and terrigenous clastic. According to the granularity of pyroclastic materials, it can be divided into aggregate structure (> > 64~2 mm), volcanic breccia structure (64 ~ 2 mm), tuff structure (2 ~ 0.05 mm) and volcanic dust structure (< 0.05 mm). Among them, volcanic dust structure (also called dust structure) can only be observed under scanning electron microscope, and it is usually used as a filler between coarse particles (tuff grade and above), so it is generally not described in detail.
Particle size/Mm pyroclastic type volcanic lava structure pyroclastic structure fused structure precipitated pyroclastic structure > 64 volcanic aggregate lava structure fused aggregate structure precipitated aggregate structure 64 ~ 2 volcanic breccia lava structure fused breccia structure precipitated breccia structure 2 ~ 0.05 volcanic tuff lava structure tuff structure fused tuff structure precipitated aggregate structure (volcanic aggregate structure) te structure) It is mainly composed of volcanic aggregate (accounting for more than 1/2 of the total amount of pyroclastic, at least > 1 If the interstitial material is mainly lava, it is a massive lava structure, and if it is mainly plastic and semi-plastic volcanic rocks, it is a molten massive structure; If there are many terrigenous clastic rocks (the content of which is less than that of volcanic rocks), they belong to sinking block structure.
Volcanic breccia structure is mainly composed of hard volcanic breccia (accounting for more than 1/2 of the total pyroclastic, at least > 1/3), and the filler is mainly volcanic ash. Its interstitial materials are mainly lava (generally accounting for more than 10% of the total rock volume), which can be defined as breccia lava structure; If volcanic breccia is mainly plastic glass debris and/or plastic rock debris, it can be defined as fused breccia structure; If volcanic breccia is mainly plastic glass debris and/or plastic rock debris, it can be defined as fused breccia structure; If there are more exogenetic sedimentary (terrigenous) breccia-grade debris (less than volcanic breccia), it can be defined as volcanic breccia structure.
The tuffaceous structure is mainly composed of tuff-level hard pyroclastic rocks (accounting for more than 1/2 of the total pyroclastic rocks, at least >: 1/3) and volcanic dust fillings. Its interstitial materials are mainly lava (generally accounting for more than 10% of the total rock volume), belonging to tuff lava structure. If the volcanic debris is mainly plastic glass debris and/or plastic rock debris, it belongs to fused tuff structure. It is characterized in that the glass fragments and rock fragments of plastic and semi-plastic are flattened and stretched into parallel stripes, lenses and flames, which are similar to fluid distribution, and rigid crystal fragments and rock fragments are often mixed between them (photo 5-39.40). Fused tuff structure, like fused breccia structure and fused aggregate structure, is a characteristic structure formed in the process of pyroclastic flow accumulation. There are many sandy-silty terrigenous detritus in tuff structure (its content is less than tuff-grade pyroclastic content), which belongs to tuff structure; If the content of terrigenous detritus is greater than that of pyroclastic, it belongs to tuffaceous sandy-silty structure and belongs to the structural type of sedimentary rocks.
The aggregate lava structure, breccia lava structure and tuff lava structure mentioned above are all characteristic structures of pyroclastic lava.
In addition to the above, when pyroclastic rocks with different particle sizes appear at the same time, they are named in the order of less before and more after, such as volcanic breccia tuff structure and fused tuff breccia structure.
2. The structure of pyroclastic rocks
Pyroclastic rocks are formed by accumulation, compaction or cementation of various pyroclastic rocks, so their structures are closer to sedimentary structures, such as layered, quasi-layered and rhythmic structures. Besides the common massive structures, there are some characteristic structures such as pseudo-ripple structure and volcanic mud sphere structure.
Pseudo-flow structure is unique to pyroclastic flow accumulation, and it is also a characteristic structure of molten pyroclastic rocks. It is characterized by the directional arrangement of plastic glass debris and plastic rock debris during the accumulation process, which is similar to the corrugated structure in lava. But it is not the texture formed by magma flow, but the plastic pyroclastic deformation, so it is called pseudo-rhyolite structure. The main difference from rhyolite structure lies in the shape and occurrence: plastic fragments are mostly irregular strips, lenses or flames, with different widths and short and discontinuous extension; Tearing at both ends or dovetail shape; Usually contains angular rigid crystal fragments (rather than complete phenocrysts) and chips; Bending or pressing in when encountering rigid fragments; The long axis direction of common crystal fragments is perpendicular or intersecting with the elongation direction of plastic fragments (photo 5-29); Pores and almonds, which are common in corrugated structures, are rarely seen in pseudo-corrugated structures.
Volcanic mud sphere structure refers to the spherical structure at the top of tuff layer or sedimentary tuff layer, which is mostly spherical, ellipsoidal or lentil-shaped, and its center is mostly tuff components such as volcanic ash, volcanic dust and plastic glass debris, sometimes mixed with terrigenous debris or silica gel. Concentric stripes composed of different particle sizes or colors can be seen on the cross section of the sphere. The diameter of the sphere varies greatly, ranging from < < 1 mm to several centimeters. It is more common in pyroclastic rocks formed by continental volcanic eruption and underwater accumulation.
When pyroclastic materials with bedding structure and grain order structure are transported in the air or water, they often appear in the form of sand grains, sand waves or bed sand bodies. Like the accumulation mechanism of normal sediments, they can form layered structures such as horizontal bedding, oblique bedding or cross bedding, but they are rarely seen in the accumulation of airborne pyroclastic materials. Different from sedimentary rock bedding, it often develops a bottom-up reverse grain order structure, especially in rocks with pyroclastic flow accumulation and partial volcanic bottom wave accumulation facies. In the process of volcanic bottom wave accumulation, climbing bedding (photo 5- 16) and accretion volcanic gravel structure are common [see section 3 (3)].
(3) Accumulation facies of pyroclastic materials (rocks)
In the fourth section of the first chapter, "Volcanic eruption facies" (15), four accumulation forms of pyroclastic materials ejected by it are mentioned. Accordingly, the accumulation facies of pyroclastic materials (rocks) can be divided into volcanic eruption cavitation accumulation facies, pyroclastic flow accumulation facies, volcanic bottom wave accumulation facies and lahar accumulation facies.
(1) The deposition stage of volcanic eruption is all products ejected into the air from the crater, including magma ejecta, lava debris in the early period of homologous magma and the accumulation of surrounding rock debris. When a volcano erupts, an eruption column composed of a large number of pyroclastic rocks and gases rises and spreads into the sky under the impact force, among which thicker and heavier fragments such as volcanic bombs, volcanic rocks and breccia are thrown by the volcano, and fall rapidly due to their own gravity, forming a volcanic cone with the lava flowing out of the crater (except lava, volcanic aggregates, volcanic breccia and clastic lava are its characteristics); However, tuff debris, such as volcanic ash and volcanic dust, because of its light weight and fineness, is mostly carried by suspended eruption clouds and carried far away by the dominant wind. When its flow energy decreases, it falls in turn according to its gravity and settlement rate, forming a pyroclastic mat dominated by tuff. In the lacustrine stage of volcanic eruption, there are plane parallel bedding and graded bedding, and the roundness becomes better away from the crater, and the sorting is mostly medium-good. The sediment thickness and the average particle size of debris gradually decrease away from the crater (Fisher and Schmincke, 1984). The subsidence and deposition of basic and intermediate basic magma mostly form volcanic slag cone, pyroclastic rock sheet dominated by tuff or lava flow dominated by lava. However, the intermediate-acid volcanic deposits mostly form pumice-like deposits, which are often accompanied by the formation of large-scale composite volcanoes. The geometry and particle size of sediment distribution depend on the height of volcanic eruption column and the direction of dominant wind in the atmosphere. The deposits in the air are widely distributed, covering all the terrain where the eruption cloud turbulence passes, which can be described as overwhelming, and this vast area is an important distinguishing sign (Liu Xiang and Xiang Tianyuan,1997; Fisher and Schminke, 1984).
(2) pyroclastic flow deposition, which is formed in the process of hot pyroclastic flow (pyroclastic density flow), and the gas expands and flows along the surface at an extremely fast speed. Volcanic debris is mainly volcanic ash and breccia, and sometimes volcanic rocks and pumice appear; Plastic glass chips, plastic rock chips and carbonized wood are phase indicator substances. Fused breccia and fused agglomerate are its representative rocks. Most of its detritus are plastic, poorly sorted, with large single-layer thickness (generally > > 1 m), well-developed graded bedding, massive structure and pseudorhyolite structure. There are usually many flow units (one flow unit refers to a debris flow deposit).
The accumulation phase of pyroclastic flow includes massive flow and ash flow formed by the collapse of lava dome and pumice flow and ash flow formed by the collapse of volcanic eruption column. Massive ash flow is a mixture of coarse homologous cuttings and fine tuffaceous matrix, and its scale is small. On the other hand, the pumice flow is large, mainly composed of plastic debris and plastic glass debris, including crystal debris (feldspar, quartz and mica), sub-circular and circular volcanic rocks (mostly intermediate-acid lava debris) and breccia. Poor sorting, false flow structure development; In many flow units, it can be seen that coarse-grained pumice debris flow has reverse graded bedding, while fine-grained debris flow has positive graded bedding. Ash flow is characterized by debris flow dominated by condensed ash (> 50%). The collapse of calc-alkaline intermediate-acid magma or volcanic eruption column with alkaline components may also form large-scale volcanic slag flow accumulation. It is characterized in that a large number of volcanic pebbles and volcanic aggregates together with volcanic ash flow form a larger-scale debris flow accumulation that is not controlled by topographic arrangement (Liu Xiang and Xiang Tianyuan, 1997).
When pyroclastic flow accumulation is produced simultaneously with pumice and volcanic ash accumulation, it can be defined as pyroclastic wave accumulation. It is characterized by plane bedding, low-angle cross bedding, and weak wave bedding. Compared with the associated debris flow, it has finer granularity and better sorting, and is rich in crystal debris and debris (Liu Xiang, Xiang Tianyuan, 1997). When there is pyroclastic wave deposition at the bottom of the flow unit, it is called ground wave, which is the precursor of pyroclastic flow and is located in front of the flow. It can be the collapse of the vertical eruption edge or the airflow brought by the front of debris flow; If the debris flow wave appears at the top of the flow unit, it is called ash cloud surge, which is caused by the turbulent low-density flow of gas carried by debris flow and volcanic ash. Fisher and Schmencke (1984) think that the gray cloud wave originates from the elutriation at the top of debris flow or the separation of coarse and fine particles in the process of turbulence.
(3) Underflow sedimentary surface is a unique product of steam magma eruption. The fiery magma explodes when it meets water (mostly surface water in water or near-surface sediments) during the rising process, and the basic wave flow generated radially spreads from the volcanic eruption column; As a part of the fundamental wave of the volcano, the condensed water vapor is mixed with the pyroclastic particles in the fundamental wave, which supports and dilutes the pyroclastic particles in the fundamental wave (Liu Xiang Hexiang Tianyuan, 1997). The basic wave flow itself is turbulent, and the so-called "pyroclastic density flow" becomes an ordinary sedimentary gravity flow with the attenuation of flow energy (Fisher, 1990). Debris such as volcanic ash and volcanic gravel migrate in suspension in its turbulent flow, which depends on the balance of shear stress and deposition rate, and finally migrate and gather in the form of basement load under the action of traction current (Dellino and La Volpe, 2000). Some people also call basal wave sediments water-based pyroclastic rocks.
Most of the base wave deposits are pyroclastic mainly composed of volcanic ash and breccia, and empty volcanic slag and volcanic ash can be superimposed in the later stage; There is also a small amount of debris from the underlying rocks. Its composition is mostly basic-intermediate acid lava debris; Crystal chips are mainly plagioclase, alkali feldspar, Yingshi and mica are also common, and olivine and pyroxene are occasionally seen. Deep inclusions such as lherzolite can be seen in the basic wave deposition caused by steam eruption of basic magma. The clastic particles of the basic wave accumulation phase are mostly sub-circular-sub-angular, and the sorting is medium-poor (or better); The thickness of a single layer is thin, mostly in the order of millimeter-centimeter, and rarely exceeds several tens of centimeters (~1m); Its sedimentary rhythm and multicycle accumulation sequence are generally developed. The bottom interface of the basic wave accumulation phase is mostly a relatively flat shear plane, and there are a lot of (large) low-angle plate bedding and cross bedding, wave bedding, dune-like structure (photo 5-1), trough-like structure (photo 5-2) and erosion groove (photo 5- 13) in the accumulation phase. What should be emphasized here is the finger marks of the basal wave accumulation phase: accretion lapilli and cross bedding. The accretionary volcanic gravel (photo 5- 15) is a concentric sphere with a diameter of 2 ~ 5 mm, and the circular structure is developed, which can be peeled off layer by layer. This is because the fine debris particles produced by the explosion of steam magma move away from the crater with the help of the basic wave current, during which the surface is bonded with granular volcanic ash and dust, forming a circular circle during the rolling process; The farther away from the crater, the more circles there are, and the larger the volcanic gravel is (Richard et al. 1983). Climbing bedding (photo 5- 16) shows that when the basic wave and current climb outward along the inner wall of the crater, the slope of the upstream surface is steep and the downstream surface is gentle, with obvious turning point; When the fundamental wave debris climbs to a certain extent along it, it can form a gradually climbing bedding (Sun Qian et al., 2006). V-shaped pits are formed by volcanic bombs, stones and pebbles thrown into the air by magma falling on unconsolidated or semi-consolidated basic wave sediments, which are often distributed continuously on the same horizontal plane. This is the result of alternating action of steam magma eruption and subsequent normal magma, indicating that it is near the crater facies. Magmatic eruption and steam magmatic eruption often occur alternately in many Quaternary basaltic volcanic areas in China. Relatively complete profiles are mostly composed of base wave sediments, fine volcanic ash, volcanic breccia, agglomerates and volcanic bombs, which shows the complexity of volcanic activity.
(4) The pyroclastic accumulation phase mixed with pyroclastic and water is formed in the process of "concrete" flowing along river valleys and lowlands. Lahar generally has low viscosity and high velocity near the crater. In the process of its flow, it also denudes and produces underlying soft substances, which are brought into the mud flow and accumulated under the influence of energy consumption or topography. The detrital composition carried by lahar can be single lithology near the crater, but other parts are often non-single; Its particle size changes greatly, and muddy-giant particles can be seen; Away from the crater, its particle size becomes smaller and smaller; The content of solid debris in lahar can account for 20% ~ 60% (up to 80% by volume and weight). The lithology of sedimentary facies in Rajal is mainly normal pyroclastic rocks near the crater, and gradually transits to pyroclastic sedimentary rocks when it is far away from the crater, mainly tuff, tuffaceous sandstone and siltstone. Mud flow accumulation facies has poor sorting, and coarse clastic rock accumulation often has undeveloped graded bedding.
Water is essential in the formation of lahar aggregation stage. Rajal will be triggered when volcanic eruption passes through snow and crater lake, or when volcanic eruption occurs in heavy rain, or when pyroclastic flows into rivers or passes through snow and ice areas. Or related to volcanic eruption, or rapid drainage caused by sudden events such as earthquakes during the intermission of volcanic eruption, may cause the formation of lahar and its sediments.
(4) Diagenesis mode and later changes of diagenesis. Diagenetic model of pyroclastic rocks
The diagenetic mode of pyroclastic rocks is unique. When pyroclastic rocks transition to lava, pyroclastic materials are mainly cemented by molten magma; During the transition to sedimentary rocks, pyroclastic materials are mainly cemented by sedimentary materials and volcanic ash secondary change products; The normal ordinary pyroclastic rocks are mainly compacted and consolidated, and a small part is supplemented by the cementation of volcanic ash secondary products or chemical cements; When the rock is mainly composed of plastic pyroclastic materials, its diagenetic mode is mainly melting or welding.
2. Late changes of diagenesis
During the formation of pyroclastic rocks, pyroclastic materials contain a large number of quasi-stable materials, which are prone to metasomatism and alteration. Therefore, in the later stage of diagenesis, they often undergo various changes, including common devitrification and metasomatic alteration.
(1) devitrification: devitrification refers to the process that glass chips (rigid and plastic), plastic chips and volcanic ash in pyroclastic rocks change from amorphous to crystalline. At the initial stage of devitrification, they only show weak photosensitivity, and then cryptocrystalline texture, microstructure, microcrystal feldspar and aging appear. The common crystallization structures of plastic and semi-plastic glass fragments are aphanitic (photo 5-3), fine and comb-like (photo 5-4). There are many types of crystallized plastic chips. The common crystallization structure is fibrous aphanitic minerals or fine microcrystals, and the edges of plastic chips grow in a comb shape in the longitudinal direction (photos 5-6 and 7). Feather-like or hair-like crystallization structure, that is, aphanitic or microcrystalline minerals are arranged in feather-like or hair-like manner; The feldspar devitrification structure has a low devitrification degree, and consists of fine fibers, extremely fine particles with a particle size less than 0.02 mm and a small amount of glass (photos 5-6). The spherulite devitrification structure consists of cross-extinction fiber spherules (photos 5-8); The mosaic crystallization structure has a high degree of crystallization, and feldspar and timely microcrystals are embedded in contact with each other and often distributed in the center of plastic debris (photos 5-7 and 9). The above devitrification structures can be produced independently, or several devitrification phenomena can occur simultaneously in a plastic fragment (photos 5-6, 7 and 9). Generally speaking, the degree of devitrification in plastic chips is higher than that in the edges. For example, it is often seen that the edge of plastic cuttings is comb-shaped and the center is inlaid with crystallization structure (photos 5-7). Some edges are comb-shaped and spherical, and the center is inlaid with crystallization structure (photos 5-9). On the same profile, the degree of devitrification in the middle is stronger than that in the top. With the strengthening of devitrification, the shape of pyroclastic is gradually blurred, and crystal chips may also form heterogeneous variants (such as the transformation of sanidine into low-temperature potash feldspar).
(2) metasomatic alteration: Late volcanic eruption and subsequent jet and hydrothermal solution often metasomatic alteration of pyroclastic materials. The result of the action is secondary quartzitization, argillization, zeolitization and carbonization.
Zeolitization is almost the only metasomatic alteration product of pyroclastic rocks, including clay minerals, siliceous minerals and chlorite. Zeolite enrichment can constitute minerals. In addition, many trace elements can be enriched during metasomatic alteration, such as gallium, lead, tin, zinc, thorium and uranium. Therefore, we should pay enough attention to it in the research.