This kind of rock contains pyroclastic materials >: 90%, and there are few normal sediments and lava materials. According to its diagenetic mode and structural characteristics, it can be divided into three subtypes: molten pyroclastic rocks, ordinary pyroclastic rocks and layered pyroclastic rocks.

(1) welding pyroclastic rocks

The viscous molten slurry rich in volatiles explodes violently, and the gas expands and foams in large quantities due to the sudden pressure drop, which makes the bubble wall thinner and finally explodes, forming a high-temperature pyroclastic flow composed of thermoplastic glass fragments, slurry fragments, crystal fragments, rock fragments and a large amount of gas, which can move at high speed along the volcanic slope, gather under certain conditions, and bond with each other under the weight of overlying sediments to form molten pyroclastic rocks. The appearance looks like lava, dense and massive, but it has pseudo-corrugated structure due to plastic pyroclastic, and sometimes there are columnar and plate joints.

It has a fusion structure, and the fragments are mainly composed of crystal fragments, plastic fragments, plastic glass fragments and volcanic dust, and there may be a small amount of rigid fragments, which have a fluid structure due to the elongated orientation of plastic fragments. According to the particle size of debris, it can be divided into fused aggregate, fused breccia and ignimbrite. It can also be further named according to Figure 2-47 and Figure 2-48, with the corresponding lava name prefix, such as rhyolite glass tuff.

Figure 2-47 Quantitative particle classification of pyroclastic rocks (according to Chengdu Institute of Geology, 1980)

Figure 2-48 Naming of "Three Kinds of Debris" in Tuff (according to Sun Shanping, 1984)

Figure 2-49 Yinganyan (Liangzhu, Hangzhou, Zhejiang, single polarized light, d=4mm)

Fused agglomerate and fused breccia are often exposed, with small distribution area, mainly occurring in volcanic neck, caldera and volcanic structural depressions. Huge pyroclastic flows and intrusive ignimbrite bodies are products of near-crater facies. For example, Ningwu Tongjing Niangniangshan produces igneous breccia, and Hangzhou Liangzhu produces dacite ignimbrite (Figure 2-49).

The degree of fusion fusion fusion fusion fusion fusion fusion fusion fusion of different parts and thicknesses of the same eruption unit is different (Figure 2-50 and Figure 2-5 1). According to the deformation characteristics of plastic glass fragments and plastic fragments, they can be divided into three levels: weak fusion, fusion and strong fusion.

Weak fused fused plastic glass fragments are slightly deformed, some edges and corners begin to become round, and some still keep curved edges and corners, which are slightly slender. Plastic cuttings are scarce and the rock flow structure is not obvious. It is often produced in the upper and lower parts of ignimbrite, and has a gradual transition relationship with ignimbrite.

Figure 2-50: Fused fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion.

Fig. 2- The internal structure and deformation characteristics of rhyolitic fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion fusion (according to Wang Dezi et al., 1982)

Fusion bonded plastic glass fragments can still restore the angular shape of the cambered surface, and plastic fragments are developed. Plastic debris is squeezed by rigid debris, and has obvious deformation orientation and obvious flow structure at its edge, especially at the compression side. It often accumulates into a huge thickness and is located in the middle and upper part of the eruption unit on the section.

There are a large number of strongly fused fused plastic glass fragments, all of which are deformed and flat, and only the glass fragments with weak deformation are occasionally seen in the spreading part of rigid fragments (usually crystal fragments). Most of the plastic fragments are in direct contact, and there is little dust. The flow structure is very obvious, and sometimes it is not easy to distinguish it from the flow structure. Generally located in the middle and lower part of the eruption unit. Generally speaking, the fusion degree between the crater and the middle and lower parts of the eruption unit layer is stronger than that between the remote crater and the middle and upper parts of the eruption unit layer.

(2) Subtypes of ordinary pyroclastic rocks

The diagenetic mode is mainly compaction, which is often superimposed with hydrochemical cementation. Cementation is often volcanic ash decomposition, which is composed of opal and clay minerals (such as montmorillonite), and after recrystallization, it becomes chalcedony and hydromica aggregate. The general layered structure is not obvious. The pyroclastic materials are mainly composed of blocks, volcanic breccia, volcanic breccia, crystal detritus and semi-plastic glass detritus, mainly rigid and semi-plastic detritus, and there is no plastic deformation such as deviation and elongation after accumulation. According to the main detritus in rocks (generally > 50%), it can be divided into agglomerate, volcanic breccia, volcanic breccia and tuff. When the contents of pyroclastic rocks with different particle sizes are mixed, the compound names can be determined according to the contents of various clastic rocks (see Figure 2-47), such as breccia tuff and conglomerate breccia. Further naming should also determine the lava composition corresponding to pyroclastic rocks according to the microcrystal combination or porphyritic characteristics in homologous cuttings, and participate in naming as prefixes, such as andesite volcanic breccia and rhyolitic tuff.

Aggregate particle size >: 64 mm, coarse pyroclastic material >: 50% rock. Debris is mainly fragments of lava, often mixed with fragments of volcanic breccia, volcanic ash and other substances. Its composition is basalt and andesite, as well as acidic and alkaline volcanic rocks. Fragments vary in size and are poorly sorted, most of which are angular, rectangular and irregular. The distribution range is narrow, mostly distributed near or filled in the crater, which is one of the signs of discovering ancient crater or ancient fire neck.

Volcanic breccia is a rock with a particle size of 2 ~ 64 mm and a volcanic breccia content of over 50%. The composition of volcanic breccia is mainly lava debris, but also crystal debris, glass debris and other debris inclusions. The size of clastic particles is uncertain, the sorting is poor, and there are often edges and corners, no bedding or no obvious bedding. Generally distributed in or near the crater, often associated with agglomerate, but also distributed in areas slightly away from the crater.

Figure 2-52 Rhyolitic glass tuff (Hebei Xuanhua, single polarization, d = 4 mm)

Tuff grain size

(3) Subclasses of layered pyroclastic rocks

Pyroclastic rocks with obvious rhythmic bedding and layered structure are generally formed by volcanic ash accumulation in water basins. Layered tuff is common, and when the normal sediment content is 10%, it will transition to tuff.

2. The pyroclastic lava that transits to lava.

This kind of rock has lava structure and massive structure. The characteristics of lava are the same as those of the corresponding extrusive rocks. The pyroclastic materials are mainly crystal debris, glass debris and rock debris, and a small amount of foreign rock debris. Its composition is usually the same as or similar to the lava that binds them. The pyroclastic content is 10% ~ 90%, which changes greatly and is cemented by molten slurry. The causes of clastic lava are varied; The solidified lava crust continues to flow in the lower part of the molten magma, and when the escaping gas explodes, it can be crushed and cemented by the lava, forming breccia lava and lump lava. When the explosion energy is insufficient, fragments are often thrown from the crater and lava overflows. Debris falling into lava can be cemented by lava to form various debris lava. When the molten magma erupts from the crater with a great impulse, most of the phenocrysts in the molten magma can be broken, forming a tuff lava dominated by crystal chips; The underground cryptoexplosion of magma often breaks the internal phenocrysts and can also form tuff lava. In tuff lava, the debris is mainly crystal debris, and there may be a small amount of rigid debris, but generally there is no glass debris.

3. Volcanic clastic rocks in transition to sedimentary rocks

This kind of rock is formed by the accumulation of pyroclastic materials and normal sediments falling into or brought into the water basin at the same time. The content of normal sediments in rocks can reach 10% ~ 90%, and the debris is cemented by chemical sediments and clay substances, or compacted and consolidated. According to the content of pyroclastic materials, it can be divided into sedimentary pyroclastic rocks and pyroclastic sedimentary rocks. The former is andesite crystalline clastic tuff and basaltic andesite clastic tuff; The latter are tuffaceous sandstone, tuffaceous mudstone and tuffaceous limestone (dolomite).

(1) sedimentary pyroclastic rock subclass

It is often associated with normal pyroclastic rocks and normal sedimentary rocks and often has a transitional relationship. According to the granularity of pyroclastic rocks, it can be divided into sedimentary agglomerate, sedimentary volcanic breccia and sedimentary tuff. The most common is tuff. Rock has layered structure and well-developed rhythmic layer. There are normal sediments such as round gravel, sand and clay in the debris, and sometimes biochemical sediments may appear.

(2) Subtypes of pyroclastic sedimentary rocks

Among them, the content of pyroclastic rocks is less, ranging from 50%- 10%, which is closer to sedimentary rocks, and often has a gradual transition relationship with sedimentary pyroclastic rocks, and the accumulation position is generally far from the crater. When naming, the basic name is the name of normal sedimentary rocks, and the prefix is pyroclastic materials, such as tuffaceous sandstone and tuffaceous conglomerate.

4. Distribution, occurrence, minerals and research significance of pyroclastic rocks.

Pyroclastic rocks are widely distributed in various geological ages and many areas, mainly occurring in volcanic rock series and sedimentary rock series in layered or quasi-layered form, and can interact or transition with normal sedimentary rock layers or volcanic rocks, and can be distributed near or inside the crater or deposited far away from the crater. It can be formed on land or underwater, and most of them exist in the active area of the earth's crust. Pyroclastic rocks are widely distributed in China, such as Precambrian strata in Qinling area, Paleozoic strata in northwest Qilian Mountain, Mesozoic volcanic rocks in eastern and southeast coastal areas, Mesozoic continental coal-bearing strata in northeast China and Mesozoic strata in southwest China (so-called "mung bean rocks"). In addition, modern pyroclastic rocks are also developed in Tengchong, Yunnan, northern Tibet and Hotan, Xinjiang.

Pyroclastic rocks, especially those near volcanic passages, usually contain many minerals, sometimes in large quantities, such as alunite, pyrophyllite, zeolite, sulfur, silver, copper, lead, zinc, iron, mercury and uranium. Most of these deposits occur in layers, and the ore bodies are in integral contact with surrounding rocks, which often form ore-bearing rock series to some extent. Ore-forming materials come from volcanoes, but the enrichment of minerals is carried out in the process of volcanic material aggregation, solidification and secondary change. Volcano provides minerals in the following ways: volcanic jet, volcanic dust adsorption, hydrothermal transportation and so on.

Some iron and copper mines in the middle and lower reaches of the Yangtze River in China, uranium mines in the southeast coast, copper mines in the northwest, iron and copper mines in Yunnan, zeolite mines and bentonite in Xinyang, Henan, etc. They are all distributed in pyroclastic rocks, and potassium-bearing vitrinite tuff in eastern Sichuan is a good raw material for potash fertilizer. Tuff is also a light and hard building material, and acidic tuff rich in glass is also a good raw material for cement mixture and expanded perlite. In addition, pyroclastic rock can be a good reservoir for oil, natural gas and water because of its porosity. The engineering performance is slightly poor.

Through the study of pyroclastic rocks, we can understand the geographical distribution, eruption mode, intensity and eruption environment (underwater or onshore) of ancient volcanoes, analyze the temporal and spatial distribution of volcanoes and explore the laws of tectonic movement. Because volcanic ash can float far away, tuff often appears in a relatively stable horizon and has a wide range. Therefore, it can be used as a "marker layer" for stratigraphic correlation. For example, "mung bean rock" (a kind of glassy tuff with mung bean-like mud ball structure), which is widely distributed in Sichuan, Guizhou and Hubei, is about 1m thick and stable in occurrence, and is a marker layer for stratigraphic division and correlation at the bottom of Middle Triassic in this area; Another example is the tuff layer in the early Cretaceous coal seam in Jixi, Heilongjiang Province, which is also a good sign of coal seam correlation.