(A) the basic characteristics of volcanic rocks
Cenozoic volcanic rocks are distributed around Whale Lake in the middle and west of the survey area, with a total exposed area of about 950 km2. Among them, the area of Xiongying Station in the southwest of the investigation area is about 170 km2, and the area of Sijiao Lake is 650km2. The area around Matiling and Laohukou in the middle of the survey area is 2km2, and the area of Crescent Lake in the east of the survey area is 130 km2.
The distribution of volcanic rocks is closely related to structural units, especially to the location of regional deep faults. Within the distribution range of volcanic rocks, deep regional faults such as Whale Lake-Mu Zi Tage Fault pass through (Figure 3-27), and the lithology and eruption cycle of volcanic rocks in different parts are quite different. Through regional geological survey, we preliminarily think that three volcanic rock areas can be divided. One is located in the southwest of the survey area and is called Xiongyingtai Rock Area, which is characterized by a large distribution area of volcanic rocks (about 86.2%), and the other is called Crescent Lake Rock Area, which is distributed in the east of the survey area, with a distribution area of about 13.6%, the thickness of volcanic rocks is 50 ~ 100 m, and the eruption mode is also fissure eruption. Thirdly, the area around Laohukou and Matiling in the middle of the survey area is called Matiling Rock Area, with a distribution area of only 0.2%. The volcanic eruption modes are central overflow and explosive eruption. The basic characteristics of Cenozoic volcanic rocks are as follows:
1. Xiongyingtai volcanic area
(1) Terrain
Volcanic rock areas have obvious characteristics in topography and aerial photographs. The satellite image shows dark gray-black variegated image, which has obvious boundary with the light gray strip image of the underlying stratum. Volcanic rocks generally occur in horizontal layers and form platform highlands on the terrain. The ground elevation of the lava platform is generally 5000 m, and lava terraces are formed due to different eruption rhythms of the lava platform. There are three levels of lava terraces around the Xiong Ying platform, showing three major eruption rhythms, each of which erupts once or more. According to the lithology and color of different volcanic rocks in each lava terrace.
(2) Lithology and structure
The ⅹ section measured in the area of Eagle Terrace is very representative (Figure 3-28). Volcanic rocks are 240.45 m thick, and * * * is divided into three layers, which represent three eruptions of volcanic rocks respectively and have three rhythms. Volcanic rocks are all stomatal basaltic andesite, but their colors, textures and structures are quite different.
The first rhythm is basaltic andesite with dark green-gray and taupe color. The rocks are dense and columnar joints are developed. The two groups of vertical joint planes are perpendicular to each other, which divides the volcanic rock into square columns of1m. The pore content is between10% and 20%, and the size is between 1 ~ 2 cm. Most of the pores are filled with crystals, which are smoky gray and colorless and transparent.
Figure 3-27 Relationship between Cenozoic Volcanic Rock Distribution and Fault Structure in Jinghu Area
The color of volcanic rocks in the second rhythm is light purple, and the rocks are loose and earthy. The pore content is 20% ~ 30%, the size is generally 2 ~ 5 cm, and some of them can reach 10cm. Pores are generally filled with crystals, which are smoky gray and colorless and transparent.
The volcanic rocks in the third rhythm are dark purple, dense, layered and without joints. The pore content is generally 10% ~ 20%, and the size is generally 1 ~ 2 cm. These holes are usually not filled with crystals.
(3) Microscopic characteristics of volcanic rocks
Grey-brown stomatal basaltic andesite: The rock has a porphyritic structure and consists of porphyritic crystals and matrix. 5% of the phenocrysts are composed of plagioclase and dark minerals, with plagioclase as the main component, taking the shape of plate (n ′ p ∧ (010) = 24, An =47), and the particle size is 0.04 mm× 0.12 mm ~ 0.4 mm×/kloc-0. The matrix accounts for 95%, and the glass-crystal staggered structure and microplate plagioclase are directionally distributed in the glass; There are 5% almonds and pores in the matrix.
Figure 3-28 Volcanic Rock Profile of Xiongyingtai
Porosity light purple gray basaltic andesite: The rock has a porphyritic structure, which is composed of porphyritic crystals and matrix. 25% of porphyritic crystals are composed of plagioclase and hypersthene, and about 65,438+05% of plagioclase porphyritic crystals are self-lath-like, with lumpy twins and intermediate feldspar, and some of them are banded, with a particle size of (0.2 ~ 0.47) mm× 60. The matrix is 75%, which is randomly distributed by nearly equiaxed plagioclase particles; There are pores and almonds in the matrix, which are arranged regularly, and almonds are composed of fluorite; The secondary mineral is apatite.
Grey-black (purple-black) stomatal basaltic andesite: porphyritic structure, consisting of phenocrysts and matrix, with phenocrysts accounting for 65,438+05% and plagioclase accounting for 65,438+00%. It is self-plate-like, with lumpy twins, some with banded structure, with melting erosion at the edge, and the grain size (0. The matrix accounts for 85%, mainly composed of microplate plagioclase, accounting for 70%, which is directionally distributed with kaolin, and altered dark minerals are distributed between plagioclase; Limonite mineralization, with magnetite particles accounting for15%; There are pores and almonds in the matrix, with more pores than almonds, and almonds are composed of fluorite.
Micro-sections show that pyroxene is widely contained in Quaternary volcanic rocks, and plagioclase is intermediate feldspar, which does not contain time. However, the edge of pyroxene is darkened and the matrix plagioclase is oriented, indicating that the rock is rapidly consolidated in the flow, which makes plagioclase oriented and pyroxene quenched.
(4) eruption rhythm and period
Cenozoic volcanic rocks in Xiongyingtai rock area are all overflow products, without explosive volcanic rocks and craters, and the eruption mode is fissure overflow. According to the regional geological survey, the Cenozoic volcanic eruption in Xiongyingtai rock area has three rhythms, each rhythm usually has one eruption and * * * has three eruptions, forming a volcanic rock with a thickness of about 240 m. There are also three rhythmic eruptions in the area around Sijiaohu, and the number of eruptions determined according to different lithologic characteristics reaches 9 times, and the thickness of volcanic rocks formed also reaches 316.72 m. The first rhythm has two eruptions, the second rhythm has six eruptions, and the third rhythm has 1 eruption. According to the different eruption rhythms, we divide the Neogene volcanic rocks in this area into three sections. Every eruption, every rhythmic volcanic eruption has a different scale, which leads to a different distribution range of volcanic rocks. The area near the eruption point of volcanic magma has a complete eruption period and a large thickness, while the area far from the eruption point has a small thickness and an incomplete eruption period. In the Neogene volcanic rock area of Xiongyingtai, the volcanic rock from Sijiaohu to the northwest of Xiongyingtai is thick (Figure 3-29), with a complete eruption period and a thickness of 200-400. The three rhythmic eruptions are relatively complete, and the number of eruptions reaches 9 times. The thickness of volcanic rocks from the southeast of Sijiao Lake to Hehua Lake is generally below 200 m, and in some places it is only a few meters to tens of meters thick. Volcanic rocks are only a few meters thick at the southernmost tip of the rock area, with only one rhythm and one eruption. It can be seen that the location of volcanic magma eruption should be in Xiongyingtai area in the north and middle of volcanic rock area, and in Huaidao Lake-Changqing Lake area, the magma flows southward after it is ejected. The flow structure of pores and almonds shows that the flow direction varies from 170 to 2 10.
Figure 3-29 Volcanic Rock Profile of Sijiaohu X.
2. Recent volcanic rock area of Crescent Lake
(1) Topography
The rock area is located in the east of Whale Lake, with a distribution area of about 130 km2, and the aerial images are dark gray mottled. Volcanic rocks are produced in horizontal layers, and the terrain is platform landform, with an altitude of about 4900 m, and there are often steep cliffs around the platform.
(2) Lithology, structure and structure
Volcanic rocks are horizontal and multi-layered, with little difference in lithology. Generally, it is andesite and basaltic andesite, but the characteristics such as color, texture and structure are quite different (Figure 3-30). The profile X measured in this rock area can be divided into six layers, indicating that the volcanic rocks in this rock area erupted six times, forming a rhythm. Volcanic rocks are not very thick, each layer is generally a few meters, with a total thickness of 23.65 m, and all contain pores, with a pore content of 10%-30% and a size of 65438+.
(3) Microscopic characteristics
Purple-gray porous rich andesite: composed of porphyritic crystals and matrix. 2% porphyritic, a little plagioclase, semi-authigenic thin plate, generally 0.3 mm×0.06 mm in size, albite flake twin, basic plagioclase and pyroxene accounting for about 2%, authigenic-semi-authigenic columnar and columnar, generally about 0.4 mm, hypersthene and a little ordinary pyroxene; 98% of the matrix is interwoven, mainly composed of plagioclase (> 53%), with semi-directional dense distribution, including glassy (25%), a small amount of amphibole (15%) and pyroxene (≤ 5%; The accessory minerals are magnetite, ilmenite and apatite, and the pore distribution of 1.5% ~ 20%.
Grey-purple porous glass-rich basaltic andesite is composed of phenocrysts and matrix. Among them, phenocrysts account for 2%, which are composed of augite, hypersthene (about 2%) and a small amount of plagioclase, and the composition of plagioclase is Labrador; The matrix is 98% with concealed structure, with plagioclase (50%) as the main component, albite and amphibole accounting for about 65,438+00%, vitreous (28% ~ 33%) and fluorite accounting for about 5% ~ 65,438+00%. The secondary accessory minerals are magnetite, ilmenite and apatite. About 5% of the pores are distributed in.
(4) eruption rhythm and period
According to the regional geological survey, the volcanic rocks in the Neogene volcanic area of Yueya Lake are all products of overflow phase, and there are no explosive volcanic rocks and craters. The eruption mode should be fissure overflow. The thickness of volcanic rocks is not large, generally only tens of meters. The northern side of the rock area is thicker, reaching nearly 100 meters, and it is thinner to the south. The volcanic rocks in ⅹ * * section are 23.65 meters long. Therefore, volcanic magma should gush along the Tage-Whale Lake deep fault in Mu Zi in the north and flow southward to form volcanic rocks. Volcanic rocks are horizontally layered with no obvious discontinuity between layers, which should be the product of eruption rhythm. There are six eruptions in the X section, in which the first eruption has a small amount of volcanic breccia at the bottom and slag lava at the top. Other eruptions only have some differences in rock structure and tectonic color, and the chemical composition of volcanic rocks is not much different, and the reaction is formed by magma overflow in the same magma chamber.
Figure 3-30 ⅶⅶⅶⅶⅶⅶⅶⅶⅶⅶⅶ85666
3. Matiling Neogene volcanic rock area
(1) Terrain
The rock area is located in the middle of the survey area, at a distance of 10 km from Hubei Province, with Quaternary alluvial fans developed. Volcanic rocks in the rock area are located on the alluvial fan, forming two volcanic cone landforms, which are annular or horseshoe-shaped. The distance between them is about 3 kilometers, and the Mu Zi Tage-Whale Lake deep fault passes nearby. Matiling Volcano Cone is well preserved, in the shape of a ring horseshoe, with a distribution area of less than 0.5 km2, a relative height difference of about 80 m, an inclination angle of 20 ~ 30 around the volcano cone, and a collapsed crater at the center. The cone of Laohukou volcano is extremely incomplete, with only cone marks, which are oval and annular, with a distribution area of about 1.5 km2 and a relative height difference of only about 20 m. It is weathered and denuded in the form of residual hills, and the center of the crater is mostly covered with Quaternary aeolian sand.
(2) Volcanic facies and lithologic structure
Both Matiling Volcanic Cone and Laohukou Residual Volcanic Cone are formed by central eruption, including volcanic eruption facies lumps, volcanic breccia and volcanic bombs. Common in Matiling volcanic cone, a small amount of overflow phase forms rope-like and slag-like lava. Due to weathering and denudation, most of the old Hukou volcanic cone retains overflow volcanic lava, and explosive agglomerates and volcanic breccia are rarely seen.
Volcanic rocks are generally alkaline basalt, leucite alkaline basalt and so on. When a volcano erupts, most of them form aggregates, breccias and volcanic bombs, and the sizes of breccias and volcanic bombs range from 2 to 10 cm. Lava flow forms rope lava and porous slag lava floating on the surface of lava flow, which is common near the crater. Away from the crater, only irregular layered volcanic lava can be seen. The lava layer inclines to the outside of the crater, and the inclination angle generally ranges from a few degrees to more than ten degrees. Pore structure is widely developed in volcanic rocks, with a general content of 20% ~ 30%, and the content of pores in slag lava can reach 80%. The pore size is generally 0.5 ~2cm, forming pore cavities of various shapes. In addition, a small amount of inclusions are found in volcanic rocks, the lithology is granite gneiss, and the size is generally 10 ~ 20cm ~ 20cm, which is irregular. Granite gneiss xenoliths are loose and micro-gneiss, which may come from the middle and upper crust.
(3) Microscopic characteristics
Slag leucite alkaline basalt: micro-mode-granular structure, slag pore structure, pore development 80%. The rocks are composed of augite, diopside, leucite and metallic minerals. The phenocrysts consist of 2% pyroxene and a small amount of leucite. Pyroxene is self-columnar, with a particle size of 0.047~0.24 mm, and leucite is in the form of circular erosion, in which inclusions are distributed. Inclusions are distributed in leucite in a ring parallel to the crystal outline, and the maximum particle size is 0.1mm; ; The matrix is also composed of pyroxene and leucite, but the content and particle size are different, and it is granular, with 78% pyroxene and 20% leucite (color swatches 3-4, 3-5 and 3-6).
Porosity leucite alkaline basalt: porphyritic structure, pore structure, composed of porphyritic crystals and matrix. Porphyry crystal is 13%, in which olivine accounts for 10%, authigenic short column, pyroxene 3%, authigenic-semi-authigenic column, similar in size to olivine, about 0.5mm；; The matrix accounts for 87%, has a microcrystalline structure and consists of pyroxene (65%-70%) and leucite (15%-20%). There are also traces of magnetite and biotite; The content of pores is 15% ~ 20%, and there is an inclusion composed of granular medium-grained feldspar and a small amount of alkali feldspar and amphibole, and the size is about1mm.
Granite gneiss (xenolith): porphyritic structure, gneiss structure, the rock is composed of porphyritic matrix, with more porphyritic crystals and less matrix. 85% of the phenocrysts are composed of plagioclase (84%), syenite (1%) and single biotite, and the phenocryst grain size is about 2.0mm；; The matrix is 15%, which consists of glass and a small amount of pyroxene; A small amount of apatite accessory minerals are round and columnar.
(4) eruption period
Cenozoic volcanic rocks in Matiling rock area were obviously formed by two eruptions, the eruption mode was central eruption, and explosive and overflow volcanic products were formed respectively. Judging from the reservation of volcanic cones, Laohukou crater erupted in front and Matiling crater erupted in the back. The two craters are not far apart, and both are near the Tage-Whale Lake fault in Mu Zi, and the volcanic rocks are similar in lithology, which indicates that the volcanic eruption of these two craters should be related to the deep fault, which may be a magma eruption in a magma chamber.
(2) Petrochemical characteristics of volcanic rocks
The petrochemical analysis results of Cenozoic volcanic rocks in Jinghu area are shown in Table 3-24. It can be seen from the table that the chemical composition of volcanic rocks in Xiongyingtai rock area and Yueyahu rock area is similar, but it is quite different from that in Matiling rock area.
Table 3-24 Petrochemical Analysis Results of Cenozoic Volcanic Rocks in Jinghu Area
sequential
Note: The unit of oxide content is%.
1. Classification and nomenclature of volcanic rocks
See TAS diagram (Figure 3-3 1) and (recommended by the International Geological Union 1989) for the petrochemical analysis results of Cenozoic volcanic rocks in Jinghu area: most samples of volcanic rocks in Xiongyingtai rock area and Yueyahu rock area are cast in rough andesite area, and only one sample is cast in basaltic rough andesite; However, most volcanic rocks in Matiling rock area are produced in Xiang Yan alkaline basalt area, and only one sample is produced in alkaline basalt area. They belong to neutral and basic volcanic rocks.
The same result can be obtained by judging volcanic rock series types with silica-alkali diagram and A-F-M diagram. As shown in Figure 3-32, the volcanic rocks in Xiongyingtai and Yueyahu rock areas are calc-alkaline volcanic rocks, and the volcanic rocks in Matiling rock area are alkaline volcanic rocks.
2. Determination of volcanic series and types
Rietmann alkalinity combination index (σ) shows that the volcanic rocks in Xiongyingtai rock area are similar to Yueyahu rock area, but quite different from Matiling rock area. The σ values of the first two are 2.83 ~ 4.67 and 2.29 ~ 3.2 1 respectively, and the latter is 15.58 ~ 28.64, so the first two should belong to calc-alkaline series, while the volcanic rocks in Matiling rock area are alkaline. K2O content K2O >Na2O content shows that the volcanic rocks in Xiongyingtai and Crescent Lake belong to potassium type, while the volcanic rocks in Matiling area belong to sodium strong alkaline volcanic rock series (Atlantic type) divided by σ value (A. Rittmam, 1962) in the rock series type table. According to the new classification standard of potassium and sodium (Le Bas, 1986), all Cenozoic volcanic rocks should be of potassium type.
Figure 3-3 1 TAS diagram of Cenozoic volcanic rocks in Whale Lake area
Figure 3-32 Silica-alkali diagram and A-F-M diagram of Cenozoic volcanic rocks in Jinghu area
3.3 calculation. CIPW standard minerals company
The calculation table of CIPW standard minerals in three rock areas is as follows (Table 3-25).
Table 3-25 CIPW standard mineral unit of Cenozoic volcanic rocks in Yuhu area:%
As can be seen from the table, the volcanic rocks in Matiling rock area do not contain Q (chronological), but contain Ne (nepheline), Fa (fayalite) and Fo (forsterite), which is consistent with olivine and leucite under rock slices and basically consistent with petrochemical composition. The differentiation index di = or+AB+NE varies from 46.08 to 49.42, while the volcanic rocks in Xiongyingtai rock belt and Yueyahu rock belt do not contain nepheline and olivine, and the seasonal (Q) content varies from 2.25 ~ 13. 17, and the differentiation index di = q+or+AB. 52.6 1 ~ 65.38, indicating that its differentiation degree is higher than that of Matiling volcanic rock.
(3) Characteristics of rare earth elements in volcanic rocks
The analysis results of rare earth elements in Cenozoic volcanic rocks in Jinghu area are shown in the following table (Table 3-26), and the distribution pattern of rare earth elements is shown in the following figure (Figure 3-33).
The characteristics of rare earth elements in Cenozoic volcanic rocks in the three areas of Jinghu Lake are similar, with the following characteristics.
The total amount of 1) rare earth is relatively high, generally between 307.49×10-6 ~ 717.45×10-6.
2) The light rare earth is enriched and the fractionation is obvious. The distribution curve leans to the right, and the ratio of light and heavy rare earths is (∑ lree/∑ hree) 15.63 ~ 40.43.
3) Slightly negative Eu anomaly, and the δEu value is generally 0.42 ~ 0.93.
Table 3-26 Rare Earth Element Composition and Digital Characteristics of Cenozoic Volcanic Rocks in Jinghu Area
Note: The unit of rare earth element content is 10-6.
Table 3-27 Table of Trace Elements Composition of Cenozoic Volcanic Rock Samples in Jinghu Area
Note: the content unit of K is%, and the content unit of other elements is 10-6.
Figure 3-33 Standardized classification of rare earth chondrites in Cenozoic volcanic rocks in Jinghu area.
4) The negative Ce is abnormal, and the δCe value is generally 0.68 ~ 0.87.
5) Although there are great differences in petrochemistry among the three segments of the volcanic rocks in Jinghu Lake, the characteristics of rare earth elements are similar, which should be the products of different differentiation stages of the same magma source.
(4) Characteristics of trace elements in volcanic rocks
The trace element composition of Cenozoic volcanic rocks in Jinghu area is shown in Table 3-27, and its trace element spider web diagram is shown in Figure 3-34.
It can be seen from the figure and table that the trace elements of Cenozoic volcanic rocks in Whale Lake area are similar. They all have the following characteristics:
1) Barium, rubidium, potassium, strontium and other large ion pre-MagmaElemental are relatively enriched.
2) Inactive elements such as niobium, zirconium, hafnium and phosphorus are relatively lacking.
The above two points reflect whether trace elements are easily soluble in fluid phase and transported by fluid. The above characteristics of Cenozoic volcanic rocks show that volcanic magma was formed by rapid upwelling and ejection after melting without large-scale fluid action, so it should have a good upwelling channel.
3) Compared with mantle rocks, the contents of compatible elements such as Sc, Cr, Ni and Co are much lower.
4) The incompatible elements La, Ce, Rb, Th, U and K are much higher than those of mantle rocks.
5) It has a large negative Ti anomaly.
There is such a huge difference between Cenozoic volcanic rocks and mantle rocks in Jinghu area, which indirectly shows that volcanic magma can not directly come from the mantle, nor can it be separated from the mantle magma, and its magma source should be produced by partial melting of crustal materials.
(5) The age and environment of volcanic rocks.
Judging from the occurrence and topography of volcanic rocks in Whale Lake area, the volcanic rocks in this area are relatively new, and most of them retain the original geomorphological features such as lava platforms and volcanic cones. We took samples from the rough andesite in Sijiaohu and Yingxiongtai, and conducted K-Ar test in the open laboratory of isotope geochemistry of China Geological Survey. The results showed that the age values were 65438±03.5Ma, 7.46Ma, 65438±03.6Ma and 65438±07.9Ma respectively.
Figure 3-34 Spider Web Diagram of Trace Elements in Cenozoic Volcanic Rocks in Jinghu Area
Figure 3-35 lgτ-lg (σ25× 100) Figure \ (The base map is based on H.K.Loffler, 1979).
There are many studies on volcanic rocks in Xiongyingtai area. Deng Wanming (199 1) took samples from the baking layer of volcanic rocks under Yingxiongtai, and obtained the analysis results of 1.08 Ma by thermoluminescence method. Wu Cailai et al. (2000) obtained the age analysis results from the K-Ar method of whole rocks and mica in Xiongyingtai volcanic rocks and Crescent Lake volcanic rocks: the age values of Xiongyingtai volcanic rocks are 1 1.05 Ma, 12.22 Ma, 12.83 Ma, and the age values of Crescent Lake volcanic rocks are 6544.
It is worth mentioning that when many scholars quoted the K-Ar dating results of the volcanic rocks in Whale Lake in the regional survey of Xinjiang (1.982) 1: 1 10,000, they all thought it was 0.69 Ma, but according to the original data of this regional survey, the result was 69 Ma, which was obviously wrong.
To sum up, the Cenozoic volcanic rocks in Jinghu area were formed in the middle Miocene, but the possibility of Pleistocene is not ruled out.
Using the method in Figure 3-35, according to the analysis results of major elements, the eruptive tectonic environment of volcanic rocks is preliminarily determined. Where τ = (Al2O3-Na2O)/TiO2, σ = (Na2O+K2O) 2/(SiO2-43). The projection results show that the sample points are concentrated in area C, but close to area B. We think that these volcanic rocks should belong to the volcanic rock type that evolved after intraplate orogeny. During the formation of volcanic rocks, crustal materials and mantle materials are mixed and melted in the "crust-mantle transition zone" to form volcanic magma. Its formation is the result of lithospheric deformation and evolution during the uplift of the Qinghai-Tibet Plateau.