Magmatic rock belt in the middle of North China landmass

This rock belt mainly includes the northern part of Shanxi Province and Henan Province and the western part of Hebei Province. It is roughly divided into Taihang rock area and Lvliang rock area with the boundary of Lingqiu-Taiyuan-Linfen line.

(1) intrusive rocks

Yanshanian diorite, syenite, granite and alkaline rocks are mainly developed in Lvliang rock area. Among them, granite and a small amount of diorite dominated in the early Yanshan period, alkaline rocks and granite dominated in the late Yanshan period, followed by diorite and syenite. Isotopic ages of main rock masses are shown in Table 3- 12. Yanshan intermediate-intermediate acidic alkaline intrusive rocks are mainly developed in Taihang Mountain area, and the main rock masses are Zhoukoudian, Handan Fushan, Wu 'an Mine, Hongshan, Yongfeng County, Pingshun-Lingchuan, Shanxi, and Anyang-Huixian, Henan.

1. Geological and Petrological Characteristics of Rock Mass

(1) Early Yanshanian

Table 3- 12 Isotopic Age Determination Results of Main Rock Mass

In Taihang Mountain rock area, neutral-intermediate acid intrusive rocks mainly developed in this period. The main rock bodies are Fushan rock body in Handan area, Dongye-Nanyao and Dongye, Shangzhuang, Dongshui and Donghuangmu rock bodies in the western area of Linxian County, and Beiluoxia, Sitou, Xigou and Shangzhanggou rock bodies in Pingshun-Lingchuan. The isotopic ages of the rocks are all greater than 140Ma, and the whole rocks are Dongye and Shangzhuang. K-Ar age of amphibole in Nanzhainao rock mass is 170Ma, etc. Rock mass mainly exists in the form of rock, rock wall and rock cover, and its exposed shape is irregular, which is obviously controlled by fault structure. For example, the Beiluoxia rock mass is SN- trending, but the central part of the main body is NNE, and the north and south ends are NNW and NNW respectively. Obviously, the rock mass follows the fault intrusion along the north-south direction, and some rock masses. The surrounding rocks invaded by rock mass are mainly caprock sedimentary rock series, such as Cambrian and Ordovician, and the contact zone of surrounding rocks is developed with skarnization, marble, epidote and chloritization.

sequential

Source: Regional Geology of Shanxi Province, 1989.

The intrusive rocks are mainly diorite, diorite, and a small amount of diorite and quartz monzonite. The rocks are light gray-grayish green, authigenic-semi-authigenic granular structure and massive structure. The main minerals are plagioclase (30% ~ 60%) and amphibole (35% ~ 55%), followed by pyroxene (0 ~ 10%) and biotite (1% ~ 65433). The grade of plagioclase varies greatly, an = 30 ~ 55, belonging to neutral plagioclase.

The early Yanshanian in Lvliang rock area can be divided into two stages. In the first stage, the intrusive rocks are mainly acidic, followed by intermediate acidity. The acidic intrusive rocks mainly include Tiewadian, Guhuayan, Pandao, Dida-Heiya, Heigoubei and Ran Zhuang, while the intermediate acidic intrusive rocks mainly include Liulingshan, Silkworm House, Xiangjiayao and Gushan. The rock masses are all in the form of rock plants and distributed in the northwest direction, but Ran Zhuang and Liulingshan are in the north-south direction, while Silkworm House and Xiangjiayao are in the east-west direction. The exposed area of each rock mass varies greatly, ranging from 0. 1 to 43km2. Among them, Heigoubei rock mass has the largest area (43km2). Rock mass can be divided into fine-grained marginal facies, medium-grained transitional facies and medium-grained central facies, which have obvious intrusive contact with surrounding rocks. The surrounding rocks are mainly Wutai Group, Jining Group and Laishui Group metamorphic rock series, and some places are Great Wall carbonate rocks.

Acid intrusive rocks are mainly adamellite, syenite and alkali feldspar granite. The difference between different rock types lies in the content change of main rock-forming minerals. Rock has a mottled medium-coarse irregular grain structure. The main rock-forming minerals are alkali feldspar (30% ~ 45%), Yanshi (25% ~ 35%), plagioclase (15% ~ 25%), followed by biotite (2% ~ 5%). Among them, alkali feldspar includes orthoclase and banded feldspar, and the latter often appears as porphyritic crystals in porphyritic structure. The grade of plagioclase in the central phase of the rock mass is an = 20 ~ 25, and the transitional phase and marginal phase are an = 15 ~ 20. The accessory minerals are zircon, apatite, sphene, anatase, pyrite, magnetite, limonite, ilmenite, monazite, chalcopyrite, molybdenite, topaz and fluorite.

The intermediate-acid intrusive rocks are mainly granodiorite and diorite, and the main difference between them is that the former has higher chronological content than the latter. The rocks are of semi-authigenic granular texture. The main rock-forming minerals are plagioclase (40% ~ 60%), Yingshi (10% ~ 20%) and alkali feldspar (5% ~ 20%), followed by amphibole (1% ~ 5%) and biotite (65438). Plagioclase is semi-self-shaped plate, with developed banded structure, an = 25 ~ 50, amphibole is semi-self-shaped column, slightly chloritization, biotite brown-green, with chloritization at the edge. The accessory minerals are magnetite, zircon, apatite, sphene, anatase, ilmenite, fluorite, rutile, chalcopyrite and scheelite.

The second stage intrusive rocks are mainly acid hypabyssal intrusive rocks, and the rock masses are mainly distributed in Hengshan, Wutai, Shenchi and Zhongtiaoshan areas. The main rock masses are Chakou, Xiaoyinchang, Boxwood, Changchenggou, Yixingzhai, Qian Shang, Damajun and Daxigou. The rock mass is composed of pebbles, bedrock and branches, with the exposed area less than 1km2, and only the Chakou rock mass is12.8km2.. The surrounding rocks of the rock mass are diverse, including Great Wall, Cambrian, Ordovician carbonate rocks, Precambrian metamorphic rocks and pre-Indosinian intrusive complexes. Late felsic dikes are interspersed among rocks, and some rocks have strong carbonation and limonitization. The main rock type is alkali-feldspar granite. The rocks are light gray and grayish white, with porphyritic texture, and the matrix is fine-grained granite structure, which is granular, pseudospherical and fine-grained. The porphyritic minerals are Yanshi (5% ~ 15%), alkali feldspar (5% ~ 30%), feldspar (2% ~ 35%) and a small amount of biotite and amphibole. Matrix minerals are similar to porphyry minerals, accounting for 10% ~ 35% and alkali feldspar 15. 3% ~ 45% plagioclase. The phenocrysts produced in the form of phenocrysts have obvious melting edges, and the phenocrysts in the matrix often occupy the pseudospherulite structure of alkali feldspar, while the phenocrysts alkali feldspar and plagioclase are semi-self-template and self-template respectively. The former is kaolinized and the latter is sericitized. Biotite is scaly, yellow-green, strong chloritization, amphibole is semi-self-columnar, yellow-green to green. The accessory minerals are magnetite, apatite, sphene, ilmenite, anatase, rutile, galena, pyrite, limonite, molybdenite, chalcopyrite, sphalerite, natural gold and fluorite.

(2) Late Yanshanian

In Taihang Mountain rock area, the intrusive rocks in this period are mainly neutral to alkaline neutral and alkaline rocks. The main rock masses are Zhoukoudian, Wu 'an Mine, Hongshan, Lizhen, Bujutou, Baixiangchuan, Jinggou-Danangou, Jiulongshan and Pingshun-Lingchuan. The isotopic age of rock mass is mostly less than 140Ma, for example, the K-Ar age of Lizhen rock mass is 160 Ma. The age of the main body of the alkaline complex still belongs to the Early Cretaceous. The K-Ar age of the whole rock mass in Ruanhe South is 103Ma. During this period, each rock mass mainly exists in the form of rock branches and dikes, and a few exist in the form of rock branches and dikes. The long axis direction of the rock mass is SN or NNE, and the intrusive surrounding rocks are Cambrian, Ordovician and Permian. The contact zone of rock mass is strongly altered, and the internal contact zone is mainly alkali metasomatism, especially albitization. The outer contact zone is dominated by skarnization and marble, followed by chloritization, silicification and asbestos.

The rock types of intrusive rocks are mainly monzonite, diorite and alkaline syenite.

The monzonite is mainly composed of monzonite and timely monzonite (porphyry), which is grayish green, light fleshy red and porphyritic. The matrix is a fine-grained granular structure and a massive structure, and the content of porphyritic crystals is generally 30% ~ 50%, including plagioclase 15% ~ 30% and potash feldspar 5% ~ 10%. 15%, biotite 0 ~ 5%. The plagioclase is sericitized, biotite is faded, and chloritization, iron and acicular rutile are precipitated.

Diorite includes diorite and diorite (porphyrite). The rocks are grayish green-dark green, semi-authigenic granular structure, porphyritic structure and massive structure. The phenocrysts are mainly plagioclase and amphibole, with a small amount of pyroxene and quartz. The mineral content of different rocks varies greatly, with plagioclase accounting for 50% ~ 75% and amphibole accounting for 65,438+00%. Plagioclase is mostly medium feldspar (an = 10 ~ 40), which is often albized, sericitized and argillaceous, and a few are replaced by calcite and chlorite. Hornblende includes tremolite and chloritization.

Alkaline syenite mainly includes aegirine syenite, porphyritic aegirine syenite and pseudoleucite syenite. The rocks are porphyritic and porphyritic, and the matrix is semi-authigenic granular structure, with rough texture and massive structure. The porphyritic crystals are composed of orthoclase, microcline or pseudoleucite which becomes sericite aggregate, and the matrix is composed of potash feldspar, aegirine, pseudoleucite, amphibole and plagioclase. Plagioclase 10% ~ 20%, aegirine 0 ~ 10%, pseudoleucite 0 ~ 15%, amphibole 0 ~ 5%, and accessory minerals are apatite, zircon, sphene, magnetite and rutile.

The late Yanshanian in Lvliang rock area can be divided into two stages. In the first stage, magmatic intrusion was intense, and intermediate basic-intermediate acidic-acidic and alkaline intrusive rocks developed. Due to multiple intrusions of magma, complex rock masses are often formed, such as Baibeibao, Laotangou, Wang Zhuang, Sunjiazhuang (south of Yixing Village), Lu Gu 'an, Teahouse, Liu Zhuang, Yushugou and Ling Chuan, which are distributed in Wutai and Hengshan areas. However, there are also rocks with simple lithology, such as Qibaigou, Liujiagou, Daquangou, Moshishan and Wangxianzhong acidic intrusions, such as Tainashui, Sunzhuang-Shijiayao, Xiaowoyan, Tanshang, Hou Yu, Jushi Mountain, Sigou Temple, Elvis Presley, Jinzhong Temple, Nantouling, Fenghuang Mouth and Yindongliang. There are also alkaline complexes with very complex lithology, such as Zijinshan, Ta 'er-Erfeng Mountain and Huyan Mountain.

Compound rock masses are all small porphyries, with exposed area less than 2km2, generally 0.06 ~1.6km2. The surrounding rocks are mostly Precambrian metamorphic rock series, and the surrounding rocks are often silicified and pyritized. Crypto-explosive breccia (such as Wang Zhuang complex rock mass) develops in some contact zones between rock mass and surrounding rock, and late lamprophyre dikes and felsic dikes often penetrate in the rock mass.

Single lithologic intrusions mainly exist in the form of small rocks, bedrock and rock wall, and the distribution of rock mass is obviously controlled by structure, with the exposed area of rock mass of 0.25 ~ 6.3km2.. The intrusive surrounding rocks are mainly caprock sedimentary rocks, such as dolomite of Gaoyuzhuang Formation of Great Wall System and limestone of Majiagou Formation of Ordovician System, followed by metamorphic rock series of Precambrian System, such as granulite of Wutai Group and biotite plagioclase gneiss. Skarnization, marble alteration and other surrounding rocks often occur in carbonate rocks.

The form and occurrence of weakly alkaline complexes are complex, including large equiaxed rocks (such as Laoshan rock mass with an exposed area of 40km2), irregular rocks (such as Qiaoergou-Yutu rock mass), complex layered rocks (such as Shengwangshan-Jiaojieshan, Lajiao and other complexes) and dikes (such as Sesame Mountain and Hanjiayan).

In this stage, the first intrusive rocks are mainly diorite and monzonite, and the second intrusive rocks are mainly quartz monzonite, syenite, syenite and alkaline syenite. The characteristics of various types of rocks are as follows.

Diorite is dark gray, semi-authigenic, fine-grained, medium-grained adamellite-like structure, with gabbro-like structure and massive structure locally. The main minerals are plagioclase (50% ~ 60%), alkali feldspar (15% ~ 20%), amphibole (10% ~ 20%) and pyroxene (. The assemblage of accessory minerals is relatively simple, mainly magnetite, sphene, apatite, zircon and so on.

The main difference between monzonite and diorite is that the alkali feldspar content of monzonite is increased to 20% ~ 35%, and the rock has monzonite structure.

The main difference among syenite, quartz monzodiorite and syenite lies in the change of plagioclase and alkali feldspar content. The rocks are light gray, grayish red, semi-self-shaped, heterogeneous, porphyritic and massive; The main minerals are plagioclase (25% ~ 40%), alkali feldspar (15% ~ 45%) and Yingshi (5% ~ 15%), followed by a small amount of biotite and amphibole. The accessory minerals are complex, mainly magnetite, apatite, sphene, zircon, pyrite, limonite, molybdenite, barite and xenotime.

Alkaline syenite is dark gray, semi-authigenic, medium-coarse-grained, porphyritic and locally massive. The main minerals are alkali feldspar (35% ~ 75%), aegirine (10% ~ 40%) and nepheline (3% ~ 20%), followed by a small amount of plagioclase (0 ~ 4%).

The second magmatic activity in Lvliang rock area mainly formed some hypabyssal or ultrahypabyssal acidic and intermediate-basic intrusive rocks. The former includes Diaoquan, Xiaoyan-Qiangtouling, Daliangshan, Sigou, Xiachegou, Qingziyao and Jinniusi alkali feldspar granite porphyries, while the latter includes diorite porphyries such as Sanmenxia, Longxushan, Nanxiangou, Laotangou, Wang Zhuang and Ciyao. Rock mass mainly exists in the form of bedrock and dike, and a few exist in the form of small rock plants. In particular, dike-like rock masses often appear in groups and are scattered, with an exposed area of 0.01~ 0.7km2. The intrusive surrounding rocks are mainly caprock sedimentary rock series, such as Wumishan Formation of Jixian System, Gaoyuzhuang Formation of Great Wall System, Ordovician System and Taiyuan Formation of Carboniferous System.

Acid porphyry is characterized by light color, porphyritic structure and fine-grained matrix. Some rocks have fine grain, graphic structure and microsphere structure, and a few have rhyolite structure. The main porphyritic minerals are Yanshi (5% ~ 15%), alkali feldspar (5% ~ 10%) and plagioclase (5%). The accessory minerals are magnetite, zircon, apatite, sphene, ilmenite, anatase, rutile, pyrite, limonite, fluorite, xenotime, copper-nickel ore, barite, chalcopyrite, galena, molybdenite, garnet and thorium ore. , belonging to zircon-apatite type.

The intermediate-basic porphyrite is porphyritic, with porphyritic crystals of intermediate feldspar (an = 30 ~ 55) and banded structure, and the matrix is plagioclase (an = 25 ~ 30), pyroxene, biotite, etc.

2. Petrochemical characteristics

The SiO2 _ 2 content of intrusive rocks in Taihang Mountain area is 48.40% ~ 67.02%, but it is mainly 50.49% ~ 6 1.40% (table omitted), and the total alkali content (Na2O+K2O) is 3.77% ~ 14.97%, among which the total alkali content of subalkaline intrusive rocks. Rietmann index is less than 4 (generally 1.65 ~ 3.92), the latter is 7.07% ~ 14.97%, and the Rietmann index is greater than 4 (generally 4.30 ~ 13.94). The calculation results of standard minerals show that the timely content of some intrusive rocks is 0.41%~17.38%; Another part of rocks with zero seasonal content contains 0.75% ~ 22.22% olivine, and a few rocks contain basic minerals such as nepheline and augite. These characteristics show that the intrusive rocks in this area are mainly neutral, followed by intermediate basicity and intermediate acidity, and the silicic acid is unsaturated to weakly saturated, and the total alkali content of rocks is high, especially some rocks that do not contain alkaline minerals but have higher alkali content than ordinary rocks of the same kind, which should belong to potash basalt series (olivine coarse rock series).

The mass fraction of SiO _ 2 in intrusive rocks in Lvliang rock area is mainly 5 1% ~ 76.8 1%, and the mass fraction of total alkali (Na2O+K2O) is 6.08% ~ 15.46%, among which the mass fraction of SiO _ 2 in subalkaline rocks is mainly 7/kloc-0. 466646Rietmann index of 466646 is 4.63 ~ 26.74. The content of SiO2 _ 2 in some rocks is 54.55% ~ 68.4%, the total alkali content is 6.43% ~ 10.09%, and the Rietmann index is 3.24 ~ 4.00.

The calculation results of CIPW standard minerals and Bart-nigri cation standard minerals show that isochron (16.05% ~ 38.64%) appears in all subalkaline rocks, which indicates that magmatic silicic acid is supersaturated, and there is no isochron or isochron with low content (0.37% ~ 6.48%) in alkaline series rocks, and olivine, augite and nepheline appear in some rocks.

3. Geochemical characteristics

The total amount of rare earth in intrusive rocks in Taihang Mountain area is 47.08× 10-6 ~ 323.32×10-6 (abbreviated), and the total amount of rare earth in pyroxene is the lowest, which is 47.08×10-6, probably because the distribution coefficient of rare earth elements in clinopyroxene in the early stage of magma evolution is less than 65438. The total amount of diorite rare earth is relatively high, ranging from 205.38×10-6 to 323.32×10-6, followed by diorite rare earth, ranging from109.68×10-6 to 222.22× 6544. The study shows that the total amount of rare earth in different rock types is mainly related to the content of amphibole and accessory minerals in rocks (Xu, 1990). The normalized curves of rare earth chondrites in all kinds of rocks are similar (Figure 3-22), all of which are right-leaning, rich in light rare earth, and have no obvious positive and negative europium anomalies. The ratio of light and heavy rare earth elements is 2.66 ~ 10.94. From pyroxenite (2.94)→ diorite (3.40)→ monzodiorite (5.27)→ quartz monzonite (10.94), the ratio of light and heavy rare earth elements increases from small to large, indicating that light rare earth elements are increasingly enriched in the process of magma evolution. The δEu value of each rock is 0.7 1 ~ 1.25.

Figure 3-22 Standardized REE Distribution Model of Intrusive Rock chondrites in Taihang Mountain Area

The total REE contents of intrusive rocks in Lvliang rock area are: magmatic carbonate rocks 733.90× 10-6, fine-grained granite porphyry veins 17.96× 10-6, and the rest rock types are generally 65.90×10-6 ~ 2009.4009.0000000000006 The total amount of rare earths in diorite, monzonite and syenite are similar, which are 206.64× 10-6, 203.60× 10-6 and 209.46×16, respectively, and the aegirine syenite and nepheline syenite are16, respectively. The normalized curves of rare earth chondrites in different rocks are similar in shape, all right-leaning, and there is no obvious europium anomaly (Figure 3-22). The δEu value is 0.86 ~ 1.2 1, and the ratio of light and heavy rare earth elements is 1.63 ~ 4.67. According to Allegre and minttr's model of partial melting crystallization of igneous rocks (1978), the La/Sm and La values of rocks are drawn on the La/Sm-La diagram, and all casting points basically fall near the partial melting curve, that is, the La/Sm ratio increases with the increase of La, indicating that rocks are generated by partial melting of the mantle, and the melting degree is less than 2% (omitted).

4. Genetic types and tectonic environment of rocks

It can be seen from the above that the intrusive rocks in Taihang Mountain rock area are mainly alkali-rich intermediate rocks and alkaline rocks, and the rock mass contains mantle-derived rock inclusions, and the consolidation index and magnesium index of the rocks are low, reflecting the deep magma source; Mineral chemistry research shows that magma mainly comes from mantle or mantle crust, and isotope geochemistry research shows that magma comes from mantle and assimilates some crustal components.

The aluminum index (Al) of intrusive rocks in Lvliang rock area is mostly less than 1.05, and the corundum or corundum content in the calculation results of standard minerals is mostly less than 1%. The (Al-Na-K)-Ca-(Fe2++Mg) diagram (Figure 3-3) shows that the mineral assemblage of intrusive rocks is plagioclase-diopside-amphibole and plagioclase. Only a few intrusive rocks (such as Ran Zhuang, Tiewadian alkali feldspar granite and Luojiawan syenite granite in the early Yanshanian, Chakou adamellite, Celadon Kiln and Liu Zhuang alkali feldspar granite in the late Yanshanian), Al > 1. 10 in standard minerals, corundum (> 1%) in mineral assemblage, plagioclase-biotite-cordierite.

The diagram of R 1-R2 shows that the intrusive rocks in the early Yanshanian mainly belong to areas 3, 4 and 6 and their vicinity, which are similar to the granitoids in the plate collision period and its uplift period. Intrusive rocks are scattered in the late Yanshan period, mainly distributed near Area 4 and Area 4 and Area 5, similar to granitoids from late orogenic period to non-orogenic period. Combined with the tectonic activities in the area, it can be inferred that the area may be in a compressive environment in the early Yanshan period, forming calc-alkaline granitoids, and in a tensile environment in the late Yanshan period, forming alkaline rocks, especially alkaline rocks.

(2) Mesozoic volcanic rocks

Mesozoic volcanic activity in this rock belt is mainly found in Lvliang rock area, and it is earlier than magmatic intrusion. Volcanic rocks are most developed in Yanshan period, followed by Indosinian period, and their characteristics are as follows (Table 3- 14, Figure 3-23 ~ Figure 3-25).

Table 3- 13 Abundance of Rare Earth Elements in Mesozoic Intrusive Rocks in Luliang Area of Central North China (wB/ 10-6)

(quoted from Lin et al., 1994)

1. Indosinian volcanic rocks

Indosinian volcanic rocks in this area belong to intermediate-acid pyroclastic rocks, which are mainly distributed in Linxian and Jixian areas, and all occur in sedimentary strata, such as the exposed crystalline tuff and glass tuff of Triassic Ermayina Formation, Tongchuan Formation and Yanchang Formation. The rocks are mainly brick red and pink, with crystalline tuffaceous structure and silty structure, and the clastic shapes are angular and sub-angular; Detrital components are mainly chronological, potash feldspar and a small amount of plagioclase; Cement is mainly volcanic ash, followed by chlorite, potassium and calcium, which are basically andesite.

2. Yanshanian volcanic rocks

Yanshanian volcanic rocks in Lvliang rock area are mainly distributed in Hunyuan, Lingqiu and Guangling, with sporadic distribution in Datong, Ningwu and Wuxiang. Volcanic activity began in the Middle Jurassic and ended in the Early Cretaceous.

(1) Main geological characteristics of volcanic rocks in each eruption period

According to the volcanic sedimentary sequence and magmatic evolution characteristics, it can be divided into eight main eruption cycles, which are described as follows.

Figure 3-23 Chemical Classification of Volcanic Rocks in Lvliang Rock Area

Figure 3-24 alkalinity ratio diagram

Figure 3-25τ-δ diagram

A. Yungang eruption period: it is distributed in the Zhecai-Guqu volcanic basins of Zuoyun Gaoshan, Ningwu, Qixian and Guxian, and occurs in the upper part of the third member of Yungang Formation. The intensity of volcanic activity is weak, the scale is small, and the thickness of eruption accumulation is not large. Lava layer is only a few meters thick, pyroclastic rocks are generally a few meters thick, and Qixian area alone is 300 meters thick. Lava facies is not developed, only 1m thick rhyolitic tuff lava is seen in the north of Yungang, Datong City, Zuoyun Mountain Basin, and the other three volcanic basins are rhyolitic tuff or rhyolitic volcanic breccia.

B. Eruption period of cascade mountain: it is distributed in Hunyuan volcanic basin and occurs in the middle and upper part of cascade mountain formation of Changshanyu Group. Overflow is the main feature, with obvious multi-rhythm characteristics, and generally 3 ~ 4 eruptions can be seen. The bottom and lower part of each eruption are dense basalt, the upper part is almond-shaped basalt, and the top is purple almond-shaped or stomatal basalt. The interval between each eruption is short, and sedimentary clastic rock interlayer is not seen or occasionally seen, generally silty mudstone or calcareous siltstone. Volcanic rock thickness 10~70m ~ 70m.

C. Houcheng eruption period: it is distributed in two volcanic basins, Hunyuan in Lingqiu and Weishan in Taibai, and occurs in the middle and upper part of Houcheng Formation in Changshanyu Group. Rhyolitic pyroclastic rocks-rhyolitic lava constitute an explosion rhythm. Volcanic activity is strong, large-scale and thick. Rhyolitic volcanic breccia is mainly formed in Hunyuan volcanic basin, with rhyolitic tuff thickness of 150m and erupting for 2 ~ 3 times. Bentonite develops at the bottom of the rhythm formed by each eruption. Guanwangpucha in Hunyuan is the center of volcanic eruption, which consists of volcanic neck facies and lava facies (mainly rhyolite). Rhyolitic tuff is mainly accumulated in Taibai Weishan volcanic basin in Lingqiu County, with occasional rhyolitic volcanic breccia and agglomerate. The extremely thick conglomerate stratum of Houcheng Formation of Changshanyu Group is the interlayer, and the volcanic rock stratum is generally about 2m thick, and two layers can be seen.

Table 3- 14 Chemical Composition of Yanshanian Major Volcanic Rocks (Lava) in Lvliang Rock Area of Central North China (wB/%)

D. Baiqi eruption period: it is mainly distributed in four volcanic basins: Hunyuan, Taibai Weishan, Tadi and Gaoshan, and is the main body of Baiqi Formation of Donglingtai Group. Generally speaking, it is composed of intermediate-basic pyroclastic rocks, intermediate-basic lava and alkaline or acidic lava, with strong volcanic activity and large scale. The middle and lower parts of Hunyuan volcanic basin are mainly alkaline basalt or basaltic andesite, and coarse andesite or trachyte is common in the upper part, with a thickness of about 180m. The bottom of Taibaiwei Mountain in Lingqiu County is mainly andesite volcanic breccia and tuff, while the middle and lower parts are pyroxene andesite and andesite, and the upper part is dacite breccia, tuff and dacite, with a thickness of about 770m .. It develops under volcanic rocks at the bottom of the tower in Lingqiu County. The lower part of volcanic eruption deposit is dacite volcanic fusion breccia and tuff, and the upper part is dacite, with a thickness of about 30m in general and up to 90m in some places. The alpine volcanic basin in Zuoyun County is composed of andesite volcanic breccia tuff in the lower part and andesite agglomerate lava in the upper part, with a total thickness of about 90m.

E. Zhangjiakou eruption period: a set of rhyolitic volcanic breccia, tuff-rhyolitic lava formed by eruption-overflow, is the main body of Zhangjiakou Formation of Donglingtai Group. Volcanic activity is strong, large-scale, thick accumulation, widely distributed in Hunyuan, Taibai Weishan, Tadi and Guangling four volcanic basins. The lower part of Hunyuan volcanic basin is mainly rhyolitic volcanic breccia and tuff, and the middle and upper part is rhyolitic agglomerate lava, spherulite rhyolite and trachyte, with perlite and andesite, generally 350 meters thick. The thickness of Hunyuan Yexigou is more than 1 000 m, and there are a lot of rhyolitic volcanic breccia and breccia tuff in the middle and lower part, and there are spherulite rhyolite, perlite and rhyolitic lava in the upper part, which is presumed to be the center of volcanic eruption. Taibaiwei Mountain in Lingqiu County is mainly composed of rhyolitic ignimbrite, rhyolitic fused agglomerate and rhyolitic fused volcanic breccia, and the rock of a single volcanic agglomerate can reach several cubic meters. The lower part of the tower in Lingqiu County is mainly rhyolitic volcanic breccia, the middle part is composed of rhyolitic volcanic breccia and tuff containing rhyolitic breccia, and the upper part is mainly rhyolite with a total thickness of 400 ~ 500 m.. In Guangling volcanic basin, dacite rhyolite with thickness over 300m is mainly formed. The K-Ar isochron ages of rhyolite in Zhangjiakou Formation are 138.3Ma (Hunyuan) and 132.2Ma (Taibai Weishan).

F. eruption period of Dabeigou: basic-intermediate basic overflow is dominant, and the deposit mainly occurs in the middle and upper part of Dabeigou Formation of Luanping Group. The intensity and scale of volcanic activity are obviously smaller than that of Baiqi and Zhangjiakou in Donglingtai eruption cycle. The thickness of volcanic deposits is relatively small, generally interbedded with river and lake sedimentary rocks, and the distribution range is small, only distributed in Hunyuan volcanic basin. The lithology of volcanic rocks has obvious vertical change: the lower part of Dabeigou Formation is generally basaltic lava, the middle part is basalt and a layer of andesite or dacite tuff or tuff, and the upper part is generally dominated by basalt or basaltic andesite. The total thickness is 30 ~ 80m.

G. Eruption period of watermelon orchard: acidic eruptive rocks are dominant, and the scale of volcanic activity is small; The rhyolitic volcanic breccia and rhyolitic breccia tuff (35m thick) in this eruption period are only found near the slope of Hunyuan Chonglou, which is the main body of the Xiguayuan Formation of Luanping Group, with 4m conglomerate and sandy mudstone under it.

H. Wang Jiagou eruption period: There are two almond-shaped basalt interlayers in the sedimentary clastic strata in the lower part of Wang Jiagou Formation of Zhongzhuangpu Group in Hunyuan volcanic basin, each layer is 2 ~ 5m thick, which is the overflow product of basic volcanic rocks during this eruption period. The upper stratum (about 100m thick) is composed of rhyolitic volcanic breccia, rhyolitic breccia tuff and rhyolitic lava, which is the product of the overflow of acidic volcanic rocks during the eruption period. During this period, the range of volcanic activity was small, only found in Wang Jiagou-Caohu bank of Hunyuan, with an area of less than 2 square kilometers. The K-Ar isochron age of rhyolite in the upper part of Wang Jiagou Formation is 105.3Ma.

(2) Petrological characteristics

A. Lava: The Mesozoic Yanshanian lava in Lvliang rock area is mainly basaltic lava, andesite lava and rhyolite lava. Basaltic lava is mainly basalt and basaltic trachyte. Basalt is grayish black, with rough texture and pore structure. Plagioclase accounts for about 45% ~ 50%, of which porphyritic plagioclase An accounts for 55 ~ 60, which is Labrador rock. Ordinary pyroxene exists in both porphyry and matrix, accounting for about 10% ~ 15% of the total rock mass. The content of basic volcanic glass is relatively high, accounting for about 30% ~ 35%. Others include a small amount of magnetite, apatite, chlorite and calcite. Coarse-grained basalt is dark gray, coarse-grained structure and pore structure. It consists of plagioclase, feldspar, augite, biotite, magnetite and apatite, in which plagioclase An accounts for 50-55% and about 30-35%. Sanidine content is relatively high, about 40% ~ 45%; The content of augite is less than that of basalt, ranging from 5% ~ 10%.

B. Clastic lava: Clastic lava in this area mainly includes basaltic andesite breccia lava, coarse basaltic breccia lava and coarse andesite aggregate. This kind of rock is a transitional rock from pyroclastic rock to lava, and its pyroclastic content changes greatly and its distribution is extremely uneven. Its main characteristics are: lava is the main factor, followed by tuff; Detrital composition is mainly pyroclastic, followed by surrounding rock detritus. Debris is distributed in a mixed way, and the particle size varies from tuff to aggregate. The size of breccia is generally 3 ~ 10 cm, and 2 ~ 3 m gravel blocks can be seen locally.

C pyroclastic rocks: widely distributed in various volcanic basins, with a wide variety of rocks. According to the types of cementation, it can be divided into lava pyroclastic rocks and compacted pyroclastic rocks.

D volcanic rocks: mainly distributed in volcanic rocks near the crater, occurring in ultra-shallow veins, dikes and bedrock, mainly secondary andesite, secondary dacite, secondary dacite rhyolite and secondary rhyolite.

(3) Petrochemical characteristics

See table 3- 14 and figure 3-23 for the petrochemical composition of Yanshanian volcanic rocks in this rock area.

It can be seen from the table that most volcanic rocks are rich in alkali but poor in calcium and magnesium: Na2O+K2O is 4.02% ~ 9.36%, with an average of 7.40%, while CaO is 0.36% ~ 7.23%, with an average of 3.84%; MgO is 0. 17% ~ 6.52%, and the average value is 1.9 1. The Rietmann index of volcanic rocks in this area is between 0.67 and 6.36, and most of them are greater than 3.3, belonging to olivine trachyte series. According to the diagram of alkalinity ratio (Figure 3-24), most volcanic lava belongs to alkaline rocks, and the rock combination is basically alkaline basalt-andesite-alkaline rhyolite.

According to the Gottili index -Rietmann index diagram (Figure 3-25), the Yanshanian volcanic rocks in this area are distributed near Area B and the boundary between Area B and Area C, which are basically the products of orogenic belts, reflecting the reactivation of the crust in Luliang area in Yanshan period.