Song and Zheng and Zhang
The original text was published in Journal of Rock Mineralogy, Volume 19, No.4, 2000; The book has been revised and supplemented as follows: ① The black-and-white photos in the original text have been changed into color photos; ② The significance of earthquake disaster events represented by K2O, ∑REE, 226Ra, U, 187Os/ 186Os, Pt and Ir in event analysis is supplemented, which is quite different from the original text.
The Proterozoic Dahongyu Formation in the Ming Tombs of Beijing has undergone a sharp change in lithofacies within less than 4 kilometers, which is really rare at home and abroad. From east to west, the tailings section consists of gray breccia silicified rock+black silty tuff shale+yellow feldspar-bearing timely sandstone, the peak section consists of impure sandstone+argillaceous dolomite, and the Deshengkou section consists of argillaceous dolomite+timely sandstone+tuff. In this paper, only three kinds of rocks of Dahongyu Formation in tailings profile are systematically studied. Macro-and micro-characteristics, geochemical and isotopic geochemical analysis data show that the gray breccia silicified rock and black silty tuff shale of Dahongyu Formation in the tailings section are sedimentary rocks formed by volcanic-earthquake events, while the yellow feldspar-bearing sandstone covering them was formed in normal coastal environment. Finally, the sedimentary rock construction model map of Dahongyu Formation containing event information is put forward.
The Ming Tombs in Beijing are famous historical sites and scenic spots. For a long time, the study of Yuanguyu in this area has accumulated a lot of data, but there are relatively few documents about Dahongyu Formation of Great Wall System. The area from Jixian to Pinggu is the main development area of Dahongyu Formation in the Oula trough of Yanshan Mountains, especially the volcanic rocks are widely distributed and thick, reaching 400 meters locally. According to the survey data from the top of the mountain where Beijing Geological Survey is located, the total thickness of Dahongyu Formation in the Ming Tombs area of Beijing is 80m, and no volcanic rocks are found. The breccia of Dahongyu Formation is very large, with melting and bending phenomena, indicating that it is affected by both volcanic and earthquake events. Because the volcanic activity in Dahongyu period occurred strongly in Pinggu-Jixian area east of the Ming Tombs, the transgression of Yanshan depression extended from east to west [8], and the breccia slumped eastward after silicification of volcanic hot water.
Table 1 measured value and corrected value of breccia dip (stratum dip152 ∠ 45).
Fig. 2 Tendency and dip map of siliceous breccia in Dahongyu Formation (corrected according to table 1)
1 Macroscopic characteristics of rocks
1. 1 gray breccia silicified rock
The silicified rock of gray breccia is located at the bottom of the tailings section, with the thickness of1.8 m. The breccia varies in size and shape, including flat, irregular and melting bending (Figure 3-a, B and C). The maximum breccia is 30cm and the minimum is 5cm. Scattered in silicified rock matrix, generally hard and massive. Silicified rock itself is also composed of rice-grained, bean-shaped and banded silicified particles with sizes ranging from 1mm to 5mm (Figure 3-d). Both breccia and matrix are basically composed of silica.
1.2 black silty tuff shale
The rock is hard and thin, with a single layer thickness of 2 ~ 3 cm and an outcrop thickness of about 12cm, which is evenly covered on the silicified breccia. On each single layer of silty tuff shale, there are staggered glue shrinkage grooves, and the width of straight grooves is 5 ~ 7 mm, which reflects the underwater glue shrinkage characteristics and is obviously different from general dry cracking. It is worth pointing out that this set of black silty tuff shale can not be seen on the peak profile. Later, the author found a set of tuff in the foundation of the construction site in Deshengkou area west of the ridge. It is unbelievable that the Dahongyu Formation produced such a completely different lithofacies combination within a distance of less than 4 kilometers from east to west.
1.3 timely sandstone containing yellow feldspar
The timely sandstone containing yellow feldspar has been quartzited, showing a medium-thick layer (about 50 cm). Because there is no direct contact with the underlying black silty tuff shale, the inclination and inclination of the two are the same, which can be regarded as an overall transition relationship. Small angle cross bedding appears in the upper part of sandstone layer. Zhao et al. [9] think that Dahongyu Formation-Gaoyuzhuang Formation was mainly formed by land tidal hydrodynamic forces, belonging to shallow beach environment, which accords with the formation conditions of small angle cross bedding.
2 Microscopic characteristics of rocks
The remains of trematode. [10, 1 1] was found in breccia silicified rock, and tuffaceous authigenic stone was found in silty tuffaceous shale [2]. The new research results are introduced as follows.
Fig. 3 Sedimentary rocks with event information in the Ming Tombs of Beijing-breccia silicified rocks and tuffaceous siltstone.
2. 1 breccia silicified rock
Silicified rocks are composed of 1 ~ 2mm silicified spherulites, and a layer of iron mineral spots is often scattered around the spherulite core, in which ilmenite automorphism is found. The periphery of silicified spherulites is composed of 0. 1 ~ 0.2 mm radial cylinders, and there are irregular banded micritic dolomite points which are alternately replaced.
The remains of trematode are only found in an "eye-shaped" silicified rock, which is estimated to be the remains of an algae nucleation stone.
2.2 black silty tuff shale
Discontinuous stripes show bedding structure, local microlayer dislocation and sand liquefaction blind veins, showing the unique sedimentary structure characteristics of earthquake events [12]. The particle size is coarse silt (0. 1 ~ 0.05 mm), and the clastic particles are feldspar crystal chips, and the tuff miscellaneous base filler accounts for more than half. Tuff is crystalline clay, and its structure is obviously different from that of Dahongyu Formation in Deshengkou. The probability diagram of particle size analysis is a suspension, and the coefficient is:
MD = 3.75φ (≈ 0.07mm)
Mz = 3.77φ (≈ 0.065mm)
σ 1=0.54
SK 1=0.05
Kilogram =0.83
The average roundness Po= 1.54, which belongs to subangular angle;
The comprehensive structural coefficient Td=9.93, which belongs to immature level [13, 14].
2.3 yellow feldspar timely sandstone
Clastic particles are mainly fine particles of 0. 15mm, with a small amount of coarse sand larger than 0.25 mm Among the rock-forming minerals, timely accounts for 85% and feldspar accounts for 15%. Feldspars are weathered potash feldspar and plagioclase, and some are fresh striped feldspar fragments. It contains a small amount of apatite and other heavy minerals, and a small amount of metamorphic tremolite appears. Granularity probability diagram is a general formula for three-level jump, and the coefficient is:
MD = 2.75 φ (~ 0.15mm)
Mz = 2.57 φ (~ 0.16mm)
σ 1=0.6
SK 1=-0.48
Kg = 1.09
The average roundness Po=3.07, which belongs to the circle category;
The comprehensive structural coefficient Td=68.29, belonging to mature sandstone [13, 14].
3 chemical composition characteristics of rocks
In order to find out the reaction of event sedimentary rocks in geochemical characteristics, this paper makes a comparative study of them by petrochemistry, rare earth elements, γ -ray spectrum, plasma spectrum and silicon isotope analysis.
3. 1 petrochemical analysis
Black silty tuff shale is the direct evidence of volcanic sedimentary events. Proterozoic volcanic activity is characterized by alkaline rocks, and K2O content is the most sensitive chemical composition. K2O content is 7.92% ~ 1 1.93% in Jixian area,10.26% in deshengkou area, and 12.73% in tailings mining area 4km away. Ren Fugen believes that Dahongyu Formation in Jixian County contains four stages of volcanic activity products, mainly lava, volcanic breccia and tuff, all of which are characterized by high potassium and low sodium [6]. According to the research of Bai Zhimin et al. (1999)[ 15], Pinggu area is the most concentrated place of volcanic rocks in Dahongyu Formation in Yanshan area. Potassium-rich volcanic rocks interbedded with timely sandstone and dolomite, with a thickness of up to 400m m, and the content of silty tuff shale in tailings is higher than that of ordinary volcanic rocks (Table 2).
Table 2 Comparison of chemical composition of volcanic rocks between the Ming Tombs in Beijing and Dahongyu Formation in Jixian County (wB/%)
3.2 Analysis of Rare Earth Elements
Zheng Jun et al. (1999) [19] discussed the characteristics of rare earth elements in the event sedimentary rocks of Dahongyu Formation. This paper only analyzes the three-layer rocks of Dahongyu Formation in Taiwei area of the Ming Tombs (Table 3).
Table 3 Rare Earth Element Analysis Data of Three Layers of Dahongyu Formation in Tailings Profile (wB/ 10-6)
It can be seen from Table 3 and Figure 4 that the total amount of rare earth in volcanic silty tuff is the highest (∑REE=222.0× 10-6), followed by breccia silicified rock (∑REE= 123.2× 10-6). Although the main diagenetic component SiO2 _ 2 is close to sandstone, the total amount of rare earth in sandstone is the least (∑REE=54.2× 10-6), which further shows that the former two are event deposits and the latter is normal deposits. Eu/eu * are 0.62, 0.64 and 1.07, respectively, which also shows the close relationship between the former two. It can also be seen from Figure 4 that both silty tuff shale and breccia silicified rock have obvious Eu loss, but sandstone is not obvious.
Fig. 4 Standardized distribution map of rare earth chondrites in the three-layer rocks of Dahongyu Formation in tailings.
(1) breccia silicified rock; (2) Silty tuff shale; (3) Timely sandstone
3.3 γ energy spectrum analysis of rocks
Multi-channel γ -ray spectrometer is used to analyze silty tuffaceous shale, breccia silicified rock and sandstone. The obtained data (Table 4) show that the contents of Th, Ra, K and U in breccia silicified rock and silty tuffaceous shale are relatively higher than those in sandstone, which also shows that the former two are event sedimentary rocks with genetic relationship, while the latter is formed in normal sedimentary environment.
Table 4 γ -ray spectrum analysis data of three-layer rocks of Dahongyu Formation in tailings section
3.4 Analysis of Precious Metal Elements
The analysis data of precious metal elements are shown in Table 5. It can be seen from Table 5 that the silty tuff shale contains higher precious metal elements, especially 187Os/ 186Os reaches 1.0 13. According to foreign research data, the closer this ratio is to 1, the more it is related to geological events [18]. Based on the data of geological research, we have reached a consistent conclusion.
Table 5 Analysis value of precious metal elements in the third layer of Dahongyu Formation in tailings section (wB/ 10-9)
3.5 Silicon Isotope (δ30Si) Analysis
The δ30Si analysis value of silicified breccia of Dahongyu Formation in tailings area is 0. 1‰, which is a sign of the genesis of volcanic hot water silicification [7], while the δ30Si analysis values of silicified nodules of Yangzhuang Formation, Wumishan Formation and Gaoyuzhuang Formation are 2.7‰, 3.4‰ and 2.04‰, respectively, which are much higher than that of silicified breccia of Dahongyu Formation.
The red, green and yellow siliceous nodules in Yangzhuang Formation are composed of terrigenous Si, in which Fe3+ and Fe2+ also come from terrigenous sources, which is consistent with terrigenous sedimentary environment such as red marl dolomite and sandy dolomite sandwiched between strata in this formation. Pseudoyeast belonging to Wumishan Formation [1 1] is a typical sign of intertidal zone, and its silicified part also comes from terrigenous Si; The silicified nucleation stone of Gaoyuzhuang Formation is formed by rolling after the stromatolite of algal mat is broken, and the silicified part is mainly terrigenous Si [7]. The above silicides are all positive values, and only the δ30Si value of Dahongyu Formation is 0. 1‰, which reflects the influence of volcanic Si.
It is worth noting that the correlation between petrochemical analysis, rare earth element analysis, γ -ray energy spectrum analysis and precious metal element analysis in the event stratum; Especially the ratio of K2O, ∑REE, 226Ra, U, 187Os/ 186Os, Pt and Ir. ①K2O content is 10% ~ 12%. It is generally high in Proterozoic sedimentary rocks in North China, even roughly consistent with volcanic rocks in surrounding areas, which is an important feature and shows the homology of volcanic eruption in Dahongyu period. ② The content of ∑ REE is high, especially ∑LREE is many times higher than that of North American Standard Shale (NASC). The author thinks that the Proterozoic potassium-rich and rare earth-rich sedimentary rocks in North China are an example of the source bed of the giant rare earth mine in Baiyun Obo, Inner Mongolia (Song et al., 2005) [16]; ③ The occurrence of 226Ra anomaly in silicified breccia of Dahongyu Formation is caused by earthquake events, which indicates that anomalies will also occur in Rn. ④ The anomaly of Th in breccia silicified rocks should be consistent with that of Ra and Rn, which represents a common phenomenon in volcanic-earthquake events. ⑤ The occurrence of 187OS/ 186OS in the black tuff siltstone is one of the reflections of the event information; ⑥ The relatively high Pt and Ir in black tuff siltstone may be caused by the increase of heavy mineral content in detritus.
4 discussion
Proterozoic Dahongyu Formation in Ming Tombs of Beijing contains geological event deposits. The breccia silicified rock exposed in the tailings mining area is the product of the comprehensive action of volcanic silicification and seismic wave impact. The black silty tuff shale covered on it is deposited by crystal debris ejected by volcano after being washed by sea waves, and the feldspar-bearing timely sandstone on it belongs to normal coastal sediments (Figure 5).
Fig. 5 Event sedimentary model of Proterozoic Dahongyu Formation in Ming Tombs, Beijing
1-dolomite; 2- silicified rocks; 3- Silty tuff shale; 4- sandstone; 5- breccia structure; 6- glue shrinkage structure; 7- staggered layer structure; 8- basement; 9- volcanic ash; 10-volcanic ash falling; 1 1- volcanic eruption; 12-Si； 13- seismic wave; 14-sea level; 15- volcano Si enters; 16- Incident seismic wave
It is worth pointing out that within a distance of less than 4km, there is such a drastic phase transition from tuff of Dahongyu Formation in Deshengkou to normal dolomite and sandstone interbedded at the top of the mountain to breccia silicified rock+silty tuff shale+timely sandstone in tailing area, which shows that the influence of volcanic activity accompanying the earthquake on shallow sea area is very uneven.
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