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Preliminary investigation on the metamorphism of Chinese coals
International Journal of Coal Geology, 2 (1982) 31--48
31
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
PRELIMINARY INVESTIGATION ON THE METAMORPHISM OF CHINESE COALS
YANG QI', REN DEYI ~ and PAN ZHIGUI'
1Bering Graduate School, Wuhan College of Geology, Beijing 100083 (People's Republic of China) 2 China Institute of Mining, Be~l'ing 100083 (People's Republic of China) (Received December 12, 1980;revised and accepted January 16, 1982)
ABSTRACT Yang Qi, Ren Deyi and Pan Zhigui, 1982. Preliminary investigation on the metamorphism of Chinese coals. Int, J. Coal Geol., 2: 31--48. The fact that most Chinese coals are of a relatively high degree of coalification indicates that the metamorphism of Chinese coals has its own characteristics. As contact metamorphism and dynamic metamorphism have influenced the coals only to a restricted extent, they are not the causes of this distinguishing feature. Though geothermal metamorphism of coal occurs universally in China, the maximum subsidence as reflected by the thickness of coal measures and their overlying rock series is so small that the palaeotemperature to which most Chinese coals were subjected was low and only low-ranking coals were formed. Hence, geothermal metamorphism is also not the principal cause of the higher ranking Chinese coals. Next to temperature, time is an important factor in geothermal metamorphism; the degree of coalification is dependent on the temperature and duration of heating that the coal has undergone. It is inferred that palaeogeothermal anomalies ought to be held responsible for most of the higher ranking Chinese coals. They are principally caused by magmatic intrusions, or by deep-seated faults, or by the uplift of the Moho with the corresponding uplift of the asthenosphere, and these three causes are interrelated. Telemagmatic metamorphism has so widely influenced Chinese coals that it accounts mainly for the majority of higherrank Chinese coals. Consequently Chinese coals have first and universally been coalified by geothermal alteration to lower rank and then partially promoted by telemagmatic metamorphism to medium and high ranks. Based upon the patterns of magmatic intrusions and associated hydrothermal processes, the telemagmatic metamorphism of Chinese coals can be tentatively divided into the following types: (1) type a, produced mainly by hypabyssal or meso-epimagmatic intrusions; (2) type b, produced mainly by hydrothermalism from blind intrusive bodies; (3) type c, produced mainly by mesoepimagmatic and epimagmatic intrusions; and (4) type d, produced mainly by hydrothermalism and epimagmatic intrusions. Characteristics of coalification zones, wall rocks and coal itself may be used as criteria to differentiate the telemagmatic metamorphism from other kinds of metamorphism. Magmatic activities which caused the telemagmatic metamorphism and its extent are controlled by structural systems, especially by the latitudinal structural systems and other structural systems associated with the former.
0166-5162/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company
32 INTRODUCTION
The investigation of coal metamorphism not only has theoretical significance, but also serves as the main basis for the prediction of coal properties and for decisions concerning mining possibilities. The metamorphism of Chinese coals may be reviewed from two aspects: (1) temporal; among late Palaeozoic coals, those of high and medium rank form a large percentage, no brown coal has ever been found; among Mesozoic coals, though brown coals do appear, low- and medium-rank bituminous one prevail, while anthracites and the high-rank bituminous coals are also found; among Tertiary kinds, there exist not only brown coals, but also low-rank bituminous coals; (2) spatial; in the region north of 3 8 ° N (roughly), including the greater parts of northeastern and northwestern China, there occur brown coals as well as low- and medium-rank bituminous coals; in north China, the area lying south of 38°N is marked by anthracites and bituminous coals of various ranks; the southwestern provinces are the regions with medium- and high-rank bituminous coals, whereas in southeastern China anthracites and high-rank bituminous coals predominate. Contact metamorphism and dynamic metamorphism have influenced the coals only to a restricted extent, therefore they cannot be held responsible for the relatively high degree of coalification of most Chinese coals. THE CHARACTERISTICS OF COALS
GEOTHERMAL
METAMORPHISM
OF CHINESE
Geothermal metamorphism of coal occurs universally in China. The effect of degree of subsidence on the geothermal metamorphism is everywhere apparent, and is more prominent in larger coal basins. In North China there are various ranks of Permo-Carboniferous coals; these were more or less affected by telemagmatic metamorphism, but evidently they are geothermally metamorphosed. In regions of low-rank bituminous coal, the thickness of coal measures and their overlying rock series, which reflects the maximum degree of subsidence, is small. For instance, in the Datong coalfield (Fig.l) the coal measures together with the overlying strata attain a thickness of only about 700 m; in Shuchou and HuainanHuaibei coalfields the thickness is n o t more than 1500 m and in the HandanXingtai coalfield it does not exceed 1800--2300 m. It is inferred that the palaeotemperatures to which the Permo-Carboniferous coals were subjected through geothermal metamorphism in these regions were low, the coal only reached the gas-coal stage, whereas in the regions of m e d i u m - a n d high-rank coals, such as in the Weichou coalfield of the Ningxia A u t o n o m o u s Region, the coal measures and their overlying strata, especially the overlying Triassic system, are much thicker. Obviously, the maximum degree of subsidence plays an important role in the degree of coal metamorphism.
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34 In the Sichuan basin, the Late Permian coal measures with their overlying strata are rather thick, for example, about 4650 m near Chonqing, where the coals were metamorphosed to medium-rank bituminous, whereas in Mianzhu, owing to the uplift of Longmenshan, the Late Permian coal measures and their overlying strata reach a total thickness of only 1200 m and the coals are still low-rank bituminous. In short, due to the differences in maximum degree of subsidence and duration of heating, the Chinese coals are coalified to various ranks (mainly low-ranking coals) under geothermal metamorphism. Generally, Tertiary coal measures are less than 1000 m thick, and brown coal was formed; Mesozoic brown coal formed when the Mesozoic coal measures and their overlying strata reach thicknesses of 1000 to 1500 m, and low-rank bituminous formed when the thickness ranges from 2000 to 2500 m. The PermoCarboniferous coals of North China and the Late Permian coals of South China are generally low-ranking bituminous when the thickness of coal measures and their overlying strata reaches 2000--2500 m; medium-rank bituminous could be formed if the thickness reached 3000--3500 m; and when the thickness reaches 4000--5000 m, which happened rarely in China, medium- to high-rank bituminous coal was formed. It is notable that under a certain geothermal regime, the duration of heating t h a t coal underwent plays an important part in the geothermal metamorphism (M. and R. Teichmfiller, 1968; Buntebarth, 1979). For example, the Late Permian coal measures of Sichuan Province were subsiding steadily in the subsiding basin until the early Yanshan Orogeny, and the coals coalified under geothermal metamorphism to the medium-rank bituminous stage. However, at the later stage of the Yanshan Orogeny, the geological conditions were very different on the eastern and western sides of the Huayinshan fracture zone and the coal measures were folded and much uplifted east of it, so that the palaeotemperature was too low to promote the coal to a higher rank. The amplitude of uplift was relatively small to the west of the Huayinshan fracture zone, where the effect of geothermal metamorphism on coal continued during a span of 140 m.y. under almost the same palaeotemperature t h a t existed before the Yanshan Orogeny, and the coals there were p r o m o t e d to high-rank bituminous, so that the differing degree of coalification in both sides of the Huayinshan fracture zone was mainly caused by difference in the duration of time. This fact confirms the opinion of M. and R. Teichmfiller that the longer the high temperature can act, the higher will be the rank of coal developed (M. and R. Teichmiiller, 1968); Suggate (1967, 1976), however, holds the opposite view about the influence of the time factor. THE TELEMAGMATIC METAMORPHISM OF CHINESE COAL
Temperature is the most important factor in coal metamorphism. Although geothermal metamorphism occurs universally and plays an important
35 role, it is not the main cause of most Chinese coal being of medium and high rank, because most of the coal measures together with their overlying strata are not thick enough. A geothermal anomaly should usually be responsible for the high degree of coalification. Geothermal anomalies are mainly due to magmatic intrusions which brought about local geothermal highs, such as the famous example of the Bramche intrusive body discussed by Teichmiiller (1968, 1975). Zhelinskiy (1980) also emphasizes the influence of Mesozoic magmatic activities on the metamorphism of Mesozoic coals in the Yakytia coalfield of the USSR. A geothermal anomaly might also be caused by deepseated faults which provided the passages for the transfer of heat and magma from depth; furthermore, it might also be caused by the uplift of the Moho and the corresponding uplift of the asthenosphere. Of course, these three causes are interrelated. For example, the Tancheng-Lujiang faults, trending NNE or NE in east China, are faulted deeply down into upper mantle. The Moho along these faults is relatively shallower (about 30--35 km) and the asthenosphere is also locally elevated to a higher level, at about 70 km beneath the Earth's surface. The high temperature (about 1500 ° C) from the elevated asthenosphere of the upper mantle, as well as the multiple magmatic intrusions and eruptions along the deep-seated faults during the Mesozoic--Cenozoic, resulted in the increase of heat flow and geothermal gradient. For example, the heat flow in the vicinity of Yinan (in Shandong Province) nearby the Tancheng-Lujiang faults is 2.5 HFU, but only 1.5 HFU at places several tens of kilometers from the faults. The geothermal gradients in coalfields neighbouring the Tancheng-Lujiang faults are also much higher, namely 3.5°C/100 m in Shungyashan, 3.4°C/100 m in Liaoyuan, and 3.6-4.6 ° C/100 m in Fushun. The coal rank is also higher as compared with those in the neighbouring regions; for instance, the Permo-Carboniferous coals in Linyi coalfield have been coalified to medium and high rank; the early Tertiary coal of the Fushun and the Yilan coalfield neighbouring the deepseated faults has become low-rank bituminous; and the Jurassic coal of the Fangzi coalfield, which is also near the Tancheng-Lujiang deep-seated faults, has become anthracite under the additional influence of magmatic intrusion. In regions with elevated Moho, both the geothermal gradients and the heat flow values are always higher. Some of these regions were affected also by Mesozoic and Cenozoic magmatism, so that coal ranks are always higher. For example, the Moho depth of the Weiyuan-Wusheng swell of Sichuan basin is about 2 km less than that of the surrounding regions (Fig.2); the average geothermal gradient there is 2.7°--2.8°C/100 m, and is 0.5°C higher than surrounding regions. Another example is the central part of the southern Liaoning Province, where a NE-trending Moho elevation is also about 2 km higher than the surrounding area (Fig.3), so the Permo-Carboniferous coals there rank higher than those of the surrounding area. In the region where high-rank coals occur, the geothermal gradient amounts to 3.56°C/ 100 m, and the volatile gradient of coal is also higher.
36
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A. Types of telemagmatic metamorphism Telemagmatic metamorphism of coal is caused by the geothermal anomaly due to radioactive elements, the heat of the magma and the hydrothermal process. Telemagmatic metamorphism facilitated the coalification of brown coal or low-rank bituminous formed by diagenesis or geothermal metamorphism to higher ranks, and it is the main cause of the formation of most Chinese medium-high-rank bituminous coals and anthracites. According to the mode of magmatic intrusion and hydrothermal process, telemagmatic metamorphism of coal in China can be tentatively divided into four types: (1) Telemagmatic metamorphism due mainly to hypabyssal and mesoepimagmatic intrusion. In this t y p e the intrusive b o d y was at a certain distance from the coal measures, so that the magmatic heat acting on the coal seam was not very high but lasted much longer, thus giving rise to mediumbituminous and also some high-rank coals. Metamorphism of Late Permian coal in Central Hunan Province and Permo-Carboniferous coal in the Handan-Xingtai coalfield are good examples of this widespread type. The Late Permian coal of Central Hunan Province shows a distinct zonal distribution involving various ranks of bituminous coal and anthracite. Coal measures together with their overlying Lower and Middle Triassic series are only 1000--2000 m thick, so the geothermal palaeotemperature affecting the Late Permian coals only coalified them to the gas-coal stage in the Late-Middle Triassic. During the Yanshan Orogeny, granitic intrusions, including blind ones, were scattered around the coalfield and along 27 ° 30' N through the central part of the coalfield, and some are of a multiple character. The isotope dates of the intrusions are about 100--200 m.y. Apparently the tremendous Mesozoic magmatic heat played an important role in promoting the degree of metamorphism of Late Permian coal on the basis of geothermal metamorphism (Fig.4). Handan-Xingtai coalfield serves as another example of this type. The Permo-Carboniferous coals in this region range from low-rank bituminous to anthracite, with an approximately east--west zonal distribution of the coal rank; the coal measures are about 200 m thick, in addition to overlying strata about 2300 m thick. The palaeotemperature exerted on the coal only coalified it to the gas-coal stage, and the higher-ranking coals are closely related to the Yanshan magmatic activities (Figs.5, 6; Table I). The Handan-Xingtai coalfield is situated at the boundary between the third uplifted fold zone and the second subsidence zone of the Neocathaysian structural system, where both the NNE-trending deep-seated faults and the east--west basement fractures developed. They provided the main passages for the uprising magmatic intrusions. Fushan, Wuan-Kangercheng, Jicun and Zishan at the northern end and Anyang and Linxian at the southern end of the coalfield both are situated at the junction of two sets of fractures where Yanshan magmatic intrusions repeatedly t o o k place. Most intrusives
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are sills several hundreds of meters in thickness, and mostly contact Middle Ordovician limestones underlying the coal measures, only in rare cases had the magma invaded the coal measures. Magmatic intrusions took place intermittently throughout the Yanshan Orogeny (the isotope dating of the intrusives being 170--64 m.y.) and greatly raised the palaeogeotherm of the Handan-Xingtai coalfield. The anthracite belt near the northern intrusion around Wuan-Kangercheng amounts to several tens of kilometers in width. Southward from Wuan-Kangercheng, the degree of coalification became gradually lower with increasing distance from the intrusion. Dioritic magma intruded the southern end of Handan-Xingtai coalfield around Anyang and Linxian, where anthracite and high-rank bituminous also occur; therefore the zonal distribution of coal metamorphism in Handan-Xingtai coalfield is characterized by the occurrence of medium-ranking coal in the middle part, while the coal rank rises towards both the northern and the southern ends of the coalfield. It is n o t e w o r t h y that a positive magnetic anomaly recorded in the middle part of the coal-
39
field with medium bituminous coal indicates that most likely a blind intrusive b o d y intruded underneath, at a depth at about 1600 m, which is much deeper than the intrusives at b o t h ends of the coalfield. Because of the effect of magmatic heat upon the regional geothermal field in Handan-Xingtai, the volatile gradient of coal there is much higher as compared with that in geothermally metamorphosed coal; for example, the volatile gradient is 3.55--5.95% per 100 m in medium-rank coal, 3.25--3.80 % per 100 m in high-rank bituminous coal, and 2.05--3.32% per 100 m in meagre coal to anthracite. 114"30
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41 TABLE I Reflectances of the Fengfeng* coals Name of mine
Rmax%
Rmin%
(Rmax'R~nin)%
Note
Huangsha
1.02
0.89
0.13
Sunzhuang Shaguoyuan Quantou
1.02--1.07 1.11--1.15 1.12
0.91-0.98 0.95--1.08 0.99
0.14 0.14 0.13
Fat coal (1.08)
Yikuang Yijing Erkuang
1.17--1.45 1.22--1.29 1.23--1.66
1.17--1.21 1.17--1.19 1.07--1.35
0.14 0.13 0.14--0.18
Fat-coking coal (1.33)
Wukuang Yangyi
1.36--1.48 1.37--1.48
1.19--1.34 1.23--1.27
0.14--0.21 0.14-0.21
Coking coal (1.42)
Sikuang Niuerzhuang Yanger
1.42--1.66 1.45--1.67 1.48--1.75
1.26--1.46 1.28--1.48 1.28--1.53
0.11--0.24 0.17---0.23 0.16--O.22
Lean coal (1.57)
Xuecun Tonger
1.76--2.14 1.27--2.04
1.47--1.79 1.11--1.80
0.23--.34 0.11--0.24
Meagre coal (1.80)
Quzhou Kangson Zhaoxian Kangyi Wannian
4.09--7.85 5.29 6.52 7.00 6.93--7.71
2.93--6.09 2.31 4.11 4.20 4.32--4.69
1.16--1.76 2.98 2.41 2.80 2.61--3,02
Anthracite (6.48)
*Fengfeng is a productive mining area of Handan-Xingtai coalfield.
(2) Telemagmatic m e t a m or phi s m due mainly to hydro-thermalism of blind intrusions: When blind intrusions were deeply buried under the coal measures, it was the h y d r o t h e r m a l solution t hat raised the t em perat ure by way o f fractures and the coalification of the coal seam was improved, so th at high-rank coals were usually formed. The Ruqigou mine, with earlymiddle Jurassic anthracite, of the Helanshan coalfield is a typical example o f this type. Taking the anthracite in Daling as a center, the volatile m at t er increases towards the north, west and southwest, although the coals are still within the range o f anthracite (Rmax = 3.04%) (Fig.7). H y d r o t h e r m a l quartz veins are well developed within the Jurassic coal measures and the underlying upper Triassic in Ruqigou. The a m o u n t of quartz veins increases downwards stratigraphically and increases towards the Daling mine. Alterations such as increase in hardness and slight metasomatism and recrystallization are observed in wallrock in c o n t a c t with the quartz veins. According to recent geophysical data, there are three magnetic anomalies to the west and nor t h of Ruqigou which poi nt out the existence
42
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of blind intrusives at depth. Evidently the high-rank coals of Ruqigou are mainly caused by the blind intrusion and the associated hydrothermal process. (3) Telemagmatic metamorphism due mainly to meso-epimagmatic and epimagmatic intrusion: The coal metamorphism of western Shandong Province may be cited as an example of this type. The total thickness of Permo-Carboniferous coal measures and their overlying rock series in western Shandong is less than 1000--1500 m. Geothermal metamorphism altered the coal to the gas-coal stage, then the region was deformed by the Yanshan Orogeny into open folds, fractures and multiple intrusions (Fig.8). The degree of coal metamorphism has increased due to the additional high palaeotemperature. Moreover, intermediate and acidic sills occur in the coal seams. It follows that not only widespread middle- to high-ranking coals are coalified by telemagmatic metamorphism, but also numerous natural cokes resulting from contact metamorphisms due to abundant epimagmatic intrusions, together with the poor confining conditions suggested by the thin covering strata and well-developed fractures during intrusion.
43
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C/-] ft
Western Shandong Province. 1 = gas-coal and fat-coal; bituminous; 4 = anthracite; 5 = natural coke; 6 = coals Cretaceous volcanic series; 8 = diorite; 9 = monzonite; 11 = f a u l t .
(4) Telemagmatic metamorphism due mainly to epimagmatic intrusion and hydrothermalism: This t y p e of coal metamorphism is rather typical in Guizhou Province where the coal measures and their overlying strata amount to 2000--4000 m. It is inferred that within the normal geothermal field, the coals had been coalified to the gas-coal--fat-coal stage. But in fact, there are abundant higher ranking coals in western Guizhou. On the other hand, there developed in western Guizhou numerous late Hercynian and Yanshan basic hypabyssal intrusions with isotopic ages of 235 m.y. and 158--115.5 m.y., respectively. Medium- and higher-ranking coals usually occur wherever there are intrusive rocks, hydrothermal deposits, or warm or hot springs. It is obvious that magmatism and hydrothermalism locally raised the geothermal field, which in turn raised the coalification of coal.
44
B. The characteristics of telemagmatic metamorphism (1) The physical and chemical-technological characteristics of coal. The coals formed by different types of telemagmatic metamorphism possess different technological properties. When the intrusives are relatively far from the coal measures, the coal coalified by hypabyssal or meso-epimagmatic telemagmatic metamorphism received heat from the magma gradually but continuously during a long time; this is favourable to the formation of middle-ranking coal with good coking property. In the case of coals metamorphosed due to meso-epimagmatic and epimagmatic intrusions, the magma is closer to the coal seam, the coals were subjected to high palaeotemperatures within much shorter intervals; the presence of fractures in overlying thin strata facilitated the escape of volatiles, so no coking coal would be formed; instead natural coke was produced. Intensely telemagmatic metamorphism of coal often results in anthracite, including meta-anthracite, with a low content of volatiles and hydrogen, a low H/C atom ratio, a high reflectance (Rma x = 6--8% ) (Table II) and an increased length of L a in the aromatic nucleus. TABLE II Characteristics of some Chinese coals Type of metamorphism
Mainly telemagmatic metamorphism
Coalfield Jiangoushan (Ningxia Auto. Region) Siwanchang (Guangdong Province) Tatian (Fujian
Age
C r (%) H r %
C~
96.96
1.15
P~
96.06
P2
C/H
V r (%)
Rmax(% )
7.01
3.45
8.83
0.79
10.15
3.82
9.81
96.87
1.63
4.15
1.85
8.02
Ji
96.34
1.30
6.18
3.47
8.84
T3
94.50
2.35
3.34
3.2--4.35
Province) Mentougou (Beijing) Mainly geothermal metamorphism
~ Sianyun t (Yunnan Province)
(2) The volatile gradient is high. The coal affected by telemagmatic metamorphism is marked by a higher volatile gradient than geothermally metamorphosed coal of the same rank. For example, in the Jixi and Muling coalfields where the magmatic processes are evident, the volatile gradient of coal is 1.61--2.86% per 100 m, while at Chengzihe coal mine, where magmatic thermal activities are lacking, the volatile gradient of same ranking coal is only 0.48% per 100m.
45
(3) Sudden change of coalification. In some coalfields where although coal measures with their overlying strata are not sufficiently thick, and the distance between upper and lower coal beds is not great, but the degree of coalification between the upper coal and the lower coals differs too much to be explained by geothermal metamorphims (for example, Early and Middle Jurassic coal measures in Tanshan mine of Ningxia Auto Region are 700 m thick), the upper coal seams are flame coal, while the lower ones are meagre coal and anthracite; this evidently results from magmatic heating. In addition, the sudden change in coal rank within a short distance in the same seam is also caused by telemagmatic metamorphism. (4) Characteristics of coalification zones. The extent of the area influenced by telemagmatic metamorphism and the width of each coalification zone depend on such factors as the scale and shape of intrusions, as well as on the number of times and depth of occurrences of intrusions. Moreover, the coalification zone is usually ring-shaped, such as the irregular ringshaped coalification zones around the dioritic stock in Shuangyashan (Heilongjiang Province) (Fig.9), but it may be perpendicular or oblique to the strike of the coal measures too, as in the case of the Handan-Xingtai coalfield where the magmatic intrusions were mainly controlled by faults which are approximately perpendicular to the strike of coal measures. Moreover, the highly telemagmatic metamorphism coal zone often consists of a single widespread anthracite zone, such as those of Ruqigou (Ningxia Auto. Region) and Jianou (Fujian Province).
Shuogyashon l t o
N
F -I'
F -14 '
'
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•
[-~-]5
Fig.9. Z o n e s d u e t o t e l e m a g m a t i c m e t a m o r p h i s m in S h u a n g y a s h a n (Heilongjian Province). 1 = long-flame coal; 2 = c o k i n g coal; 3 = m e a g r e coal; 4 = a n t h r a c i t e ; 5 = diorite.
46 (5) The alteration of wan rocks occurs commonly in anthracite zones coalified by telemagmatic metamorphism. Examples are recrystallization in sandstone, quartz-sandstone changed into quartzite, argillites turned into sericite slates, and limestone changed into cryptocrystalline limestone, or even marble. The development of hydrothermal quartz veins in wall rocks is the main criterion of telemagmatic metamorphism, e.g. in Ruqigou in Ninxia Auto. Region and in Tianhushan in Fujian Province. Sometimes there are also calcite-barite veins. Fluorite veins and asbestos veins of hydrothermal origin also develop in wall rocks. (6) Paragenesis with endogenic mineralization. In north and east China the Yanshan diorite often intruded into the Ordovician limestone and formed magnetic iron ore deposits of skarn type, whereas the overlying Permo-Carboniferous coal is usually highly coalified by telemagmatic metamorphism with evident coalification zones. In western Guizhou Province, medium- to high-rank bituminous coals are found, often with mesothermal or epithermal deposits. The telemagmatic metamorphism of coal was often caused by blind intrusives, many of which have not yet been well studied. In defining the type of metamorphic coal and in prognoses of coal qualities, it is significant to study the presence of blind intrusions and their scale and shape as well as the intrusive depth. In exploring the blind bodies it will be more efficient to make use of the characteristics mentioned above as complementary criteria.
C. The distribution o f telemagmatic metamorphosed coals in China Magmatic activities which caused the telemagmatic metamorphism and their extent are controlled by structural systems. Within the area of latitudinal structural systems and where these are combined with other structural systems, intensive Mesozoic magmatic activity frequently took place, especially during the Yanshan Orogeny; the coals were usually coalified to medium- to high-rank bituminous grade and anthracite. Along the YinshanTianshan latitudinal structure zone, the Early and Middle Jurassic coals as well as Carboniferous coals in Northern Xinjiang Auto. Region, Neimenggu Auto. Region, Liaoning and Jiling Provinces were metamorphosed to medium- to high-ranking bituminous coals and anthracites due to Yanshan magmatic intrusions. Along 37--38°N lat. high-ranking coals due to telemagmatic metamorphism are distributed intermittently on the western border of Southern Xinjiang, eastwards through the eastern end of the Kunlun Mts., Qinghai Province, Sh~nxi Province to not far from the shoreline of Shandong Province. Along 27--28°N lat., in Hunan, Jiangxi and Fujiang Provinces, the extensive intrusions of the Yanshan granitic magma (also Yinzhi granitic magma) brought about the widespread Late Permian and Late Triassic--Early Jurassic coals of higher ranks. A great quantity of coals was metamorphosed to high ranking anthracite in Fujiang Province, where
47
a latitudinal structural zone had combined with the Neocathaysian structures. In the Nanling latitudinal structural zone, especially in its central and eastern parts, Mesozoic intrusives were widespread and were often of a multiple nature; this changed the Early Carboniferous and Late Permian, as well as Triassic--Jurassic coals of southern Hunan, southern Kiangxi, southern Fujiang and eastern Guangdong Provinces into high-ranking coals, mostly into high ranking anthracites. The Helanshan-Liupanshan meridional structural zones, and the frontal arc of Huiyang epsilon structure, especially the section along the Yangtze River, where deep-seated faults are well developed, are areas of intensive Mesozoic magmatism, and the Late Palaeozoic coals as well as Early Mesozoic coals were highly metamorphosed. CONCLUSIONS
Based upon the discussions given above, the characteristics of metamorphism of Chinese coals may be preliminarily summarized as follows: (1) The trends of coalification zones vary in different regions of China. The coalification zones of Jurassic coals trend northwestward or approximately latitudinally in northwestern China; those of Late Jurassic--Early Cretaceous coal in northeastern China trend northeastward to north-northeastward and in these two regions brown coals and low-medium coalified bituminous coals predominate. In the central portion of northern China, high-rank Permo-Carboniferous coals predominate but they decrease in rank towards its bordering areas. In southern China Late Palaeozoic coals are mainly medium- to high-rank bituminous and anthracite, and the coalification zones in southeastern provinces trend NNE and NE. In short, the degree of coalification of Chinese coals varies in different regions, lower ranking in the north, higher in the south and highest in the southeast part of China. (2) As contact metamorphism and dynamic metamorphism have influenced the coal only to a restricted extent, they cannot be held responsible for the relatively high coalification of most Chinese coals. (3) Although coals are universally geothermally metamorphosed, most of the coal measures and their overlying strata which reflect the maximum degree of subsidence are not sufficiently thick, therefore the ranks of most Chinese coals are too high to be explained by geothermal metamorphism alone. (4) Time is an important factor in geothermal metamorphism, and the degree of coalification is dependent on the temperature and duration of heating that the coal had undergone. (5) The high ranks of most Chinese coals are related to palaeogeothermal anomaly which might be caused by: (a) magmatic intrusion and hydrothermal processes; (b) deep-seated faults and basement fracture zones which provide the passage for magmatic intrusion and the transfer of internal heat; and (c) the elevation of the Moho as well as of the corresponding asthenosphere which results in raising the geothermal gradient. Of course,
48
there are still other causes that can raise the geothermal gradient, such as the upheaval of basement rocks and the heat conductivities of rock, but they are less important. (6) Telemagmatic metamorphism has influenced Chinese coals so widely that it is the main cause of most of the coals higher than of low rankings. Not only anthracites and highly coalified bituminous coals in the southeastern provinces, but also most coals higher than low rank in north China, east China, and southwestern China are due to telemagmatic metamorphism. (7) Based on the patterns of magmatic intrusion and associated hydrothermal process, the telemagmatic metamorphism of coal in China can be divided into four types: (a) due mainly to hypabyssal or meso-epimagmatic intrusion; (b) due mainly to hydrothermalism from blind intrusive bodies; (c) due mainly to meso-epimagmatic and epimagmatic intrusion; (d) due mainly to hydrothermalism and epimagmatic intrusion. (8) Most of the higher ranking coals in China had been coalified by geothermal metamorphism and this was facilitated by telemagmatic metamorphism. ACKNOWLEDGEMENT
The authors wish to thank the Coal Prospecting Companies belonging to the Ministry of Coal Industry, for providing the reference materials.
REFERENCES Buntebarth, G., 1979. Eine empirische Methode zur Berechnung yon palh'ogeothermischen Gradienten aus dem Inkohlungsgrad organischer Einlagerungen in Sedimentgesteinen mit Anwendung auf den Mittleren Oberrhein-Graben. In: Inkohlung und Geothermik. Van Acken, Krefeld, pp. 97--108. Geothermal Group of the Geological Institute of the Academy of Sciences of China, 1978. Monograph on geothermics (in Chinese). Huang Jiqing et al., 1977. The elemental contour of Chinese tectonics. J. Geol., 2 (in Chinese). Lee, J.S., 1973. Crustal structure and crustal movement. Sci. Sin., XVI (4): 520--559. Suggate, R.P., 1967. Metamorphic rank in Taiwan Coals. Bull. Geol. Surv. Taiwan, No. 19, July 1967. Suggate, R.P., 1976. Coal ranks and geological history of the Nottinghamshire-Yorkshire coalfield. Mercian Geol., 6 (1): 1--24. Teichmiiller, M. and Teichmiiller, R., 1968. Geological aspects of coal metamorphism. In: D.G. Murchison and T.S. Westoll (Editors), Coals and Coal-bearing Strata. Oliver and Boyd, Edinburgh, pp. 233--267, Teichm/iller, M. and Teichmiiller, R., 1975. The geological basis of coal formation. In: E. Stach et al., Stach's Textbook of Coal Petrology. Gebriider Borntraeger. BerlinStuttgart, pp. 5--54. Wang Qianshan and Liu Yuanlong, 1976. The contour of crustal structure of South Liaoning. J. Geophys., 3 (in Chinese). Wang Zuquan, 1956. The zone distribution of coal rank and its geological causes in North China. Colliery Technique, 7 (in Chinese). Zhelinsky, V.M., 1980. Catagenesis of terrigenous rocks and coal metamorphism of South Yakytia. In: Lithology and Mineral Deposits. Sceenie, Moscow, pp. 99--114.
31
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
PRELIMINARY INVESTIGATION ON THE METAMORPHISM OF CHINESE COALS
YANG QI', REN DEYI ~ and PAN ZHIGUI'
1Bering Graduate School, Wuhan College of Geology, Beijing 100083 (People's Republic of China) 2 China Institute of Mining, Be~l'ing 100083 (People's Republic of China) (Received December 12, 1980;revised and accepted January 16, 1982)
ABSTRACT Yang Qi, Ren Deyi and Pan Zhigui, 1982. Preliminary investigation on the metamorphism of Chinese coals. Int, J. Coal Geol., 2: 31--48. The fact that most Chinese coals are of a relatively high degree of coalification indicates that the metamorphism of Chinese coals has its own characteristics. As contact metamorphism and dynamic metamorphism have influenced the coals only to a restricted extent, they are not the causes of this distinguishing feature. Though geothermal metamorphism of coal occurs universally in China, the maximum subsidence as reflected by the thickness of coal measures and their overlying rock series is so small that the palaeotemperature to which most Chinese coals were subjected was low and only low-ranking coals were formed. Hence, geothermal metamorphism is also not the principal cause of the higher ranking Chinese coals. Next to temperature, time is an important factor in geothermal metamorphism; the degree of coalification is dependent on the temperature and duration of heating that the coal has undergone. It is inferred that palaeogeothermal anomalies ought to be held responsible for most of the higher ranking Chinese coals. They are principally caused by magmatic intrusions, or by deep-seated faults, or by the uplift of the Moho with the corresponding uplift of the asthenosphere, and these three causes are interrelated. Telemagmatic metamorphism has so widely influenced Chinese coals that it accounts mainly for the majority of higherrank Chinese coals. Consequently Chinese coals have first and universally been coalified by geothermal alteration to lower rank and then partially promoted by telemagmatic metamorphism to medium and high ranks. Based upon the patterns of magmatic intrusions and associated hydrothermal processes, the telemagmatic metamorphism of Chinese coals can be tentatively divided into the following types: (1) type a, produced mainly by hypabyssal or meso-epimagmatic intrusions; (2) type b, produced mainly by hydrothermalism from blind intrusive bodies; (3) type c, produced mainly by mesoepimagmatic and epimagmatic intrusions; and (4) type d, produced mainly by hydrothermalism and epimagmatic intrusions. Characteristics of coalification zones, wall rocks and coal itself may be used as criteria to differentiate the telemagmatic metamorphism from other kinds of metamorphism. Magmatic activities which caused the telemagmatic metamorphism and its extent are controlled by structural systems, especially by the latitudinal structural systems and other structural systems associated with the former.
0166-5162/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company
32 INTRODUCTION
The investigation of coal metamorphism not only has theoretical significance, but also serves as the main basis for the prediction of coal properties and for decisions concerning mining possibilities. The metamorphism of Chinese coals may be reviewed from two aspects: (1) temporal; among late Palaeozoic coals, those of high and medium rank form a large percentage, no brown coal has ever been found; among Mesozoic coals, though brown coals do appear, low- and medium-rank bituminous one prevail, while anthracites and the high-rank bituminous coals are also found; among Tertiary kinds, there exist not only brown coals, but also low-rank bituminous coals; (2) spatial; in the region north of 3 8 ° N (roughly), including the greater parts of northeastern and northwestern China, there occur brown coals as well as low- and medium-rank bituminous coals; in north China, the area lying south of 38°N is marked by anthracites and bituminous coals of various ranks; the southwestern provinces are the regions with medium- and high-rank bituminous coals, whereas in southeastern China anthracites and high-rank bituminous coals predominate. Contact metamorphism and dynamic metamorphism have influenced the coals only to a restricted extent, therefore they cannot be held responsible for the relatively high degree of coalification of most Chinese coals. THE CHARACTERISTICS OF COALS
GEOTHERMAL
METAMORPHISM
OF CHINESE
Geothermal metamorphism of coal occurs universally in China. The effect of degree of subsidence on the geothermal metamorphism is everywhere apparent, and is more prominent in larger coal basins. In North China there are various ranks of Permo-Carboniferous coals; these were more or less affected by telemagmatic metamorphism, but evidently they are geothermally metamorphosed. In regions of low-rank bituminous coal, the thickness of coal measures and their overlying rock series, which reflects the maximum degree of subsidence, is small. For instance, in the Datong coalfield (Fig.l) the coal measures together with the overlying strata attain a thickness of only about 700 m; in Shuchou and HuainanHuaibei coalfields the thickness is n o t more than 1500 m and in the HandanXingtai coalfield it does not exceed 1800--2300 m. It is inferred that the palaeotemperatures to which the Permo-Carboniferous coals were subjected through geothermal metamorphism in these regions were low, the coal only reached the gas-coal stage, whereas in the regions of m e d i u m - a n d high-rank coals, such as in the Weichou coalfield of the Ningxia A u t o n o m o u s Region, the coal measures and their overlying strata, especially the overlying Triassic system, are much thicker. Obviously, the maximum degree of subsidence plays an important role in the degree of coal metamorphism.
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34 In the Sichuan basin, the Late Permian coal measures with their overlying strata are rather thick, for example, about 4650 m near Chonqing, where the coals were metamorphosed to medium-rank bituminous, whereas in Mianzhu, owing to the uplift of Longmenshan, the Late Permian coal measures and their overlying strata reach a total thickness of only 1200 m and the coals are still low-rank bituminous. In short, due to the differences in maximum degree of subsidence and duration of heating, the Chinese coals are coalified to various ranks (mainly low-ranking coals) under geothermal metamorphism. Generally, Tertiary coal measures are less than 1000 m thick, and brown coal was formed; Mesozoic brown coal formed when the Mesozoic coal measures and their overlying strata reach thicknesses of 1000 to 1500 m, and low-rank bituminous formed when the thickness ranges from 2000 to 2500 m. The PermoCarboniferous coals of North China and the Late Permian coals of South China are generally low-ranking bituminous when the thickness of coal measures and their overlying strata reaches 2000--2500 m; medium-rank bituminous could be formed if the thickness reached 3000--3500 m; and when the thickness reaches 4000--5000 m, which happened rarely in China, medium- to high-rank bituminous coal was formed. It is notable that under a certain geothermal regime, the duration of heating t h a t coal underwent plays an important part in the geothermal metamorphism (M. and R. Teichmfiller, 1968; Buntebarth, 1979). For example, the Late Permian coal measures of Sichuan Province were subsiding steadily in the subsiding basin until the early Yanshan Orogeny, and the coals coalified under geothermal metamorphism to the medium-rank bituminous stage. However, at the later stage of the Yanshan Orogeny, the geological conditions were very different on the eastern and western sides of the Huayinshan fracture zone and the coal measures were folded and much uplifted east of it, so that the palaeotemperature was too low to promote the coal to a higher rank. The amplitude of uplift was relatively small to the west of the Huayinshan fracture zone, where the effect of geothermal metamorphism on coal continued during a span of 140 m.y. under almost the same palaeotemperature t h a t existed before the Yanshan Orogeny, and the coals there were p r o m o t e d to high-rank bituminous, so that the differing degree of coalification in both sides of the Huayinshan fracture zone was mainly caused by difference in the duration of time. This fact confirms the opinion of M. and R. Teichmfiller that the longer the high temperature can act, the higher will be the rank of coal developed (M. and R. Teichmiiller, 1968); Suggate (1967, 1976), however, holds the opposite view about the influence of the time factor. THE TELEMAGMATIC METAMORPHISM OF CHINESE COAL
Temperature is the most important factor in coal metamorphism. Although geothermal metamorphism occurs universally and plays an important
35 role, it is not the main cause of most Chinese coal being of medium and high rank, because most of the coal measures together with their overlying strata are not thick enough. A geothermal anomaly should usually be responsible for the high degree of coalification. Geothermal anomalies are mainly due to magmatic intrusions which brought about local geothermal highs, such as the famous example of the Bramche intrusive body discussed by Teichmiiller (1968, 1975). Zhelinskiy (1980) also emphasizes the influence of Mesozoic magmatic activities on the metamorphism of Mesozoic coals in the Yakytia coalfield of the USSR. A geothermal anomaly might also be caused by deepseated faults which provided the passages for the transfer of heat and magma from depth; furthermore, it might also be caused by the uplift of the Moho and the corresponding uplift of the asthenosphere. Of course, these three causes are interrelated. For example, the Tancheng-Lujiang faults, trending NNE or NE in east China, are faulted deeply down into upper mantle. The Moho along these faults is relatively shallower (about 30--35 km) and the asthenosphere is also locally elevated to a higher level, at about 70 km beneath the Earth's surface. The high temperature (about 1500 ° C) from the elevated asthenosphere of the upper mantle, as well as the multiple magmatic intrusions and eruptions along the deep-seated faults during the Mesozoic--Cenozoic, resulted in the increase of heat flow and geothermal gradient. For example, the heat flow in the vicinity of Yinan (in Shandong Province) nearby the Tancheng-Lujiang faults is 2.5 HFU, but only 1.5 HFU at places several tens of kilometers from the faults. The geothermal gradients in coalfields neighbouring the Tancheng-Lujiang faults are also much higher, namely 3.5°C/100 m in Shungyashan, 3.4°C/100 m in Liaoyuan, and 3.6-4.6 ° C/100 m in Fushun. The coal rank is also higher as compared with those in the neighbouring regions; for instance, the Permo-Carboniferous coals in Linyi coalfield have been coalified to medium and high rank; the early Tertiary coal of the Fushun and the Yilan coalfield neighbouring the deepseated faults has become low-rank bituminous; and the Jurassic coal of the Fangzi coalfield, which is also near the Tancheng-Lujiang deep-seated faults, has become anthracite under the additional influence of magmatic intrusion. In regions with elevated Moho, both the geothermal gradients and the heat flow values are always higher. Some of these regions were affected also by Mesozoic and Cenozoic magmatism, so that coal ranks are always higher. For example, the Moho depth of the Weiyuan-Wusheng swell of Sichuan basin is about 2 km less than that of the surrounding regions (Fig.2); the average geothermal gradient there is 2.7°--2.8°C/100 m, and is 0.5°C higher than surrounding regions. Another example is the central part of the southern Liaoning Province, where a NE-trending Moho elevation is also about 2 km higher than the surrounding area (Fig.3), so the Permo-Carboniferous coals there rank higher than those of the surrounding area. In the region where high-rank coals occur, the geothermal gradient amounts to 3.56°C/ 100 m, and the volatile gradient of coal is also higher.
36
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A. Types of telemagmatic metamorphism Telemagmatic metamorphism of coal is caused by the geothermal anomaly due to radioactive elements, the heat of the magma and the hydrothermal process. Telemagmatic metamorphism facilitated the coalification of brown coal or low-rank bituminous formed by diagenesis or geothermal metamorphism to higher ranks, and it is the main cause of the formation of most Chinese medium-high-rank bituminous coals and anthracites. According to the mode of magmatic intrusion and hydrothermal process, telemagmatic metamorphism of coal in China can be tentatively divided into four types: (1) Telemagmatic metamorphism due mainly to hypabyssal and mesoepimagmatic intrusion. In this t y p e the intrusive b o d y was at a certain distance from the coal measures, so that the magmatic heat acting on the coal seam was not very high but lasted much longer, thus giving rise to mediumbituminous and also some high-rank coals. Metamorphism of Late Permian coal in Central Hunan Province and Permo-Carboniferous coal in the Handan-Xingtai coalfield are good examples of this widespread type. The Late Permian coal of Central Hunan Province shows a distinct zonal distribution involving various ranks of bituminous coal and anthracite. Coal measures together with their overlying Lower and Middle Triassic series are only 1000--2000 m thick, so the geothermal palaeotemperature affecting the Late Permian coals only coalified them to the gas-coal stage in the Late-Middle Triassic. During the Yanshan Orogeny, granitic intrusions, including blind ones, were scattered around the coalfield and along 27 ° 30' N through the central part of the coalfield, and some are of a multiple character. The isotope dates of the intrusions are about 100--200 m.y. Apparently the tremendous Mesozoic magmatic heat played an important role in promoting the degree of metamorphism of Late Permian coal on the basis of geothermal metamorphism (Fig.4). Handan-Xingtai coalfield serves as another example of this type. The Permo-Carboniferous coals in this region range from low-rank bituminous to anthracite, with an approximately east--west zonal distribution of the coal rank; the coal measures are about 200 m thick, in addition to overlying strata about 2300 m thick. The palaeotemperature exerted on the coal only coalified it to the gas-coal stage, and the higher-ranking coals are closely related to the Yanshan magmatic activities (Figs.5, 6; Table I). The Handan-Xingtai coalfield is situated at the boundary between the third uplifted fold zone and the second subsidence zone of the Neocathaysian structural system, where both the NNE-trending deep-seated faults and the east--west basement fractures developed. They provided the main passages for the uprising magmatic intrusions. Fushan, Wuan-Kangercheng, Jicun and Zishan at the northern end and Anyang and Linxian at the southern end of the coalfield both are situated at the junction of two sets of fractures where Yanshan magmatic intrusions repeatedly t o o k place. Most intrusives
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are sills several hundreds of meters in thickness, and mostly contact Middle Ordovician limestones underlying the coal measures, only in rare cases had the magma invaded the coal measures. Magmatic intrusions took place intermittently throughout the Yanshan Orogeny (the isotope dating of the intrusives being 170--64 m.y.) and greatly raised the palaeogeotherm of the Handan-Xingtai coalfield. The anthracite belt near the northern intrusion around Wuan-Kangercheng amounts to several tens of kilometers in width. Southward from Wuan-Kangercheng, the degree of coalification became gradually lower with increasing distance from the intrusion. Dioritic magma intruded the southern end of Handan-Xingtai coalfield around Anyang and Linxian, where anthracite and high-rank bituminous also occur; therefore the zonal distribution of coal metamorphism in Handan-Xingtai coalfield is characterized by the occurrence of medium-ranking coal in the middle part, while the coal rank rises towards both the northern and the southern ends of the coalfield. It is n o t e w o r t h y that a positive magnetic anomaly recorded in the middle part of the coal-
39
field with medium bituminous coal indicates that most likely a blind intrusive b o d y intruded underneath, at a depth at about 1600 m, which is much deeper than the intrusives at b o t h ends of the coalfield. Because of the effect of magmatic heat upon the regional geothermal field in Handan-Xingtai, the volatile gradient of coal there is much higher as compared with that in geothermally metamorphosed coal; for example, the volatile gradient is 3.55--5.95% per 100 m in medium-rank coal, 3.25--3.80 % per 100 m in high-rank bituminous coal, and 2.05--3.32% per 100 m in meagre coal to anthracite. 114"30
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41 TABLE I Reflectances of the Fengfeng* coals Name of mine
Rmax%
Rmin%
(Rmax'R~nin)%
Note
Huangsha
1.02
0.89
0.13
Sunzhuang Shaguoyuan Quantou
1.02--1.07 1.11--1.15 1.12
0.91-0.98 0.95--1.08 0.99
0.14 0.14 0.13
Fat coal (1.08)
Yikuang Yijing Erkuang
1.17--1.45 1.22--1.29 1.23--1.66
1.17--1.21 1.17--1.19 1.07--1.35
0.14 0.13 0.14--0.18
Fat-coking coal (1.33)
Wukuang Yangyi
1.36--1.48 1.37--1.48
1.19--1.34 1.23--1.27
0.14--0.21 0.14-0.21
Coking coal (1.42)
Sikuang Niuerzhuang Yanger
1.42--1.66 1.45--1.67 1.48--1.75
1.26--1.46 1.28--1.48 1.28--1.53
0.11--0.24 0.17---0.23 0.16--O.22
Lean coal (1.57)
Xuecun Tonger
1.76--2.14 1.27--2.04
1.47--1.79 1.11--1.80
0.23--.34 0.11--0.24
Meagre coal (1.80)
Quzhou Kangson Zhaoxian Kangyi Wannian
4.09--7.85 5.29 6.52 7.00 6.93--7.71
2.93--6.09 2.31 4.11 4.20 4.32--4.69
1.16--1.76 2.98 2.41 2.80 2.61--3,02
Anthracite (6.48)
*Fengfeng is a productive mining area of Handan-Xingtai coalfield.
(2) Telemagmatic m e t a m or phi s m due mainly to hydro-thermalism of blind intrusions: When blind intrusions were deeply buried under the coal measures, it was the h y d r o t h e r m a l solution t hat raised the t em perat ure by way o f fractures and the coalification of the coal seam was improved, so th at high-rank coals were usually formed. The Ruqigou mine, with earlymiddle Jurassic anthracite, of the Helanshan coalfield is a typical example o f this type. Taking the anthracite in Daling as a center, the volatile m at t er increases towards the north, west and southwest, although the coals are still within the range o f anthracite (Rmax = 3.04%) (Fig.7). H y d r o t h e r m a l quartz veins are well developed within the Jurassic coal measures and the underlying upper Triassic in Ruqigou. The a m o u n t of quartz veins increases downwards stratigraphically and increases towards the Daling mine. Alterations such as increase in hardness and slight metasomatism and recrystallization are observed in wallrock in c o n t a c t with the quartz veins. According to recent geophysical data, there are three magnetic anomalies to the west and nor t h of Ruqigou which poi nt out the existence
42
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of blind intrusives at depth. Evidently the high-rank coals of Ruqigou are mainly caused by the blind intrusion and the associated hydrothermal process. (3) Telemagmatic metamorphism due mainly to meso-epimagmatic and epimagmatic intrusion: The coal metamorphism of western Shandong Province may be cited as an example of this type. The total thickness of Permo-Carboniferous coal measures and their overlying rock series in western Shandong is less than 1000--1500 m. Geothermal metamorphism altered the coal to the gas-coal stage, then the region was deformed by the Yanshan Orogeny into open folds, fractures and multiple intrusions (Fig.8). The degree of coal metamorphism has increased due to the additional high palaeotemperature. Moreover, intermediate and acidic sills occur in the coal seams. It follows that not only widespread middle- to high-ranking coals are coalified by telemagmatic metamorphism, but also numerous natural cokes resulting from contact metamorphisms due to abundant epimagmatic intrusions, together with the poor confining conditions suggested by the thin covering strata and well-developed fractures during intrusion.
43
1f7 °
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Fig.8. L a t e P a l e o z o i c c o a l s in 2 = coking coal; 3 = high-rank with various ranks; 7 = Lower 10 = gabbro and gabbro-diorite;
C/-] ft
Western Shandong Province. 1 = gas-coal and fat-coal; bituminous; 4 = anthracite; 5 = natural coke; 6 = coals Cretaceous volcanic series; 8 = diorite; 9 = monzonite; 11 = f a u l t .
(4) Telemagmatic metamorphism due mainly to epimagmatic intrusion and hydrothermalism: This t y p e of coal metamorphism is rather typical in Guizhou Province where the coal measures and their overlying strata amount to 2000--4000 m. It is inferred that within the normal geothermal field, the coals had been coalified to the gas-coal--fat-coal stage. But in fact, there are abundant higher ranking coals in western Guizhou. On the other hand, there developed in western Guizhou numerous late Hercynian and Yanshan basic hypabyssal intrusions with isotopic ages of 235 m.y. and 158--115.5 m.y., respectively. Medium- and higher-ranking coals usually occur wherever there are intrusive rocks, hydrothermal deposits, or warm or hot springs. It is obvious that magmatism and hydrothermalism locally raised the geothermal field, which in turn raised the coalification of coal.
44
B. The characteristics of telemagmatic metamorphism (1) The physical and chemical-technological characteristics of coal. The coals formed by different types of telemagmatic metamorphism possess different technological properties. When the intrusives are relatively far from the coal measures, the coal coalified by hypabyssal or meso-epimagmatic telemagmatic metamorphism received heat from the magma gradually but continuously during a long time; this is favourable to the formation of middle-ranking coal with good coking property. In the case of coals metamorphosed due to meso-epimagmatic and epimagmatic intrusions, the magma is closer to the coal seam, the coals were subjected to high palaeotemperatures within much shorter intervals; the presence of fractures in overlying thin strata facilitated the escape of volatiles, so no coking coal would be formed; instead natural coke was produced. Intensely telemagmatic metamorphism of coal often results in anthracite, including meta-anthracite, with a low content of volatiles and hydrogen, a low H/C atom ratio, a high reflectance (Rma x = 6--8% ) (Table II) and an increased length of L a in the aromatic nucleus. TABLE II Characteristics of some Chinese coals Type of metamorphism
Mainly telemagmatic metamorphism
Coalfield Jiangoushan (Ningxia Auto. Region) Siwanchang (Guangdong Province) Tatian (Fujian
Age
C r (%) H r %
C~
96.96
1.15
P~
96.06
P2
C/H
V r (%)
Rmax(% )
7.01
3.45
8.83
0.79
10.15
3.82
9.81
96.87
1.63
4.15
1.85
8.02
Ji
96.34
1.30
6.18
3.47
8.84
T3
94.50
2.35
3.34
3.2--4.35
Province) Mentougou (Beijing) Mainly geothermal metamorphism
~ Sianyun t (Yunnan Province)
(2) The volatile gradient is high. The coal affected by telemagmatic metamorphism is marked by a higher volatile gradient than geothermally metamorphosed coal of the same rank. For example, in the Jixi and Muling coalfields where the magmatic processes are evident, the volatile gradient of coal is 1.61--2.86% per 100 m, while at Chengzihe coal mine, where magmatic thermal activities are lacking, the volatile gradient of same ranking coal is only 0.48% per 100m.
45
(3) Sudden change of coalification. In some coalfields where although coal measures with their overlying strata are not sufficiently thick, and the distance between upper and lower coal beds is not great, but the degree of coalification between the upper coal and the lower coals differs too much to be explained by geothermal metamorphims (for example, Early and Middle Jurassic coal measures in Tanshan mine of Ningxia Auto Region are 700 m thick), the upper coal seams are flame coal, while the lower ones are meagre coal and anthracite; this evidently results from magmatic heating. In addition, the sudden change in coal rank within a short distance in the same seam is also caused by telemagmatic metamorphism. (4) Characteristics of coalification zones. The extent of the area influenced by telemagmatic metamorphism and the width of each coalification zone depend on such factors as the scale and shape of intrusions, as well as on the number of times and depth of occurrences of intrusions. Moreover, the coalification zone is usually ring-shaped, such as the irregular ringshaped coalification zones around the dioritic stock in Shuangyashan (Heilongjiang Province) (Fig.9), but it may be perpendicular or oblique to the strike of the coal measures too, as in the case of the Handan-Xingtai coalfield where the magmatic intrusions were mainly controlled by faults which are approximately perpendicular to the strike of coal measures. Moreover, the highly telemagmatic metamorphism coal zone often consists of a single widespread anthracite zone, such as those of Ruqigou (Ningxia Auto. Region) and Jianou (Fujian Province).
Shuogyashon l t o
N
F -I'
F -14 '
'
'
•
[-~-]5
Fig.9. Z o n e s d u e t o t e l e m a g m a t i c m e t a m o r p h i s m in S h u a n g y a s h a n (Heilongjian Province). 1 = long-flame coal; 2 = c o k i n g coal; 3 = m e a g r e coal; 4 = a n t h r a c i t e ; 5 = diorite.
46 (5) The alteration of wan rocks occurs commonly in anthracite zones coalified by telemagmatic metamorphism. Examples are recrystallization in sandstone, quartz-sandstone changed into quartzite, argillites turned into sericite slates, and limestone changed into cryptocrystalline limestone, or even marble. The development of hydrothermal quartz veins in wall rocks is the main criterion of telemagmatic metamorphism, e.g. in Ruqigou in Ninxia Auto. Region and in Tianhushan in Fujian Province. Sometimes there are also calcite-barite veins. Fluorite veins and asbestos veins of hydrothermal origin also develop in wall rocks. (6) Paragenesis with endogenic mineralization. In north and east China the Yanshan diorite often intruded into the Ordovician limestone and formed magnetic iron ore deposits of skarn type, whereas the overlying Permo-Carboniferous coal is usually highly coalified by telemagmatic metamorphism with evident coalification zones. In western Guizhou Province, medium- to high-rank bituminous coals are found, often with mesothermal or epithermal deposits. The telemagmatic metamorphism of coal was often caused by blind intrusives, many of which have not yet been well studied. In defining the type of metamorphic coal and in prognoses of coal qualities, it is significant to study the presence of blind intrusions and their scale and shape as well as the intrusive depth. In exploring the blind bodies it will be more efficient to make use of the characteristics mentioned above as complementary criteria.
C. The distribution o f telemagmatic metamorphosed coals in China Magmatic activities which caused the telemagmatic metamorphism and their extent are controlled by structural systems. Within the area of latitudinal structural systems and where these are combined with other structural systems, intensive Mesozoic magmatic activity frequently took place, especially during the Yanshan Orogeny; the coals were usually coalified to medium- to high-rank bituminous grade and anthracite. Along the YinshanTianshan latitudinal structure zone, the Early and Middle Jurassic coals as well as Carboniferous coals in Northern Xinjiang Auto. Region, Neimenggu Auto. Region, Liaoning and Jiling Provinces were metamorphosed to medium- to high-ranking bituminous coals and anthracites due to Yanshan magmatic intrusions. Along 37--38°N lat. high-ranking coals due to telemagmatic metamorphism are distributed intermittently on the western border of Southern Xinjiang, eastwards through the eastern end of the Kunlun Mts., Qinghai Province, Sh~nxi Province to not far from the shoreline of Shandong Province. Along 27--28°N lat., in Hunan, Jiangxi and Fujiang Provinces, the extensive intrusions of the Yanshan granitic magma (also Yinzhi granitic magma) brought about the widespread Late Permian and Late Triassic--Early Jurassic coals of higher ranks. A great quantity of coals was metamorphosed to high ranking anthracite in Fujiang Province, where
47
a latitudinal structural zone had combined with the Neocathaysian structures. In the Nanling latitudinal structural zone, especially in its central and eastern parts, Mesozoic intrusives were widespread and were often of a multiple nature; this changed the Early Carboniferous and Late Permian, as well as Triassic--Jurassic coals of southern Hunan, southern Kiangxi, southern Fujiang and eastern Guangdong Provinces into high-ranking coals, mostly into high ranking anthracites. The Helanshan-Liupanshan meridional structural zones, and the frontal arc of Huiyang epsilon structure, especially the section along the Yangtze River, where deep-seated faults are well developed, are areas of intensive Mesozoic magmatism, and the Late Palaeozoic coals as well as Early Mesozoic coals were highly metamorphosed. CONCLUSIONS
Based upon the discussions given above, the characteristics of metamorphism of Chinese coals may be preliminarily summarized as follows: (1) The trends of coalification zones vary in different regions of China. The coalification zones of Jurassic coals trend northwestward or approximately latitudinally in northwestern China; those of Late Jurassic--Early Cretaceous coal in northeastern China trend northeastward to north-northeastward and in these two regions brown coals and low-medium coalified bituminous coals predominate. In the central portion of northern China, high-rank Permo-Carboniferous coals predominate but they decrease in rank towards its bordering areas. In southern China Late Palaeozoic coals are mainly medium- to high-rank bituminous and anthracite, and the coalification zones in southeastern provinces trend NNE and NE. In short, the degree of coalification of Chinese coals varies in different regions, lower ranking in the north, higher in the south and highest in the southeast part of China. (2) As contact metamorphism and dynamic metamorphism have influenced the coal only to a restricted extent, they cannot be held responsible for the relatively high coalification of most Chinese coals. (3) Although coals are universally geothermally metamorphosed, most of the coal measures and their overlying strata which reflect the maximum degree of subsidence are not sufficiently thick, therefore the ranks of most Chinese coals are too high to be explained by geothermal metamorphism alone. (4) Time is an important factor in geothermal metamorphism, and the degree of coalification is dependent on the temperature and duration of heating that the coal had undergone. (5) The high ranks of most Chinese coals are related to palaeogeothermal anomaly which might be caused by: (a) magmatic intrusion and hydrothermal processes; (b) deep-seated faults and basement fracture zones which provide the passage for magmatic intrusion and the transfer of internal heat; and (c) the elevation of the Moho as well as of the corresponding asthenosphere which results in raising the geothermal gradient. Of course,
48
there are still other causes that can raise the geothermal gradient, such as the upheaval of basement rocks and the heat conductivities of rock, but they are less important. (6) Telemagmatic metamorphism has influenced Chinese coals so widely that it is the main cause of most of the coals higher than of low rankings. Not only anthracites and highly coalified bituminous coals in the southeastern provinces, but also most coals higher than low rank in north China, east China, and southwestern China are due to telemagmatic metamorphism. (7) Based on the patterns of magmatic intrusion and associated hydrothermal process, the telemagmatic metamorphism of coal in China can be divided into four types: (a) due mainly to hypabyssal or meso-epimagmatic intrusion; (b) due mainly to hydrothermalism from blind intrusive bodies; (c) due mainly to meso-epimagmatic and epimagmatic intrusion; (d) due mainly to hydrothermalism and epimagmatic intrusion. (8) Most of the higher ranking coals in China had been coalified by geothermal metamorphism and this was facilitated by telemagmatic metamorphism. ACKNOWLEDGEMENT
The authors wish to thank the Coal Prospecting Companies belonging to the Ministry of Coal Industry, for providing the reference materials.
REFERENCES Buntebarth, G., 1979. Eine empirische Methode zur Berechnung yon palh'ogeothermischen Gradienten aus dem Inkohlungsgrad organischer Einlagerungen in Sedimentgesteinen mit Anwendung auf den Mittleren Oberrhein-Graben. In: Inkohlung und Geothermik. Van Acken, Krefeld, pp. 97--108. Geothermal Group of the Geological Institute of the Academy of Sciences of China, 1978. Monograph on geothermics (in Chinese). Huang Jiqing et al., 1977. The elemental contour of Chinese tectonics. J. Geol., 2 (in Chinese). Lee, J.S., 1973. Crustal structure and crustal movement. Sci. Sin., XVI (4): 520--559. Suggate, R.P., 1967. Metamorphic rank in Taiwan Coals. Bull. Geol. Surv. Taiwan, No. 19, July 1967. Suggate, R.P., 1976. Coal ranks and geological history of the Nottinghamshire-Yorkshire coalfield. Mercian Geol., 6 (1): 1--24. Teichmiiller, M. and Teichmiiller, R., 1968. Geological aspects of coal metamorphism. In: D.G. Murchison and T.S. Westoll (Editors), Coals and Coal-bearing Strata. Oliver and Boyd, Edinburgh, pp. 233--267, Teichm/iller, M. and Teichmiiller, R., 1975. The geological basis of coal formation. In: E. Stach et al., Stach's Textbook of Coal Petrology. Gebriider Borntraeger. BerlinStuttgart, pp. 5--54. Wang Qianshan and Liu Yuanlong, 1976. The contour of crustal structure of South Liaoning. J. Geophys., 3 (in Chinese). Wang Zuquan, 1956. The zone distribution of coal rank and its geological causes in North China. Colliery Technique, 7 (in Chinese). Zhelinsky, V.M., 1980. Catagenesis of terrigenous rocks and coal metamorphism of South Yakytia. In: Lithology and Mineral Deposits. Sceenie, Moscow, pp. 99--114.