Petrography and geochemistry of tonsteins from the 4th Member of the Upper Triassic Xujiahe formation in southern Sichuan Province, China

International Journal of Coal Geology 49 (2002) 1 – 17 www.elsevier.com/locate/ijcoalgeo

Petrography and geochemistry of tonsteins from the 4th Member of the Upper Triassic Xujiahe formation in southern Sichuan Province, China Kurt Burger a, Yiping Zhou b, Youliang Ren c,* a Lothringenstrasse 8, D-45259 Essen, Germany Kunming Institute of Coal Sciences, Kunming, Yunnan Province, China c Colorado School of Mines, 2203 South Holly Street, Suite 7, Denver, CO 80222, USA b

Received 31 January 2001; accepted 8 October 2001

Abstract Petrographic and geochemical studies on five layers of tonsteins from the 4th Member of the Upper Triassic Xujiahe formation (T3xj4) in the southern Sichuan Province of China are presented in this paper. The results suggest that these tonsteins were derived from synsedimentary acid volcanic ash fallout, which was subsequently altered into relatively pure kaolinitic rocks through hydrolysis and diagenesis in a peat-bog environment (for definition of tonsteins, refer to [Geol. Soc. Am., Spec. Pap. 285 (1993)]). The petrographic texture is mainly of dense and granular types, with a subordinate crystalline type. Each tonstein layer is characterized by its unique petrographic texture, mineral content, and assemblage of lithophile elements, which is related to the composition of its parent magma. The trace element geochemistry of these tonsteins is elaborated in this paper, which also suggests an origin as silicic volcanic ash fallout. Typically, these features are relatively consistent within the same tonstein layer over a broad lateral extent, and are characteristic of each specific tonstein layer. Therefore, some tonsteins in this region serve as distinct marker beds for correlation of coal seams within the framework of regional chronostratigraphy. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Tonstein; Petrography; Geochemistry; Coal geology; Triassic; Sichuan Province

1. Introduction The Upper Triassic Xujiahe formation in the southern Sichuan Basin consists of massive layers of sandstones intercalated with siltstones, mudstones, and coal seams, with a total thickness in the range of

*

Corresponding author. Fax: +1-303-334-1679. E-mail address: [email protected] (Y. Ren).

450 – 600 m. Based on its lithology and coal distribution, it can be divided into six sections. Sections 1, 3, and 5 are mainly massive sandstones, while sections 2, 4, and 6 are coal-bearing mudstones. The workable coal seams within sections 2 and 4 contain kaolinitic tonsteins (Yang, 1996; Han and Yang, 1980). Fig. 1 shows the geographic location of the study area. The study area stretches in an east – west direction from Hechuan to Leshan (Fig. 2), 270 km apart, covering an area of approximately 10,000 km2. The

0166-5162/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 5 1 6 2 ( 0 1 ) 0 0 0 5 3 - 2

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3. Analytical techniques

Fig. 1. Index map showing the location of the study area.

4th Member of Xujiahe formation (T3xj4) is 50 – 130 m thick. It contains three to five layers of coal seams, among which K7 and K6 have a thickness of 0.6 – 1.0 m. They are separated by an interval of about 7 m and have a relatively continuous lateral extent. These two seams contain one to three layers of tonsteins (Fig. 2). The 4th Member of the Upper Triassic Xujiahe formation is of lacustrine deltaic sedimentary facies. The ranks of coal therein are mainly of fat coal and bituminous coal, according to the Chinese coal ranking classification.

2. Sample distribution and collection Samples were collected from tonsteins of K7 and K6 in five underground workings, including two samples from the K6 seam and 11 samples from the K7 seam. Various petrographic textures, characteristic of individual tonsteins, served as a basis for identification of equivalent layers. Closely coupled doublet or triplet tonstein sequence and its lateral variation in interval thinning or thickening were carefully traced for sampling purposes. In addition, samples of subdivided layers from individual tonsteins were separately collected based on their distinctive macroscopic petrographic types (Fig. 2).

This paper presents the results of petrologic studies on five tonstein layers from K7 and K6 coal seams across five stratigraphic profiles of the study area. Channel-sampling procedures were strictly followed and any adhering coal were carefully removed to provide uncontaminated representative samples. Each sample was pulverized and split four ways; one of these splits was incinerated to determine the loss on ignition (LOI) (Zhou et al., 1982). Samples prepared for microscopic studies are thin sections. X-ray diffraction and chemical analyses were performed on a total of 13 tonstein samples. The contents of rare-earth elements (REE) and 20 more trace elements were determined for all samples using techniques including polarography, X-ray fluorescence spectra (XRF), and induction-coupled plasma mass spectrometry (ICP-MS). In addition, analyses of infrared absorption spectra were performed on two of these samples.

4. Petrographic studies 4.1. Macroscopic petrographic criteria Tonsteins from coal mining workings or cores are mostly brownish-black in color, but they weather white on outcrops. The polished specimens show different types of textures. Tonsteins of dense texture are characterized by smooth surfaces and conchoidal fracture. The fine- to coarse-grained tonsteins contain light-colored kaolinitic aggregates, varying in size, scattered in the much darker matrix. The crystalline tonsteins are typically packed with whitish vermicular stacks of kaolinite crystals. Most tonsteins have sharp contacts with the enclosing coal beds, with the exception of the K7b tonstein in this area, which shows a partially transitional relationship. 4.2. Microscopic studies 4.2.1. Types of petrographic texture The petrographic classification is based on the publications of Schu¨ller (1951) and Schu¨ller and Hoehne (1956). Microscopic studies revealed that all the samples that were examined are tonsteins, which refer to

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Fig. 2. Diagrams showing the structure of coal seams K7 and K6 and the distribution of sampling locations in the study area.

non-marine, generally kaolinitic layers derived from the in situ alteration of volcanic ash fallout (Bohor and Triplehorn, 1993). Based on their petrographic characteristics, they can be classified into three major types, including four sub-types, as follows (Fig. 3). (a) Crystalline tonstein. a1—Crystal tonstein with lenses of cryptocrystalline kaolinitic matrix: the kaolinite crystals (tablets, columns and fragments) are pseudomorphs after biotite. Special investigations of Upper Carboniferous tonsteins of the Ruhr Basin, Germany (Burger, in preparation) prove that these typical shapes have been caused by the splitting of biotites. Additional minerals

are quartz, b-quartz, feldspar, and pseudomorphs after feldspar and zircon (Plate 2a –d,g,h). (b) Dense tonstein. b1—Dense tonstein with abundant crystals: spread within the cryptocrystalline kaolinite matrix are abundant kaolinite crystals and kaolinitic pseudomorphs after feldspar and biotite, columns of kaolinite up to 1.4 mm in length, and scattered vermicular forms (Plate 1h) occur. Fragments of vesicular ash (Plate 4c), quartz, b-quartz, feldspar, and zircon also abound (Plates 1g,h and 2e,f ). b2—Dense tonstein with minor crystals: some kaolinite crystals and pseudomorphs are scattered within the cryptocrystalline kaolinite matrix. Sporadic

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Fig. 3. Profiles showing lateral changes in petrographic texture types of tonsteins K7a, K7b, and K7c.

columns of kaolinite, up to 1.4 mm in length, and some vesicular volcanic relics are observed in samples 84 and 87 (Plate 4h). Additional minerals are quartz, rare b-quartz, feldspar, anorthite (sample 87, Plate 1d), pseudomorphs after feldspar, and zircons up to 0.180 mm (Plates 1a –f, Plates 2b, Plates 3a – d). (c) Granular tonstein. c1—Fine- to coarse-grained tonstein: scattered within the cryptocrystalline kaolinite matrix are numerous light-colored microcrystalline kaolinite aggregates. These aggregates are typically ovoid in shape, with their long axes oriented parallel with the bedding of coal seams, often showing vertical gradation in abundance and size. Small, elongated kaolinite crystals are striking (Plate 3f) and represent devitrified vitroclastic components. Extremely compacted vesicular particles are hard to identify, their original shape has changed due to swelling. Similar vitroclastic constituents have been reported and documented with

micrographs from tonsteins of the Upper Carboniferous of the Ruhr Basin, Germany, and of the Saar – Lorraine area, France by Burger (1990, 1992). Scattered minerals in sample 83 are quartz, b-quartz, pseudomorphs after feldspar, and fragments of zircon (Plate 3e– h). 4.2.2. Accessory minerals The amount of accessory minerals account for less than 3% of the total content. They are mainly quartz splinters and beta-form (high temperature) quartz (Plate 4b), with subordinate sharp-edged anorthite fragments (Plate 1d). Zircon is present in relatively low abundance as elongate bipyramidal euhedral crystals in tonsteins, and are extremely resistant to alteration. Apatite is rare and occurs as euhedral crystals. In addition, some thin sections revealed well-preserved volcanic glass bubble inclusions (Plate 4g). Polished surfaces were examined to verify the presence of pyrite

Plate 1. Showing the types of petrographic textures of tonsteins revealed by the microscopic studies on thin sections from the following samples and locations. Seam K7, tonstein 7a + b, sample 87: Location 1. (a) Isotropic, dense matrix of kaolinite with opaque root casts and humic relics. Grains of feldspar and quartz sporadically; 1/4 nicols. (b) Root remains infiltrated with kaolinite in kaolinitic matrix; 1/4 nicols. (c) Small area with vitroclastic structure, bedding indicated by coalified plant remains; 1/4 nicols. (d) Scattered anorthites with cleavage cracks (001) and (010), partly kaolinized at the margins. Seam K6, tonstein K6a, sample 86: Location 2. (e) Kaolinized fragments of biotite and column of kaolinite distributed sporadically in kaolinitic matrix which is interstratified with humic particles; 1/4 nicols. (f) Root remains destroyed by massive isotropic kaolinite; 1/4 nicols. Seam K6, tonstein K6b, sample 85: (g) Columnar crystals of kaolinite and their fragments close to a root remain which has been infiltrated with isotropic kaolinite. (h) Extremely bent column of kaolinite with lamellar structure (length 1.4 mm). Matrix of kaolinite with humic remains and grains of hematite. The distance between locations 1 and 2 is approximately 25 km.

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in samples 80– 89. Scattered crystals of pyrite ( < 5 mm) were observed in all samples. Minute pyrite crystals between the lamellae of kaolinized biotites occur frequently in some samples, as well as concentrated pyrite aggregates in the coaly ‘‘envelopes’’ of root relics. The secondary pyrite was formed by bacterial activity during the weathering processes. In summary, the characteristics of accessory minerals in tonsteins in this region are similar to those of Carboniferous – Permian coal-bearing formations in North China, and those of the Late Permian formations in Southwest China. It is indicated that the distal volcanic ash fallout forming most tonsteins were derived mainly from a rhyolitic magma source (Feng, 1989; Zhou et al., 1982, 2000). 4.2.3. Different types of petrographic textures observed in tonstein profiles Five tonstein layers with different types of petrographic textures and their distribution characteristics are described in descending order as follows. K7a and K7b are located at the top and middle portions of seam K7, respectively. These two tonsteins merge into one single layer when they extend towards the central and eastern parts of the study area. The combined layer becomes thinner upon merging, but can still be separated as two layers. On the other hand, K7b branches out into two layers towards the west part of the study area, with a thin coal seam (8 cm thick) intercalated in between. K7a is of dense type (type b1), containing kaolinite crystals and pseudomorphs (after biotite) in Profiles 4 and 1. K7b is of granular type. It changes locally into dense type with minor crystals (type b2).

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K7c is located in the lower part or near the bottom of seam K7. It does not appear in Profile 1. It is possible that this tonstein layer passes beyond the bounds of the paleo-swamp environment and extends diagonally through the floor of the coal seam. It branches out as doublets in Profile 3, with an intercalation of a thin layer of dull coal (8 mm thick) in between. The upper tonstein layer is granular in texture (type c1), while the lower layer is crystalline (type a1) and changes to dense tonstein with minor crystals (type b2) in Profile 5. K6a (10 cm thick) is atop seam K6. It is dense, with minor crystals (type b2). K6b occurs in the middle of seam K6, with a thickness of 11 cm. This tonstein demonstrates a triple sub-layer sequence, of which the upper 4.5 cm and the lower 2.5 cm are dense, while the middle 4 cm is distinctly granular in texture. The texture of tonsteins, as seen in thin sections, has been the primary basis for their classification. In addition, closely coupled or triple-layered tonstein sequences are often useful in field correlation. The petrographic texture and mineral constitution of tonsteins, as characterized by the authigenic kaolinite minerals and replaced pyroclastic components, are related to the composition of their parent synsedimentary volcanic ash fallout. Kaolinite is the most common and abundant clay mineral in tonsteins. Kaolinite in tonsteins is derived mainly from the alteration of volcanic glass, resulting in a dense type of tonsteins. On the other hand, feldspar, biotite, and other minerals also alter to kaolinite. Kaolinite may be pseudomorphic after these minerals, leading to the formation of a granular type of tonsteins. In addition, pseudohexagonal platelets of

Plate 2. Showing the types of petrographic textures of tonsteins revealed by the microscopic studies on thin sections from the following samples and locations. Seam K7, tonstein K7a + b, sample 81: Location 3. (a) Columnar and tabular fragments of kaolinized biotites, humic matrix; 1/4 nicols. (b) Lower part of the layer with kaolinite gel-lumps, microcrystalline-granular pseudomorphs after feldspar and quartz in humic matrix; 1/4 nicols. Seam K7, tonstein K7c, sample 80. (c) Elongated and bent columns and fragments of kaolinite in various sizes and shapes, sporadic quartz, humic matrix; 1/4 nicols. (d) Crystals of kaolinite in the form of columns, tablets and fragments, densely packed; sporadic quartz, humic matrix; 1/4 nicols. Seam K7, tonstein K7a + b, sample 89: Location 4. (e) Columns and fragments of kaolinite, pseudomorphs after biotite and feldspar in kaolinitic matrix which is interstratified with humic matter; scattered quartz; 1/4 nicols. (f) Kaolinite pseudomorph after biotite, quartz and pseudomorphs after feldspar; kaolinitic matrix with humic matter; 1/2 nicols. Seam K7, tonstein K7c, sample 88. (g) Closely packed crystals of kaolinite in the form of columns, tablets and fragments, sporadic quartz, brown matrix of kaolinite with humic relics; 1/4 nicols. (h) Columns and fragments of kaolinite in various sizes, brownish kaolinitic matrix; 1/4 nicols. The distance between locations 3 and 4 is approximately 45 km.

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kaolinite can be formed by the replacement of biotite, which resulted in a crystalline type of tonsteins as characterized by the large stacks of vermicules visible to the naked eye. However, a certain amount of kaolinite can be chemically precipitated without any obvious precursor, as typical of the microcrystalline or cryptocrystalline kaolinite which fills in plant cells to form the clay matrix of different types of tonsteins. Individual tonsteins may have gradual lateral changes in texture. However, different layers of tonsteins are characterized by their unique properties and can be distinguished from each other over a lateral extent. It is clear that the mineral composition and texture of tonsteins are functions of original ash composition, bed thickness and geochemical conditions in the depositional environment, as well as the courses of subsequent diagenesis. 4.3. X-ray diffraction and infrared absorption spectra analyses X-ray diffraction analyses of tonsteins from this area show kaolinite of a low degree of crystallinity (i.e. in terms of order – disorder of crystal structure) as the dominant component of the clay fraction. The quartz content is low. A comparison of infrared absorption spectra shows similar features between the two samples from this area and those from Xuanwei, eastern Yunnan Province (Zhou et al., 1982). However, they are somewhat different from the well-crystallized kaolinite from the tonstein of Suzhou, Jiangsu Province (Han and Chen, 1982). As shown by the infrared absorption spectra (Fig. 4), the tonstein from Suzhou shows a peak at 3655 cm-1 between two strong absorption peaks at 3700 and 3000

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cm
1 within the high-frequency spectra interval. In the medium-frequency spectra interval within the 1200 – 1000-cm
1 range, there appear three absorption peaks, but the tonstein from Suzhou shows an additional sharp peak at 1117 cm
1. In summary, the analytical results of infrared absorption spectra are in conformity with those of X-ray diffraction patterns of typical tonsteins. Silicates with highly ordered crystal structure show more distinct, sharp band passes on infrared absorption spectra. By the same token, the well-crystallized kaolinite also shows sharp basal peaks and prism reflection of X-ray diffraction pattern, as indicative of an authigenic origin (Zhou et al., 1982).

5. Chemical composition The analytical data of chemical composition on the 13 samples are given in Table 1. The results of chemical analyses show that the organic matter content of the tonsteins is high and quite variable, ranging from 3% to 25%, as is characteristic of a peat-swamp environment. SiO2 and Al2O3 account for about 93% of the total content. The ratio of SiO2/Al2O3 (weight) is in the range of 1.19– 1.30, slightly higher than the theoretical value for kaolin (1.18), indicative of a relatively pure kaolinite with minor free SiO2 (quartz). TiO2 content ranges from 0.13% to 0.88%. The most useful indicator of provenance is the ratio of TiO2/Al2O3 (weight), which is typically < 0.02 for silicic volcanic rocks. Our data show TiO2/Al2O3 ratios ranging from 0.0226 to 0.0035, which is consistent with a silicic volcanic origin (Addison et al., 1983). The contents of Fe, Mg, Ca and alkaline metal oxides are very low.

Plate 3. Showing the types of petrographic textures of tonsteins revealed by the microscopic studies on thin sections from the following samples and locations. Seam K7, tonstein K7a, sample 84: Location 5. (a) Microcrystalline-granular to dense kaolinitic matrix with scattered kaolinites and humic matter; 1/4 nicols. (b) Root casts filled with kaolinite, brownish in parts. Seam K7, tonstein K7c, sample 82. (c) Slightly bent column of kaolinite and anomalously shaped kaolinites in brownish kaolinitic matrix, opaque humic matter; 1/4 nicols. (d) Column of kaolinite with adsorption of organic substances, scattered kaolinite crystals, brownish kaolinitic matrix with humic matter; 1/4 nicols. Seam K7, tonstein K7b, sample 83. (e) Crystal of kaolinite with columnar cleavage, weakly birefringent, surrounded by anomalously shaped kaolinites, sporadic quartz, humic matrix; 1/4 nicols. (f) Clear elongated relics as pseudomorphs of kaolinite after volcanic glass. Oval shapes caused by swelling. Sporadic kaolinized biotite and quartz, humic matrix; 1/4 nicols. (g) Fragment of quartz surrounded by vitroclasts (compaction 4:1), humic matrix; 1/2 nicols. (h) Kaolinized biotite split in columns and fragments, surrounded by pseudomorphic relics of volcanic glass; humic matrix; 1/4 nicols.

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In summary, our data from the study area support a scenario that the finest vitric and labile primary mineral components of a distal rhyolitic ash layer were

deposited in a peat-forming swamp, dissolved and precipitated as kaolinite. Most of the alkaline metals and alkaline earth metals were leached out by large

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Fig. 4. Infrared absorption spectra of tonstein sample 86 from the study area, compared to those of the well-developed kaolin crystals (sample a) from Suzhou, Jiangsu Province.

volume of low pH water containing humic compounds from the peat. Diffusion replacement of larger and less-soluble glass clasts obviously occur in tonsteins. Pseudomorphs of glass splinters, feldspars, and biotite by kaolinite are evidence of this replacement reaction.

6. Trace element geochemistry The analytical results of trace elements are listed in Tables 2 and 3. U and Th were measured using X-ray fluorescence (XRF) spectra. Sn and W were deterK7a K7b K7a + b K7c K6a K6b

mined by catalytic polarography. The rest of the elements were determined by induction-coupled plasma mass spectrometry (ICP-MS). 6.1. Rare-earth elements (REE) Studies of the abundance of REE in tonsteins are useful for the determination of volcanic origin and weathering schemes. In general, Pthe gross concentrations of rare-earth elements ( REE) of individual tonsteins are quite variable, reflecting the different compositions of these tonstein layers (Fig. 5):

255.0 – 273.2  10
6, averaging 265.1  10
6 647.1 – 1090.4  10
6, averaging 830.1  10
6 420.6  10
6, with an average value within the range of K7a and K7b 153.4 – 430.1  10
6, averaging 307.8  10
6, far below the average value of the adjacent K7b 64.2  10
6 and 318.3  10
6

Plate 4. Showing the types of petrographic textures of tonsteins revealed by the microscopic studies on thin sections from the following samples and locations. (a) Hypidiomorphic zircon of 115 mm length; 1/4 nicols, location 5, sample 84. (b) Magmatic quartz showing liquid inclusions; + nicols, location 3, sample 81. (c) Volcanic glass relic with vesicular structures, altered to microcrystalline-granular kaolinite; + nicols, location 4, sample 89. (d) Feldspar, kaolinized at the margin; 1/4 nicols, location 1, sample 86. (e) Microcrystalline-granular pseudomorphs of kaolinite after feldspar and one fresh feldspar; + nicols, location 3, sample 81. (f) Angular pseudomorphs after feldspar and two partially kaolinized feldspars; + nicols, location 5, sample 84. (g) Vesicular volcanic relic with illite frame, filled with almost isotropic kaolinite; 3/4 nicols, location 5, sample 83. (h) Vesicle fragment of 210 mm length; 1/2 nicols, location 1, sample 83.

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Table 1 Chemical composition and characteristic geochemical values of tonsteins Tonstein horizon

K7a

K7a + b K7b

K7c

K6a K6b

Sample code

0

87 1 0 89 1 84 81 0 87 2 0 89 2 83 0 80 1 0 80 2 88 82 86 85

Thickness (cm)

5 4 7 10 4 7 9 2 3.5 12 7 10 11

Chemical composition x/10
2

Characteristic geochemical values

SiO2

Fe2O3

Al2O3

TiO2

CaO

MgO

Na2O

K2O

SO3

Burning loss

SiO2/ Al2O3

TiO2/ Al2O3

51.31 51.38 51.98 53.29 51.97 52.31 52.07 57.10 54.22 54.66 51.53 51.94 50.36

1.07 1.07 0.96 1.17 0.89 1.04 0.69 2.80 2.16 1.73 0.60 1.43 1.28

43.02 42.42 42.87 38.75 42.06 41.98 43.58 30.97 37.24 39.76 43.48 40.06 42.12

0.44 0.77 0.52 0.70 0.50 0.76 0.37 0.70 0.13 0.43 0.88 0.82 0.59

0.11 0.08 0.08 0.18 0.14 0.07 0.12 0.28 0.32 0.14 0.11 0.26 0.18

0.12 0.36 0.12 0.54 0.21 0.53 0.18 1.09 0.54 0.33 0.15 0.28 0.55

0.12 0.30 0.16 0.28 0.08 0.27 0.13 – 0.40 0.32 0.12 0.05 0.05

0.45 0.36 0.35 1.12 0.45 0.31 0.24 – 1.70 0.36 0.24 1.20 0.75

0.23 0.31 0.30 0.42 0.37 0.32 0.50 0.24 0.29 0.36 0.34 0.44 0.34

18.78 25.98 22.44 23.31 27.00 22.97 28.70 21.96 18.50 40.20 21.97 18.10 16.93

1.19 1.21 1.21 1.38 1.24 1.25 1.19 1.84 1.46 1.37 1.19 1.30 1.20

0.010 0.018 0.012 0.018 0.012 0.018 0.009 0.023 0.004 0.011 0.020 0.020 0.014

Weight percentage on dry basis, after LOI calibration.

REE abundance normalized to chondrites (Zhou et al., 2000) for different layers of tonsteins are shown in Plate 2. They all have a high content of light REE, with their distributive curves dipping toward the right. Note the negative europium anomalies characteristic P of silicic volcanism. In addition, an increase in TR increases the abundance of light REE. Another notable feature is that dCe has relatively slight positive anomalies, while dEu shows pronounced negative anomalies (0.12 – 0.37) as shown in Table 2. dEu decreases from 0.36 for K6b (lower stratigraphic sequence) to 0.2 for K7b and K7a (higher sequence), indicating that K7b and K7a had rather heavier loss of Eu (Fig. 6). The REE concentrations and their distributive patterns of tonsteins from this region bear a strong resemblance to those of the A-type granites of Indosinian – Yanshanian Age (Xing and Tang, 1990). As shown in Fig. 5, the difference in REE pattern of various tonsteins provides an aid in correlation of tonstein stratigraphy. 6.2. Other trace elements As shown in Table 3, the contents and assemblage of lithophile elements (Li, Be, Se, Y, Zr, Th, and U), the chalcophile elements (Cu, Zn, and Ga), and the

elements in the W and Mo family show convergence of evidence indicative of a silicic volcanic origin of the tonsteins listed therein. In addition, the contents of Be, W, Sn, and Ga are even higher than the average values for the granites of Yanshanian age from South China (Liu, 1984). It is noteworthy that the contents and assemblage of the immobile elements such as Y, Zr, Th, Ga, and Ti, along with the REE patterns of tonsteins, can be used to determine the composition of the original ash, and thus the type of source magma. These features are also observed in the Late Permian coal-bearing formation in southwestern China (Zhou and Ren, 1994; Zhou et al., 1982, 2000).

7. Conclusions The properties of tonsteins are profoundly modified by the combined effect of many factors, such as the original ash composition, geochemical conditions in the depositional environment, and the subsequent diagenesis. The coal-forming environment is well suited for preservation of ash fallout. The 4th Member of Upper Triassic Xujiahe formation (T3xj4) in the southern Sichuan Basin contain two major workable coal seams, K7 and K6, which bear five layers of relatively pure kaolinitic tonsteins.

Tonstein horizon

K7a

K7a + b K7b

K7c

K6a K6b

Sample code

8701 8901 84 81 8702 8902 83 8001 8002 88 82 86 85

Thickness (cm)

5 4 7 10 4 7 9 2 3.5 12 7 10 11

REE x/10
6 La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

50.7 53.7 64.3 84.9 223.0 129.0 164.0 78.1 43.9 79.9 29.6 11.8 58.6

113.0 115.0 125.0 177.6 491.0 284.0 346.0 132.6 107.9 186.7 64.5 24.2 145.1

10.5 10.9 11.0 17.6 49.5 28.6 35.0 11.5 11.8 19.3 5.8 1.5 14.1

44.5 43.6 42.1 68.7 200.0 115.0 139.0 37.8 51.6 82.7 25.0 7.6 59.9

10.0 9.7 8.9 14.5 41.4 24.2 28.0 5.6 11.5 18.8 6.1 2.5 12.6

0.46 0.41 0.63 0.71 1.40 1.10 1.60 0.44 1.02 2.02 0.44 0.32 1.30

8.0 8.5 6.5 14.7 27.1 18.1 17.9 4.6 9.1 13.9 6.0 3.2 9.1

1.3 1.8 1.1 2.7 4.3 3.1 2.0 0.9 1.1 1.9 1.1 0.9 1.5

8.0 12.1 6.8 17.7 25.6 20.0 12.8 4.6 4.3 11.8 7.4 5.4 8.0

1.6 2.5 1.3 3.6 4.8 4.0 2.3 1.0 0.7 2.3 1.5 1.1 1.6

3.8 6.9 3.0 8.8 11.2 9.8 5.0 2.2 1.6 5.4 3.2 2.9 3.7

0.5 0.9 0.3 1.3 1.5 1.2 0.5 0.4 0.2 0.7 0.3 0.4 0.5

3.0 6.0 2.1 7.0 9.1 8.3 3.4 2.0 1.1 4.3 2.3 2.2 3.0

0.3 0.5 0.2 0.8 0.5 0.7 0.3 0.2 < 0.1 0.4 0.2 0.2 0.3

Characteristic geochemical values P P REE LREE/ P HREE 255.0 267.1 273.2 420.6 1090.4 647.1 752.8 281.9 245.8 430.1 153.4 64.2 318.3

8.9 6.9 11.8 6.4 12.0 8.9 14.3 16.7 12.6 9.6 6.0 2.9 10.5

dCe

dEu

1.12 1.08 1.03 1.05 1.08 1.08 1.05 0.95 1.12 1.10 1.16 1.19 1.18

0.158 0.135 0.244 0.148 0.121 0.155 0.205 0.258 0.295 0.366 0.252 0.345 0.355

LREE and HREE stand for light rare-earth elements and heavy rare-earth elements, respectively. dEu stands for the ratio Eu/Eu*. Eu* is the interpolation based on the values measured for its adjacent elements (i.e. Sm and Gd) and plotted on logarithmic coordinate diagrams, as shown in Fig. 5. The computational method for determining dCe, the ratio of Ce/Ce*, is similar.

K. Burger et al. / International Journal of Coal Geology 49 (2002) 1–17

Table 2 Rare-earth element concentrations (ppm) and characteristic geochemical values of tonsteins

13

14

Tonstein horizon

K7 a

K7 a + b K7 b

K7 c

K6 a K6 b

Sample code

8701 8901 84 81 8702 8902 83 8001 8002 88 82 86 85

Thickness (cm)

5 4 7 10 4 7 9 2 3.5 12 7 10 11

Element x/10
6

Characteristic geochemical values

Li

Be

Sc

Y

Zr

Th

U

Ti

V

Cr

Ni

W

Mo

Cu

Zn

Ga

Ga  104/ Al

Y/ TR

Ti/ Y

115.2 150.3 181.3 175.1 99.4 147.8 146.5 – 197.5 160.8 167.0 166.1 283.2

5.7 5.9 4.5 8.8 6.6 6.3 4.6 – 4.4 6.2 4.3 4.6 3.6

2.8 7.1 3.1 6.3 5.1 8.9 6.4 9.6 4.1 12.0 5.9 4.3 13.8

33.3 66.3 29.6 84.2 98.6 93.3 49.0 22.5 17.3 49.3 33.0 31.1 32.0

189.4 274.5 147.8 642.0 397.4 335.1 267.3 – 88.3 253.8 332.0 206.7 251.5

68.0 74.0 40.0 97.0 135.0 95.0 77.0 30.0 29.0 57.0 61.0 55.0 68.0

11.0 8.0 13.0 15.0 28.0 14.0 11.0 9.0 < 5.0 10.0 12.0 7.0 8.0

2387 4523 2817 4484 2552 4329 1917 3779 1030 2415 4799 3760 3437

3.5 24.6 9.0 19.4 3.7 23.8 6.2 – 7.7 6.2 12.2 15.6 19.0

6.8 14.9 9.6 16.6 4.3 15.2 8.8 – 11.3 20.1 9.2 11.5 6.7

4.1 4.2 < 4.0 5.7 7.1 < 4.0 < 4.0 – 6.2 7.2 < 4.0 11.8 4.2

3.2 6.8 9.7 8.3 3.8 6.5 4.0 8.4 2.0 4.6 6.8 6.5 3.7

< 4.0 4.5 < 4.0 < 4.0 < 4.0 10.0 < 4.0 – 6.6 < 4.0 5.9 4.7 8.2

50.1 38.5 38.3 37.4 46.3 42.4 43.7 – 34.7 39.6 31.1 21.9 46.9

82.3 44.1 34.7 60.6 112.8 31.4 29.3 – 36.3 28.8 43.3 16.9 118.1

58.4 52.8 52.3 54.2 44.7 51.1 46.9 – 46.4 39.5 55.9 58.5 37.7

3.1 2.8 3.3 2.6 2.8 2.4 3.0 – 2.5 2.6 3.0 3.8 1.8

0.13 0.25 0.11 0.20 0.09 0.14 0.07 0.09 0.07 0.12 0.22 0.48 0.10

72 68 95 53 26 46 39 177 60 49 145 121 107

K. Burger et al. / International Journal of Coal Geology 49 (2002) 1–17

Table 3 Trace element concentrations and characteristic geochemical values of tonsteins

K. Burger et al. / International Journal of Coal Geology 49 (2002) 1–17

15

Fig. 5. Diagrams showing REE distributive curves of tonsteins (normalized to C1 chondrites).

The petrographic texture of these tonsteins is mainly of dense and granular types, with a subordinate crystalline type. Although the tonstein layers (or sub-layers) change in petrographic texture to some extent laterally, most distal tonsteins contain a limited suite of primary volcanic minerals. The primary minerals, such as feldspar and biotite, along with minor euhedral beta-quartz paramorphs, anorthite, idiomorphic zircon, and other accessory minerals are characteristic of each specific silicic magma eruption event. Chemical analyses of these tonsteins revealed a low Ti content, as well as a low ratio of TiO2/Al2O3, which are consistent with a silicic volcanic ash origin (Bohor and Triplehorn, 1993).

The analytical results of the contents of more than 30 trace elements and their assemblage characteristics in tonsteins also point to a silicic magmatic source. The content and assemblage of trace elements are characteristic of individual tonstein layers and are quite stable over a broad lateral extent. The geochemical characteristics of trace elements in tonsteins are similar to those of rhyolitic ashes. In short, tonsteins are isochronous stratigraphic units and can be used for correlating coal beds locally, as well as for mapping corresponding stratigraphic sequences on a regional scale. We have identified five layers of tonsteins from two coal seams less than 10 m apart. The close chemical similarity between these kaolinitic clay partings

16

K. Burger et al. / International Journal of Coal Geology 49 (2002) 1–17

Fig. 6. Correlative graphs showing the contents of TR, Th, Y, and the ratios of Ga  104/Al, Y/TR, and dEu values of different tonstein layers.

implies that the ashes originated from a single, massive, silicic volcanic source. They cover the study area of about 10,000 km2 and far beyond. Their relatively uniform thin thickness indicates that they are distal fallout ashes. It is clear that volcanic eruption activities

in the vicinity of the Sichuan Basin during the Late Triassic period were very strong and frequent. Silicic volcano eruptions of Plinian type (Newell and Walker, 1981; Zhou and Kyte, 1988), which expelled significant volumes of ash into the upper atmosphere and

K. Burger et al. / International Journal of Coal Geology 49 (2002) 1–17

spread ash deposits over many hundreds or even thousands of kilometers have been documented. Additional research is needed to determine the extent of these eruptions and whether a hypothesis of the Plinian type of event is worthy of consideration (Francis, 1985).

Acknowledgements The authors want to thank Dr. Bruce F. Bohor of the US Geological Survey for reviewing the paper and making valuable suggestions.

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