Palaeogeography, Palaeoclimatology, Palaeoecology 297 (2010) 452–464

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Geological and geochemical aspects of a Devonian siliceous succession in northernThailand: Implications for the opening of the Paleo-Tethys

Hidetoshi Hara a,⁎, Toshiyuki Kurihara b, Junichiro Kuroda c, Yoshiko Adachi d, Hiroshi Kurita e, Koji Wakita a,Ken-ichiro Hisada f, Punya Charusiri g, Thasinee Charoentitirat g, Pol Chaodumrong h

a Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japanb Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japanc Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Kanagawa 237-0061, Japand Center for Transdisciplinary Research, Niigata University, Niigata 950-2181, Japane Department of Geology, Faculty of Science, Niigata University, Niigata 950-2181, Japanf Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japang Earthquake and Tectonic Geology Research Unit (EATGRU), Chulalongkorn University, Bangkok 10330, Thailandh Department of Mineral Resources, Bangkok 10400, Thailand

⁎ Corresponding author. Tel.: +81 298 61 3981; fax:E-mail address: hara-hide@aist.go.jp (H. Hara).

0031-0182/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.palaeo.2010.08.029

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 April 2010Received in revised form 11 August 2010Accepted 30 August 2010

Keywords:GeochemistryRedox indicatorChertBlack shaleDevonianPaleo-Tethys

The opening of the Paleo-Tethys are reconstructed, including the depositional setting and redox conditions,based on an analysis of radiolarian fossils and the geochemistry of aDevonian siliceous succession in the ChiangDao area of northern Thailand. The succession is subdivided into the following five rock types (in ascendingstratigraphic order): black shale (Lower Devonian), siliceous shale (Middle Devonian), tuffaceous chert andtuff (Middle/Upper Devonian), and chert (Upper Devonian). The succession was deposited in continentalmargin and pelagic environments between the Sibumasu Block and the Indochina–North China blocks. Theconcentrations of terrestrial-derived elements (Al2O3, TiO2, Rb, and Zr) suggest that the succession (except forthe chert) was supplied with terrigenous material and volcanic ash from the adjacent continent, depositedwithin a SiO2-rich environment. Geochemical indicators of redox conditions (total organic carbon and the Th/Uratio) reveal a gradual change from anoxic to oxic oceanic conditions between the black shale and chert. Takinginto account the interpreted depositional setting and redox conditions, the initial Paleo-Tethys developed asa small, closed anoxic–suboxic oceanic basin during the Early to Middle Devonian, located close to thecontinental margin. Black shale and siliceous shale were deposited in the basin at this time. Opening of thePaleo-Tethys started around the Middle and Upper Devonian boundary, marked by voluminous volcanicactivity. The tuffaceous chert was deposited under oxic conditions, suggesting that ash and pumice withinthe chert were derived from a continental source. After the Late Devonian, the Paleo-Tethys developed as adeep, broad ocean in which pelagic chert was deposited.

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1. Introduction

The Paleo-Tethys, which opened in response to Devonianseparation of the North China, South China, Indochina and Tarimblocks from Gondwana, occupied a large area around the equatorfrom the Devonian to Triassic, where carbonates, chert, and basaltwere deposited in a pelagic domain (e.g., Metcalfe, 1999, 2006). ThesePaleo-Tethyan rocks, characterized by an ocean plate stratigraphy(OPS), were subducted beneath the Indochina Block during thePermian–Triassic (Metcalfe, 2002; Wakita and Metcalfe, 2005; Haraet al., 2009); however, the depositional environment and style ofPaleo-Tethys opening have yet to be clarified.

Here, the tectonics of the opening of the Paleo-Tethys werereconstructed, focusing on a Devonian siliceous succession that waslocated in the convergence zone of Paleo-Tethyan rocks within theInthanon Zone, northern Thailand. This Devonian to Lower Carbonif-erous siliceous succession has been described as chert in previousstudies and is informally named the ‘Fang Chert’ where it occurs innorthern Thailand (e.g., Bunopas, 1981; Sashida et al., 1993;Wonganan and Caridroit, 2005; Wonganan et al., 2007). In the ChiangDao area of northern Thailand, black shale is exposed at the base of thesuccession, bearing Early Devonian graptolites (Kobayashi and Igo,1966; Jaeger et al., 1968, 1969). The succession from black shale tochert records the change in depositional environment related toopening of the Paleo-Tethys (Randon et al., 2006). In addition, thesimilar Devonian siliceous succession with graptolite-bearing blackshale has been reported in the western Yunnan area of south Chinaalong the convergence zone of the Paleo-Tethyan rocks (Feng and Liu,

Fig. 1. Geology of the Chiang Dao area in northern Thailand, and surrounding area. (A) Tectonic map of Thailand and the surrounding region. (B) Simplified geological map of theChiang Dao area. The map is based on the Geological Map of Thailand (1:1,000,000), published by Department of Mineral Resources (1999).

453H. Hara et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 297 (2010) 452–464

1993; Wang et al., 2003, 2006). The characteristic succession is animportant sediment for the understanding of the depositionalenvironment at the initial Paleo-Tethys.

The aim of this paper is to reconstruct the depositional environ-ment during the opening of the Paleo-Tethys, as recorded in theDevonian siliceous succession around the Chiang Dao area, northernThailand. The reconstruction is based on new lithological observationsand radiolarian fossil data, geochemical analyses, measurements oftotal organic carbon (TOC), and analyses of Th/U ratios. Geochemicalanalyses of siliceous rocks are useful in terms of understanding thedepositional setting and redox conditions (e.g., Adachi et al., 1986;Murray, 1994; Kametaka et al., 2005). In particular, analyses of TOCand Th/U are commonly employed to clarify redox conditions in thepaleo-ocean (e.g., Stein, 1986;Wignall and Twitchett, 1996; Algeo andMaynard, 2004). By combining information on the depositional settingand redox conditions for the Devonian siliceous succession, alongwithage control provided by radiolarian fossils, we reconstruct the openingand development of the Paleo-Tethys.

2. Geological outline of northern Thailand

A tectonic scheme has recently been proposed for northernThailand (Ueno and Hisada, 2001; Ueno, 2003; Sone and Metcalfe,

2008; Kamata et al., 2009), based on Paleozoic and Mesozoicbiostratigraphy and paleo-biogeography (e.g., foraminifers and radi-olarians), as well as correlations between northern Thailand andthe western Yunnan area of south China. In this scheme, northernThailand is divided into the following four geotectonic units (fromwest to east): the Sibumasu Block, the Inthanon Zone, the SukhothaiZone, and the Indochina Block (Fig. 1). The Devonian siliceoussuccession analyzed in the present study is located within theconvergence zone of Paleo-Tethyan rocks in the Inthanon Zone.

The Sibumasu Block, which is eastern part of Cimmerian continent,is characterized by a Gondwanan stratigraphy, including a LowerCarboniferous hiatus, Upper Carboniferous to Lower Permian glacio-genic diamictites with Gondwanan fauna and flora, and Middle–Upper Permian platform carbonates (Metcalfe, 1988, 2006; Ueno,2003).

The Inthanon Zone, originally proposed by Barr and Macdonald(1991), is characterized by Paleo-Tethyan oceanic rocks, pre-Devoni-an basement rocks, and Late Triassic and Early Jurassic S-typegranitoids and gneissic rock. The Paleo-Tethyan rocks consist ofpelagic Carboniferous–Permian seamount-type carbonate rocks (theDoi Chiang Dao Limestone) associated with basaltic rocks, MiddleDevonian–Middle Triassic radiolarian chert, and mélange-type rocksrelated to subduction of the Paleo-Tethys beneath the Indochina Block

454 H. Hara et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 297 (2010) 452–464

(Ueno and Hisada, 2001; Hara et al., 2009; Kamata et al., 2009; Uenoet al., 2010).

The pre-Devonian basement rocks consist of metamorphic rocksof unknown age, Cambrian sandstone, and Ordovician limestone,corresponding to the rocks of the Sibumasu Block. The Cambriansandstone and Ordovician limestone indicate that marginal sedimentsof the Sibumasu Block are imbricated with Paleo-Tethyan rocks in theInthanon Zone. The Inthanon Zone is interpreted to represent nappesof Paleo-Tethyan rocks thrust westward over a marginal part of theSibumasu Block (Caridroit et al., 1992; Ueno and Hisada, 2001).

The Sukhothai Zone, which largely corresponds to the SukhothaiZone of Barr and Macdonald (1991) and the Sukhothai fold belt ofBunopas (1981), is dominated by deformed Paleozoic–Mesozoicsedimentary rocks, volcanic rocks, and Early Permian to Triassic I-type granitoids. The Sukhothai Zone is considered to represent avolcanic arc that developed along the margin of the Indochina Block,related to subduction of the Paleo-Tethys (Ueno and Hisada, 2001;Sone and Metcalfe, 2008).

The Indochina Block is part of the South China or IndochinaSuperterrane (Metcalfe, 2000, 2006) and has remained within thepaleo-equatorial region since its Early Devonian breakaway fromGondwana. In eastern Thailand, Upper Paleozoic shallow-marinecarbonate rocks, containing highly diversified Tethyan faunas, arewidely distributed along the margin of the Indochina Block. The Nan–Uttaradit Suture, dividing the Sukhothai Zone and the IndochinaBlock, is interpreted as the remnant of a back-arc basin (Ueno andHisada, 2001; Sone and Metcalfe, 2008).

3. Lithology and mineralogy of the Devonian siliceous succession

The analyzed section (Fig. 2), located at the 106 km point on Route107 from Chiang Mai to Fang, has been investigated previously byWonganan and Caridroit (2005) and Randon et al. (2006), whodetermined the age of deposition based on radiolaria and conodonts.The section corresponds to Section 1 of Wonganan and Caridroit(2005) and the lower half of section A of Randon et al. (2006).

CD22

CD08CD09

CD23

30

30 85

graptolites

8060

50W(25)

8085

454030

50 30

60

NS

50W(40)

5085

5060

80

80

4075

5040

CD05

CD03

100 m

19° 34’ 51.1’’99° 06’ 13.3’’

siliceous shale

gray chert

R107

Locality of radiolarian fossils

CD03 Sample for geochemical analysis

S-type fold

CD01

To Chiang Dao

interbedded black shale and siliceous shale

Fig. 2. Route map of the studied section, showing sa

Based on the new field and microscopic observations, the siliceoussuccession is divided into the following five rock types (in increasingstratigraphic order): black shale, siliceous shale, tuffaceous chert, tuff,and chert. All the contacts in the succession are conformable. Therocks are faulted and folded. Field photographs of representativeoutcrops of each rock type are shown in Fig. 3.

The mineral compositions of the siliceous rocks were determinedby X-ray diffractometer (XRD) analysis using a RINT2000 housed atthe Geological Survey of Japan, Tsukuba, under operating conditionsof CuKα radiation at 40 kV and 100 mA, a step scan speed of 0.05° 2θ/s, using the≤2 μm clay fraction. The obtained XRD patterns are shownin Fig. 4, and representative photomicrographs of each rock are shownin Fig. 5.

The black shale is dark black in color, although locally yellow toyellowish brown on weathered surfaces, and is 10 m thick (Fig. 3A, B).The shale is composed mainly of illite/smectite and quartz, withcharacteristic pyrite (Fig. 4). Rhythmic laminations are commonlyobserved with intervals of several millimeters (Fig. 5A). Pyritecommonly occurs as framboidal aggregates of less than 0.01 mm indiameter (Fig. 5B). The organic matter in the shale is dominated bythose of terrigenous origin, with little contribution from marineorganisms to the organic matter assemblage, as revealed bytransmitted light observations on organic residues concentrated bytreatment with HCl and HF and subsequent heavy liquid separationusing ZnBr2. Most of the organic matter consists of opaque or semi-opaque grains that are generally rectangular in shape (Fig. 5C). Someof them exhibit mesh-like internal structures that are attributable tocellular structures of vascular land plants (Fig. 5C). Algal remains orchitinozoans were not observed in the organic residues. EarlyDevonian fossils (graptolites and brachiopods) within the blackshale indicate a Gedinnian (Lochkovian) to Emsian age (Kobayashiand Igo, 1966; Jaeger et al., 1968, 1969; Baum et al., 1970).

The siliceous shale is dark gray to black in color, 6 m thick (Fig. 3C),and consists of illite/smectite and quartz (Fig. 4). The shale is laminatedand contains claymineralswith a shape-preferred orientation, silt-sizeddetrital grains (Fig. 5D), and abundant radiolarian fossils (Fig. 5D, E)

Chert

Tuffaceous chert

Siliceous shale

Tuff

tuff with pumice

40

70

25

20

Black shale

CD14CD15

5060

2040

8070

3030

4570

gentle fold

S-type fold

7040

106 km point from Chaing Mai

7080W

(20)

7080

40507555

CD21

2050

irregular bedding

5060 10E

(35)1020

gentle fold

R107CD11

CD18green chert

interbedded tuffaceous chert and siliceous shale

green chert

CD13

19° 35’ 04.1’’99° 06’ 20.8’’

gentle fold

CD19

To Fang

CD20

CD17

mpling localities (see Fig. 1B for map location).

Fig. 3. Outcrop photographs of the Devonian siliceous succession. (A) Black shale exposed in a road cutting (samples CD08 and CD09). (B) Boulders of black shale bearing graptolites(CD09). (C) Folded siliceous shale (CD01). (D) Tuffaceous chert (CD19). (E) Pumice on bedding surface of tuff (CD13). (F) Chert (CD11). See Fig. 2 for outcrop localities.

455H. Hara et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 297 (2010) 452–464

that indicate a Middle Devonian age, as described in the followingsection.

The tuffaceous chert, which is light gray to light green in color and40 m thick (Fig. 3D), is the dominant lithology within the Devoniansiliceous succession. The chert is well bedded at a scale of 5–10 cmand is generally interbedded with siliceous shale. The chert containsillite/smectite and quartz, being similar in lithology to the siliceousshale (Fig. 4). Quartz and clay domains are recognized under themicroscope, as are radiolarian fossils (Fig. 5F). The quartz domain ischaracterized by cryptocrystalline to microcrystalline quartz, whilethe clay domain is laminated and consists of alternating layers ofquartz and clay minerals with thicknesses of less than 0.1 mm.Clay minerals in the clay domain have a prominent shape-preferredorientation.

The tuffaceous chert contains a 2-m-thick tuff layer (Fig. 2). Thetuff is acidic, light green in color, andweakly foliated. The tuff containsflattened pumice fragments (up to 2 cm in length) aligned parallel to

bedding (Fig. 3E). The tuff consists of illite/smectite, chlorite, andquartz, with the chlorite being a characteristic feature (Fig. 4).Microscopic observations reveal that the tuff is commonly graded andconsists of clay minerals with a prominent shape-preferred orienta-tion and silt-sized quartz grains, although without accessory mineralssuch as zircon (Fig. 5G). The tuff also contains large acicular and wavymica grains (mainly chlorite) of up to 0.05 mm in length (Fig. 5H).These grains were possibly derived from altered volcanic glass or ash.

The chert is well bedded, usually dark green to dark gray in color,and 6 m thick (Fig. 3F), consisting entirely of cryptocrystalline tomicrocrystalline quartz, with no terrigenous material but containingradiolarian fossils (Figs. 4, 5I). According to Kamata et al. (2009), thechert of the Chiang Dao area ranges in age from Late Devonian to EarlyCarboniferous, and from Permian to Triassic, and was deposited in apelagic environment, as indicated by its lithology. In the presentstudy, a Late Devonian radiolarian assemblage was obtained from thetuffaceous chert and chert, as described in the following section.

Siliceous shale (CD-18)

Black shale (CD-09)

Pyrite

Chert (CD-21)Qtz

Qtz

Qtz

10 4030202 (°)

Tuff (CD-23)

Illite+

ChlChl

Illite Illite

Illite/Sme

Tuffaceous chert (CD-14)

Illite/Sme

Illite/Sme

θ

Fig. 4. X-ray diffractometer patterns of the siliceous rocks. Qtz: quartz, Chl: chlorite, andSme: smectite.

456 H. Hara et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 297 (2010) 452–464

4. Radiolarian fauna and age constraints

Twenty-one samples (black shale, siliceous shale, tuff, tuffaceouschert, and chert) were collected from the study section (Fig. 2) withthe aim of determining the age of deposition based on radiolarianfauna. Moderately well-preserved radiolarians were recovered fromsiliceous shale (sample CD03), tuffaceous chert (CD22), and chert(CD11, 21).

A sample of dark gray to black siliceous shale (CD03), character-ized by abundant radiolarian shells and silt-sized detrital grains,yielded a restricted radiolarian fauna (Fig. 6) including Eoalbaillellalilaensis Feng and Liu, Stigmosphaerostylus hystricuosa (Aitchison),Stigmosphaerostylus profundisulcus (Aitchison), Trilonche tanheensisLuo, Aitchison, and Wang, Trilonche chiangdaoensis Wonganan andCaridroit, Trilonche elegans (Hinde), Trilonche echinata (Hinde), andTrilonche davidi (Hinde). Among these species, E. lilaensis is animportant age-diagnostic ceratoikiscid species that has a triangularframework with a conical lamellar shell. This species shows largeintraspecific variation in the shape and length of the lamellar shell(Feng and Liu, 1993; Wang et al., 2003). The present materials arevery similar to specimens having a shorter conical lamellar shell (e.g.,pl. 1, Figs. 4–6 in Wang et al., 2003). According to a detailed review ofthe age assigned to the “Eoalbaillella lilaensis fauna” (Wang et al.,

2003), the fauna is given a Middle Devonian (Givetian) age, based onthe biostratigraphy of the Lila, Huiku, and Shaijingpo sections in westand southwest Yunnan, China. Other entactiniid species have beenreported from many Middle–Upper Devonian localities worldwide,such as the Givetian in China (Luo et al., 2002) and the Frasnian inAustralia (Aitchison, 1993). Wonganan and Caridroit (2005) reporteda radiolarian assemblage including abundant Tlecerina sp. from thestratigraphically lowest sample (Th180) in the Section 1, probablycorresponding to a horizon close to CD03, and assigned this sample aLate Eifelian to Early Givetian age. However, without confirmation ofthe internal spicule (Furutani, 1983), these taxonomic and agedeterminations remain uncertain. Here, we consider it reasonable toassign a Givetian age to the fauna in sample CD03, placing emphasison the occurrence of Eoalbaillella.

An entactiniid-dominated radiolarian fauna were recognizedfrom light gray to green tuffaceous chert (CD22) and dark greento gray chert (CD11 and CD21). Among the analyzed samples,CD21 contains the best-preserved fossil assemblage (Fig. 6), fromwhich we identified Trilonche minax (Hinde), Trilonche vetusta(Hinde), T. elegans, Trilonche palimbola (Foreman), T. echinata,Trilonche sp. aff. “Heliosoma echinatum” of Hinde (1899), Helioentacti-nia sp., Palaeoscenidium cladophorum Deflandre and Ceratoikiscum sp.T. minax and other Trilonche species are the representative of theradiolarian fauna reported by Hinde (1899). The fauna described byHinde (1899) has been reinvestigated by Aitchison and Stratford(1997), and subsequently by Aitchison et al. (1999), who named it theT. minax assemblage, as found at several localities in the Gamilaroiterrane in eastern New South Wales, Australia.

Based on the species composition, the radiolarian fauna recognizedfrom tuffaceous chert (CD22) and chert (CD11 and CD21) in thepresent study clearly corresponds to the T. minax assemblage.Aitchison et al. (1999) assigned an age of latest Givetian to EarlyFrasnian to this assemblage, based on age constraints provided byconodonts in strata above and below the assemblage. Randon et al.(2006) reported late Frasnian conodonts from a chert bed located at~27 m from the base of chert in the measured section A, which isconsistent with the age of radiolarians identified in the present study.

In conclusion, the biostratigraphy of the studied section indicatesthat the siliceous succession from siliceous shale to tuffaceous chertand chert ranges in age fromMiddle Devonian (Givetian) to early LateDevonian (Frasnian), based on the ages assigned to the two faunasrecognized in the section.

5. Methods of geochemical analyses

Eleven samples collected from the Devonian siliceous succession ofthe Chiang Dao area were crushed to powder for geochemicalanalyses. The concentrations of 10 major elements were determinedby X-ray fluorescence (XRF) analysis of fused glass beads using aPhilips PW-1404 housed at the Geological Survey of Japan, Tsukuba.Total Fe content is reported as Fe2O3. Loss on ignition (LOI) wasmeasured by weighing the samples before and after 2 h of calcinationat 850 °C.

Concentrations of seven trace elements (V, Cr, Ni, Rb, Sr, Zr, andPb) were determined by analyses of fused glass beads by XRF (RigakuRIX3000) at Niigata University, Japan. Concentrations of three traceelements (Co, Zn, and Nb) and rare earth elements (REEs) wereanalyzed by inductively coupled plasma mass spectrometry (ICP-MS)using an Agilent 7500a housed at Niigata University, calibrated usingthe reference values for BHVO-1 (U.S. Geological Survey; Eggins et al.,1997). Variations in sensitivity during the analytical run werecorrected using an internal standard. Samples were prepared usinga combined acid digestion procedure (HCl and HF) and alkali fusion bydissolution with a combination of HF–NHO3 and HF–HCl at 150 °Cafter adding anhydrous Na2CO3. Analytical precision, as estimated by

Fig. 5. Photomicrographs of rocks of the Devonian siliceous succession. (A) Black shale (CD09) under plane polarized light. (B) Pyrite within black shale (CD09). Positive image underepi-illumination (left photo). Negative image (right photo). (C) Organic matter within black shale (CD08) under transmitted light. Cellular structures of vascular land plants areindicated by arrowhead. (D) Siliceous shale (CD03) under plane polarized light. Arrowheads indicate radiolarian fossils. (E) Close-up view of a radiolarian fossil within siliceous shale(CD03). (F) Tuffaceous chert (CD17) under plane polarized light. Arrowheads indicate radiolarian fossils. (G) Tuff (CD13) under plane polarized light. Arrow indicates gradedbedding. (H) Close-up view of acicular and wavy chlorite grains (arrowheads) (CD13). (I) Chert composed of cryptocrystalline to microcrystalline quartz (CD20), under planepolarized light. See Fig. 2 for sampling localities.

457H. Hara et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 297 (2010) 452–464

Fig. 6. Selected Middle and Late Devonian radiolarians from the siliceous rocks. (1–4) Eoalbaillella lilaensis Feng and Liu, sample CD03. (5) Stigmosphaerostylus hystricuosa(Aitchison), CD03. (6) Stigmosphaerostylus profundisulcus (Aitchison), CD03. (7) Trilonche tanheensis Luo, Aitchison, and Wang, CD03. (8) Trilonche chiangdaoensis Wonganan andCaridroit, CD03. (9, 14) Trilonche elegans (Hinde), (9) CD03, (14) CD21. (10, 16) Trilonche echinata (Hinde), (10) CD03, (16) CD21. (11) Trilonche davidi (Hinde), CD03. (12) Triloncheminax (Hinde), CD21. (13) Trilonche vetusta (Hinde), CD21. (15) Trilonche palimbola (Foreman), CD21. (17) Palaeoscenidium cladophorum Deflandre, CD21.

458 H. Hara et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 297 (2010) 452–464

relative deviation values using the geological reference material W-2(U.S. Geological Survey; Eggins et al., 1997), was less than 10%.

For determining the TOC content, and Th and U concentrations, 10samples were selected for analysis from among the samples analyzedfor major element concentrations. After the removal of carbonatesusing 1 N HCl in powdered samples, TOC contents were determinedusing a Thermo Finnigan Flash EA1112 housed at the Japan Agency forMarine-Earth Science and Technology (JAMSTEC), Yokosuka.

Concentrations of Th and U were analyzed by inductively coupledplasma quadrupole mass spectrometry (ICP-QMS) using an Agilent7500cs housed at JAMSTEC. An external calibration method wasapplied with an in-house standard solution. The signal instability ofthe ICP was corrected with a Bi internal standard. Samples weredissolved with a combination of HF and NHO3 at 120 °C. Based onrepeated measurements of Th and U concentrations in the referencematerial JCh-1 (Geological Survey of Japan; Imai et al., 1996),analytical precision and accuracy were better than 5%.

6. Results of geochemical analyses

The results of geochemical analyses are listed in Table 1, andstratigraphic variations in the compositional data are shown in Fig. 7.All of the samples from the analyzed succession have high SiO2

contents, in excess of 80% (Fig. 7). Al2O3 contents vary from 3.56 to7.21%, although the mean value for the tuff is 10.9% (Fig. 7). The tuffalso has the highest contents of TiO2, K2O, and MgO within thesiliceous succession.

A clear negative correlation was found between SiO2 and Al2O3

(Fig. 8). TiO2 and K2O contents all show a positive correlation withAl2O3 (Fig. 8). The black shale is characterized by relatively high P2O5

contents (mean value of 1.7%, Table 1) and the chert containsrelatively high MnO contents (mean value of 0.78%, Table 1). Theblack shale is characterized by high V, Cr, and Ni contents, while thetuff contains high Rb and Zr contents in excess of 100 ppm (Table 1).

Fig. 9 shows REE patterns normalized to chondrite. The REE patternsof the siliceous shale, tuffaceous chert, tuff, and chert are similar tothe pattern of NASC (North American Shale Composite; Gromet et al.,1984;McLennan, 1989),whereas theblack shale has lowconcentrationsof middle REEs (MREEs), with a minimum at Eu (CD08), and highconcentrations of REEs (CD09).

The cerium anomaly (Ce/Ce*) was estimated by the equation Ce/Ce*=2Cen/(Lan+Prn), where the subscript n indicates a NASC-normalized value (De Baar et al., 1985; Murray et al., 1992). Theobtained Ce anomalies range from 0.62 to 1.19, showing an increasefrom the black shale to tuffaceous chert and a small positive anomalyfor the chert, with an overall upward increasing trend within thesiliceous succession (Fig. 7).

TOC values for the two samples of black shale are 5.5% and 8.3%,representing the highest values in the succession (Fig. 8). Thesiliceous shale has TOC values of 0.19% and 0.57%. Values for thetuffaceous chert and tuff range from 0.02% to 0.06%, and the values forchert are 0.02%, representing the lowest values in the succession. TOCvalues decrease from the bottom (black shale) to the top (chert) of thesuccession (Fig. 7). The TOC in the chert is just 1% of that in the blackshale.

The obtained values of Th/U are 0.06–0.15 for the black shale,0.39–2.08 for the siliceous shale, 1.09–5.26 for the tuffaceous chert,2.08 for the tuff, and 3.70–5.26 for the chert, showing an upwardincreasing trend and a negative correlation with TOC values (Fig. 7).

7. Depositional setting and redox conditions inferred fromgeochemical data

All of the rocks in the siliceous succession, excluding the tuff,have an SiO2 content exceeding 85%, consistent with geochemicaldata published for chert (Adachi et al., 1986; Yamamoto, 1987). Thesiliceous rocks contained abundant radiolarian fossils, suggested thathigh SiO2 contents mostly originated from biogenic silica (Hori et al.,

Table 1Results of geochemical analyses of the Devonian siliceous succession.

Number CD03 CD05 CD08 CD09 CD11 CD13 CD14 CD15 CD18 CD21 CD23Lithology Sil. shale Tuff. chert Black shale Black shale Chert Tuff Tuff. chert Tuff. chert Sil. shale Chert Tuff

Major element (wt.%)SiO2 96.71 91.65 88.14 86.77 95.34 82.91 92.77 92.20 91.37 91.34 81.90TiO2 0.11 0.24 0.28 0.30 0.12 0.50 0.31 0.21 0.22 0.08 0.61Al2O3 3.77 6.79 7.20 7.21 3.87 10.59 6.61 4.12 6.20 3.56 11.24Fe2O3* 1.06 1.19 1.11 3.34 2.11 1.85 0.81 4.53 1.06 4.70 1.62MnO 0.01 0.01 0.00 0.01 0.34 0.01 0.02 0.01 0.00 1.22 0.01MgO 0.12 0.44 0.43 0.54 0.24 1.08 0.48 0.15 0.39 0.38 1.06CaO 0.06 0.04 0.03 0.06 0.04 0.04 0.04 0.04 0.04 0.03 0.05Na2O b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1K2O 0.55 1.47 1.17 1.28 0.73 3.01 1.60 0.65 1.32 0.87 3.04P2O5 0.03 0.04 2.90 0.55 0.03 0.05 0.04 0.06 0.17 0.02 0.05Total 102.42 101.87 101.25 100.08 102.81 100.03 102.68 101.97 100.78 102.22 99.58LOI (%) 1.30 2.10 9.70 15.40 1.60 3.70 1.80 1.40 2.90 2.00 3.40

Trace element (ppm)V 77.1 30.4 920.7 667.3 45.6 47.6 48.2 98.7 55.6 21.6 105.7Cr 27.6 15.1 328.7 214.4 3.8 33.2 16.8 17.7 22.0 2.5 54.9Co+ 78.9 26.8 9.7 17.1 64.2 3.0 29.5 39.6 27.6 43.8 3.0Ni n.d 8.9 102.2 348.4 6.8 7.8 7.8 n.d 17.5 19.9 18.4Zn+ 8.6 18.3 36.6 352.2 24.3 18.3 7.5 3.6 7.5 25.7 14.6Rb 23.3 65.0 51.3 58.3 34.6 138.8 69.5 32.0 57.1 36.4 128.2Sr 36.1 43.4 74.3 193.0 23.0 43.2 53.1 79.6 73.0 21.0 77.1Zr 28.0 49.2 73.8 97.7 24.8 97.7 55.1 43.6 45.5 19.7 107.9Nb+ 3.9 7.5 7.8 7.7 3.7 12.8 7.2 5.8 7.5 2.5 16.6Pb 9.4 12.7 16.8 31.0 6.9 16.3 21.3 33.6 8.9 2.3 23.6

REE (ppm)La 5.98 24.83 14.44 55.62 13.77 33.01 18.11 13.62 12.63 6.25 43.36Ce 10.56 46.78 17.14 91.88 30.60 64.85 38.32 32.38 19.67 15.82 76.23Pr 1.54 5.69 2.43 14.40 3.39 7.06 3.89 3.42 2.08 1.33 8.43Nd 5.85 21.23 9.95 58.11 11.24 26.84 14.98 15.04 8.21 5.14 31.48Sm 1.22 4.20 1.55 12.46 1.92 4.82 2.74 3.64 1.24 1.09 5.06Eu 0.23 0.76 0.28 3.00 0.37 0.84 0.50 0.75 0.21 0.21 0.89Gd 1.17 3.60 1.40 14.81 1.54 3.98 2.39 3.49 1.26 1.02 4.46Tb 0.18 0.57 0.24 2.46 0.22 0.59 0.34 0.50 0.23 0.15 0.69Dy 1.15 3.11 1.51 15.99 1.12 3.48 1.93 2.56 1.58 0.95 4.05Ho 0.28 0.57 0.39 3.65 0.22 0.70 0.40 0.46 0.39 0.21 0.90Er 0.80 1.55 1.20 10.85 0.57 2.03 1.21 1.33 1.14 0.55 2.65Tm 0.12 0.22 0.23 1.54 0.08 0.32 0.18 0.18 0.20 0.08 0.44Yb 0.82 1.30 1.85 9.65 0.57 2.03 1.14 1.10 1.11 0.54 2.60Lu 0.12 0.18 0.32 1.40 0.08 0.30 0.17 0.17 0.17 0.07 0.40Ce/Ce* 0.76 0.86 0.62 0.71 0.98 0.92 0.99 1.03 0.82 1.19 0.86Lan/Cen 1.29 1.21 1.92 1.38 1.03 1.16 1.08 0.96 1.47 0.90 1.30TOC (%) 0.19 0.02 5.5 8.3 0.02 n.a. 0.02 0.06 0.57 0.02 0.03Th 0.41 2.75 0.19 4.03 1.99 n.a. 1.9 2.09 2.23 1.23 7.79U 1.04 0.52 2.99 26.8 0.55 n.a. 0.81 1.91 1.07 0.23 2.08Th/U 0.39 5.26 0.06 0.15 3.70 n.a. 2.33 1.09 2.08 5.26 3.70

Fe2O3* is total iron as Fe2O3. LOI: weight loss on ignition. n.d.: not detected. n.a.: not analyzed. +: Co, Zn, Nb were analyzed by ICP-MS. Ce/Ce* is the Ce anomaly (De Baar et al., 1985).Lan and Cen indicate NASC-normalized values. TOC: Total organic carbon.

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2007). The SiO2 content of the black shale (mean value of 87.5%) ishigher than that of NASC (Gromet et al., 1984). The tuff has a meanSiO2 content of 82.4%, which is higher than the contents in acidic tuffwithin the Japanese Island Arc (Yamamoto et al., 1986) and the FishCanyon Tuff, USA (Bachman et al., 2002), which are generally 65–75%.These high SiO2 contents suggest that the black shale and tuff containbiogenic silica derived from a SiO2-rich oceanic environment.

Al2O3 and TiO2 contents within sedimentary rocks are supplied fromterrigenous material such as clay minerals, and are insensitive to theeffects of diagenesis (Tada and Iijima, 1992; Murray, 1994). Theoccurrence of Rb, Zr, Nb, and Th is also indicative of a supply ofterrestrial detritus (Jones and Manning, 1994; Hori et al., 2007). Withinthe siliceous succession analyzed in the present study, Al2O3 and Zrcontents are relatively high in the black shale and tuff, and the contentsof TiO2, K2O, Rb, and Th are relatively high in the tuff (Figs. 7, 8).Microscopic observations of the black shale and tuff, combined withmineral composition data, indicate that these rock typeswere deposited

within an SiO2-rich environment, into which terrigenous materialand ash (as clay minerals) were transported and deposited.

Murray (1994) proposed a discrimination diagram for the deposi-tional environment of chert, based on Al2O3, Fe2O3, TiO2, Lan, and Cen.When plotted in this diagram, the present siliceous succession rangesfrom a continental margin to a pelagic environment (Fig. 10). Inparticular, the black shale and tuff plot within the continental marginfield, whereas the chert plots within the pelagic field in the Al2O3/(Al2O3+Fe2O3) versus Fe2O3/TiO2, diagram. These findings suggestthat the black shale and tuff received a relatively high input ofterrigenous material.

TOC values show an upward decrease from the black shale (5.5%and 8.3%) to the chert within the siliceous succession (Fig. 7). Stein(1986) examined the relationship between TOC and sedimentationrate for Miocene–Recent and Upper Cretaceous sediments undervarious redox conditions. Based on the thickness of the black shale(10 m) in the present study and the interval over which it was

Fig. 7. Lithological column and stratigraphic variations in geochemistry within the Devonian siliceous succession. Geological ages are from the time scale of Gradstein et al. (2004).Ce/Ce* is the Ce anomaly (De Baar et al., 1985). TOC: Total organic carbon.

Fig. 8. Al2O3 versus SiO2, TiO2, K2O, Rb, Zr, Th, V, Cr, and U for the Devonian siliceous succession.

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1

10

100

1000

Ele

men

t/cho

ndrit

e

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

NASC

1

10

100

1000E

lem

ent/c

hond

rite

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

CD09

CD08

CD18

CD03

NASC

Black shale

Sili. shale

CD05

CD11

CD13

CD14CD15

CD21

CD23

Tuff. chert

Tuff

Chert

Fig. 9. Chondrite-normalized rare earth element (REE) patterns for the Devoniansiliceous succession. NASC: North American Shale Composite (Gromet et al., 1984;McLennan, 1989).

0 0.2 0.4 0.6 0.8 1.01

10

100

1000

Al2O3/(Al2O3+Fe2O3)

Fe 2O

3/T

iO2

Continental Margin

Pelagic

Ridge

ChertTuffTuffaceous chertSiliceous shaleBlack shale

Al2O3/(Al2O3+Fe2O3)

Lan/

Ce n

0 0.2 0.4 0.6 0.8 1.0

1

0

2

3

4

Ridge

Pelagic

Continental Margin

Fig. 10. Plots of Al2O3/(Al2O3+Fe2O3) versus Fe2O3/TiO2 and Lan/Cen showing data forthe Devonian siliceous succession. The fields of Ridge, Pelagic, and Continental Marginare from Murray (1994).

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deposited (Early Devonian), the sedimentation rate is estimated to be0.54 cm ky−1, similar to the rate reported previously for oceanic chert(e.g., Matsuda and Isozaki, 1991). The TOC values and sedimentationrate obtained for the black shale plot within the anoxic field in Stein's(1986) discrimination diagram.

V, Cr, Co, Ni, Cu, Zn, Mo, Pb, and U are known to be redox-sensitiveelements (Algeo and Maynard, 2004; Rimmer, 2004). These elementsare typically enriched in anoxic sediments. In the present study, theblack shale is enriched in V, Cr, Ni, Zn and U, and the contents of theseelements show an upward decreasing trend throughout the succes-sion (Figs. 7, 8).

Several indicators of redox conditions have been proposed inprevious geochemical studies (Hatch and Leventhal, 1992; Jones andManning, 1994; Rimmer, 2004). Here, we apply the Th/U ratio inseeking to understand the redox conditions of the environment inwhich the siliceous succession was deposited, as this ratio has beenused as a proxy for the redox conditions of the depositionalenvironment (Wignall and Twitchett, 1996; Kimura and Watanabe,2001). Th is unaffected by redox conditions and is supplied fromterrigenous material. In contrast, U is a redox-sensitive element foundin organic matter.

Wignall and Twitchett (1996) proposed that Th/U ratios vary from0 to 2 in anoxic environments, 2 to 7 in oxic environments, and are inexcess of 7 in strongly oxidizing environments. According to thisscheme, the black shale in the present study was deposited underanoxic conditions (Th/U ratios of 0.06 and 0.15), while the chert wasdeposited under oxic conditions (Th/U ratios of 3.70 and 5.26). TheTh/U ratios indicate a gradual change from anoxic to oxic conditionsfrom the black shale to the chert, showing no influence by theterrigenous material within the tuff (Fig. 7).

The cerium anomaly (Ce/Ce*) in ocean sediments is controlled byseveral factors, including redox conditions (De Baar et al., 1985;Bellanca et al., 1997; Holser, 1997; Pattan et al., 2005), depositionalsetting (Matsumoto et al., 1988; Murray et al., 1990, 1991), sedimen-tation rate (Sholkovitz et al., 1994), and variation of the Ce contentwithin the ocean (Thomson et al., 1984). Within the siliceoussuccession analyzed in the present study, Ce/Ce* values show anupward increase (toward the chert), consistent with the obtained Th/Uratios as an indicator of redox conditions (Fig. 7). In the case of ananoxic ocean, seawater generally shows a positive anomaly due to theelution of Ce into the ocean, whereas bottom sediment shows no suchanomaly. In contrast, the sediment in an oxic ocean shows a positiveanomaly due to the precipitation of Ce, and seawater is depleted in Ce(Holser, 1997). The trend in Ce/Ce* values observed within the presentsiliceous succession supports the trend in redox conditions inferredfrom TOC values and Th/U ratios. Together, these redox indicatorssuggest a gradual upward (i.e., toward the chert) change fromanoxic tooxic conditions within the succession.

8. Opening of the Paleo-Tethys

Here, the opening of the Paleo-Tethys was reconstructed, based onthe depositional setting and redox conditions of the Devoniansiliceous succession in the Chiang Dao area of northern Thailand(Fig. 11). In the Early Devonian (Lochkovian–Emsian), graptolite-bearing black shale was deposited in the initial Paleo-Tethys upon acontinental margin between the Sibumasu Block and the Indochina–North China blocks. The Paleo-Tethys at this time was a closed, anoxicbasin (Fig. 11-1), as indicated by the high TOC values and low Th/Uratios in the black shale.

The black shale also contains fine-grained framboidal pyrite(Fig. 3B). The occurrence of pyrite in organic carbon-rich sedimenthas been reported from various depositional settings (Berner andRaiswell, 1983; Tada and Iijima, 1992; Kuroda et al., 2005; Wignallet al., 2005), suggesting that an enhanced flux of organic matterresulted in the enhanced reduction of microbial sulfate in anoxic

Fig. 11. Schematic model of the opening of the Paleo-Tethys, showing the depositional setting and redox conditions. Right-hand panels show a paleogeographic reconstruction of thePaleo-Tethys during the Devonian, modified from Metcalfe (2006). Left-hand panels show cross-sections (along the red line in the right-hand panels) through the Paleo-Tethys,showing the depositional setting, based on geochemical data and redox indicators. See the text for details.

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parts of the water column and sediments (Berner and Raiswell,1983). The morphology of pyrite within the black shale of the presentstudy indicates anoxic conditions within the initial Paleo-Tethys,although sulfur isotope data are required to clarify this interpretation.The rhythmic laminations observed in the black shale (Fig. 5A)indicate very low biogenetic activity, with no bioturbation, alsosuggesting that the oceanic floor was anoxic at the time the blackshale was deposited (Droser and Bottjer, 1988; Zhuravlev and Wood,1996).

The black shale is also characterized by the deposition of organicmatter derived from both plant material and a marine biogenic sourcesuch as graptolites. The anoxic conditions in the initial Paleo-Tethyswere probably caused by the input of terrestrial plant material andproductivity in the seawater.

The black shale was conformably overlain by siliceous shale duringthe Middle Devonian (Givetian), when the redox conditions changedfrom anoxic to suboxic (Fig. 11-2). The black shale and siliceous shale

are interbedded, indicating that the siliceous succession was depos-ited continuously during the Early–Middle Devonian, even thoughno fossils of Eifelian age have been reported from these rocks. Duringthe Early–Middle Devonian, the Paleo-Tethys developed as a small,closed, anaerobic oceanic basin located around the continentalmargin, from where terrigenous material (clay and plant material)was supplied.

Subsequently, during the widening of the Paleo-Tethys in thelatest Middle to early Late Devonian (latest Givetian to EarlyFrasnian), tuffaceous chert was deposited under oxic conditions (asindicated by low TOC values and high Th/U ratios), suggesting that ashand pumice were supplied from the continent into an open sea rich inbiogenic silica (Fig. 11-3). The ash was probably derived from felsicvolcanoes that formed along a rift zone associated with the openingof the Paleo-Tethys.

The mixing of mafic and felsic magmas is observed along ancientand modern rift zones, such as the Androy massif, which was located

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within the rift zone between Madagascar and India during the LateCretaceous (Mahoney et al., 2008); the Askja central volcano,within an active rift zone in Iceland (Sigurdsson and Sparks, 1981);the Alid Volcanic Center in Eritrea (Lowenstern et al., 2006); andthe Great Olkaria Volcanic Complex in Kenya (Macdonald et al.,2008). However, no such volcanoes have been reported from theSibumasu and Indochina blocks in northern Thailand during theDevonian. It is possible that such a Devonian volcano existed buthas been eroded away, with the sole remnant being the tuffdeposited in the Paleo-Tethys. The volcanic activity representedby the tuff probably occurred around the Middle and UpperDevonian boundary, as the initial chert deposition occurredmainly in the Upper Devonian, indicating the same initial depositionage for the tuffaceous chert and chert, as indicated by radiolarianfossils.

During the Late Devonian, pelagic chert was widespread withinthe oxic deep ocean of the Paleo-Tethys (Fig. 11-4). The Paleo-Tethyscontinued to develop during the Late Devonian to Early Triassic,culminating in subduction beneath the Indochina Block (Hara et al.,2009).

9. Summary and conclusions

Based on the geology and geochemistry of the Devonian siliceoussuccession in the Chiang Dao area, northern Thailand, the depositionalsetting and redox conditions were reconstructed at the time ofthe opening of the Paleo-Tethys. Based on new data, the siliceoussuccession is subdivided into the following five rock types (inincreasing stratigraphic order): black shale, siliceous shale, tuffaceouschert, tuff, and chert. The main findings of the study are summarizedas follows.

1) The first occurrence of a Middle Devonian (Givetian) radiolarianassemblage is reported from siliceous shale, characterized byEoalbaillella lilaensis. In addition, the Trilonche minax assemblage ofradiolarian fossils, indicating a latest Middle to early Late Devonian(latest Givetian to Early Frasnian) age, was obtained fromtuffaceous chert and chert.

2) The Devonian siliceous succession in northern Thailand wasdeposited in continental margin to pelagic environments betweenthe Sibumasu Block and the Indochina–North China blocks.Based on microscopic observations and analyses of land-derivedelements (Al2O3, TiO2, Rb, Zr, and Th) and mineral compositiondata, we infer that the siliceous succession (except for the chert)was deposited in an SiO2-rich environment into which terrigenousmaterial and volcanic ash were transported from the adjacentcontinent. The black shale and tuff contain a large terrigenousinput, whereas the chert contains no terrigenous material.

3) Data on TOC and Th/U ratios, which are indicators of redoxconditions, indicate a gradual change from anoxic to oxic oceanicconditions between the black shale and chert, showing noinfluence by terrigenous material in the tuff. This interpretationis supported by Ce anomaly (Ce/Ce*) data, and V and Cr contents.

4) During the Early Devonian, graptolite-bearing organic black shalewas deposited in the initial Paleo-Tethys under anoxic conditions.Siliceous shale was conformably deposited over the black shaleduring the Middle Devonian, marking a shift in redox conditionsfrom anoxic to suboxic. During the Early–Middle Devonian, thePaleo-Tethys developed as a small, closed anoxic–suboxic oceanicbasin located around the continental margin. The start of thewidening of the Paleo-Tethys was marked by volcanic activityaround the Middle and Upper Devonian boundary. Tuffaceouschert was deposited under oxic conditions, suggesting that ash andpumice were derived from the continent. After the Late Devonian,the Paleo-Tethys developed as a deep, broad ocean, in whichpelagic chert was deposited.

Acknowledgements

We would like to thank Professor. K. Ueno and Dr. Y. Kamata fortheir valuable comments on the geology and tectonics of Thailand;Dr. Y. Ishizuka for suggestions regarding the description of tuff andXRF analyses; Ms. R. Nohara, Dr. Y. Kurihara, and Dr. N. Kimurafor their support with ICP-MS and XRD analyses; and Mr. A. Owada,Mr. T. Sato, and Mr. K. Fukuda for their expertise in preparing thinsections. We thank Drs. N.O. Ogawa and N. Ohkouchi for technicalsupports with organic carbon analysis. Thanks are also due to theeditor Professor F. Surlyk, Professor I. Metcalfe and an anonymousreviewer for their constructive and valuable comments of themanuscript.

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