泰國北部Inthanon帶外來硅質(zhì)巖塊的中、晚二疊世放射蟲：對與古特提斯消減有關(guān)的混雜巖形成時代的約束

泰國北部Inthanon帶外來硅質(zhì)巖塊的中、晚二疊世放射蟲：對與古特提斯消減有關(guān)的混雜巖形成時代的約束

Middle and Late Permian Radiolarians from Allochthonous Chert Blocks in the Inthanon Zone, Northern Thailand: Constraints for the Formation Age of Mélange Fabric Related to Paleo-Tethys Subduction

Yoshihito KAMATA1)*, Hidetoshi HARA2), Katsumi UENO3), Apsorn SARDSUD4),Thasinee CHAROENTITIRAT5), Punya CHARUSIRI5), Ken-ichiro HISADA1)

1)Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki, Japan,;

2)Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki305-8567, Japan;

3)Department of Earth System Science, Faculty of Science, Fukuoka University, Fukuoka814-0180,Japan;

4)Bureau of Geological Survey, DMR, Bangkok10400,Thailand;

5)Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok10330,Thailand

* Corresponding author. E-mail: yoshi_kamata@geol.tsukuba.ac.jp.

The Inthanon Zone of Northern Thailand, originally proposed by Barr and Macdonald (1991), is characterized by the occurrence of Paleo-Tethyan pelagic sediments including Carboniferous–Permian seamount-type carbonate associated with oceanic basaltic rocks and Middle Devonian–Middle Triassic radiolarian chert (Ueno, 1999; Ueno and Hisada, 2001;Ueno and Charoentitirat, 2011). These pelagic rocks have been mainly studied from the viewpoint of biostratigraphy to clarify the duration of their deposition.These studies concluded the Paleo-Tethys to be a vast ocean basin once existed between the Indochina and Sibumasu continental blocks during Devonian–Triassic times.

In the accretionary complex, these pelagic rocks often occur as allochthonous blocks within a muddy or siliciclastic matrix. Mélange represented by block-in-matrix texture is one of major products in association with the subduction of the oceanic lithosphere. The mélange fabrics provide crucial evidence for understanding concrete accretionary process. For example, asymmetrical shear fabrics of mélange show the shear direction along a décollement and subducting direction of the oceanic plate. Geological ages of blocks and matrix in mélange also supply important information. Age of matrix gives temporal constraint of mélange formation, while the depositional age of blocks provides information of the subducted oceanic plate. Recently, Hara et al. (2009) examined mélange fabrics and metamorphic temperature in the Inthanon Zone, and clarified that accretionary complexes of the zone was formed at shallow burial levels within the accretionary prism. Moreover, they suggested that kinematics shown by asymmetric fabrics in the mélange indicates northward subduction of the Paleo-Tethys beneath the Indochina Continental Block.

As mentioned above, it is important to clarify geological ages of both blocks and matrix in mélange to understand its formation process. In this paper, we describe the occurrence of some mélange outcrops in the Inthanon Zone, Northern Thailand and report the geological age of chert blocks based on radiolarian biostratigraphy. Detailed lithology and depositional age of matrix of the outcrops will be described by Hara et al. (this issue).

We studied chert blocks in two exposures(BPK-14 and BMS-05) that exhibit typical block-in-matrix occurrence, located in the Inthanon Zone, Northern Thailand.

(a) BPK-14

Location: Along Highway R1226, northeast of Mae Hong Son, Northern Thailand, coordinates:19?38’54”N, 98?13’28”E.

Occurrence and lithology: A lense-shaped, bedded black chert block embedded within mixed rocks of sheared mudstone and sandstone. The matrix probably originates from alternation of shale and sandstone. The chert block is about 100 cm in size in the long axis.This chert clearly shows stratification ranging from a few cm to 10 cm and looks glassy in fresh surface.Under the microscope, it consists of densely packed radiolarian tests within microcrystalline quartz with clay mineral matrix. The lithology of this chert corresponds to that of the Type 1 chert by Kamata et al.(2009), representing pelagic radiolarian chert. This outcrop has illustrated in Kamata et al. (2009) and Hara et al. (2009).

Depositional age of chert: The chert block yields moderately preserved radiolarians. The Albaillellaria such asAlbaillella excelsaIshiga, Kimoto and Imoto,A. lautaKuwahara,A. flexaKuwahara,A. angusta,Kuwahara,Neoalbaillelasp. and the Latentifistularia includingLatentifistula? sp.,Ishigaumsp. andRaciditorsp. are obtained. These albaillellarians have been documented from Upper Permian radiolarian chert successions in Southwest Japan (Kuwahara and Sakamoto, 1992; Kuwahara et al., 1998; Kuwahara,1999), and theAlbaillellaspecies identified herein have ranges from the upper part of theNeoalbaillella ornithoformisto the lower part ofNeoalbaillella optimaassemblage zones (Kuwahara et al., 1998; Kuwahara, 1999). Thus, the radiolarian assemblage from this chert block is referable to the Changhsingian (Late Permian).

(b) BMS-05

Location: Along Highway R1226, northeast of Mae Hong Son, Northern Thailand, coordinates:19?32’07”N, 98?06’38”E.

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Occurrence and lithology: A rectangular-shaped chert block within siliciclastic matrix consisting of sandstone and shale. The chert is well-bedded (3～5cm thick) and black in color. Some part of the block exhibits chevron fold as well as its hinge parts. Lithology of this chert is very similar to that of BPK-14 and thus,it is also clearly included in the Type 1 plagic radiolariam chert of Kamata et al. (2009).

Depositional age of chert: Radiolarian fauna from this chert block is represented byFollicucullus scholasticusOrmiston and Babcock,F.cf.scholasticus,F.ventricosusOrmiston and Babcock,F. dilatatusRudenko, and some latentifistularians. TheseFollicucullusspecies are diagnostic in theF. scholasticus–F.ventricosusZone (Caridroit in De Wever et al., 2001),which indicates the late Guadalupian (Capitanian) of the Middle Permian, and reported worldwide.

Unfortunately, sedimentary age of mélange matrix has yet been clarified neither from these localities nor elsewhere in the Inthanon Zone. Generally in accretionary complexes, however, muddy matrix is younger than any oceanic allochthonous blocks such as pelagic chert and limestone. Thus, it can be concluded that the siliciclastic matrices in the studied outcrops are at least younger than the Middle Permian and Late Permian, respectively. Recently, Hara et al.(2009) demonstrated that the deformation style of mélange matrix in the Inthanon Zone is commonly of hydrofracturing of sandstone associated with mud injection, suggesting that the mélange in this zone was highly probably formed before the siliciclastic matrix had consolidated. Therefore, we conclude that the mélange formation of the studied outcrops is probably very shortly after the sedimentation of matrices

Northern Thailand; Mélange; Radiolarian; Permo-Carboniferous

10.3975/cagsb.2012.s1.18