Microsoft word - 18-kamata et al 2.doc

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泰国北部 Inthanon 带外来硅质岩块的
中、晚二叠世放射虫：对与古特提斯消减 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, Ibaraki 305-8567, Japan; 3) Department of Earth System Science, Faculty of Science, Fukuoka University, Fukuoka 814-0180, Japan; 4) Bureau of Geological Survey, DMR, Bangkok 10400, Thailand; 5) Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand The Inthanon Zone of Northern Thailand, origi- fabrics and metamorphic temperature in the Inthanon nally proposed by Barr and Macdonald (1991), is Zone, and clarified that accretionary complexes of the characterized by the occurrence of Paleo-Tethyan pe- zone was formed at shallow burial levels within the lagic sediments including Carboniferous–Permian accretionary prism. Moreover, they suggested that seamount-type carbonate associated with oceanic ba- kinematics shown by asymmetric fabrics in the mé- saltic rocks and Middle Devonian–Middle Triassic lange indicates northward subduction of the Pa- radiolarian chert (Ueno, 1999; Ueno and Hisada, 2001; leo-Tethys beneath the Indochina Continental Block. Ueno and Charoentitirat, 2011). These pelagic rocks As mentioned above, it is important to clarify have been mainly studied from the viewpoint of bio- geological ages of both blocks and matrix in mélange stratigraphy to clarify the duration of their deposition. to understand its formation process. In this paper, we These studies concluded the Paleo-Tethys to be a vast describe the occurrence of some mélange outcrops in ocean basin once existed between the Indochina and the Inthanon Zone, Northern Thailand and report the geological age of chert blocks based on radiolarian biostratigraphy. Detailed lithology and depositional In the accretionary complex, these pelagic rocks age of matrix of the outcrops will be described by Hara often occur as allochthonous blocks within a muddy or siliciclastic matrix. Mélange represented by We studied chert blocks in two exposures block-in-matrix texture is one of major products in (BPK-14 and BMS-05) that exhibit typical association with the subduction of the oceanic litho- block-in-matrix occurrence, located in the Inthanon sphere. The mélange fabrics provide crucial evidence for understanding concrete accretionary process. For example, asymmetrical shear fabrics of mélange show Location: Along Highway R1226, northeast of the shear direction along a décollement and subducting Mae Hong Son, Northern Thailand, coordinates: direction of the oceanic plate. Geological ages of blocks and matrix in mélange also supply important Occurrence and lithology: A lense-shaped, bed- information. Age of matrix gives temporal constraint ded black chert block embedded within mixed rocks of of mélange formation, while the depositional age of sheared mudstone and sandstone. The matrix probably blocks provides information of the subducted oceanic originates from alternation of shale and sandstone. The plate. Recently, Hara et al. (2009) examined mélange chert block is about 100 cm in size in the long axis. * Corresponding author. E-mail: yoshi_kamata@geol.tsukuba.ac.jp. Yoshihito KAMATA et al.: Middle and Late Permian Radiolarians from Allochthonous Chert Blocks… This chert clearly shows stratification ranging from a sandstone and shale. The chert is well-bedded (3~5cm few cm to 10 cm and looks glassy in fresh surface. thick) and black in color. Some part of the block ex- Under the microscope, it consists of densely packed hibits chevron fold as well as its hinge parts. Lithology radiolarian tests within microcrystalline quartz with of this chert is very similar to that of BPK-14 and thus, clay mineral matrix. The lithology of this chert cor- it is also clearly included in the Type 1 plagic radio- responds to that of the Type 1 chert by Kamata et al. (2009), representing pelagic radiolarian chert. This Depositional age of chert: Radiolarian fauna from outcrop has illustrated in Kamata et al. (2009) and this chert block is represented by Follicucullus scho- lasticus Ormiston and Babcock, F. cf. scholasticus, F. Depositional age of chert: The chert block yields ventricosus Ormiston and Babcock, F. dilatatus moderately preserved radiolarians. The Albaillellaria Rudenko, and some latentifistularians. These Follicu- such as Albaillella excelsa Ishiga, Kimoto and Imoto, cullus species are diagnostic in the F. scholasticus–F. A. lauta Kuwahara, A. flexa Kuwahara, A. angusta, ventricosus Zone (Caridroit in De Wever et al., 2001), Kuwahara, Neoalbaillela sp. and the Latentifistularia which indicates the late Guadalupian (Capitanian) of including Latentifistula? sp., Ishigaum sp. and Racid- the Middle Permian, and reported worldwide. itor sp. are obtained. These albaillellarians have been Unfortunately, sedimentary age of mélange ma- documented from Upper Permian radiolarian chert trix has yet been clarified neither from these localities successions in Southwest Japan (Kuwahara and Sa- nor elsewhere in the Inthanon Zone. Generally in ac- kamoto, 1992; Kuwahara et al., 1998; Kuwahara, cretionary complexes, however, muddy matrix is 1999), and the Albaillella species identified herein younger than any oceanic allochthonous blocks such have ranges from the upper part of the Neoalbaillella as pelagic chert and limestone. Thus, it can be con- ornithoformis to the lower part of Neoalbaillella op- cluded that the siliciclastic matrices in the studied tima assemblage zones (Kuwahara et al., 1998; Ku- outcrops are at least younger than the Middle Permian wahara, 1999). Thus, the radiolarian assemblage from and Late Permian, respectively. Recently, Hara et al. this chert block is referable to the Changhsingian (Late (2009) demonstrated that the deformation style of mé- lange matrix in the Inthanon Zone is commonly of hydrofracturing of sandstone associated with mud in- Location: Along Highway R1226, northeast of jection, suggesting that the mélange in this zone was Mae Hong Son, Northern Thailand, coordinates: highly probably formed before the siliciclastic matrix had consolidated. Therefore, we conclude that the mé- Occurrence and lithology: A rectangular-shaped lange formation of the studied outcrops is probably chert block within siliciclastic matrix consisting of very shortly after the sedimentation of matrices
Key words: Northern Thailand; Mélange; Radiolarian; Permo-Carboniferous
References:
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