184 K. Yonemura, Y. Osanai, N. Nakano, T. Adachi, P. Charusiri and Tun Naing ZawJournal of Mineralogical and Petrological Sciences, Volume 108, page 184─188, 2013

doi:10.2465/jmps.121019aK. Yonemura, yonemura-kazuhiro@jogmec.go.jp Corresponding

authorY. Osanai, osanai@scs.kyushu-u.ac.jp

LETTER

EPMA U-Th-Pb monazite dating of metamorphic rocks from the Mogok Metamorphic Belt, central Myanmar

Kazuhiro Yonemura*,§, Yasuhito Osanai*, Nobuhiko Nakano*, Tatsuro Adachi*, Punya Charusiri** and Tun Naing Zaw***

*Division of Earth Sciences, Faculty of Social and Cultural Studies, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

**Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand***Geology Department, Yangon University, Yangon, Myanmar

§Present address: Japan Oil, Gas and Metals National Corporation

The Mogok Metamorphic Belt (MMB), situated in central Myanmar, contains an assemblage of high-grade metamorphic rocks believed to have been formed during a regional Early Eocene-Oligocene metamorphic event. We newly found Grt-Opx granulite in the Mogok area, which was formed under pressure-temperature (P-T ) conditions estimated as being 6.5-8.7 kbar and 800-950 °C. Based on the results of EPMA U-Th-Pb Mnz dating, the central MMB, which runs through the Mogok, Mandalay, and Meikthila areas, is characterized by an Eocene to Oligocene deformation and fluid infiltration event. In addition, several Mnz grains from the Meikthila and Mandalay areas record ages of the Late Triassic (~ 200 Ma) and Cretaceous (~ 80 Ma and 110 Ma) geologic events.

Keywords: Garnet-orthopyroxene granulite, EPMA U-Th-Pb monazite dating, Mogok Metamorphic Belt, Myanmar

INTRODUCTION

A vast tract of the continental crust of Southeast Asia is considered to have been formed through two important Phanerozoic tectonic events: an initial Permian-Triassic accretionary stage involving the subduction/collision/ac-cretion of a series of small microcontinents (e.g., Metcal-fe, 1996; Osanai et al., 2008), and the subsequent conti-nent-continent collision between India and Asia that began in the Early Eocene (e.g., Dewey et al., 1989). In the western part of Southeast Asia, the Mogok Metamor-phic Belt (MMB) occurs as a narrow (~ 40 km wide) and elongated (~ 1500 km long) belt of metamorphic rocks (e.g., Searle et al., 2007), lying parallel to the western margin of the Sibumasu Block, where it is juxtaposed with the West Burma Block (Fig. 1a) (e.g., Searle and Ba Than Haq, 1964). The formation of the MMB is consid-ered to be related to the Indian-Asian continental collision

events (Barley et al., 2003; Searle et al., 2007). Neverthe-less, the MMB has been also proposed that it is linked to the Inthanon Zone (Searle et al., 2007), which was formed by the continental collision between the Sibumasu and In-thanon blocks during the Late Triassic (e.g., Sone and Metcalfe, 2008).

In Southeast Asia, it is considered that several granu-lites were generated by continental collision (e.g., Trans Vietnam Orogenic belt; Osanai et al., 2008). Therefore, it is also possible that the granulites from the MMB were formed by such a collisional event. Detailed petrological and geochronological studies of the MMB are therefore important in an analysis of the growth process of the con-tinental crust of Southeast Asia. In this paper, we report several petrological features of a newly-found Grt-Opx granulite from the Mogok area (Fig. 1b) and constrain the timing of metamorphism of the MMB using age-dating from granulite and other high-grade metamorphic rocks. In addition, we briefly discuss the geological and tectonic implications of these new data. All mineral abbreviations used in this paper are after Whitney and Evans (2010).

185Metamorphic rocks from central Myanmar: monazite dating

GEOLOGICAL SETTING AND SAMPLE DESCRIPTIONS

This study focuses on the metamorphic rocks from the central part of the MMB, which includes the Mogok area (Fig. 1b), the Mandalay area, and the Meikthila area (Fig. 1c). The bedrock in the Mogok area consists mainly of Bt gneiss, along with khondalite, Grt-Sil gneiss (Iyer, 1953) and Grt-Opx granulite, in addition to localized intercala-tions of coarse-grained calc-silicate rocks that occur con-cordantly with the gneissic foliation, and which character-istically contain Spl and Crn (Iyer, 1953; Mitchell et al., 2007). Rock types in the vicinity of the Mandalay area in-

clude marble and calc-silicate rocks, which for the most part occur only in that region, and also there contains the minor amounts of paragneisses (e.g., Grt-Sil-Bt gneiss, Bt gneiss, And-Bt-Ms schist), orthogneisses, schists, and migmatites (Mitchell et al., 2007; Searle et al., 2007). Metamorphic rocks in the Meikthila area consist mainly of schist, quartzite, and augen gneiss, with small amounts of marble; and some exhibit pronounced mylonitization. Granitoids also intrude into the lithologies in the study area (Mitchell et al., 2007).

For age determination, the following were examined in detail: pelitic or felsic metamorphic rocks from each field area, including Grt-Opx granulites from the Mogok area, Grt- and Sil-bearing gneisses from the Mandalay area; and Bt gneisses from the Mandalay and Meikthila areas. The mineral chemistry of the Grt-Opx granulite was determined using a JEOL JXA-8530F hyperprobe housed at Kyushu University, Japan. For each analysis, the analytical conditions were set at an accelerating volt-age of 15 kV, a probe current of 12 nA and a beam diame-ter of 2 µm. Representative chemical compositions are listed in Appendix 1 (Appendix 1 is available online from http://japanlinkcenter.org/DN/JST.JSTAGE/jmps/ 121019a).

Grt-Opx granulite (samples 1177 and 1158). The main mineral assemblage is Grt, Opx, Kfs, Pl, and Qz with minor amounts of Mnz, Zrn, and Ilm. Grt and Opx grains exhibit granoblastic textures (Fig. 2a). Ep and Chl grains occur as secondary phases. The Chl replace rim of the Opx. Some light-rare earth elements-carbonate-fluo-ride (LREE-carbonate-fluoride: bastnäsite?) and Py occu-py several cracks in Ep and Qz, respectively (Fig. 2b). Coarse-grained (up to 1 cm) Grt grains are in contact with Opx, Pl, and Qz. In sample 1177, Grt shows a homoge-neous chemical composition (Alm56−61Prp27−29 Sps3Grs7−13), Opx grains are anhedral and have relatively low and ho-mogeneous Al contents (XMg = 0.53-0.56, Al = 0.11-0.17 p.f.u.), and the chemical composition of Pl is also homo-geneous (An34−37). Based on Grt-Opx geothermometers (Herley, 1984; Sen and Bhattacharya, 1984) and Grt-Opx-Pl-Qz geobarometers (Bohlen et al., 1983; Newton and Perkins, 1982), the estimated peak metamorphic P-T conditions of the Grt-Opx granulite are 6.5-8.7 kbar and 800-950 °C.

Grt- and Sil-bearing gneiss (samples 2204B, 2203A, and 2308A). The main mineral assemblage in this rock type is Grt, Bt, Sil, Pl, and Qz (Fig. 2c), with minor amounts of Mnz, Zrn, Ap, Tur, Ilm, and Gr. And and Ms are also observed only in specimen 2308A. Grt porphy-roblasts in this rock are medium-grained, (up to ~ 0.5 mm), and contain inclusions of Pl and Qz. Sil occurs as fine-grained fibrolite that defines the gneissosity with Bt

MALAYSIA

CAMBODIA

Fig. 1c

Fig. 1b

InthanonZone

MogokMetamorphicBelt

NujiangZone

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MYANMAR

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VIETNAM

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South China blockIndochina blockInthanon blockSibumasu blockWest Burma blockTrans VietnamOrogenic Belt

Collision boundary

Metamorphic rocks

JAPAN

AUSTRALIAEAST TIMOR

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GUINEA

PALAU

Mapped area

4 km

96º35’E

23º00’N

22º55’N

Yeni chaung

Nam lan

g

MOGOK

(b) 96º30’E

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Ultramafic rocks

Granititoid rocks

Syenites

Marble and Calc-silicates

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96º00’E

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ing

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t

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g Fa

ult

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1177

1158

2602A

2203A

2302A

2308A

2601B

2204B

Sampling points

Figure 1. Tectonic domain map and geological maps of the study area. (a) Tectonic domain map showing the distribution of micro-continental blocks and belts of metamorphic rocks in Southeast Asia, after Osanai et al. (2010). (b) Geological map of the Mogok area, after Mitchell et al. (2007). (c) Geological map of the Man-dalay-Meikthila area, after Mitchell et al. (2007).

186 K. Yonemura, Y. Osanai, N. Nakano, T. Adachi, P. Charusiri and Tun Naing Zaw

(Fig. 2d). These oriented Bt, Ms, and Sil grains surround the And and Grt porphyroblasts (Fig. 2d).

Bt gneiss (samples: 2302 from the Mandalay area, 2601B and 2602A from the Meikthila area). These rocks consist mainly of Bt, Pl, and Qz, with minor amounts of Zrn, Mnz, Ap, and opaque minerals (Fig. 2e). Bt and Sil are also observed only in samples 2602A and 2601B, respectively. Chl and Ms also occur only as sec-ondary phases. Sample 2602A has a weak mylonitic tex-ture (Fig. 2f).

EPMA U-Th-Pb MONAZITE DATING

EPMA U-Th-Pb Mnz dating was performed using the same JEOL JXA-8530F hyperprobe as described above, and the analytical procedures employed in this study closely followed those outlined in Adachi et al. (2012). Determination of the chemical composition of Mnz in this study was carried out using thin sections of specimens, with an accelerating voltage of 15 kV, a probe current of 50 nA, and a 1 µm beam diameter. The results are listed in Appendix 2 (Appendix 2 is available online from http://japanlinkcenter.org/DN/JST.JSTAGE/jmps/121019a). PbO contents in the analyzed Mnz are low (0.003-0.108

wt%), especially in the young grains. Since errors of dates were calculated based on concentrations of UO2, ThO2 and PbO, each date has a relatively large error (14-93 Ma at the 2σ confidence level).

The Mnz grain in the Grt-Opx granulite is in contact with Ep and Qz, which have LREE-carbonate-fluoride and Py in their inner cracks (Fig. 2b). The Mnz grain has an unzoned chemical composition. In the Grt- and Sil-bearing gneiss from the Mandalay area, the Mnz grains appear both in the matrix and as inclusions in Grt, and the chemical composition of the Mnz grains indicates both an homogeneous and heterogeneous internal texture (Fig. 3a). Many of the Mnz grains are chemically homogeneous within the Bt gneiss from the Mandalay area (Fig. 3b), while some grains clearly display zonal structures. Mnz grains in the Sil-Bt gneisses from the Meikthila area were found as fine-grained and homogeneous chemical compo-sitions and the Mnz grains in the mylonitic Bt gneiss from the Meikthila area show both an homogeneous, and chemically heterogeneous, internal texture (Figs. 3c and 3d).

The results of age determination indicate that the U-

Th-Pb ages of Mnz grains in the Grt-Opx granulite are less than 50 Ma. A slightly younger age of ~ 30 Ma was obtained from the Mnz grains in the Grt- and Sil-bearing gneisses from the Mandalay area (Figs. 4a and 4b). Mnz in the Bt gneiss from the Mandalay area yielded three dis-tinct age peaks at ~ 30 Ma, 80 Ma, and 110 Ma (Fig. 4c). Mnz grains in the Sil-Bt gneisses from the Meikthila area yielded an age of ~ 50 Ma (Fig. 3d). Zoned cores of Mnz

Figure 2. Photomicrographs and BSE image of metamorphic rocks from the Mogok Metamorphic Belt (a) Grt-Opx granulite from the Mogok area. (b) BSE image of Mnz occurrence in the Grt-Opx granulite. (c) Grt- and Sil-bearing gneiss (Grt-Sil-Bt gneiss) from the Mandalay area. (d) Grt- and Sil-bearing gneiss (Grt-Sil-And-Bt-Ms gneiss) from the Mandalay area. (e) Bt gneiss from the Mandalay area. (f) Mylonitic Bt gneiss from the Meikthila area.

Figure 3. Representative analyzed monazite BSE images and age results. The scale bar in the BSE images is 10 µm. Dark circles are analyzed points. Numbers in BSE images are the ages of each analyzed point. (a) Zoned Mnz grain in 2203A: the age is similar in each portion. (b) Mnz grain in 2302A, the ages show patchy rejuvenation. (c) Chemically zoned Mnz grains in 2602A: the zoned core shows an age of ~ 200 Ma. (d) Patchy chemically- zoned Mnz grain in 2602A: there is no clear age difference be-tween the light and dark portions.

187Metamorphic rocks from central Myanmar: monazite dating

in the mylonitic Bt gneiss from the Meikthila area yielded the oldest ages of ~ 200 Ma (Figs. 3c and 4e), while other grains yielded younger ages of ~ 50 Ma (Figs. 3d and 4e).

INTERPRETATION OF U-Th-Pb MONAZITE AGES

Based on the existence of the LREE-carbonate-fluoride and Py in the inner cracks, the secondary Chl in the ma-trix, and the occurrence of Mnz grains in contact with the inner cracks, it is possible to believe that the Mnz in the Grt-Opx granulite was affected by a younger deformation and fluid-infiltration event than that of 50 Ma (Eocene). The U-Th-Pb ages for Mnz grains from the three Grt- and Sil-bearing gneisses, and the Bt gneiss from the Man-dalay area, are of the Oligocene (~ 30 Ma) age. In addi-tion, Cretaceous age peaks (~ 80 Ma and 110 Ma) were determined from Bt gneiss samples from the Mandalay area. Eocene Mnz grains are dominant in Bt gneiss from the Meikthila area, which has a weak mylonitic texture, as described above, and the Eocene rims overlap several Late Triassic Mnz cores. The Eocene age was reported as being the starting age of the continent-continent collision between the Indian and Asian continents (e.g., Dewey et al., 1989). Moreover, widespread areas across Southeast Asia (e.g., the Red River Shear Zone) underwent intense shearing during the Late Oligocene to Miocene deforma-tion associated with the India-Asia collision (e.g., Leloup

et al., 1995). Therefore, the Eocene to Oligocene thermal event recorded within Mnz, might indicate a deformation and fluid-infiltration event that is related to the Indian and Asian continent collision. However, in the western part of Southeast Asia, the thermal event related to the subduc-tion and subsequent continental collision tectonic setting between the Sibumasu and Inthanon blocks (e.g., Charu-siri et al., 1993; Sone and Metcalfe, 2008; Osanai et al., 2010), has been acknowledged as occurring in the Late Triassic age. Although the Late Triassic age, which was noted from the core of Mnz from the Meikthila area, might be related to this tectonic event, petrological and geological evidence is still absent. Although the implica-tion of the Cretaceous Mnz from the Mandalay area is not known, they could be correlated with the Cretaceous magmatism in the Mandalay area (e.g., Mitchell et al., 2012).

Our findings related to the three events suggest that more geological, petrological, and geochronological in-vestigations in this region and in the neighboring terrain could open a new window in the understanding of the Late Triassic to Paleogene tectonic history in Southeast Asia.

ACKNOWLEDGMENTS

We would like to thank Kyaing Sein, Zaw Naing Oo, Zaw Win Ko, and the Myanmar Geoscience Society for the field surveys carried out in the study area. We would also like to thank two anonymous reviewers for their construc-tive reviews, and Prof. T. Tsunogae for his editorial sup-port. This work was partly supported by a Grant-in-Aid for Scientific Research (nos. 21253008 and 22244063 to Y. Osanai) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

DEPOSITORY MATERIAL

Appendixes are available online from http://japanlinkcen ter.org/DN/JST.JSTAGE/jmps/121019a.

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Manuscript received October 19, 2012Manuscript accepted April 16, 2013

Manuscript handled by Toshiaki Tsunogae